Biology Unit 3 Topic 1-6

Science

Dissolved substances

Cells need to take in useful substances and remove other substances, such as waste, in order to function effectively. Exchange of materials occurs between the cell and its surrounding environment across the cell membrane. Depending on the circumstances, this is achieved via three transport processes - diffusion, osmosis and active transport. To make exchange as efficient as possible, larger organisms have evolved specialised exchange surfaces.

Diffusion

Diffusion is the process by which particles of a substance spread out from each other, moving from a region where they are in high concentration to a region of low concentration. In the same way as a ball will roll from a high point to a low point down a gradient [gradient: Another word for 'slope'. On a graph, the gradient is defined as being the change in the y value divided by the change in the x value, and defines how steep a line is.] , particles of a substance will move down a concentration gradient [concentration gradient: The difference in the concentration of a chemical across a membrane.]  until they are evenly spread.

In order to do this, particles of a substance must be free to move. This is the case for particles of a gas or particles of a dissolved substance.

Diffusion allows substances to pass into or out of cells across the cell membrane - but they must be dissolved and there must be a concentration gradient present between the solutions on either side of the cell membrane.

Diffusion is a very important exchange process which is widely used in living organisms.

Examples of diffusion

The table below lists some examples of the process of diffusion.

Example	Substance(s) involved	Moved from (high concentration region)	Moved to (low concentration region)
Gas exchange in lungs	Oxygen	Inhaled air (inside thealveolus [alveoli:Tiny air sacs in the lungs, where gas is exchanged during breathing.] )	Blood circulating through lungs
Gas exchange in lungs	Carbon dioxide	Blood circulating through lungs	Air inside the alveolus
Gas exchange in a leaf	Carbon dioxide	Air outside the leaf	Air spaces inside the leaf, and then into leaf cells
Exchange in the small intestine	Digested food molecules, egamino acids [amino acid:Complex molecules, which form the building-blocks of proteins.]  andglucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]	Small intestine	Blood in villi capillaries [villi:Minute hair-like projections which cover the lining of the small intestine. They aid digestion by greatly enlarging the gut's surface area.]

Diffusion	Active transport
Transports dissolved substances from high to low concentration	Transports dissolved substances from low to high concentration
Requires no additional energy input	Requires energy from respiration
Does not necessarily require protein carriers in the cell membrane	Requires protein carriers in the cell membrane

Artificial ventilator	Uses	Advantages	Disadvantages
Negative pressure	Developed and used from the 1920s to treat polio sufferers	·         Effective at treating many polio [polio: An infectious viral disease that affects the nervous system and can cause paralysis.]  patients over the years	·         Patient is confined to the machine ·         The vacuum on full-body machines can affect the abdomen, leading to the pooling of blood in lower parts of the body
Positive pressure	Used extensively since the 1950s	·         Useful during operations, where surgeons need access to the body ·         Effective at ventilating the lungs	·         Long-term ventilation requires the tube to be surgically inserted into the trachea through the neck

Gas	% of inhaled air	% of exhaled air
Oxygen	21	16
Carbon dioxide	0.04	4
Nitrogen	79	79

Adaptation	Purpose
Flattened shape	Larger surface area to absorb light and carbon dioxide
Thin	Short diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  distance for carbon dioxide to diffuse into leaf cells, and oxygen to diffuse out of leaf cells
Stomata [stomata: Tiny holes in the epidermis (skin) of a leaf - usually on the undersides of leaves. They control water loss and gas exchange by openng and closing. Singular is stoma.]	Can open to allow diffusion of carbon dioxide into the leaf from the atmosphere, and the diffusion of oxygen and water vapour out of the leaf

Adaptation	Purpose
Internal air spaces in spongy mesophyll layer	Increases surface area of leaf to absorb more carbon dioxide
Guard cells around stomata	Allows the size of stomata to be adjusted (eg they close the stomata to prevent wilting)

	Advantages	Disadvantages
Biological valves	·         Do not damage red blood cells as they pass through the open valves	·         Prone to becoming hardened over the course of several years ·         For patients with long life expectancy, there is a higher chance of further operations to replace the valves (any operation carries risks)
Mechanical valves	·         Very strong and durable - able to last a lifetime	·         Damage red blood cells as they pass through the open valves ·         Require the patient to take anti-blood clotting drugs for the rest of their life ·         Some people say they can hear the valves opening and closing

Substance type	Substance	Moved from	Moved to
Nutrients	Soluble products of digestion	Small intestine	Organs of the body
Waste	Carbon dioxide	Organs of the body	Lungs
Waste	Urea	Liver	Kidneys

Factor	Description	Explanation
Light	Transpiration increases in bright light	The stomata [stomata: Tiny holes in the epidermis (skin) of a leaf - usually on the undersides of leaves. They control water loss and gas exchange by openng and closing. Singular is stoma.]  open wider to allow more carbon dioxide into the leaf for photosynthesis. More water is therefore able toevaporate [evaporate: The process in which a liquid turns into a gas.] .
Temperature	Transpiration is faster in higher temperatures	Evaporation and diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  are faster at higher temperatures.
Wind	Transpiration is faster in windy conditions	Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf.
Humidity	Transpiration is slower in humid conditions	Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.

Removal of waste and water controlRemoving waste products
Production and removal of waste products

Waste product	Why is it produced?	How is it removed?
Carbon dioxide	It is a product of aerobic respiration [aerobic respiration: Respiration that requires oxygen.]	Through the lungs when we breathe out
Urea	It is produced in the liver when excessamino acids [amino acids: Complex molecules which form the building blocks of proteins.]  are broken down	The kidneys remove it from the blood and make urine - which is temporarily stored in the bladder

Water balance
How is the water balance maintained?
The role of the kidney
Stage 1: Filtration
Stage 2: Selective reabsorption
Stage 3: The formation of urine
Kidney dialysis
How dialysis works
Dialysis summary
Kidney transplants
Precautions against rejection
Transplants versus dialysis

	Advantages	Disadvantages
Kidney transplants	·         Patients can lead a more normal life without having to watch what they eat and drink ·         Cheaper for the NHS overall	·         Must take immune-suppressant drugs which increase the risk of infection ·         Shortage of organ donors ·         Kidney only lasts 8-9 years on average ·         Any operation carries risks
Kidney dialysis	·         Available to all kidney patients (no shortage) ·         No need for immune-suppressant drugs	·         Patient must limit their salt and protein intake between dialysis sessions ·         Expensive for the NHS ·         Regular dialysis sessions – impacts on the patient’s lifestyle

Temperature ControlMaintaining body temperature
How our body maintains a constant temperature
The skin and temperature control - Higher tier
Responses to an increase in body temperature
Responses to a decrease in body temperature
Controlling temperature
Too cold
Too hotBlood Sugar ControlControlling rising blood sugar
Diabetes
Controlling Type 1 diabetes

	Advantages	Disadvantages
Injecting insulin multiple times throughout the day	·         Equipment is cheaper ·         More discrete as needles, insulin and blood glucose monitor are easy to conceal	·         Uses more insulin ·         Does not control blood glucose levels as well, leading to more swings in blood glucose levels (which can lead to health effects). ·         Requires more careful control of diet and exercise
Insulin pump	·         Allows the delivery of more precise volumes of insulin – and therefore offers better control of blood glucose level ·         Reduced risk of long-term effects of diabetes (due to better control ·         Uses less insulin ·         More flexibility over diet and exercise ·         Computer keeps accurate record of insulin usage history	·         Equipment is more expensive ·         Pump may be uncomfortable to wear and may present problems for some activities, eg contact sports ·         Users may have to do more blood glucose tests per day to identify if pump is working effectively ·         Increased amounts of scar tissue around injection sites

Controlling falling blood sugar level – Higher tier
The effect of insulin
The effect of glycogen

Air pollutant	Source	Typical effect
Smoke	Incomplete combustion of fossil fuels, especially coal.	Deposits soot on buildings and trees, causing them damage. Permeates the air - which can cause breathing problems in living creatures.
Sulfur dioxide	Combustion of fossil fuels with sulfur impurities in them, eg coal.	Contributes to acid rain. This can cause weathering of buildings, the release of toxic metals from the soil, damage to aquatic ecosystems and to forests.
Carbon dioxide	Combustion ofhydrocarbon [hydrocarbons:A group of organic compounds made up entirely of hydrogen and carbon.] fuels.	Greenhouse gas  [greenhouse gas: Carbon dioxide, methane and other gases that absorb infrared radiation in the atmosphere.]  that contributes to global warming [global warming: The gradual increase in the average temperature of the Earth.] .
Methane	Rice fields, cows,anaerobic [aerobic respiration: Respiration that requires oxygen.] decomposition of landfill waste.	Greenhouse gas that contributes to global warming.

Country	Problem	Solution
Cooler country (eg UK)	Temperatures below optimum slow the respiration rate of bacteria resulting in slower biogas production.	Bury the biogas generator or build the biogas generator with thick walls to insulate  [insulate: To help maintain the temperature by reducing heat loss.]  the generator and keep the inside warmer than the external temperature.
Hotter country (eg India)	Temperatures above optimum begin to denature [denature: Disable by changing the original qualities or nature of something.] bacterial enzymes, resulting in slower biogas production.	Bury the biogas generator in the ground. The ground helps to insulate the biogas generator to keep it cool during the day and warm at night.

Active transport

Active transport is a transport process which is used to move dissolved molecules from low concentration to high concentration, against a concentration gradient. This process requires energy fromrespiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.]  in order to take place.

Active transport is carried out by a series of protein carriers [carrier protein:A carrier protein is a protein which is responsible for transporting specific substances through the cell membrane and into the cell.]  within the cell membrane. These have a binding site, allowing a specific dissolved substance to bind to the side of the membrane where it is at a lower concentration.

Energy from respiration then changes the shape of the protein so that it releases the substance onto the other side of the membrane.

Active transport has the advantage of allowing cells to absorb dissolved substances from very dilute solutions, which is otherwise an impossible process.

Examples of active transport

·         Root hair cells in plant roots use active transport to absorb mineral ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons.]  (such as nitrates) from the soil - even though there are lower concentrations of minerals in the soil than there are within the root hair cell.

·         Small intestine villi cells use active transport alongside diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  to maximise the absorption of  [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.] glucose and other substances, eg minerals.

Differences between diffusion and active transport

The table below shows the major differences between active transport and diffusion.

Osmosis

Osmosis only applies to the movement of water into or out of a cell. The definition of osmosis is the movement of water molecules from a dilute solution (with a high proportion of water molecules) to a more concentrated solution (with a low proportion of water molecules) across a partially permeable [partially permeable: Allowing some particles to pass through but not others.]  membrane.

A partially permeable membrane allows small, soluble molecules like water to pass through it freely - but prevents larger molecules from doing so. In a cell, the cell membrane acts as a partially permeable membrane.

Osmosis experiment

The experiment below demonstrates osmosis happening:

On the left-hand side of the partially permeable membrane is pure water [pure water: Water that has been physically processed to remove impurities.] . On the right-hand side is a solution which is more concentrated, containing minerals, for example.

Water molecules move across the membrane at random in both directions, but the higher concentration of water molecules on the left-hand side means that more water moves by osmosis from left to right than moves in the opposite direction. The overall (net) effect is that water levels on the left drop, whilst they rise on the right due to the overall movement of water from left to right.

Eventually, the concentrations of water on either side of the partially permeable membrane become equal. This means that the molecules of water moving in either direction are equal, so effectively there is no overall (net) movement of water.

The effect of osmosis on cells

If a cell has a more dilute solution inside it than outside it, then the overall movement of water is out of the cell. In animal cells this would cause the cell to shrivel up, whilst in plant cells this would cause the membrane andcytoplasm [cytoplasm: The living substance inside a cell (not including the nucleus).]  to shrink away from the cell wall, causing the plant cell to becomeflaccid (limp).

Osmosis effects – plant cells

If a cell has a more concentrated solution inside it than outside it, then the overall movement of water is into the cell. In plant cells this causes the cell to begin to swell, and the cytoplasm and membrane push against the cell wall. The strong cell wall then resists further expansion, supporting the cell which becomes turgid (fully inflated).

However, animal cells do not have a cell wall so any large movement of water into the cell causes it to burst. For this reason, it is important that the concentration of water outside cells is constant.

Osmosis effects – animal cells

Organ systems

All living organisms rely on exchanges with the environment to survive. However, diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  only works efficiently if the distance over which the substances have to diffuse is small and the organism has a large surface area [surface area: The area of the surface of an organism or membrane.]  compared to its volume [volume: A measurement of the amount of three-dimensional space something takes up.] . This is the case for small organisms.

For larger, more complex organisms – which have a small surface area:volume ratio and a bigger distance from the surface to the cells inside the body - diffusion alone is insufficient to meet the needs of all cells.

As larger organisms evolved, specialised organ systems - with surfaces across which substances could be exchanged efficiently - also evolved. These specialised organ systems were needed in order to transport substances around the organisms.

Adaptations of exchange surfaces

Common features of exchange surfaces include:

·         having a large surface area for greater exchange – achieved by having a folded surface

·         having a thin exchange surface for a short diffusion distance

Animals also further maximise the efficiency of exchange by:

·         having a good blood supply due to an extensive capillary [capillary:Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins.]  network in exchange organs – this distributes the exchanged materials to all cells of the body and can help to maintain a concentration gradient [concentration gradient: The difference in the concentration of a chemical across a membrane.] 

·         maintaining ventilation of the surface (at gas exchange surfaces) through breathing - this always ensures that a concentration gradient is maintained

Exchange in the digestive system

Villi are folds within the wall of the small intestine across which digested food molecules are exchanged between the gut and the bloodstream. This exchange takes place by diffusion and active transport [active transport: When energy is used to move a chemical across a membrane, from an area of low concentration to an area of higher concentration. This occurs against the concentration gradient.] .

Absorption into bloodstream

Villi are adapted for the maximum absorption of digested food molecules because:

·         the folded villi greatly increase the surface area of the intestine

·         the villi are made of a single layer of thin cells

·         beneath the villi is an extensive blood capillary network to distribute the absorbed food molecules

For more information about the role of villi and other parts of the digestive system visit the Health section of the BBC website.

Sports drinks

During extended periods of exercise, an athlete’s body changes:

·         the athlete uses up much of the glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.] in their body during respiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.] 

·         the athlete generates heat as they respire more

·         the athlete sweats more to try to cool themselves down (this sweating results in the loss of water and mineral ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons.] , eg sodium)

Maintaining the correct balance of mineral ions is essential for cells to function efficiently and effectively. If the water and ion content of the body changes, it can cause too much water to move into or out of its cells - possibly leading to them becoming damaged.

During prolonged exercise, not only are ions and water lost, but the loss of water occurs at a faster rate than the loss of ions - which can disturb this balance and lead to cells dehydrating.

It is therefore important that athletes replace the lost water and mineral ions and replenish the glucose which has been used during respiration.

Water is only able to rehydrate the body. It does not replace the lost ions and glucose. Most soft drinks contain water, sugar and mineral ions, but not at the concentrations which are most effective at maintaining an athlete’s performance.

However, sports drinks contain water, sugar and mineral ions at levels which are most effective at maintaining performance - rehydrating the athlete as well as replacing the glucose and maintaining the correct ion/water balance for cells to function effectively. This helps the athlete to continue exercising for longer.

The effects of drinking sports drinks

Evaluating sports drinks

Sports drinks manufacturers often make claims about the performance benefits of using their branded sports drinks, but it is important that these claims are evaluated based on valid data from controlled trials [controlled trial: A study in which researchers attempt to measure the effectiveness of a drug or treatment by comparing it with a placebo (the control).]  of a large sample of athletes.

Different manufacturers put slightly different amounts of sugar and mineral ions in their sports drinks, and therefore each brand will potentially have differing effects on an athlete’s performance.

Science

Gaseous exchange in the lungs

To supply the cells of our body with a continuous supply of oxygen for respiration and to remove the carbon dioxide generated by respiration, we have evolved a specialised exchange surface for gas exchange within the breathing system. The efficiency of this system is further improved by ventilation of this exchange surface and by having an efficient blood supply - both of which maintain a suitable concentration gradient.

The lungs

The lungs are part of the breathing system which is adapted for two functions:

·         ventilation – the movement of air into and out of the lungs

·         gas exchange – the 'swapping’ of gases between the alveolar [alveolar air: The air in the alveoli.]  air and the blood

The lungs are located within the upper part of your body called the thorax. They are surrounded by the ribcage (which protects them) and in between the ribs areintercostal muscles which play a role in ventilating the lungs.

Beneath the lungs is a muscular sheet called the diaphragm. This separates the lungs from the abdomen [abdomen: In humans, the lower part of the torso (body). In insects, the segment of the body furthest from the head.]  of the body and also plays a role in ventilating the lungs.

Diagram of the lungs

Within the lungs is a network of tubes through which air is able to pass. Air is firstly warmed, moistened and filtered as it travels through the mouth and nasal passages. It then passes through the trachea [trachea: The windpipe or tube from the back of the mouth to the top of the lungs.]  and down one of the twobronchi [bronchi: The plural of 'bronchus'. The bronchi are the two major air tubes in the lungs.]  and into one of the lungs.

After travelling into the many bronchioles [bronchioles: The many small, branching tubules into which the bronchi subdivide.] , it finally passes into some of the millions of tiny sacs called alveoli, which have the specialised surfaces for gas exchange.

Ventilation

When you inhale:

1.   The intercostal muscles [intercostal muscle: Muscles between the ribs which raise the ribcage by contracting and lower it by relaxing.]  contract, expanding the ribcage outwards and upwards.

2.   The diaphragm [diaphragm: A large sheet of muscle that separates the lungs from the abdominal cavity, and that is pulled down to cause inhalation.]  contracts, pulling downwards to increase the volume of the chest.

3.   Pressure inside the chest is lowered and air is sucked into the lungs.

When you exhale:

1.   The intercostal muscles relax, the ribcage drops inwards and downwards.

2.   The diaphragm relaxes, moving back upwards, decreasing the volume of the chest.

3.   Pressure inside the chest increases and air is forced out.

Mechanical ventilation

When a person stops breathing on their own, mechanical ventilation can be used until the patient is able to recover and again breathe independently. This is done by machines called ventilators - which fall into two main types:

1.   Negative pressure ventilators - the patient is placed in an airtight machine from the neck down, and a vacuum is created around thethorax [thorax: The chest area of a human, or the middle segment of an insect's body.] . This creates a negative pressure, which leads to the expansion of the thorax and a decrease in pressure. As a result, air is drawn into the lungs. As the vacuum is released, the elasticity of the lungs, diaphragm and chest wall cause exhalation.

2.   Positive pressure ventilators - air is forced into the lungs through a tube which is inserted into the trachea [trachea: The windpipe or tube from the back of the mouth to the top of the lungs.] . As the ventilator pumps air in, the lungs inflate. When the ventilator stops, the elasticity of the lungs, diaphragm and chest wall cause exhalation.

The table below lists some of the pros and cons of using artificial ventilators.

Gas exchange

Within the alveoli [alveoli: Tiny air sacs in the lungs, where gas is exchanged during breathing.] , an exchange of gases takes place between the gases inside the alveoli and the blood.

Blood arriving in the alveoli has a higher carbon dioxide concentration which is produced during respiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.]  by the body’s cells. However, the air in the alveoli has a much lower concentration of carbon dioxide, meaning there is a concentration gradient [concentration gradient: The difference in the concentration of a chemical across a membrane.]  which allows carbon dioxide to diffuse [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  out of the blood and into the alveolar air.

Similarly, blood arriving in the alveoli has a lower oxygen concentration (as it has been used for respiration by the body’s cells), while the air in the alveoli has a higher oxygen concentration. Therefore, oxygen moves into the blood by diffusion and combines with the haemoglobin [haemoglobin: The red protein found in red blood cells that transports oxygen round the body.]  in red blood cells to form oxyhaemoglobin [oxyhaemoglobin: A chemical formed when haemoglobin bonds to oxygen.] .

This table shows the differences (approximate figures) in the composition of inhaled and exhaled air.

Adaptations of the alveoli

To maximise the efficiency of gas exchange, the alveoli have several adaptations:

·         They are folded, providing a much greater surface area [surface area: The area of the surface of an organism or membrane.]  for gas exchange to occur.

·         The walls of the alveoli are only one cell thick. This makes the exchange surface very thin - shortening the diffusion distance across which gases have to move.

·         Each alveolus is surrounded by blood capillaries [capillaries: Extremely narrow tubes, which carry blood around a body's tissues.]  which ensure a good blood supply. This is important as the blood is constantly taking oxygen away and bringing in more carbon dioxide - which helps to maintain the maximum concentration gradient between the blood and the air in the alveoli.

·         Each alveolus is ventilated [ventilation: Breathing.] , removing waste carbon dioxide and replenishing oxygen levels in the alveolar air. This also helps to maintain the maximum concentration gradient between the blood and the air in the alveoli.

Science

Exchange system in plants

Like all living organisms, plants must exchange materials with their environment. These exchanges include absorbing water and minerals from the soil and absorbing carbon dioxide from the air for photosynthesis. Therefore plants have specialised exchange surfaces which maximise the efficiency of these exchanges.

Exchanges in the roots

The role of the roots is to absorb water from the soil by osmosis [osmosis: The net movement of water molecules across a partially-permeable membrane from a region of low solute concentration to a region of high solute concentration.] and dissolve mineral ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons.]  from the soil by active transport [active transport: When energy is used to move a chemical across a membrane, from an area of low concentration to an area of higher concentration. This occurs against the concentration gradient.] .

The mineral ions are transported around the plant where they serve a variety of functions, whilst the water is transported to be used as a reactant inphotosynthesis [photosynthesis: A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.] , as well as to cool the leaves by evaporation [evaporate: The process in which a liquid turns into a gas.]  and support the leaves and shoots by keeping cells rigid [rigid:Inflexible. Unable to bend or be forced out of shape.] .

To maximise the efficiency of absorption, roots have specialised cells calledroot hair cells which are found just behind the tip of the root.

Root hair cells have several adaptations:

·         the tube-like protrusion provides a greater surface area [surface area: The area of the surface of an organism or membrane.]  across which water and mineral ions can be exchanged

·         the tube-like protrusion can penetrate between soil particles, reducing the distance across which water and mineral ions must move

·         the root hair cell contains lots of mitochondria [mitochondria: Structures in the cytoplasm of all cells where respiration takes place (singular is mitochondrion).] , which release energy from glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  during respiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.]  in order to provide the energy needed for active transport

Diffusion in the leaves

One of the main functions of leaves is as a major site of photosynthesis – to produce glucose from water and carbon dioxide with the input of energy from sunlight.

To perform this function effectively, leaves are adapted to maximising the absorption of carbon dioxide and sunlight.

The internal structure of leaves is also adapted to maximise the efficiency of exchange.

Internal structure of a leaf

Note that the guard cells open and close the stomata depending upon the amount of potassium ions present in the fluid in the cell. The more potassium ions that are present, the more the cells become turgid (swollen) and the bigger the opening.

The size of the opening is used by the plant to control the rate of transpiration and therefore limit the levels of moisture in the leaf which prevents it from wilting.

Science

The blood system

Even with specialised exchange surfaces, the size of larger organisms means that they must still have a system to transport substances between the exchange surface and the cells of the body. In humans and large animals, this is achieved through the circulatory system.

The circulatory system

The circulatory system consists of:

·         the heart - which is the muscular pump that keeps the blood moving around the body

·         the blood - which carries the substances around the body

·         the arteries - which carry blood away from the heart

·         the veins - which return blood to the heart

·         the capillaries - which are tiny blood vessels that are close to the body’s cells where exchanges can happen

As the diagram shows, humans have a double circulatory system. This means that there is one circulation solely for the lungs (in order to oxygenate the blood) and one for the rest of the body. On its journey around the body, blood must go through both circulations.

The circulatory system

Note that oxygenated blood is shown in red and deoxygenated blood in blue.

The heart

The heart is the organ responsible for pumping blood around the circulatory system. The walls of the heart are made from muscle tissue which cancontract [muscle contraction: A shortening or tensing of the muscle.]  to put the blood under pressure, forcing its movement.

The heart consists of two separate sides, and the blood does not mix between the two. The right-hand side of the heart only pumps blood to the lungs to pick up oxygen, whilst the left-hand side of the heart pumps blood to the rest of the body. This double circulation allows the oxygenated blood to become re-pressurised before being sent around the body.

Each side of the heart is made up of two chambers - meaning that there are four in total. The atrium is at the top and the ventricle is at the bottom. Blood enters the heart through a vein [vein: Thin-walled, valved tubes which carry blood back to the heart.]  and collects in the left atrium (remembering that you always describe the heart from the perspective you view it from).

The first part of a heart beat causes the atrium wall to contract, which puts the blood under pressure - forcing it through a one-way valve [valve: Structures containing a flap or flaps to ensure one-way flow of liquid.]  into the ventricle. The second part of a heart beat then causes the muscular wall of the ventricle to contract - forcing the blood out through an artery [artery: Thick-walled muscular tube, which carries blood away from the heart.]  under pressure.

The one-way valve prevents the blood flowing back to the atrium. The artery also contains a valve to stop blood flowing back to the ventricle when the ventricle relaxes.

The passage of blood through the heart

Deoxygenated blood arrives at the left-hand side of the heart:

1.   It enters the heart through the vena cava [vena cava: The major vein that carries deoxygenated blood to the right side of the heart from the body tissues.] .

2.   Blood flows into the right atrium.

3.   Blood is pumped into the right ventricle.

4.   Blood is pumped out of the heart, along the pulmonary artery [pulmonary artery: The major blood vessel leaving the right side of the heart, carrying deoxygenated blood to the lungs.] , to the lungs.

Oxygenated blood arrives at the right-hand side of the heart:

1.   It enters the heart through the pulmonary vein [pulmonary vein: The major blood vessel returning to the left side of the heart from the lungs, carrying oxygenated blood.] .

2.   Blood flows into the left atrium.

3.   Blood is pumped into the left ventricle.

4.   Blood is pumped out of the heart, along the aorta [aorta: The major artery that leaves the left side of the heart, carrying oxygenated blood to the body tissues.] , to the rest of the body.

Replacement heart valves and heart transplants

Artificial heart valves

Occasionally, some people’s heart valves [valve: Structures containing a flap or flaps to ensure one-way flow of liquid.]  become stiff or leaky, which prevents the valves from functioning properly to prevent the backflow of blood. In these circumstances, it is possible to replace the faulty valves with either valves from a biological source (eg human donor or animal) or by using mechanical (man-made) valves.

Artificial heart valve

Both types of artificial heart valve have advantages and disadvantages. The table below shows some of the main pros and cons.

Artificial hearts

In cases where a patient has severe heart disease/damage/failure, a heart transplant is necessary. However, there is often a shortage of compatible heart donors available - meaning that many people die while on the waiting list.

Artificial (man-made) hearts provide an alternative as they replicate the function of the heart. But current designs have not proved to be very successful in the long term, and are prone to blood clotting within them. Therefore, artificial hearts are only used as a short-term measure to keep patients alive until a biological donor heart can be found.

Artificial heart

Arteries and veins

Blood flows from the heart to the body’s other organs through arteries [artery:Thick-walled muscular tube, which carries blood away from the heart.] . In the organs, the arteries repeatedly branch into a network of smaller blood vessels called capillaries. These then branch back together to form veins [veins: Thin-walled, valved tubes which carry blood back to the heart.] , which then carry blood back to the heart.

Remember it like this:

Artery – carries blood Away from the heart

VeIN – carries blood back INto the heart

With both arteries and veins, their structure is related to their function.

Arteries and veins

Arteries

Blood in the arteries is under high pressure generated by the heart. The arteries therefore have:

·         thick walls - to resist the high pressure of the blood

·         a thick layer of elastic fibres – to allow the artery to stretch when a surge of blood passes through it, and then recoil in between heart beats to maintain blood pressure

·         a thick layer of muscle within the wall – to allow blood to be diverted to where it is needed in the body

Veins

Blood in the veins is under less pressure. The veins therefore have:

·         thin walls as they have blood with a lower pressure flowing through them

·         one-way valves [valve: Structures containing a flap or flaps to ensure one-way flow of liquid.]  in them to prevent blood flowing back in the opposite direction

Cross-section of a vein

Stents

In order to keep beating, the heart muscle has its own artery called thecoronary artery, which supplies the heart with glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  and oxygen. For patients who have heart disease, arteries can become narrower due to the build-up of fatty deposits within the wall of the artery. This has the effect of narrowing the lumen [lumen: The central cavity of a hollow structure in an organism or cell.]  of the artery, reducing the amount of oxygenated blood that can be supplied to the heart muscle.

Stents are metal grids which can be inserted into an artery to maintain blood flow by keeping the artery open.

To insert a stent, a catheter [catheter: A thin tube that can be inserted into a body cavity, duct, or vessel to treat diseases or perform a surgical procedure.] with a balloon attached to it is inserted into a blood vessel in the leg. The balloon has the metal stent on it. The catheter is directed to the coronary artery. When the narrowed section of artery is found, the balloon is inflated which causes the stent to expand, and it becomes lodged in the artery.

The stent then acts to keep the artery open so that the heart continues to receive enough oxygen to function effectively.

Stents are good alternatives to more risky operations, like by-pass surgery [by-pass surgery: Surgery designed to by-pass (get around) the narrowed sections of coronary arteries, to improve blood supply to the heart.] , providing the patient’s heart disease is not too serious. However, fatty deposits may build up on the stent over time - meaning that blood flow to the heart muscle may be reduced again.

Capillaries

Capillaries are the smallest type of blood vessel, and are adapted to allow the effective exchange of substances between the blood and the tissues [tissue:Group of cells of the same type doing a particular job, eg the blood (a liquid tissue).]  of the body.

Capillaries

Capillaries are made of thin cells, meaning that some parts of the blood can easily leave the capillary, bathing the cells in a fluid known as tissue fluid.

Useful substances within the tissue fluid - including glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.] , oxygen and amino acids [amino acids: Complex molecules which form the building blocks of proteins.]  - can then diffuse [diffuse: When particles spread out from a region of higher concentration to a region of lower concentration.]  into the cells down a concentration gradient [concentration gradient: The difference in the concentration of a chemical across a membrane.] . The concentration gradient is always maintained as the useful substances are constantly being used up by the cell.

Waste substances generated by the cell diffuse out of the cell, and back into the tissue fluid. Most of the tissue fluid is then reabsorbed back into the blood, and with it the waste substances – such as carbon dioxide and urea [urea: A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine.]  – which are taken away to be excreted [excreted:Discharged as waste.] .

A concentration gradient is always maintained as the cell constantly generates more waste substances, and the blood constantly takes them away.

Science

Blood

Blood is made of four constituent parts - red blood cells, white blood cells, platelets and plasma. Each part plays a vital role in ensuring that blood can meet its two primary roles, to transport substances around our body and to defend against infection by potential pathogens.

Blood

Blood is used to transport materials around the body and to protect against disease.

Blood is a tissue which includes liquid, cells, cell fragments andsolutes [solute: A solute is the material that dissolves in a solvent to form a solution.] .

Red blood cells

Red blood cells are tiny, nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom.] -free cells which carry oxygen from the lungs to tissues.

Oxygen transport is efficient because:

·         there are huge numbers of red blood cells

·         the cells are tiny so they can pass through narrow capillaries [capillary:Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins.] 

·         the cells have a flattened disc shape to increase surface area - allowing rapid diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  of oxygen

·         the cells contain haemoglobin - which transports oxygen and carbon dioxide around the body

Blood appears bright red when oxygenated and dark red when deoxygenated.

In oxygen-rich environments (ie the lungs), haemoglobin combines with oxygen to form oxyhaemoglobin. In low-oxygen environments (such as body cells), oxyhaemoglobin releases the oxygen to become haemoglobin again.

This process is summarised here:

White blood cells

Different types of white blood cells exist. Some white blood cells can engulfbacteria [bacterium: A type of single-celled microorganism.]  and otherpathogens [pathogen: Microorganism that can cause disease.]  byphagocytosis [phagocytosis: The process of the ingestion of bacteria or other material by phagocytes.] . They can change shape easily and produceenzymes [enzyme: Proteins which catalyse or speed up chemical reactions inside our bodies. Enzymes are a vital in chemical digestion of food in the gut.] which digest the pathogens.

Other types of white blood cell secrete antibodies [antibody: A protein produced by the body's immune system that attacks foreign organisms (antigens) that get into the body.]  and antitoxins to help destroy pathogens.

Blood plasma

Plasma is a straw-coloured liquid which makes up about 55 per cent of blood. It transports dissolved substances around the body. These include:

·         hormones [hormone: Chemical messengers produced in cells or glands and carried by the blood to specific organs in the body.] 

·         antibodies

·         nutrients - such as water, glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.] ,amino acids [amino acids: Complex molecules which form the building blocks of proteins.] , minerals and vitamins

·         waste substances - such as carbon dioxide and urea [urea: A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine.] 

The table below gives more detail about the transportation of nutrients and waste products in plasma.

Platelets

Platelets are small fragments of cells, but they do not possess a nucleus. They are involved in the process of forming clots at sites where there is a wound, eg a cut or graze.

Science

Transport systems and processes in plants

Plants have two systems for the transportation of substances - using two different types of transport tissue. Xylem transports water and solutes from the roots to the leaves, while phloem transports food from the leaves to the rest of the plant. Transpiration is the process by which water evaporates from the leaves, which results in more water being drawn up from the roots. Plants have adaptations to reduce excessive water loss.

Xylem and phloem

Plants have two transport systems to move food, water and minerals through their roots, stems and leaves. These systems use continuous tubes called xylem and phloem, and together they are known as vascular bundles.

Plant root

Plant root

Xylem

Xylem vessels are involved in the movement of water through a plant - from its roots to its leaves via the stem.

During this process:

1.   Water is absorbed from the soil through root hair cells.

2.   Water moves by osmosis [osmosis: The net movement of water molecules across a partially-permeable membrane from a region of low solute concentration to a region of high solute concentration.]  from root cell to root cell until it reaches the xylem.

3.   It is transported through the xylem vessels up the stem to the leaves.

4.   It evaporates [evaporate: The process in which a liquid turns into a gas.] from the leaves (transpiration).

The xylem tubes are made from dead xylem cells which have the cell walls removed at the end of the cells, forming tubes through which the water and dissolved mineral ions can flow. The rest of the xylem cell has a thick, reinforced cell wall which provides strength.

Plant root

Phloem

Phloem vessels are involved in translocation. Dissolved sugars, produced during photosynthesis [photosynthesis: A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.] , and other soluble food molecules are moved from the leaves to growing tissues (eg the tips of the roots and shoots) and storage tissues (eg in the roots).

In contrast to xylem, phloem consists of columns of living cells. The cell walls of these cells do not completely break down, but instead form small holes at the ends of the cell. The ends of the cell are referred to as sieve plates. The connection of phloem cells effectively forms a tube which allows dissolved sugars to be transported.

Plant root

Transpiration

Water on the surface of spongy and palisade cells (inside the leaf)evaporates [evaporate: The process in which a liquid turns into a gas.]  and then diffuses [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.]  out of the leaf. This is called transpiration.

Leaf

More water is drawn out of the xylem cells inside the leaf to replace what has been lost. Water molecules have a tendency to stick together – so as water leaves the xylem to enter the leaf, more water is pulled up behind it. This produces a continuous flow of water and dissolved minerals moving up the xylem tube from the roots, up the stem, and into the leaves. This is known as the transpiration stream.

Movement of water through the roots

The movement of water up the xylem means more water must be drawn in through the roots from the soil. To do this, water passes from root cell to root cell by osmosis [osmosis: The net movement of water molecules across a partially-permeable membrane from a region of low solute concentration to a region of high solute concentration.] .

The pathway of water across a root

As water moves into the root hair cell down the concentration gradient [concentration gradient: The difference in the concentration of a chemical across a membrane.] , the solution inside the root hair cell becomes more dilute. This means that there is now a concentration gradient between the root hair cell and adjacent [adjacent: Next to or adjoining something else.] root cells, so water moves from the root hair cell and into the adjacent cells by osmosis.

This pattern continues until the water reaches the xylem vessel within the root - where it enters the xylem to replace the water which has been drawn up the stem.

Transpiration is part of the water cycle, and it is the loss of water vapor from parts of the plant

Factors that affect transpiration rate

Factors that speed up transpiration will also increase the rate of water uptake from the soil. If the loss of water is faster than the rate at which it is being replaced by the roots, then plants can slow down the transpiration rate by closing some of their stomata. This is regulated by guard cells, which lie on either side of a stoma [stoma: A hole in the outer surface of a living thing. For example, a hole in the underside of a leaf to allow gaseous exchange, or a hole surgically added to the trachea through the front of the neck to enable breathing despite damage to the throat.] .

Plants can slow down the transpiration rate by closing some of their stomata

If the guard cells are turgid [turgid: Having turgor; enlarged and swollen with water.] , then they curve forming ‘sausage-shaped’ structures with a hole between them. This is the stoma.

However, if the guard cells are flaccid [flaccid: Lacking turgor. Lacking in stiffness or strength.]  due to water loss, they shrivel up and come closer together, closing the stoma. This is turn reduces the water loss due to transpiration, and can prevent the plant from wilting.

Science

The conditions inside our body must be very carefully controlled if the body is to function effectively. Waste is constantly being generated in the body and must be removed in order to stop waste levels becoming toxic. Water and mineral ion content must also be kept constant for our cells to work effectively. This is the role of the kidneys. Those who suffer from kidney failure cannot control their water and mineral ion levels, and must therefore undergo kidney dialysis or have a kidney transplant.

Waste products are constantly being produced by the body and must therefore be excreted.  [excreted: Discharged as waste.]  If they are not, they will increase in concentration and may interfere with chemical reactions or damage cells. Waste products that must be removed include carbon dioxide and urea.

Our bodies take in water from food and drinks. We even get some water when we respire [respire: To engage in respiration, the energy-producing process inside living cells.]  by burning glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  to release energy. We lose water in sweat, faeces, urine and when we breathe out. On a cold day you can see this water as it condenses [condenses:Condensation is a change of state in which gas becomes liquid by cooling.]  into vapour.

For the cells of our body to work properly, it is important that the water and mineral ion content in our body is maintained at the correct level. This is an example of homeostasis [homeostasis: The maintenance of a constant internal environment inside a living organism.] . If the water and ion content was to change, this would cause too much water to move into or out of cells - leading to them becoming damaged.

Our body must maintain a balance between the water we take in and the water we lose. This is done by the kidneys.

Our body must maintain a balance between the water we take in and the water we lose

The kidneys maintain our water balance by producing urine of different concentrations.

When the water level of our blood plasma [plasma: Liquid, non-cellular part of the blood.]  is low, more water is reabsorbed back into the blood and the urine becomes more concentrated. When the water level of our blood plasma is high, less water is reabsorbed back into the blood and our urine is more dilute.

The level of water in the blood plasma can vary depending on:

·         External temperature - when it is hot, we sweat more and lose water, which makes the blood plasma more concentrated.

·         Amount of exercise - if we exercise, we get hot and increase our sweating, so we lose more water and the blood plasma becomes more concentrated.

·         Fluid intake - the more we drink, the more we dilute the blood plasma. The kidneys respond by producing more dilute urine to get rid of the excess water.

·         Salt intake - salt makes the plasma more concentrated. This makes us thirsty, and we drink more water until the excess salt has beenexcreted [excreted: Discharged as waste.]  by the kidneys.

Each kidney contains over one million microscopic filtering units callednephrons [nephron: Filtration unit of the kidney, also called a kidney tubule.] . Each nephron is made of a tubule and is responsible for ‘cleaning’ the blood by removing urea [urea: A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine.]  and excess water and mineral ions.

How the kidney works

This process takes place in stages:

As blood passes through the capillary  [capillary: Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins.]  at the start of the nephron, small molecules are filtered out and pass into the nephron [nephron: Filtration unit of the kidney, also called a kidney tubule.]  tubule. These small molecules include glucose, urea, ions andwater. However, large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood.

Having filtered out small molecules from the blood - many of which are essential to the body - the kidneys must reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine. Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.

The reabsorbed molecules include:

·         all of the glucose which was originally filtered out

·         as much water as the body needs to maintain a constant water level in the blood plasma

·         as many ions as the body needs to maintain a constant balance of water and mineral ions in the blood plasma

The reabsorption of water takes place by osmosis. [osmosis: The net movement of water molecules across a partially-permeable membrane from a region of low solute concentration to a region of high solute concentration.]  The reabsorption of glucose and mineral ions - from the nephron to the blood capillary - takes place by active transport.  [active transport: When energy is used to move a chemical across a membrane, from an area of low concentration to an area of higher concentration. This occurs against the concentration gradient.] 

The cells which make up the wall of the nephron are adapted by having a folded membrane (providing a large surface area [surface area: The area of the surface of an organism or membrane.] ) and a large number ofmitochondria [mitochondria: Structures in the cytoplasm of all cells where respiration takes place (singular is mitochondrion).]  (to supply the energy for active transport).

The molecules which are not selectively reabsorbed (the urea and excess water and ions) continue along the nephron tubule as urine . This eventually passes down to the bladder.

In carrying out these processes, the kidney is able to fulfil its functions of regulating the water and ion balance of the blood plasma, as well as keeping the level of urea low.

Kidney failure has serious consequences as it means that the water andion [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.]  balance cannot be regulated, and the levels of toxic urea [urea: A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine.]  build up in the body. This would ultimately be fatal if not treated.

One method of treatment is kidney dialysis. In this procedure, patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and ion balance of the blood.

‘Dirty’ blood (high in urea) is taken from a blood vessel in the arm, mixed with blood thinners to prevent clotting, and pumped into the machine. Inside the machine - separated by a partially permeable [partially permeable: Allowing some particles to pass through but not others.]  membrane the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists.

How dialysis works

Dialysis fluid contains:

·         a glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  concentration similar to a normal level in the blood

·         a concentration of ions similar to that found in normal bloodplasma [plasma: Liquid, non-cellular part of the blood.] 

·         no urea

As the dialysis fluid has no urea in it, there is a large concentration gradient - meaning that urea moves across the partially permeable membrane, from the blood to the dialysis fluid, by diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.] .

As the dialysis fluid contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists.

And, as the dialysis fluid contains an ion concentration similar to the ideal blood plasma concentration, movement of ions across the membrane only occurs where there is an imbalance.

·         If the patient’s blood is too low in ions , they will diffuse from the dialysis fluid into the blood, restoring the ideal level in the blood.

·         If the patient’s blood is too high in ions , the excess ions will diffuse from the blood to the dialysis fluid.

The overall effect of this is that the blood leaving the machine and returning into the patient’s arm will have:

·         greatly reduced levels of urea – it is ‘cleaned blood’

·         no overall change in blood glucose levels

·         the correct water and ion balance maintained or restored (with only excess ions removed)

Kidney dialysis requires highly specialised and expensive machinery. The patient must be connected to this machinery 2-3 times a week for periods (on average) of between 4-6 hours at a time.

As the filtration  [filtration: Method used to separate an insoluble solid from a liquid.]  only works when they are connected, kidney patients must monitor their diet carefully in between dialysis sessions. They need to avoid eating foods with a high salt content or a high protein content as excess amino acids are broken down into urea.

So although dialysis is a life-saving treatment, it does have a significant effect on a person’s lifestyle.

Kidney transplantation is an alternative method for treating kidney failure. This procedure involves implanting a kidney from an organ donor [donor: An organism that provides something.]  into the patient’s body to replace the damaged kidney.

As with all cells, the donor kidney cells will have protein antigens on their surface. Antigens are unique to each of us (with the exception of identical twins), and allow our body to identify our own cells from those of potentialpathogens. [pathogen: Microorganism that can cause disease.] 

Differences in the antigens of the donor kidney cells and those of the patient receiving the transplant would mean that the patient’s immune system would quickly form antibodies [antibody: A protein produced by the body's immune system that attacks foreign organisms (antigens) that get into the body.] against the kidney cell antigens, and would ultimately destroy the kidney. This is known as organ rejection.

Two precautions can be taken to reduce organ rejection:

1.   Tissue typing - only giving the kidney to patients who have antigens that are very similar to the antigens of the donor kidney. This can lead to long waits for a transplant for many kidney patients while compatible donors become available - during which time patients must undergo dialysis.

2.   Immuno-suppressant drugs – these drugs must be taken by transplant patients for the rest of their lives. They suppress the immune system, greatly reducing the immune response against the donor kidney. The negative effect of this is that it also suppresses the immune response against pathogens which enter the body, increasing the risk of getting infections.

Even with these two precautions, most donor kidneys will only survive for an average period of 8-9 years before the patient will require a further transplant or a return to dialysis.

The table below shows some of the pros and cons for both dialysis and kidney transplants

Science

Our body temperature must be controlled within a very narrow range so that our body can function properly. A constant core temperature of around 37ºC needs to be maintained. The thermoregulatory centre of the brain triggers changes in effectors, such as sweat glands and muscles, in order to constantly balance our temperature gains and temperature losses.

Temperature control is the process of keeping the body at a constant core temperature close to 37°C.

Our body can only stay at a constant temperature if the heat we generate is balanced and equal to the heat we lose.

Although our core temperature must be close to 37ºC , our fingers and toes can be colder. This is because energy is transferred from the blood as it travels to our fingers and toes.

The warm thermogram (l) shows the body at normal temperature 37°C (red) - the extremities are cooler (peach and pink areas). The cool thermogram (r) illustrates how the body diverts heat to the core organs to aid survival - the extremities are the coldest areas below 25°C (dark blue).

Temperature receptors in the skin detect changes in the external temperature.Sensoryand relay neurones [neurones: Nerve cells. They carry an electrical message or impulse when they are stimulated.]  transmit this information asimpulses [impulse: A nervous impulse is a signal that is transmitted along a neuron or series of neurons.]  to the thermoregulatory centre of the brain – the area of the brain responsible for monitoring and controlling temperature.

The thermoregulatory centre also has temperature receptors which detect changes in the temperature of the blood flowing through the brain.

In the event of a change in temperature away from 37oC, the thermoregulatory centre sends electrical impulses to effectors [effectors: organs which have an effect when stimulated (eg muscles or glands)]  (predominantly in the skin) which bring about responses that correct the temperature back to 37oC.

When the body is too cold:

·         The blood vessels supplying the skin capillaries [capillary: Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins.]  constrict [constrict: To get narrow.] , causing less blood to flow nearer the surface of the skin, the skin to become pale in appearance, and a reduction of heat loss.

·         The body shivers - the twitching of muscles generates additional heat as their contraction  [muscle contraction: A shortening or tensing of the muscle.]  causes the muscles to respire  [respire: To engage in respiration, the energy-producing process inside living cells.]  thus releasing energy to warm the body.

When the body is too hot:

·         The blood vessels supplying the skin capillaries dilate [dilate: Widened or expanded.]  causing more blood to flow nearer the surface of the skin, the skin to become red in appearance, and an increase in heat loss.

·         The body sweats - which increases heat loss due to the large amount of heat energy required to evaporate [evaporate: The process in which a liquid turns into a gas.]  the water.

Note that we sweat more in hot conditions, so more water is lost from the body. This water must be replaced through food or drink to maintain the balance of water in the body. Ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons.]  such as sodium ions and chloride ions are also lost when we sweat. They must be replaced through food and drink.

1.   If the body temperature rises, the thermoregulatory centre’s receptors detect this and coordinate responses which lower the temperature back to 37oC.

2.   Sweat glands secrete sweat onto the skin. The evaporation [evaporation:The process in which a liquid turns into a gas]  of sweat requires heat energy, which in turn cools the skin down.

3.   Vasodilation occurs – the muscles in the wall of the blood vessels supplying the skin capillaries relax causing the blood vessel to dilate. This increases the flow of blood into the capillaries and allows more blood to flow near the surface of the skin. This in turn increases the amount of heat lost by radiation and results in the skin appearing red and flushed.

4.   Vasodilation occurs – the muscles in the wall of the blood vessels supplying the skin capillaries relax causing the blood vessel to dilate  [dilate:Widened or expanded.] . This increases the flow of blood into thecapillaries [capillaries: Extremely narrow tubes, which carry blood around a body's tissues.]  and allows more blood to flow near the surface of the skin. This in turn increases the amount of heat lost by radiation [radiation:Energy carried by particles from a radioactive substance, or spreading out from a source.]  and results in the skin appearing red and flushed.

1.   If the body temperature falls, the thermoregulatory centre’s receptors detect this and coordinate responses which raise the temperature back to 37oC.

2.   To do this, electrical impulses are sent, via relay and motor neurones [neurones: Nerve cells. They carry an electrical message or impulse when they are stimulated.] , to effectors [effectors: organs which have an effect when stimulated (eg muscles or glands)]  effectors in the skin and muscles. This causes muscles attached to our skeleton to start to shiver. Shivering - the rapid contraction of muscles - requires muscles to increase the rate ofrespiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.] . This increase in respiration generates more waste heat to warm the body back up.

3.   Vasoconstriction occurs – the muscles in the wall of the blood vessels supplying the skin capillaries contract causing the blood vessel toconstrict [constrict: To get narrow.] . This reduces the flow of blood into the capillaries and allows less blood to flow near the surface of the skin. This in turn decreases the amount of heat lost by radiation and results in the skin appearing pale.

Note that the capillaries themselves do not constrict/dilate – it is the blood vessels supplying the capillaries that do this. Nor do the blood vessels move closer to/further from the skin surface. These are two common mistakes made in exams.

A - Hair muscles pull hairs on end.

D - Hair muscles relax. Hairs lie flat so heat can escape.

B - Erect hairs trap air

E - Sweat secreted by sweat glands. Cools skin by evaporation.

C - Blood flow in capillaries decreases.

F - Blood flow in capillaries increases.

Science

The concentration of glucose in our blood is important and must be carefully regulated. This is done by the pancreas, which releases hormones that regulate the usage and storage of glucose by cells. Type 1 diabetics are unable to make sufficient quantities of one of these hormones – insulin - and must therefore control their blood sugar levels by injecting insulin, as well as by carefully controlling their diet and exercise levels.

It is important that blood glucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  level is kept within a narrow range due to its importance as an energy source forrespiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.]  - but also because of the effects it could have in causing the movement of water into and out of cells by osmosis [osmosis: The net movement of water molecules across a partially-permeable membrane from a region of low solute concentration to a region of high solute concentration.] 

Having eaten a meal containing sugars or starch [starch: A type of carbohydrate. Plants can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose when it is needed for respiration.]  (eg sweets, potatoes, bread, rice or pasta), the starch and large sugars are digested down into glucose and absorbed across the small intestine wall into the bloodstream. This triggers a rise in blood glucose concentration.

The pancreas [pancreas: large gland located in the abdomen near the stomach which produces digestive enzymes and the hormone insulin]  monitors and controls the concentration of glucose in the blood. In response to an increase in blood glucose level above the normal level, the pancreas produces ahormone [hormone: Chemical messengers produced in cells or glands and carried by the blood to specific organs in the body.]  called insulin which is released into the bloodstream.

Insulin causes glucose to move from the blood into cells, where it is either used for respiration or stored in liver and muscle cells as glycogen. [glycogen: The storage form of glucose in animal cells.]  The effect of this is to lower the blood glucose concentration back to normal.

The animation below shows how this works.

There are two main types of diabetes:

·         Type 1 which usually develops during childhood

·         Type 2 which is usually develops in later life

This syllabus focuses on Type 1 diabetes - which is caused when the pancreasdoes not produce enough insulin. [insulin: A hormone that regulates the level of sugar in the blood. It is produced in the Islets of Langerhans in the Pancreas.] The body is therefore unable to lower blood sugar level when it rises too high.

Sufferers of Type 1 diabetes can help to control their blood glucose  [glucose:A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  level by being careful with their diet (eating foods which will not cause big spikes in their blood sugar level) and by exercising (which can lower blood glucose levels due to increased respiration in the muscles).

However, Type 1 diabetics must also inject insulin to control their blood glucose level. This requires a person to conduct a blood test to provide a reading of their blood glucose level (using a blood glucose meter), from which they can then work out the dose of insulin they are required to inject.

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Traditionally, diabetics have had to inject themselves with multiple injections of insulin throughout the day to try to regulate their blood sugar level.

However, some diabetics now wear an insulin pump. This supplies insulin continuously at low levels and can be programmed to adjust the supply at meal times or times of exercise.

The table below shows some of the advantages and disadvantages of each method.

Read on if you're taking the higher paper.

Following periods of exercise, or when you have not eaten for a while, the bloodglucose [glucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration.]  level might fall below a normal level.

The pancreas detects the fall in the blood glucose level and releases another hormone, glucagon. This causes the cells in the liver to turn some of the stored [glycogen: The storage form of glucose in animal cells.]  back into glucose which can then be released into the blood. The blood sugar levels will then rise back to a normal level.

Science

Waste from human activity

The rapid increase in the human population and improvements in living standards during recent years have resulted in an increased demand for land, energy and resources. It has also lead to greater quantities of waste being generated, which, in turn, has led to the pollution of land, water and air. This pollution has changed the environment in many eco-systems, making it harder for many species to survive.

Human population

Like all living things, humans exploit their surroundings for resources. Before the beginning of agriculture - around 10,000 years ago - small groups of humans wandered across large areas, hunting and gathering just enough food to stay alive. Population numbers were kept low because of the difficulty of finding food.

Over time, the development of agriculture led to increases in population around the world. But it was not until the 20th century that population numbers began to explode, and this steep rise was accelerated by huge improvements in [hygiene: A set of behaviours or practices that aim to minimise threats to health.]  hygiene and healthcare.

Human population growth over the last 10,000

In October 2011, the human population on Earth reached 7 billion, and continues to increase. Watch this BBC News item.

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Standards of living

In addition to the huge rise in population, there has been a big rise in thestandard of living [standard of living: How much wealth a group of people have and the goods and services available to them. Life expectancy and literacy rate may also be taken into account.]  standard of living, especially in thedeveloped [developed: The end point of the process of development.]  world. People in the developed world now enjoy a high standard of living - with abundant food, cars and comfortable housing. People in the developing world have a lower standard of living, but many countries are catching up quickly.

Impact of humans

The growth in the human population and the improvements in the standard of living are putting strains on the global environment. Here are some of the ways in which this is happening:

·         Non-renewable energy resources (such as coal, oil and natural gas) are being used up rapidly.

·         Raw materials are being used up rapidly.

·         More land is being used for buildings and transport networks,quarrying [quarrying: Removing a useful mineral (eg limestone) from the ground in large open pits, usually excavated by blasting.] , farming and dumping waste - reducing the amount of land available to other animals and plants.

·         More waste is being produced - causing more pollution.

Land and water pollution

Pollution is the addition of substances to the environment that may be harmful to living organisms  [organism: A living being - plant, animal, fungus or bacterium.] . Population growth and a higher standard of living cause more waste to be produced. If this waste is not handled correctly, it leads to pollution.

Land pollution

In order to improve the yield [yield: The yield in a reversible reaction is usually expressed as the percentage of product in the reaction mixture.]  from their land, most farmers spray their crops with chemicals includingherbicides [herbicide: A chemical that kills unwanted plants.]  andpesticides. [pesticide: Chemicals used to kill insects, weeds and micro-organisms that might damage crops.] 

A farmer sprays a field with pesticide

Herbicides increase crop yield by killing or inhibiting the growth of weeds, reducing the competition for resources such as minerals, space and sunlight. Pesticides increase yield by killing off pests, such as small insects or plantpathogens, [pathogen: Microorganism that can cause disease.]  which would otherwise feed on or damage the crops.

However, some of these chemicals can remain in the soil for long periods, polluting the land, and they may also be washed into rivers, lakes and seas. There can also be consequences further up food chains within an eco-system [ecosystem: A community of animals, plants and microorganisms, together with the habitat where they live.] - with pollution disrupting food chains or accumulating to toxic [toxic: Poisonous.]  levels.

Most rubbish is buried in landfill sites and some of it may be unsafe. Even common household items can contain toxic chemicals such as poisonous metals. Industrial waste is also discharged onto the land.

Water pollution

Water pollution is caused by the discharge of harmful substances into rivers, lakes and seas.

Fertilisers are used by farmers to increase their crop yield, supplying extra minerals to their plants so they grow better. However, these minerals can run off into waterways and lead to a process called eutrophication [eutrophication:'Hyper-nutrition' resulting from fertiliser pollution of aquatic ecosystems. Results in oxygen depletion and reduced ability to support life.] . This involves the over-growth of algae and ultimately leads to oxygen depletion [depletion: Using up a useful chemical to the point at which it is too low.]  from the water and the death of invertebrates [invertebrate: An animal without a backbone.]  and fish. This causes food chains within the eco-system to collapse.

River pollution: dead fish due to lack of oxygen

Sewage may also pollute waterways. Sewage contains high mineral levels and can promote the process of eutrophication (see above). It may also contain harmful pathogens.

Toxic chemicals from industries and mining can also pollute waterways. These chemicals might be highly toxic, or might accumulate in food chains to toxic levels.

Red Tide in the Red Sea

Air pollution

The most common source of air pollution is the combustion [combustion: The process of burning by fire.]  of fossil fuels. This usually happens in vehicle engines and power stations. However, there are other sources of atmospheric pollution (see table below).

Common air pollutants

Science

Deforestation and the destruction of areas of peat

The growth of the human population has created a greater demand for food, energy sources, natural resources and land. New land is increasingly being found by cutting down rainforests, allowing agriculture to expand. But this has negative consequences for both the environment and for biodiversity. This exploitation of natural resources is also occurring with peat bogs - again with negative consequences.

Deforestation

The world’s forests, especially rainforests, are vital in that they provide unique habitats for many unique species. They also act as a ‘carbon sink’, trapping away lots of carbon in their biomass [biomass: The dry mass of an organism.] that was previously absorbed for photosynthesis.  [photosynthesis: A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.] 

Humans have been cutting down trees for thousands of years. However, this clearing of forests has accelerated in recent decades and is now being carried out on a large scale. This is known as deforestation.

The world's forests 8000 BC

The world's forests 2000 CE

The reasons for deforestation:

·         To provide timber as a fuel or a building material.

·         To provide extra land for agriculture. This agricultural land is often used to grow rice in paddy fields or to rear cattle in order to satisfy the increasing demand for food. However, increasingly this land is being used to grow crops for biofuel [biofuel: A fuel that is produced using crops as a raw material rather than fossil fuels.]  production (based around bioethanol [bioethanol: Ethanol that has been produced from crops. Bioethanol is an example of a biofuel.]  ) in order to satisfy the increasing demand for energy.

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Consequences of deforestation

Deforestation has some important consequences:

·         It reduces the rate at which carbon dioxide is absorbed and ‘locked away’ in the plant biomass by photosynthesis, as there are fewer trees.

·         As timber is burnt to clear space, it increases the release of carbon dioxide into the atmosphere. The remaining parts of the tree (eg the roots) are then decomposed by microorganisms. [microorganisms: Microscopic (too small to see) organisms such as bacteria and viruses.]  This adds further carbon dioxide to the atmosphere and so contributes to global warming. [global warming: The gradual increase in the average temperature of the Earth.] 

·         Forest habitats are destroyed and biodiversity [biodiversity: Variety in and between organisms, species and ecosystems.]  is reduced.

·         Cattle are often reared on the land, producing  [methane: Chemical compound with the formula CH4, the simplest alkane.] methane. Methane, a greenhouse gas, contributes more to global warming than carbon dioxide.

·         Rice fields - created to satisfy the need for food production due to the growing population - are grown on previously deforested land and also produce methane when the crop rots.

Biodiversity

The term biodiversity refers not only to the number of differentspecies. [species: Used in the classification of living organisms, referring to related organisms capable of interbreeding.]  It also refers to all the variations within and between species, and all the differences between the habitats [habitat: The physical space in which a given species lives.]  andecosystems [ecosystems: communities of animals, plants and microorganisms, together with the habitats where they live]  that make up the Earth’sbiosphere. [biosphere: All of the living things on Earth.] 

The loss of forests reduces biodiversity and we run the risk of losing organisms that might have been useful in the future - for example, as sources of new medicines. There is also a moral responsibility to look after the planet and its resources.

Destruction of peat bogs

What is peat?

Peat is formed in waterlogged, acidic [acid: A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.] fens  [fen: One of the six main types of wetland, usually fed by mineral-rich surface or ground water.]  and bogs [peat bog: Poorly drained areas made up of partially decomposed organic matter due to water logging.]  over thousands of years by the growth of mosses and other plants, which absorb and ‘lock away’ carbon dioxide during photosynthesis. [photosynthesis: A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.]  When the moss dies, the waterlogged bog providesanaerobic [aerobic respiration: Respiration that requires oxygen.]  conditions which, together with the acidity of the bog, prevent the total decomposition [decomposition : A reaction in which substances are broken down, by heat, electrolysis or a catalyst.]  of the moss. It accumulates in the bogs in a partially-decomposed state, forming peat.

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The importance of peat

Peat bogs cover nearly 2-3% of the Earth’s surface and are an important carbon sink [carbon sink: Anything that absorbs more carbon that it releases, whether natural or artifical.] ,containing more ‘locked-away’ carbon than the Earth’s forests.

However, the amount of biomass [biomass: The dry mass of an organism.]  it contains means it can be dried and burnt as a fuel, which makes it an important energy source in some countries. Peat also has valuable properties when mixed in with soil - including improved soil structure, mineral retention, water retention and acidity - making it valuable in agriculture and gardening. Many peat bogs have been drained to allow the peat to be extracted.

Peat Extraction

However, the use of peat causes problems. Burning the peat releases its stored carbon back into the atmosphere as carbon dioxide. Similarly, as peat is mixed in with soil it is exposed to aerobic [aerobic: With oxygen.]  conditions and begins to decompose - which again causes the release of its trapped carbon as carbon dioxide. This is in addition to the carbon dioxide released in extracting the peat.

Therefore, the destruction of peat bogs contributes to global warming [global warming: The gradual increase in the average temperature of the Earth.]  as well as destroying important habitats. [habitat: The physical space in which a given species lives.] 

Consequences for gardeners

Since the 1950s, many gardeners have bought peat-based composts due to their perceived benefits (see above).

However, the impact of peat extraction in terms of global warming and habitat destruction has seen a rise in the number of gardeners opting for ‘peat-free’ composts which contain sustainable alternatives to the use of peat. This trend has been supported by government targets for reducing the use of peat in compost.

Science

Biofuels

The rising demands for energy from a growing population have led to the burning of increasing amounts of fossil fuels, which generate carbon dioxide emissions. These emissions, along with other greenhouse gas emissions, are leading to global warming - which is predicted to have very serious consequences for the environment in the coming decades. Fossil fuels are also non-renewable. An alternative to fossil fuels, which is being investigated, is the use of biofuels.

Global warming

The presence of certain greenhouse gases [greenhouse gas: Carbon dioxide, methane and other gases that absorb infrared radiation in the atmosphere.]  in our atmosphere naturally results in the Earth being warmer than it should be, as the gases trap some of the Sun’s heat and prevent it escaping from our atmosphere. This is called the greenhouse effect.

Global warming is the term which is used to describe the increase in the Earth’s temperature above the natural greenhouse effect. This increase is caused by additional greenhouse gases being released.

Carbon dioxide from burning fuels causes global warming

The two main greenhouse gases which are increasing in the atmosphere arecarbon dioxide and methane [methane: Chemical compound with the formula CH4, the simplest alkane.] .

Carbon dioxide is a process capable of changing the world’s climate significantly

The graphs indicate a strong correlation [correlation: A relationship between two sets of data, such that when one set changes you would expect the other set to change as well.]  between the rise in carbon dioxide levels and the rise in global temperature. Many scientists believe this rise is due to human activity.

Why are greenhouse gas levels increasing?

Carbon dioxide levels are increasing because:

·         Humans are burning more fossil fuels [fossil fuel: Fuel, such as coal, oil and natural gas, made from the remains of ancient plants and animals.]  to provide energy.

·         Humans are cutting down forests - reducing the number of trees that can absorb carbon dioxide.

·         Humans are destroying peat bogs  [peat bog: Poorly drained areas made up of partially decomposed organic matter due to water logging.]  – and the process of destroying them releases carbon dioxide.

Methane levels are increasing because:

·         Humans are rearing more cattle to supply food. During their digestive process, cows produce a lot of methane.

·         Humans are planting more rice paddy fields to supply food. These grow in water, creating anaerobic [aerobic respiration: Respiration that requires oxygen.]  conditions - and therefore as plants rot, methane is produced.

·         Humans are producing more waste - which produces methane as it decays anaerobically.

Why do greenhouse gases cause global warming?

Graph showing change in global temperature over 100 year period

1.   Heat from the Sun enters the Earth’s atmosphere and warms the Earth’s surface.

2.   The Earth’s surface becomes hotter and radiates heat back out.

3.   Some of this heat is absorbed by greenhouse gases. These gases then radiate the heat back towards Earth.

4.   The Earth becomes warmer as a result.

Consequences of global warming

A major area of uncertainty is what will happen over the next century as greenhouse gas levels in the atmosphere continue to rise. Computer models indicate that the temperature will rise - but different models generate slightly different values for this temperature, as assumptions about the future vary slightly.

However, a rise of only a few degrees Celsius may:

·         cause big changes in the Earth’s climate and weather patterns

·         cause ice caps on land to melt causing a rise in sea level - resulting in flooding and low lying areas being submerged

·         reduce biodiversity [biodiversity: Variety in and between organisms, species and ecosystems.]  as habitats [habitat: The physical space in which a given species lives.]  are lost and organisms [organism: A living being - plant, animal, fungus or bacterium.]  fail to adapt to the changed environment

·         cause changes in the migration [migrate: To travel long distances in search of a new habitat.]  patterns of birds and other organisms

·         result in changes to the distribution of species [species: Used in the classification of living organisms, referring to related organisms capable of interbreeding.]  (ie where they are found) as some species move to cooler areas to cope with the increase in global temperatures

Carbon dioxide sequestering

The oceans, lakes and ponds of planet Earth are important as they absorb and ‘lock away’ over a quarter of the carbon dioxide that humans  [emit: To give or send out.]  emit into the atmosphere. The process by which they absorb and lock away the carbon dioxide is known as sequestration. This occurs due to:

1.   Carbon dioxide being soluble and dissolving directly in the water.

2.   Phytoplankton [phytoplankton: Microscopic aquatic plants.]  performing photosynthesis which absorbs carbon dioxide, trapping the carbon within their biomass. [biomass: The dry mass of an organism.] 

This sequestering plays an important role in removing carbon dioxide from the atmosphere. As carbon dioxide levels in the atmosphere rise, it is likely that more would be sequestered in the oceans, rivers and ponds.

Many organisations and companies are also looking at how more carbon dioxide can be sequestered by enhancing natural sequestration (eg getting the phytoplankton to do more photosynthesis) or by using artificial sequestration.

Biofuels

With fossil fuels being non-renewable and contributing to global warming, biofuels are increasingly considered to be a possible alternative for the future. Biofuels are produced from natural products, often plant biomass [biomass:The dry mass of an organism.]  containing carbohydrate [carbohydrate: Food belonging to the food group consisting of sugars, starch and cellulose. It is vital for energy in humans, and is stored as fats if eaten in excess. In plants, carbohydrates are important for photosynthesis.] . As biofuels are produced from plants, they are renewable and theoretically carbon neutral. [carbon neutral: When a whole process does not make a net contribution of carbon dioxide to the atmosphere. For example, burning biofuels might be described as carbon neutral because it only releases the same amount of carbon dioxide as it absorbed by photosynthesis when the crop was grown.] 

Some biofuels are produced by using microorganisms [microorganisms:Microscopic (too small to see) organisms such as bacteria and viruses.] toanaerobically ferment  [anaerobically ferment: Ferment in the absence of air.]  carbohydrate in the plant material, as is the case with bioethanol and biogas production (each process uses different microorganisms).

Bioethanol

Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful as a fuel. It is usually mixed with petrol for use in cars and other vehicles.

Ethanol can be made by a process called fermentation. This converts sugar into ethanol and carbon dioxide if conditions are anaerobic. Single-celled fungi, called yeast, contain enzymes that are natural catalysts [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself.]  for making this process happen:

In some countries, eg Brazil, the source of sugar is sugar cane - which yeast can directly ferment into ethanol. In other countries, plants such as maize are used. Because maize contains starch rather than sugar, the enzymeamylase [amylase: An enzyme that breaks down starch into sugars. It is present in human saliva.]  must first break down the starch into sugar before the yeast can ferment it into ethanol.

The ethanol produced by yeast only reaches a concentration of around 15 per cent before the ethanol becomes toxic to the yeast. In order to make it sufficiently concentrated to be burnt as a fuel, the ethanol must bedistilled. [distil: A liquid that has been evaporated and then condensed in order to purify it.] 

Disadvantages of bioethanol

There are some disadvantages to growing biofuel crops, such as sugar cane and maize, to be used as bioethanol:

·         The demand for biofuel crops means greater demand on rainforest land.

·         Crops grow slowly in parts of the world that have lower light levels and temperatures, so growing biofuel crops in these countries would not satisfy the demand for fuel.

·         For bioethanol to be burnt in a car engine, some engine modification is needed. Modern petrol engines can use petrol containing up to 10 per cent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol

·         Although biofuels are in theory carbon neutral, this does not take into account the carbon dioxide emissions associated with growing, harvesting and transporting the crops, or producing the ethanol from them. Therefore, overall, more carbon dioxide is emitted than is absorbed - which means that it contributes to global warming.  [global warming: The gradual increase in the average temperature of the Earth.] 

·         Some people morally object to using food crops to produce fuels. For example, it could cause food shortages or increases in food prices.

Biogas

Biogas is a biofuel produced from the anaerobic fermentation of carbohydrates in plant material or waste (eg food peelings or manure) by  [bacterium: A type of single-celled microorganism.] bacteria.

It is mainly composed of methane [methane: Chemical compound with the formula CH4, the simplest alkane.] , with some carbon dioxide and other trace gases. However, the proportion of methane within the biogas can vary between 50% and 80%, depending on whether some oxygen is able to enter at the beginning or during the process. If some oxygen is present, the bacteria will respire aerobically [aerobic: With oxygen.]  and will produce a gas with a higher proportion of carbon dioxide and a lower proportion of methane.

Biogas can be produced on a small scale in a biogas generator/digester,which can be made of simple materials.

The carbohydrate-containing materials are fed in, and a range of bacteria anaerobically ferment the carbohydrate into biogas. The remaining solids settle to the base of the digester and can be run off to be used asfertiliser [fertiliser: A substance added to the soil to increase the soil fertility.]  for the land. These types of biogas generator are most commonly used in the developing world to satisfy the needs of a small family.

Biogas typically refers to a gas produced by the breakdown of organic matter in the absence of oxygen

The optimum [optimum: The most favourable.]  temperature for biogas production is between 32oC and 35oC. Temperatures above and below this optimum can result in less biogas being produced, which can be a problem in hotter and cooler countries (see table below).

If a bigger, more sophisticated biogas generator is used, biogas can also be produced on a large scale.

Biogas is naturally produced in landfill sites as bacteria anaerobically break down our rubbish, but normally the methane escapes into the atmosphere where it contributes to global warming. If a pipe network with holes in it can be built into the landfill site - and the methane is prevented from escaping into the atmosphere by covering the site - then the methane can be collected via the pipe network.

The methane can then be used as a fuel to generate electricity or heat buildings, eg care homes, hospitals and schools. This is an example of biogas generation on a commercial scale.

Biogas extraction well

Science

Food Production

A growing population brings with it a necessity to produce more food. However, the potential impact on the local and global environment must be considered. Part of the solution lies in careful management to reduce energy losses in food chains, as well as looking to new food sources. It is necessary to find a compromise between the priority of obtaining food and the priority of protecting ecosystems.

Efficiency of food production

Both biomass [biomass: The dry mass of an organism.]  and the energy within it decrease up a food chain. [food chain: A sequence (usually shown as a diagram) of feeding relationships between organisms, showing who eats what and the movement of energy through trophic levels.]  At each level in the chain, energy/biomass is lost through waste (eg faeces) or throughrespiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.]  and associated processes (such as movement and maintaining body temperature).

The efficiency of food production can be improved by reducing the number of levels in the food chain. This is because fewer energy losses occur along a shorter food chain, meaning a greater proportion of the energy that entered the food chain is available to humans and more people can be fed.

The amount of available energy decreases at every step in a food chain

The efficiency of food production from animals can be improved by reducing the amount of energy lost to the surroundings. This can be done by:

·         Preventing animals moving around too much - this conserves energy which can be used to increase biomass.

·         Keeping their surroundings warm - this preserves the energy which would have been used to maintain their body temperature, so that it can be used to increase biomass.

Such practices are known as factory farming.

Pig farm

The main advantages for keeping animals in warm sheds with little space to move are that it results in more efficient food production - and therefore cheaper food. However, there are disadvantages in terms of reduced animal welfare, increased risk of injury, and increased risk of diseases (eg salmonella amongst chickens).

A balance must be reached between the needs of farmers and consumers and the welfare of the animals.

Calculating energy efficiency

Calculating energy efficiency

This bullock has eaten 100 kJ of stored energy in the form of grass, andexcreted  [excreted: Discharged as waste.]  63 kJ in the form of faeces, urine and gas. The energy stored in its body tissues is 4 kJ. So how much has been used up in respiration?

The energy released by respiration

= 100 - 63 - 4 = 33 kJ

Only 4 kJ of the original energy available to the bullock is available to the next stage in the food chain, which might be humans.

The efficiency of this energy transfer is:

⁴/₁₀₀ × 100 = 4%

Mycoprotein

The increasing demand for food, especially protein-based food, to feed the growing world population has also led scientists to investigate alternative ways of obtaining food from microorganisms.  [microorganisms: Microscopic (too small to see) organisms such as bacteria and viruses.] 

Mycoprotein is a high-protein food produced from the fungal biomass [biomass:The dry mass of an organism.]  of a soil fungus [fungus: A large group of eukaryotic organisms that contain single celled yeasts, moulds and mushrooms.]  called Fusarium . It also has a high fibre content, and is low in fat with no cholesterol. This makes it a healthy, vegetarian alternative to meat.

Mycoprotein is grown in a fermenter  [fermenter: Vessels used to cultivate microorganisms on a large scale.]  - an apparatus for growing cultures on a large scale.

Mycoprotein is a high-protein food produced from the fungal biomass of a soil fungus called Fusarium

The fermenter is aseptically [aseptic: Containing nothing that could cause disease, such as bacteria, viruses or fungi.]  filled with a sterile [sterile: A sample that contains no forms of life.]  broth containing glucose syrup, obtained from the breakdown of plant starch [starch: A type of carbohydrate. Plants can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose when it is needed for respiration.]  by amylase enzymes. To this is added a small starter culture [culture: In microbiology, a colony of microbes, typically on an agar plate.]  of the Fusarium fungus.

Sterile glucose syrup, ammonia and air (containing oxygen) are then added continuously for a period of six weeks, so that the fungus has the correct nutrients and conditions to grow.

The role of the ammonia is to provide a nitrogen source for the fungus to produce amino acids - the building blocks of protein - while the air ensures that the conditions in the fermenter are aerobic [aerobic: With oxygen.]  as well as mixing the broth to ensure it is uniform throughout.

During the six-week period, the fungus  [fungus: A large group of eukaryotic organisms that contain single celled yeasts, moulds and mushrooms.]  and grows, doubling its biomass every five hours. The cooling coils remove the excess heat generated by the fungus during respiration, keeping the temperature inside the fermenter constant at its optimum level.

At the end of the six-week period, the fungal biomass is harvested andpurified by heating it to 65oC to remove harmful substances, and spinning it dry using a centrifuge [centrifuge: A device that spins around very quickly in order to separate mixtures according to the densities of their constituents. For example, if a blood sample is centrifuged, the red blood cells end up at the bottom, then the white blood cells, and the plasma remains on top.] . The yellow solid substance which is obtained can then be flavoured and shaped into different products.

Sustainable fishing

Fish are an important part of the human diet, accounting for a worldwide average of 15 per cent of protein intake. Most of these fish are caught wild.

As the population has increased, so has the demand for fish. If fish are caught at a faster rate than the remaining fish can reproduce, the numbers of fish – the fish stock – will decline. Trying to harvest more fish than the sea can produce is an example of unsustainability. [unsustainability: An activity which uses up resources or damages the environment so that it cannot be continued in the future.] 

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North Sea cod have been overfished since the 1960s. Increasing numbers of boats - using increasingly sophisticated technology - were able to catch more and more cod. At first, catches continued to rise each year. However, the size of catches then started to decline as cod populations fell, leaving fewer and fewer breeding fish to maintain cod numbers.

The graph shows the decline of North Sea cod stocks since the 1960's

In order to prevent the disappearance of certain fish species [species: Used in the classification of living organisms, referring to related organisms capable of interbreeding.]  in some areas, it is important to maintain fish stocks at a level that allows breeding to occur and ensures that fish populations remain at asustainable [sustainable: Activity which does not use up or destroy resources or the environment, so that it can continue to be done in the future.]  level. As a result of the near collapse of some fish populations, the European Union introduced regulations to conserve [conserve: To keep something the same, or to protect it from being reduced.]  fish stocks.

These regulations included:

·         Setting fishing quotas for EU countries and for individual fishing vessels, which limited the amount of each species of fish which could be caught. By catching fewer fish, more are left to breed, so in time the population should recover.

·         Limiting mesh size of the nets. By increasing the size of the holes in nets, only mature, full-sized fish can be caught and immature fish can escape and eventually breed, allowing the population to recover.

In spite of these measures, stocks of cod and some other fish remain dangerously low.

Food miles

Humans must also consider the impact of their food production on the environment. In order to supply cheap produce all year round, many supermarkets import food from other countries around the world - where it is cheaper to produce or grows more plentifully. Some developing countries rely on food exports to the UK to generate income.

The distance that food travels from the farm where it is produced to the consumer is referred to as ‘food miles’. Locally grown produce has far fewer food miles than produce grown in other countries.

The greater the distance the food has travelled, the greater the impact on the environment. This is due to the pollution from carbon dioxide emissions, generated by the transporting vehicles.

A compromise must be found between the monetary cost to the consumer, the impact on developing economies and the environmental cost of the pollution associated with transporting food over such long distances.