TGS Revision Year 10 Biology

a) Most animal cells have the following parts:

• a nucleus, which controls the activities of the cell
• cytoplasm, in which most of the chemical reactions take place
• a cell membrane, which controls the passage of substances into and out of the cell
• mitochondria, which is where most energy is released in respiration
• ribosomes, which is where protein synthesis occurs.

Most human cells are like most other animal cells.

b) In addition to the above, plant cells often have:

• chloroplasts, which absorb light energy to make food
• a permanent vacuole filled with cell sap.

Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

c) A bacterial cell consists of cytoplasm and a membrane surrounded by a cell wall; the genes are not in a distinct nucleus; some of the genes are located in circular structures called plasmids.

d) Yeast is a single-celled organism. Yeast cells have a nucleus, cytoplasm and a membrane surroundedby a cell wall.

e) Cells may be specialised to carry out a particular function.Candidates should be able, when provided with appropriate information, to relate the structure of different types of cell to their function in a tissue, an organ, or the whole organism.

Example: Red blood cells: No nucleus, more room for haemoglobin

a) Diffusion is the spreading of the particles of any substance in solution, or particles of a gas, resulting in a net movement from a region where they are of a higher concentration to a region with a lower concentration. The greater the difference in concentration, the faster the rate of diffusion.

b) Dissolved substances can move into and out of cells by diffusion.

c) Oxygen required for respiration passes through cell membranes by diffusion.

d) Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.

e) Differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

 Candidates should be familiar with experiments related to diffusion and osmosis as well as the terms

isotonic, hypotonic, hypertonic, turgor and plasmolysis.

f) Substances are sometimes absorbed against a concentration gradient. This requires the use of energy

from respiration. The process is called active transport.

g) Active transport enables plants to absorb ions from very dilute solutions, eg by root hair cells. Similarly,

sugar may be absorbed from low concentrations in the intestine and from low concentrations in the

kidney tubules.

h) A single-celled organism has a relatively large surface area to volume ratio. All the necessary exchanges

occur via its surface membrane. 

The size and complexity of an organism increase the difficulty of exchanging materials.

i) In multicellular organisms many organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by:

• having a large surface area that is thin, to provide a short diffusion path
• (in animals) having an efficient blood supply
• (in animals, for gaseous exchange) being ventilated.

Candidates should be able to explain how the small intestine and lungs in mammals, and the roots and leaves in plants, are adapted for exchanging materials.

j) Gas and solute exchange surfaces in humans and other organisms are adapted to maximise effectiveness. 

Candidates should be able, when provided with appropriate information, to explain how gas and solute exchange surfaces are adapted to maximise effectiveness.

a) The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body. Each chromosome carries a large number of genes.

b) Many genes have different forms called alleles, which may produce different characteristics.

c) In body cells the chromosomes are normally found in pairs. 

d) Body cells divide by mitosis to produce additional cells during growth or to produce replacement cells.

e) When a body cell divides by mitosis:

• copies of the genetic material are made
• the cell then divides once to form two genetically identical body cells.

f) Cells in reproductive organs divide to form gametes.

g) A cell divides to form gametes by meiosis.

h) When a cell divides to form gametes:

• copies of the genetic information are made
• the cell then divides twice to form four gametes, each with a single set of chromosomes.

i) Gametes join at fertilisation to form a single body cell with new pairs of chromosomes. This cell repeatedly divides by mitosis to form many cells. As an organism develops, these cells differentiate to form different kinds of cells.

j) Most types of animal cell differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.

a) Large multicellular organisms develop systems for exchanging materials. During the development of a multicellular organism, cells differentiate so that they can perform different functions.

b) A tissue is a group of cells with similar structure and function.

c) Organs are made of tissues. One organ may contain several tissues. 

d) Organ systems are groups of organs that perform a particular function. 

Candidates should develop an understanding of size and scale in relation to cells, tissues, organs and systems.

a) Examples of animal tissues include:

• muscular tissue, which can contract to bring about movement
• glandular tissue, which can produce substances such as enzymes and hormones
• epithelial tissue, which covers some parts of the body. 

b) An example of an animal organ is the stomach, which contains:

• muscular tissue, to allow contents to move through the digestive system
• glandular tissue, to produce digestive juices
• epithelial tissue, to cover the outside and the inside of the stomach. 

c) An example of an animal organ system is the digestive system, a system in which humans and other

• mammals exchange substances with the environment. The digestive system includes:
• glands, such as the pancreas and salivary glands, which produce digestive juices
• the stomach and small intestine, where digestion occurs
• the liver, which produces bile
• the small intestine, where the absorption of soluble food occurs
• the large intestine, where water is absorbed from the undigested food, producing faeces

a) Examples of plant tissues include:

• epidermal tissues, which cover the plant
• palisade mesophyll, which carries out photosynthesis
• spongy mesophyll, which has air spaces to facilitate diffusion of gases
• xylem and phloem, which transport substances around the plant.

b) Plant organs include stems, roots and leaves.

Details of the internal structure of these organs are limited to the leaf and to the position of the xylem and phloem in a dicotyledonous primary root and primary stem.

a) All carbohydrates are made up of units of sugar. Carbohydrates that contain only one sugar unit, eg glucose, or two sugar units, eg sucrose, are referred to as simple sugars. Complex carbohydrates, eg starch and cellulose, are long chains of simple sugar units bonded together.

b) Lipids are molecules consisting of three molecules of fatty acids joined to a molecule of glycerol.

c) Protein molecules are made up of long chains of amino acids. These long chains are folded to produce a specific shape that enables other molecules to fit into the protein. Proteins act as:

• structural components of tissues such as muscles
• hormones
• antibodies
• enzymes

a) Enzymes are biological catalysts. Catalysts increase the rate of chemical reactions. 

b) The shape of an enzyme is vital for the enzyme's function. High temperatures denature the enzyme, changing the shape of the active site.

c) Different enzymes work best at different pH values.

d) Some enzymes work outside the body cells. The digestive enzymes are produced by specialised cells in glands and in the lining of the gut. The enzymes then pass out of the cells into the gut, where they come into contact with food molecules. They catalyse the breakdown of large molecules into smaller molecules.

e) Some microorganisms produce enzymes that pass out of the cells. These enzymes have many uses in the home and in industry.

f) In the home:

• biological detergents may contain protein-digesting and fat-digesting enzymes (proteases and lipases)
• biological detergents are more effective at low temperatures than other types of detergents.

g) In industry:

• proteases are used to ‘pre-digest’ the protein in some baby foods
• carbohydrases are used to convert starch into sugar syrup
• isomerase is used to convert glucose syrup into fructose syrup, which is much sweeter than glucose and therefore can be used in smaller quantities in slimming foods.

a) The respiratory (breathing) system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream and carbon dioxide can diffuse out of the bloodstream into the air. The lungs are in the upper part of the body (thorax), protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.

Candidates should be able to recognise the following on a diagram of the respiratory system: ribs, intercostal muscles, diaphragm, lungs, trachea, bronchi, bronchioles, alveoli.

b) To inhale:

• the intercostal muscles contract, pulling the ribcage upwards
• at the same time the diaphragm muscles contract, causing the diaphragm to flatten
• these two movements cause an increase in the volume of the thorax
• the consequent decrease in pressure to below that of the air surrounding the body results in
• atmospheric air entering the lungs.

To exhale:

• the intercostal muscles relax, allowing the rib cage to move downwards
• at the same time the diaphragm muscles relax, allowing the diaphragm to resume its domed shape
• these two movements cause a reduction in the volume of the thorax
• the consequent increase in pressure results in air leaving the lungs.

c) The alveoli provide a very large surface area, richly supplied with blood capillaries, so that gases can readily diffuse into and out of the blood.

d) A healthy person breathes automatically twenty four hours each day. However, spontaneous breathing may stop due to disease or injury. If this happens the patient can be helped to breathe by using a mechanical ventilator. There are two main types of mechanical ventilator:

• negative pressure ventilators, which cause air to be ‘drawn’ into the lungs
• positive pressure ventilators, which force air into the lungs.

a) Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to release energy.

b) During aerobic respiration chemical reactions occur that use glucose (a sugar) and oxygen and release energy.

c) Aerobic respiration is summarised by the equations:

glucose + oxygen carbon dioxide + water (+ energy)

C6H12O6 + 6O2=6CO2+ 6H2O (+ energy)

d) Aerobic respiration takes place continuously in both plants and animals.

e) Most of the reactions in aerobic respiration take place inside mitochondria.

f) The energy that is released during respiration may be used by the organism in a variety of ways:

• to build larger molecules from smaller ones
• in animals, to enable muscles to contract
• in mammals and birds, to maintain a steady body temperature in colder surroundings
• in plants, to build up sugars, nitrates and other nutrients into amino acids, which are then built up
• into proteins.

g) During exercise the human body needs to react to the increased demand for energy. A number of changes take place:

• the heart rate increases, increasing blood flow to the muscles
• the rate and depth of breathing increase
• glycogen stored in the muscles is converted back to glucose.

h) These changes increase the supply of glucose and oxygen to, and increase the rate of removal of carbon dioxide from, the muscles.
i) If insufficient oxygen is reaching the muscles, energy is produced by anaerobic respiration. glucose lactic acid (+ energy) C6H12O6=2C3H6O3(+ energy)
j) Anaerobic respiration in muscles is the incomplete breakdown of glucose, which causes a build-up of lactic acid. An oxygen debt needs to be repaid to oxidise the lactic acid to carbon dioxide and water.
k) Because the breakdown of glucose is incomplete, much less energy is released in anaerobic respiration than during aerobic respiration.
l) During long periods of vigorous activity muscles become fatigued and stop contracting efficiently. One cause of muscle fatigue is the build-up of lactic acid in the muscles. Blood flowing through the muscles eventually removes the lactic acid.
Candidates will be expected to interpret data relating to the effects of exercise on the human body.
m) Anaerobic respiration in plant cells and in some microorganisms results in the production of ethanol and carbon dioxide.

a) Substances are transported from where they enter the body to the cells, or from the cells to where they are removed from the body, by the circulatory system (the heart, the blood vessels and the blood).
b) The heart is an organ that pumps blood around the body. Much of the wall of the heart is made from muscle tissue.
c) The heart has four main chambers (right and left atria and right and left ventricles).
d) The natural resting heart rate is controlled by a group of cells that act as a pacemaker, located in the right atrium. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate.
e) In coronary heart disease layers of fatty material build up inside the coronary arteries, narrowing them. This reduces the flow of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle. Stents are used to keep the coronary arteries open.

f) In some people heart valves may become faulty. There are two main faults – the heart valve tissue might stiffen, preventing the valve from opening fully, or the heart valve might develop a leak. Faulty heart valves can be replaced using:

• biological valves – valves from humans or other mammals
• mechanical valves.

g) Artificial hearts are occasionally used to keep patients alive whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.
h) Blood enters the atria of the heart. The atria contract and force blood into the ventricles. The ventricles contract and force blood out of the heart. Valves in the heart ensure that blood flows in the correct direction. 
i) Blood flows from the heart to the organs through arteries and returns through veins. There are two separate circulation systems, one for the lungs and one for all other organs of the body.

Knowledge of the blood vessels associated with the heart is limited to aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries.

j) Arteries have thick walls containing muscle and elastic fibres. Veins have thinner walls and often have valves to prevent back-flow of blood.
k) In the organs, blood flows through very narrow, thin-walled blood vessels called capillaries. Substances needed by the cells in body tissues pass out of the blood and substances produced by the cells pass into the blood, through the walls of the capillaries.
l) Blood is a tissue consisting of a fluid called plasma, in which the white blood cells, platelets and red blood cells are suspended.

m) Blood plasma transports:

• carbon dioxide from the organs to the lungs
• soluble products of digestion from the small intestine to other organs
• urea from the liver to the kidneys.

n) Red blood cells have no nucleus. They are packed with a red pigment called haemoglobin. Red blood cells transport oxygen from the lungs to the organs. In the lungs haemoglobin combines with oxygen to form oxyhaemoglobin. In other organs oxyhaemoglobin splits up into haemoglobin and oxygen. 

o) White blood cells have a nucleus. They form part of the body’s defence system against microorganisms. 
p) Platelets are small fragments of cells. They have no nucleus. Platelets help blood to clot at the site of a wound.
q) Blood clotting is a series of enzyme-controlled reactions, resulting in the change of fibrinogen to fibrin, which forms a network of fibres trapping blood cells and forming a clot.
r) Antigens are proteins on the surface of cells.
s) In organ transplants a diseased organ is replaced with a healthy one from a donor. The recipient’s antibodies may attack the antigens on the donor organ as they do not recognise them as part of the recipient’s body. To prevent rejection of the transplanted organ:

• a donor organ with a ‘tissue-type’ similar to that of the recipient is used
• the recipient is treated with drugs that suppress their immune system.

Candidates should be able, when provided with appropriate information, to evaluate the advantages and disadvantages of treating organ failure by mechanical devices or transplant.

t) There are four main types of human blood: O, A, B and AB. Blood group O is the universal donor.

Candidates should understand:

• the need for blood typing
• the ABO compatibility table.

a) Starch (a carbohydrate), proteins and fats are insoluble. They are broken down into soluble substances so that they can be absorbed into the bloodstream in the wall of the small intestine. In the large intestine much of the water mixed with the food is absorbed into the bloodstream. The indigestible food which remains makes up the bulk of the faeces. Faeces leave the body via the anus. Candidates should be able to recognise the following on a diagram of the digestive system: salivary glands, oesophagus, stomach, liver, gall bladder, pancreas, duodenum, small intestine, large intestine, anus.

b) The enzyme amylase is produced in the salivary glands, the pancreas and the small intestine. Amylase catalyses the breakdown of starch into sugars in the mouth and small intestine.

c) Protease enzymes are produced by the stomach, the pancreas and the small intestine. These enzymes catalyse the breakdown of proteins into amino acids in the stomach and the small intestine.

d) Lipase enzymes are produced by the pancreas and small intestine. These enzymes catalyse the breakdown of lipids into fatty acids and glycerol in the small intestine.

e) The stomach also produces hydrochloric acid. The enzymes in the stomach work most effectively in acid conditions.

f) The liver produces bile, which is stored in the gall bladder before being released into the small intestine. Bile neutralises the acid that was added to food in the stomach. This provides alkaline conditions in which enzymes in the small intestine work most effectively. Bile also emulsifies fats (breaks large drops of fats into smaller droplets). This increases the surface area of fats for lipase enzymes to act upon.

a) The nervous system enables humans to react to their surroundings and to coordinate their behaviour.

b) Information from receptors passes along cells (neurones) as impulses to the central nervous system (the brain or the spinal cord). The brain coordinates the response.

c) The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions:

• the cerebral cortex is concerned with consciousness, intelligence, memory and language
• the cerebellum is concerned mainly with the coordination of muscular activity
• the medulla is concerned with unconscious activities such as heartbeat and breathing.

d) Scientists have been able to map the regions of the brain to particular functions by studying patients with brain damage, electrically stimulating different parts of the brain and using MRI scanning techniques.

e) Reflex actions are automatic and rapid. They often involve sensory, relay and motor neurones.

f) In a simple reflex action such as a pain-withdrawal reflex: impulses from a receptor pass along a sensory neurone to the central nervous system at a junction (synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released that causes an impulse to be sent along a relay neurone a chemical is then released at the synapse between a relay neurone and motor neurone in the central nervous system, causing impulses to be sent along a motor neurone to the effector the effector is either a muscle or a gland: a muscle responds by contracting and a gland responds by releasing (secreting) chemical substances.

g) Effectors include muscles and glands. Candidates should be able, when provided with appropriate information, to analyse a particular given example of behaviour in terms of: stimulus-> receptor ->co-ordinator ->effector-> response

a) Automatic control systems in the body keep conditions inside the body relatively constant.
b) Control systems include:

• cells called receptors, which detect stimuli (changes in the environment)
• coordination centres that receive and process information from receptors
• effectors, which bring about responses.

c) Receptors are found in many organs, including:

• the eyes – sensitive to light
• the ears – sensitive to sound, and to changes in position (which enables us to keep our balance) 
• the tongue and in the nose – sensitive to chemicals (enable us to taste and to smell)
• the skin – sensitive to touch, pressure, pain and to temperature changes
• the brain – sensitive to blood temperature and the concentration of water in the blood
• the pancreas – sensitive to the concentration of glucose in the blood.

Knowledge and understanding of the structure and functions of sense organs such as the eye and the eye is NOT required

d) Coordination centres include the brain and spinal cord and the pancreas. 

Many processes are coordinated by chemical substances called hormones. Hormones are secreted by glands and are usually transported to their target organs by the bloodstream.

e) Internal conditions that are controlled include:

• temperature
• the water content of the body
• the ion content of the body
• blood glucose levels.

a) Water leaves the body via the lungs when we breathe out and the skin when we sweat. Excess water is removed via the kidneys in the urine.

b) Urea and ions are lost via the skin when we sweat. Excess ions are removed via the kidneys in the urine.

c) In the liver:

• excess amino acids are deaminated to form ammonia, which is converted into urea for excretion
• poisonous substances are detoxified, and the breakdown products excreted in the urine via the kidneys
• old blood cells are broken down and the iron is stored.

d) In a healthy kidney:

• the blood is filtered
• all the glucose is reabsorbed
• the dissolved ions needed by the body are reabsorbed
• as much water as the body needs is reabsorbed
• urea, excess ions and excess water are released as urine.

Knowledge of other parts of the urinary system, the structure of the kidney and the structure of a nephron is NOT required

e) If the water content of the blood is too low, the pituitary gland releases a hormone called ADH into the blood. This causes the kidneys to reabsorb more water and results in a more concentrated urine. 

f) If the water content of the blood is too high, less ADH is released into the blood. Less water is reabsorbed in the kidneys, resulting in a more dilute urine.

g) People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis,which restores the concentrations of dissolved substances in the blood to normal levels. Dialysis has to be carried out at regular intervals. In a dialysis machine a person’s blood flows between partially permeable membranes. The dialysis fluid contains the same concentration of useful substances as the blood of a person without kidney disease. This ensures that glucose and useful mineral ions are not lost but that harmful substances such as urea pass out from the blood into dialysis fluid.

Candidates should be able, when provided with appropriate information, to evaluate the advantages and disadvantages of treating organ failure by mechanical devices or transplants

a) Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This centre has receptors sensitive to the temperature of the blood flowing through the brain. The name of the centre in the brain (hypothalamus) is notrequired. 

b) Temperature receptors in the skin send impulses to the thermoregulatory centre, giving information about skin temperature.

c) If the core body temperature is too high: blood vessels supplying the skin capillaries dilate so that more blood flows through the capillaries and more heat is lost sweat glands release more sweat, which cools the body as it evaporates. 

d) Sweating helps to cool the body. More water is lost when it is hot, and more fluid has to be taken through drink or food to balance this loss.

e) If the core body temperature is too low: blood vessels supplying the skin capillaries constrict to reduce the flow of blood through the capillaries muscles may ‘shiver’ – their contraction needs respiration, which releases some energy to warm the body.

a) The blood glucose concentration is monitored and controlled by the pancreas. Much of the glucose is stored as glycogen in the liver and muscles. When these stores are full, excess glucose is stored as lipid.

b) If blood glucose levels are too high, the pancreas produces the hormone insulin, which allows the glucose to move from the blood into the cells.

c) When blood glucose levels fall, the pancreas produces a second hormone, glucagon. This causes glycogen to be converted into glucose and released into the blood.

d) In Type 1 diabetes a person’s blood glucose level may be too high because the pancreas does not produce enough of the hormone insulin. Type 1 diabetes may be controlled by careful diet, exercise, and by injecting insulin.

e) Type 2 diabetes develops when the body does not respond to its own insulin. Obesity is a significant factor in the development of Type 2 diabetes. Type 2 diabetes can be controlled by careful diet, exercise and by drugs that help the cells to respond to insulin.

a) Microorganisms that cause infectious disease are called pathogens.

b) Bacteria and viruses may reproduce rapidly inside the body. Bacteria may produce poisons (toxins) that make us feel ill. Viruses live and reproduce inside cells, causing damage. Knowledge of the structure of viruses is not required.

c) White blood cells help to defend against pathogens by:

• ingesting pathogens
• producing antibodies, which destroy particular bacteria or viruses
• producing antitoxins, which counteract the toxins released by the pathogens.

d) The immune system of the body produces specific antibodies to kill a particular pathogen. This leads to immunity from that pathogen. In some cases, dead or inactivated pathogens stimulate antibody production. If a large proportion of the population is immune to a pathogen, the spread of the pathogen is very much reduced.

e) People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body (vaccination). Vaccines stimulate the white blood cells to produce antibodies that destroy the pathogen. This makes the person immune to future infections by the microorganism, because the body can respond by rapidly making the correct antibody, in the same way as if the person had previously had the disease. The MMR vaccine is used to protect children against measles, mumps and rubella.

Details of vaccination schedules and side effects associated with specific vaccines are notrequired.

Candidates should be able, when provided with appropriate information, to evaluate the advantages and disadvantages of being vaccinated against a particular disease.

f) Some medicines, including painkillers, help to relieve the symptoms of infectious disease, but do not kill the pathogens.

g) Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics. The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. 

h) Antibiotics cannot kill viral pathogens.

Candidates should be aware that it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

i) Mutations of pathogens produce new strains. Antibiotics kill individual pathogens of the non-resistant strain but individual resistant pathogens survive and reproduce, so the population of the resistant strain rises. Antibiotics and vaccinations may no longer be effective against a new resistant strain of the pathogen. The new strain will then spread rapidly because people are not immune to it and there is no effective treatment.

Knowledge of development of resistance in bacteria is limited to the fact that pathogens mutate, producing resistant strains.

j) Many strains of bacteria, including MRSA, have developed resistance to antibiotics. Overuse and inappropriate use of antibiotics has increased the rate of development of antibiotic-resistant strains of bacteria. Antibiotics are not currently used to treat non-serious infections such as mild throat infections, in order to slow down the rate of development of resistant strains.

k) The development of antibiotic-resistant strains of bacteria necessitates the development of new antibiotics.

l) Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.

For this:

• Petri dishes and culture media must be sterilised before use to kill unwanted microorganisms
• inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame
• the lid of the Petri dish should be secured with adhesive tape to prevent microorganisms from the air contaminating the culture, and stored upside down to stop bacteria falling onto the agar surface.

m) In school and college laboratories, cultures should be incubated at a maximum temperature of  25 °C, which greatly reduces the likelihood of the growth of pathogens that might be harmful to humans.

n) In industrial conditions higher temperatures can produce more rapid growth.

a) Photosynthesis is summarised by the equations:

light energy

carbon dioxide + water glucose + oxygen

light energy

6CO2+ 6H2O=C6H12O6+ 6O2

b) During photosynthesis:

• light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and in algae this energy is used to convert carbon dioxide (from the air) and water (from the soil) into sugar (glucose)
• oxygen is released as a by-product.

c) The rate of photosynthesis may be limited by: 

• low temperature 
• shortage of carbon dioxide
• shortage of light.

These factors interact and any one of them may be the factor that limits photosynthesis.

Candidates should be able to relate the principle of limiting factors to the economics of enhancing thefollowing conditions in greenhouses:

• temperature
• carbon dioxide concentration
• light intensity

d) The glucose produced in photosynthesis may be:

• used for respiration
• converted into insoluble starch for storage
• used to produce fat or oil for storage
• used to produce cellulose, which strengthens the cell wall
• used to produce proteins.

e) To produce proteins, plants also use nitrate ions that are absorbed from the soil.

f) Carnivorous plants such as the Venus Fly Trap are adapted to live in nutrient-poor soil as they obtain most of their nutrients from the animals, such as insects, that they catch.

a) In flowering plants:

• carbon dioxide enters leaves by diffusion through the stomata
• most of the water and mineral ions are absorbed by roots.

b) The surface area of roots is increased by root hairs, and the surface area of leaves is increased by the flattened shape and internal air spaces.

c) Plants have stomata to obtain carbon dioxide from the atmosphere and to remove oxygen produced in photosynthesis.

d) Plants mainly lose water vapour from their leaves. Most of the loss of water vapour takes place through the stomata. Evaporation is more rapid in hot, dry and windy conditions. If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

e) The size of stomata is controlled by guard cells, which surround them

f) Flowering plants have separate transport systems:

• xylem tissue transports water and mineral ions from the roots to the stem and leaves
• the movement of water from the roots through the xylem and out of the leaves is called the
• transpiration stream
• phloem tissue carries dissolved sugars from the leaves to the rest of the plant, including the growing
• regions and the storage organs. 

This process is called translocation.

a) Plants are sensitive to light (phototropism), moisture (hydrotropism) and gravity (gravitropism):

• their shoots grow towards light and against the force of gravity
• their roots grow towards moisture and in the direction of the force of gravity.

b) Plants produce hormones to coordinate and control growth. The hormone auxin controls phototropism and gravitropism (geotropism).

c) The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of auxin, causing unequal growth rates.

d) Plant growth hormones are used in agriculture and horticulture as weed killers and as rooting hormones.

Names of specific weed killers and rooting hormones are not required.

a) Sexual reproduction in flowering plants involves:

• the production of male and female gametes
• the transfer of the male gametes to the female ovules in a process called pollination
• fertilisation, after which ovules grow into seeds within a fruit. 

b) The processes involved in sexual reproduction include:

• the anther produces the male gametes in pollen grains 
• the pollen grains attach to the stigma on top of a carpel, in which the female gametes (ovules) are located
• a pollen tube grows through the carpel's style into the ovule
• the nuclei from the pollen grain migrate into the ovule to fertilise the egg cell nucleus and endosperm nuclei
• the resulting zygote develops into an embryo
• the endosperm and the female tissues of the ovule give rise to seed
• the ovary then grows into a fruit, which surrounds the seed(s).

Candidates should be able to recognise the structures listed above on a diagram.

a) Differences in the characteristics of individuals of the same kind may be due to differences in:

• the genes they have inherited (genetic causes)
• the conditions in which they have developed (environmental causes)
• a combination of the above.

b) The information that results in plants and animals having similar characteristics to their parents is carried by genes, which are passed on in the sex cells (gametes) from which the offspring develop.

c) The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body. Chromosomes are normally found in pairs.

d) In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (**); in males the sex chromosomes are different (XY).

e) Different genes control the development of different characteristics of an organism. Some characteristics are controlled by a single gene. Each gene may have different forms called alleles.

Candidates should understand that genes operate at a molecular level to develop characteristics that can be seen.

f) If both chromosomes in a pair contain the same allele of a gene, the individual is homozygous for that gene. If the chromosomes in a pair contain different alleles of a gene, the individual is heterozygous for that gene.

g) An allele that controls the development of a characteristic when it is present on only one of the chromosomes is called a dominant allele. An allele that controls the development of a characteristic only if the dominant allele is not present is called a recessive allele.

Candidates should be familiar with principles used by Mendel in investigating monohybrid inheritance in peas. They should understand that Mendel’s work preceded the work by other scientists which linked Mendel’s ‘inherited factors’ with chromosomes.

Candidates should be able to construct genetic diagrams of monohybrid crosses and to predict the outcomes of monohybrid crosses. They should be able to use the terms homozygous, heterozygous, phenotype and genotype.

Candidates should understand that genetic diagrams are biological models which can be used to predict the outcomes of crosses.

Candidates should be able to interpret genetic diagrams, including family trees.

h) There are two forms of reproduction:

• sexual reproduction – the joining (fusion) of male and female gametes. The mixture of the genetic information from two parents leads to variety in the offspring
• asexual reproduction – no fusion of gametes and only one individual is needed as the parent. There is no mixing of genetic information and so no genetic variation in the offspring. These genetically identical individuals are known as clones.

i) Chromosomes are made up of large molecules of DNA (deoxyribonucleic acid). DNA contains the coded information that determines inherited characteristics. 

j) A gene is a small section of DNA. Each gene codes for a particular combination of amino acids, to make a specific protein.

k) DNA is made of very long strands, twisted to form a double helix, which contain four different compounds, called bases.  Candidates are not expected to know the names of the four bases or how complementary pairs of bases enable DNA replication to take place.

l) A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.

a) Some disorders are inherited.

b) Polydactyly (having extra fingers or toes) is caused by a dominant allele and can therefore be passed on by only one parent who has the disorder.

c) Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele and must therefore be inherited from both parents. It can be passed on by parents who may be carriers of the disorder without actually having the disorder themselves.

d) Sickle-cell anaemia (a disorder affecting red blood cells) is caused by a recessive allele. People who are heterozygous for the sickle-cell anaemia gene are less likely to get malaria than people who are homozygous for the dominant gene.

e) Some inherited conditions are caused by inheritance of abnormal numbers of chromosomes, eg Down’s Syndrome is caused by the presence of an extra chromosome. 

f) Embryos can be screened for the alleles that cause many genetic disorders:

• DNA is isolated from embryo cells
• a gene probe that will bind onto the gene for a specific disorder is produced. The probe usually has a fluorescent chemical attached to it
• the probe is added to a mixture containing the DNA sample from the embryo
• UV light is used to detect the probe attached to the gene for the disorder.

g) Concerns about embryo screening include:

• the risk of miscarriages
• the reliability of the information from the screening procedure
• decisions about terminating pregnancy.

Candidates should be able, when provided with appropriate information, to evaluate and make informed judgements about issues concerning embryo screening.

a) Modern cloning techniques include:

• tissue culture – using small groups of cells from part of a plant embryo transplants – splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers 
• adult cell cloning – the nucleus is removed from an unfertilised egg cell and the nucleus from an adult body cell, eg a skin cell, is inserted into the egg cell. An electric shock then acts as the catalyst for the egg cell to begin to divide to form embryo cells. These embryo cells contain the same genetic information as the adult skin cell. When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

b) In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms:

• enzymes are used to isolate the required gene
• this gene is inserted into a vector, usually a bacterial plasmid or a virus
• the vector is used to insert the gene into the required cells.

c) Genes can also be transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics. Crops that have had their genes modified in this way are called genetically modified crops (GM crops). Genetically modified crops include ones that are resistant to insect attack or to herbicides. Genetically modified crops generally show increased yields.

d) Concerns about GM crops include the effect on populations of wild flowers and insects, and uncertainty about the effects of eating GM crops on human health. 

Candidates should be able, when provided with appropriate information, to interpret information about cloning techniques and genetic engineering techniques and to make informed judgements about issues concerning cloning and genetic engineering, including GM crops.