Cell structures and their functions

New cells are produced only by the division of existing cells.

Specialised cells

In multicellular organisms, cells differentiate to carry out particular functions. For each, link a feature to its function.

Levels of organisation

Size of specimens: magnification

• Image size and actual size must be in the same units before dividing.
• Measure the image with a ruler in mm (to the nearest mm).
• Supplement Convert between millimetres and micrometres: mm → µm multiply by 1000; µm → mm divide by 1000.

Diffusion

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.

• The energy for diffusion comes from the kinetic energy of random movement of molecules and ions. No energy from respiration is needed (it is a passive process).
• Some substances move into and out of cells by diffusion through the cell membrane.
• Importance to organisms: oxygen and carbon dioxide exchange in alveoli and leaves; absorption of digested food in the small intestine; movement of urea from cells into blood.

Factors that increase the rate of diffusion

Water as a solvent

Water dissolves many substances, which matters for:

• Digestion: enzymes and food molecules are in solution so reactions can happen.
• Excretion: urea and excess salts are removed dissolved in urine.
• Transport: glucose, ions and hormones dissolve in blood plasma; mineral ions and sucrose dissolve in xylem and phloem sap.

Osmosis

• Water diffuses through partially permeable membranes by osmosis. The cell membrane is partially permeable.
• Supplement Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
• Pure water has the highest water potential. Adding solute lowers the water potential.

Effects on plant and animal cells

• Supplement Turgor pressure: the pressure of the cell contents pushing outwards against the inelastic cell wall. It keeps plant cells firm and supports non-woody stems and leaves. Loss of turgor causes wilting.
• Supplement Plants absorb water because the cell sap in root hair cells has a lower water potential than the soil water, so water moves into root hairs by osmosis.
• Supplement Animal tissues must be bathed in fluid of the right water potential (the blood plasma and tissue fluid), otherwise cells are damaged.

Required practical: osmosis in potato (or dialysis tubing)

With dialysis (Visking) tubing: fill with sugar solution, tie it, place in water and watch the tubing swell or a liquid level rise in an attached capillary tube as water enters by osmosis.

Active transport

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration.

• Example: root hair cells take up mineral ions (e.g. nitrate) from the soil, where they are at a lower concentration than inside the cell.
• Other examples: absorption of glucose in the small intestine and reabsorption of glucose in the kidney.
• Supplement Protein carriers in the membrane carry out active transport: the particle binds to the carrier; energy from respiration changes the carrier's shape; the particle is released on the other side.
• Cells that do a lot of active transport have many mitochondria. Anything that stops respiration (no oxygen, cyanide, low temperature) stops active transport.

Comparing the three processes

Chemical elements

Large molecules from small units

Large biological molecules are built by joining many smaller units together. Digestion breaks them back down.

Food tests

Food samples are usually ground up with water first to make a solution or suspension.

Supplement Structure of DNA

• A DNA molecule is made of two strands coiled together to form a double helix.
• Each strand contains chemicals called bases. There are four bases: A, T, C and G.
• Bonds between pairs of bases hold the two strands together.
• The bases always pair in the same way: A with T and C with G. This is complementary base pairing.
• The sequence of bases along a strand forms the genetic code.

If one strand reads ATTGCA, the complementary strand reads TAACGT. If a DNA sample is 20% A, it is also 20% T, leaving 60% for C + G, so 30% C and 30% G.

Key definitions

• A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.
• Enzymes are proteins that function as biological catalysts and are involved in all metabolic reactions.
• Enzymes are important because they keep reaction rates fast enough, at body temperature, to sustain life. Without them, reactions such as respiration and digestion would be far too slow.

How an enzyme works

• The same model works for breaking molecules down (e.g. amylase: starch → maltose) and for building them up (e.g. joining glucose to make glycogen).
• Supplement Specificity: each enzyme only catalyses one reaction (or one type of reaction) because the shape of its active site is complementary to the shape of only one substrate. Amylase cannot digest protein because protein molecules do not fit its active site.

Effect of temperature

• As temperature rises towards the optimum, the rate increases.
• Above the optimum, the rate falls quickly and eventually stops.
• Most human enzymes have an optimum of about 37°C.
• Supplement Rising to the optimum: molecules gain kinetic energy and move faster, so there are more frequent effective collisions between enzyme and substrate, and more enzyme-substrate complexes form.
• Supplement Above the optimum: the enzyme vibrates so much that bonds holding its shape break. The active site changes shape, so the substrate no longer fits and fewer enzyme-substrate complexes form. The enzyme is denatured; this is permanent.
• At low temperatures enzymes are inactive but not denatured; activity returns on warming.

Effect of pH

• Each enzyme has an optimum pH at which its activity is greatest.
• Pepsin in the stomach works best at around pH 2; most enzymes in the small intestine, such as trypsin and lipase, work best at around pH 7 to 8.
• Supplement Moving away from the optimum pH (either more acidic or more alkaline) changes the shape of the active site, so the substrate fits less well and fewer enzyme-substrate complexes form. Extreme pH denatures the enzyme.

Required practical: investigating temperature or pH

A typical method uses amylase and starch, testing for the disappearance of starch with iodine.

Control variables: volume and concentration of starch and amylase; pH (when varying temperature); temperature (when varying pH); sampling interval. Other suitable investigations include catalase breaking down hydrogen peroxide (measure the volume of oxygen collected) or protease acting on milk or egg white.

Photosynthesis: the definition

Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light.

• Raw materials: carbon dioxide (from the air, through stomata) and water (from the soil, through roots and xylem).
• Chlorophyll is a green pigment found in chloroplasts. It absorbs light energy and transfers light energy into chemical energy in molecules, for the synthesis of carbohydrates.
• Oxygen is released as a by-product.

Equations

What the plant does with glucose

Mineral ions

Practical: testing a leaf for starch

Starch in a leaf is evidence that photosynthesis has happened. Plants are destarched first by leaving them in the dark for 48 hours so any starch present afterwards must be new.

Showing each factor is needed

• Chlorophyll: use a variegated leaf (green and white). Only green areas turn blue-black.
• Light: cover part of a leaf with black paper or foil. The covered area stays orange-brown.
• Carbon dioxide: enclose a leaf or plant in a sealed bag or flask with soda lime or sodium hydroxide (absorbs CO₂). Control: identical set-up without the absorber.

Practical: rate of photosynthesis

• Use an aquatic plant such as pondweed (Elodea) in water with added sodium hydrogencarbonate (a source of CO₂).
• Count oxygen bubbles per minute, or collect gas in a measuring cylinder or gas syringe.
• Light intensity: move a lamp to different distances. Use a heat shield (a beaker of water) or LED lamp so temperature stays constant.
• CO₂ concentration: vary the concentration of sodium hydrogencarbonate.
• Temperature: use water baths at different temperatures.
• Allow the plant to acclimatise for a few minutes at each new condition before counting; repeat and calculate a mean.

Hydrogencarbonate indicator (gas exchange)

Supplement Limiting factors

A limiting factor is something present in the environment in such short supply that it restricts life processes. For photosynthesis the main limiting factors are light intensity, carbon dioxide concentration and temperature.

• On a graph, while the line is rising, the factor on the x-axis is limiting.
• When the line levels off (plateaus), something else is now limiting.
• Temperature affects the enzymes that control photosynthesis: rate rises to an optimum, then falls as enzymes denature.
• Growers use this in glasshouses: paraffin heaters raise temperature and CO₂; artificial lighting extends the day.

Leaf structure

• Supplement Leaves are also broad (large surface area to absorb light) and thin (short diffusion distance for gases).

Diet

A balanced diet contains all the nutrients in the correct proportions, plus fibre and water, to meet the body’s needs. Energy needs depend on age, activity level and pregnancy.

Deficiency diseases

The digestive system

Key definitions

Physical digestion

Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. It increases the surface area for enzymes to act on.

• Incisors: chisel-shaped, for biting and cutting.
• Canines: pointed, for gripping and tearing.
• Premolars and molars: broad, ridged surfaces for chewing and grinding.
• Tooth structure: enamel (hardest outer layer), dentine, pulp cavity with nerves and blood vessels, cement anchoring the root in the jaw bone, surrounded by the gum.
• The stomach churns food with its muscular walls, mixing it with gastric juice.
• Supplement Bile emulsifies fats: it breaks large fat droplets into smaller droplets, increasing the surface area for lipase to act on. This is physical, not chemical, digestion.

Chemical digestion

Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. This is needed so they can be absorbed into the blood and used by cells.

Acid and alkali

• Hydrochloric acid in gastric juice kills harmful microorganisms in food and gives the acidic pH (about 2) that is optimum for pepsin.
• Supplement Bile is alkaline. It neutralises the acidic mixture of food and gastric juice entering the duodenum, giving a suitable pH for enzymes in the small intestine.

Absorption

• The small intestine is the main region where nutrients are absorbed.
• Water is absorbed in both the small intestine (most) and the colon.
• The inner wall is folded and covered in villi; each epithelial cell has microvilli. Both greatly increase the surface area for absorption.

Supplement Structure of a villus

Xylem and phloem

Position in the plant

• Root: xylem forms a central star or X shape; phloem sits in the gaps between the arms.
• Stem: vascular bundles arranged in a ring near the edge; xylem on the inside, phloem on the outside.
• Leaf: in the veins (vascular bundles); xylem towards the upper surface, phloem towards the lower surface.

Supplement Xylem structure and function

• Thick walls strengthened with lignin: support the plant and stop vessels collapsing under tension.
• No cell contents (dead cells): nothing blocks the flow of water.
• Cells joined end to end with no cross walls: form a continuous hollow tube.

Water uptake

• Root hair cells are the site of water and mineral ion uptake. Water enters by osmosis.
• Their long, thin extension gives a large surface area, increasing the rate of uptake of water by osmosis and mineral ions by active transport.

Pathway of water

Practical: tracing the pathway

• Stand a celery stalk or white flower in water containing a dye (e.g. food colouring or eosin) for a few hours.
• Cut thin cross-sections and view with a hand lens or microscope: only the xylem is stained.

Transpiration

Transpiration is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata.

• Supplement The mesophyll cells have a large surface area and the leaf interior has air spaces, so a lot of water evaporates; the air spaces become saturated with water vapour.

Supplement How water moves up the plant

Factors affecting transpiration rate

Practical: investigating transpiration

• Use a potometer: a leafy shoot sealed into a water-filled tube with a capillary and air bubble.
• Cut the shoot under water and assemble under water so no air enters the xylem; seal joints with petroleum jelly.
• Measure the distance the air bubble moves in a set time. A potometer actually measures water uptake, which is close to the rate of water loss.
• Vary temperature (e.g. in rooms of different temperatures) or wind speed (a fan at different distances or settings), keeping other factors constant.
• Alternative: weigh a potted plant over time with the soil surface covered by a plastic bag.

Supplement Wilting

• If a plant loses water faster than it is taken up, cells lose water by osmosis and become flaccid.
• The cells no longer press against each other, so the leaves and stem lose support and droop.

Supplement Translocation

Translocation is the movement of sucrose and amino acids in phloem from sources to sinks.

• Some parts act as a source at one time and a sink at another. A potato tuber is a sink in summer (storing starch) and a source in spring (supplying sugar to new shoots before leaves can photosynthesise).

Circulatory systems

The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood.

• Fish have a single circulation: blood passes through the heart once per circuit (heart → gills → body → heart).
• Supplement Mammals have a double circulation: blood passes through the heart twice per circuit. The pulmonary circuit goes to the lungs; the systemic circuit goes to the rest of the body.
• Supplement Advantages of double circulation: blood returns to the heart after the lungs so it can be pumped at high pressure to the body, delivering oxygen and glucose quickly; lower pressure in the lungs avoids damaging delicate capillaries.

Structure of the heart

• Muscular wall of cardiac muscle that contracts to pump blood.
• Septum separates the left and right sides, so oxygenated and deoxygenated blood do not mix.
• Right and left atria (upper chambers) receive blood; right and left ventricles (lower chambers) pump blood out.
• One-way valves prevent backflow.
• Coronary arteries on the surface supply the heart muscle with oxygen and glucose.
• Supplement Ventricles have thicker walls than atria because they pump blood out of the heart, not just into the ventricles below.
• Supplement The left ventricle wall is thicker than the right because it pumps blood at high pressure all around the body; the right only pumps to the nearby lungs.
• Supplement Atrioventricular valves lie between atria and ventricles; semilunar valves lie at the bases of the aorta and pulmonary artery.

Blood flow through the heart

Supplement The cardiac cycle

• The atria contract, pushing blood through the open atrioventricular valves into the ventricles.
• The ventricles contract: pressure rises, closing the atrioventricular valves (preventing backflow into the atria) and forcing open the semilunar valves so blood enters the arteries.
• The ventricles relax: pressure falls, and the semilunar valves close, preventing backflow from the arteries into the ventricles.

Heart activity and exercise

• Heart activity can be monitored by an ECG, by measuring pulse rate, and by listening to the sounds of valves closing with a stethoscope.
• Physical activity increases heart rate.
• Supplement Explanation: muscles respire faster during exercise and need more oxygen and glucose, and more carbon dioxide must be removed. A faster heart rate delivers blood to the muscles more quickly. If oxygen supply is insufficient, anaerobic respiration produces lactic acid, and heart rate stays high after exercise to remove it.

Coronary heart disease (CHD)

• CHD is caused by blockage of the coronary arteries (fatty deposits build up in the artery walls), so heart muscle gets less oxygen and glucose. Heart muscle cells cannot respire and may die, causing a heart attack.
• Risk factors: diet high in saturated fat and salt, lack of exercise, stress, smoking, genetic predisposition, increasing age, and sex (males are at higher risk).
• Supplement Reducing risk: a diet low in saturated fat and salt reduces fatty deposits and blood pressure; regular exercise strengthens heart muscle, reduces blood pressure and helps maintain a healthy body mass.

Blood vessels

Main blood vessels

Blood

Supplement Blood clotting

• Clotting prevents blood loss and stops pathogens entering through the wound.
• At a wound, platelets trigger a chain of reactions converting soluble fibrinogen into insoluble fibrin.
• Fibrin forms a mesh of fibres that traps red blood cells, forming a clot which dries into a scab.

Key definitions

• Pathogens include bacteria, viruses, fungi and protoctists.
• Not all diseases are transmissible. Some are caused by the immune system attacking the body’s own cells, e.g. type 1 diabetes, where the insulin-secreting cells of the pancreas are destroyed.

How pathogens are transmitted

Body defences

Mechanical barriers

• Skin: a tough outer layer that pathogens cannot easily cross.
• Hairs in the nose: trap larger particles in inhaled air.

Chemical barriers

• Mucus in the airways traps pathogens; cilia sweep it away from the lungs.
• Stomach acid (hydrochloric acid) kills pathogens in food and drink.

Cells

• White blood cells: phagocytes carry out phagocytosis; lymphocytes produce antibodies.

• Vaccination also protects the body by giving immunity in advance.

Controlling the spread of disease

• A clean water supply.
• Hygienic food preparation: washing hands, cooking thoroughly, keeping raw and cooked foods apart, refrigeration.
• Good personal hygiene: handwashing, covering coughs and sneezes.
• Waste disposal: collecting rubbish so it does not attract flies and rats.
• Sewage treatment: prevents human waste contaminating drinking water.

Supplement Antibodies and antigens

• Antigens are molecules (often proteins) on the surface of pathogens. Each pathogen has its own antigens with a specific shape.
• Antibodies are proteins made by lymphocytes that bind to antigens.
• Each antibody has a specific shape that is complementary to one particular antigen, so it fits only that antigen.
• Antibodies work by directly destroying pathogens (or clumping them together) or by marking pathogens for destruction by phagocytes.

Supplement Active immunity

Active immunity is defence against a pathogen by antibody production in the body. It is gained after an infection by a pathogen or by vaccination, and is long-term because memory cells are produced.

How vaccination works

• Vaccination is important in controlling the spread of disease: if enough people are immune, the pathogen cannot pass easily between hosts, protecting those who are not vaccinated (herd immunity).

Supplement Passive immunity

Passive immunity is short-term defence against a pathogen by antibodies acquired from another individual, e.g. across the placenta or in breast milk.

• Memory cells are not produced, so the protection fades as the antibodies are broken down.
• Breast-feeding is important because it passes antibodies to the infant, protecting it while its own immune system is still developing.

Supplement Cholera

Features of gas exchange surfaces

Gas exchange happens by diffusion. In humans the exchange surface is the lining of the alveoli. Any efficient exchange surface has:

Structures of the breathing system

Inspired and expired air

• Test for CO₂: bubble the gas through limewater; it turns from colourless to milky/cloudy. Expired air turns limewater cloudy much faster than inspired air.
• Supplement Explaining the differences: O₂ is lower in expired air because it diffuses into the blood and is used in aerobic respiration; CO₂ is higher because it is produced by respiration and diffuses from the blood into the alveoli; water vapour is higher because water evaporates from the moist alveolar lining.

Investigating physical activity and breathing

• Count breaths per minute at rest (rate) and note depth, e.g. using a spirometer or chest expansion.
• Exercise for a fixed time (e.g. 2 minutes of step-ups), then measure again straight away and every minute until back to resting.
• Control: same person, same exercise intensity and duration; repeat for reliability.
• Result: both rate and depth of breathing increase during and after exercise.

Supplement Muscles respire faster during exercise, producing more CO₂. The brain detects the increased CO₂ concentration in the blood and sends impulses to the intercostal muscles and diaphragm, increasing the rate and depth of breathing. This removes CO₂ faster and supplies more O₂ for aerobic respiration.

Supplement Ventilation: breathing in and out

The internal and external intercostal muscles are antagonistic: when one set contracts the other relaxes.

Supplement Protecting the breathing system

• Goblet cells in the lining of the trachea and bronchi secrete mucus, which traps pathogens and dust particles.
• Ciliated cells have cilia that beat to sweep the mucus up to the throat, where it is swallowed.
• This stops pathogens and particles reaching the alveoli.

Key definitions

• Respiration: the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
• Aerobic respiration: the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
• Anaerobic respiration: the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.

Respiration is controlled by enzymes, so its rate is affected by temperature. It happens in all living cells, all the time, including plant cells.

Uses of energy in the body

• Muscle contraction
• Protein synthesis
• Cell division
• Active transport
• Growth
• Passage of nerve impulses
• Maintaining a constant body temperature

Aerobic respiration

Word equation

glucose + oxygen → carbon dioxide + water

Supplement Balanced symbol equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partly broken down.

Supplement Balanced symbol equation for yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Aerobic vs anaerobic

Supplement Lactic acid and the oxygen debt

During vigorous exercise the heart and lungs cannot supply oxygen to muscles fast enough, so muscles also respire anaerobically. Lactic acid builds up in the muscles and blood, causing an oxygen debt: the oxygen needed afterwards to break the lactic acid down.

Practical: temperature and yeast respiration

• Mix yeast with glucose solution and place tubes in water baths at different temperatures (e.g. 20, 30, 40, 50, 60 °C).
• Measure the rate by counting CO₂ bubbles per minute, measuring gas volume in a syringe, or timing how long limewater (or hydrogencarbonate indicator) takes to change colour.
• Control variables: mass of yeast, concentration and volume of glucose, time allowed to acclimatise.
• Expected result: rate rises to an optimum (around 35–40 °C) and then falls as the enzymes are denatured.

Respiration in germinating seeds or small invertebrates can also be shown with a respirometer; a soda-lime absorbs CO₂ so that movement of a liquid bead shows oxygen uptake.

What is excretion?

• Excretion: the removal of the waste products of metabolism and substances in excess of requirements.
• Do not confuse this with egestion, the removal of undigested food as faeces; faeces were never part of metabolism.

The urinary system

In a section through a kidney you should identify the outer cortex, the inner medulla and the ureter leaving it. The renal artery brings blood in; the renal vein takes filtered blood away.

Supplement The liver and deamination

• The body cannot store excess amino acids.
• Deamination: the removal of the nitrogen-containing part of amino acids to form urea.
• The rest of the amino acid can be respired for energy or converted to carbohydrate and stored.
• The liver also assimilates amino acids by converting them into proteins, including plasma proteins such as fibrinogen.
• Urea travels in the blood plasma to the kidneys.
• Why excretion matters: urea and other nitrogenous wastes are toxic; if they build up they damage cells. Excess CO₂ also lowers blood pH.

Supplement The nephron

Each kidney contains about a million tubules called nephrons. Parts: glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.

Comparing blood plasma, filtrate and urine

The nervous system

• The mammalian nervous system is made of the central nervous system (CNS), the brain and spinal cord, and the peripheral nervous system (PNS), the nerves outside the CNS.
• It coordinates and regulates body functions using electrical impulses that travel along neurones.
• Sensory neurones carry impulses from receptors to the CNS; relay neurones (inside the CNS) connect sensory and motor neurones; motor neurones carry impulses from the CNS to effectors (muscles or glands).
• Reflex action: a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).

The reflex arc

Supplement Synapses

• A synapse is a junction between two neurones, with a narrow gap called the synaptic cleft.
• An impulse arriving at the end of the first neurone causes vesicles to release neurotransmitter molecules into the cleft.
• The neurotransmitter diffuses across the gap and binds to receptor proteins on the next neurone, stimulating a new impulse.
• Synapses ensure impulses travel in one direction only, because only the first neurone releases neurotransmitter and only the second has receptors.

Sense organs and the eye

Sense organs are groups of receptor cells that respond to specific stimuli: light, sound, touch, temperature and chemicals.

• Pupil reflex: in bright light the pupil gets smaller; in dim light it gets larger.
• Supplement The iris contains two antagonistic muscles. In bright light the circular muscles contract and radial muscles relax, making the pupil narrower. In dim light the radial muscles contract and circular muscles relax, widening the pupil.
• Supplement Accommodation (focusing). Near object: ciliary muscles contract, suspensory ligaments slacken, lens becomes fatter (more convex). Distant object: ciliary muscles relax, suspensory ligaments tighten, lens is pulled thinner.
• Supplement Rods work in dim light and give black-and-white vision; they are spread across the retina. Cones give colour vision in bright light; there are three types, sensitive to different colours (red, green, blue). Cones are concentrated in the fovea, which gives the sharpest image.

Hormones

Hormone: a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Supplement Adrenaline also increases blood glucose concentration, providing more glucose for respiration in muscles.

Homeostasis

• Homeostasis: the maintenance of a constant internal environment.
• Supplement It works by negative feedback: when a condition moves away from the set point, changes are made to return it to the set point.

Supplement Blood glucose control

• Blood glucose too high: the pancreas secretes insulin; the liver and muscles convert glucose to glycogen, so blood glucose falls.
• Blood glucose too low: the pancreas secretes glucagon; the liver converts glycogen to glucose, so blood glucose rises.
• Type 1 diabetes: the pancreas does not produce enough insulin. Symptoms include high blood glucose, glucose in urine, thirst, tiredness and weight loss. Treated by insulin injections, monitoring blood glucose and controlling carbohydrate intake.

Temperature control

Identify on a skin diagram: hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue. Fatty tissue insulates.

Supplement The brain monitors blood temperature and coordinates these responses.

Tropic responses in plants

• Gravitropism: a response in which parts of a plant grow towards or away from gravity. Roots are positively gravitropic; shoots are negatively gravitropic.
• Phototropism: a response in which parts of a plant grow towards or away from the direction from which light is coming. Shoots are positively phototropic.
• Investigate using seedlings lit from one side, seedlings on their side, or a clinostat (which rotates to cancel the effect of gravity).
• Supplement Auxin is made in the shoot tip and diffuses down the shoot. In one-sided light it collects on the shaded side, where it stimulates cell elongation, so the shoot bends towards the light.
• Supplement Synthetic auxins such as 2,4-D are used as weedkillers: they cause uncontrolled growth in broad-leaved weeds.

What is a drug?

• Drug: any substance taken into the body that modifies or affects chemical reactions in the body.
• Drugs can be medicinal (e.g. antibiotics, painkillers) or recreational. For this syllabus the focus is on antibiotics.

Antibiotics

• Antibiotics target structures and processes found in bacterial cells, such as the cell wall or bacterial enzymes and ribosomes.
• Because these targets are not found in human cells, the antibiotic can harm the bacteria without damaging the patient’s cells.

Supplement Why antibiotics do not affect viruses

• Viruses are not cells: they have no cell wall, no ribosomes of their own and no metabolism.
• Viruses reproduce inside host cells, using the host’s own machinery.
• So there is nothing for an antibiotic to act on, and killing the virus would mean damaging the host cell.
• This is why antibiotics are not prescribed for colds or influenza.

Supplement Antibiotic resistance

Some bacteria are resistant to antibiotics. A well-known example is MRSA (methicillin-resistant Staphylococcus aureus), which is resistant to many common antibiotics and is hard to treat, especially in hospitals.

Reducing the development of resistant strains

• Use antibiotics only when essential, e.g. not for viral infections or minor infections the body can fight off.
• Complete the full course as prescribed, so that all the bacteria are killed and none are left to develop or spread resistance.
• Other helpful measures: reduce use of antibiotics in farming, good hygiene in hospitals to stop resistant strains spreading.

Asexual and sexual reproduction

• Asexual reproduction: a process resulting in the production of genetically identical offspring from one parent. Examples: binary fission in bacteria, potato tubers, runners in strawberry plants, budding in yeast.
• Sexual reproduction: a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other.
• Fertilisation: the fusion of the nuclei of gametes.
• Gametes are haploid (one set of chromosomes); the zygote is diploid (two sets).

Crop growers use asexual methods (e.g. cuttings) to keep desirable varieties, and sexual reproduction to breed new varieties.

Sexual reproduction in plants

• Pollination: the transfer of pollen grains from an anther to a stigma.
• Self-pollination: transfer to a stigma of the same flower or another flower on the same plant. Cross-pollination: transfer to a stigma of a flower on a different plant of the same species.
• Supplement Self-pollination gives less variation and depends less on pollinators; cross-pollination gives more variation, so the population is better able to adapt to change, but depends on insects or wind.
• Supplement After pollination a pollen tube grows down the style to an ovule, and the male nucleus fuses with the female nucleus (fertilisation).

Germination practical: seeds need water, oxygen and a suitable temperature. Test by setting up tubes of seeds with one condition missing each time (e.g. dry cotton wool, boiled water with an oil layer, a fridge) and comparing with a control that has all three.

Sexual reproduction in humans

• Sperm adaptations: flagellum for swimming; many mitochondria to release energy; acrosome containing enzymes to digest through the jelly coat of the egg.
• Egg adaptations: energy stores in the cytoplasm; jelly coat that changes after fertilisation so only one sperm can enter.
• Supplement Compared with eggs, sperm are much smaller, made in far greater numbers, and are mobile (eggs are moved by cilia in the oviduct).

Fertilisation and pregnancy

• Supplement The amniotic sac and amniotic fluid protect the fetus from bumps.
• Supplement The placenta exchanges materials between mother and fetus by diffusion: glucose, amino acids, oxygen, water and ions pass to the fetus; carbon dioxide and urea pass back to the mother. The maternal and fetal blood do not mix.
• Supplement The umbilical cord contains blood vessels that connect the fetus to the placenta.
• Supplement Some pathogens (e.g. rubella virus) and toxins (e.g. nicotine) can cross the placenta and harm the fetus.

Sex hormones

• Testosterone (testes) causes male secondary sexual characteristics at puberty: growth of facial and body hair, deeper voice, increased muscle, sperm production.
• Oestrogen (ovaries) causes female secondary sexual characteristics: breast development, wider hips, body hair, start of the menstrual cycle.
• The menstrual cycle lasts about 28 days: menstruation (days 1–5), uterus lining rebuilds, ovulation around day 14, lining maintained ready for implantation.

Sexually transmitted infections

• Sexually transmitted infection (STI): an infection that is transmitted through sexual contact.
• HIV is a virus that causes an STI; HIV infection may lead to AIDS.
• Transmitted by unprotected sex, sharing needles, infected blood, and from mother to baby.
• Control: using condoms, limiting sexual partners, testing and treatment, not sharing needles, screening blood.

Chromosomes, genes and alleles

• Chromosome: a thread-like structure of DNA, carrying genetic information in the form of genes.
• Gene: a length of DNA that codes for a protein.
• Allele: an alternative form of a gene.
• Sex is determined by one pair of chromosomes: females are XX, males are XY. Every egg carries an X; half of sperm carry X and half carry Y, so there is a 50% chance of either sex at each fertilisation.
• Supplement Haploid nucleus: a nucleus containing a single set of chromosomes (e.g. gametes). Diploid nucleus: a nucleus containing two sets of chromosomes (e.g. body cells). In humans the diploid number is 46 (23 pairs); the haploid number is 23.

Supplement From gene to protein

• The sequence of bases in a gene determines the sequence of amino acids in a protein.
• Different sequences of amino acids give different shapes of protein, so different functions (e.g. enzyme active sites, antibodies, receptors).
• All body cells contain the same genes, but most cells express only some of them. A cell only makes the proteins it needs, which is why a muscle cell differs from a liver cell.

Mitosis

Mitosis: nuclear division giving rise to genetically identical cells.

• Used for growth, repair of damaged tissues, replacement of cells and asexual reproduction.
• Supplement Before mitosis, each chromosome is replicated (copied) so it consists of two identical strands. During mitosis the copies separate and move to opposite poles, so each daughter cell gets a full, identical set of chromosomes.
• Supplement Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
• Supplement Uncontrolled cell division by mitosis can result in cancer.

Meiosis

Meiosis: a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.

• Meiosis produces gametes (sperm, eggs, pollen nuclei, ovules).
• Halving the number means fertilisation restores the diploid number.
• The gametes are genetically different from each other, which is a source of variation.

Monohybrid inheritance: key terms

Homozygous individuals only ever pass on one type of allele, so two identical homozygous parents are pure-breeding. Heterozygous individuals pass on either allele with equal probability.

Laying out a genetic diagram

• Heterozygous × heterozygous gives a 3 : 1 ratio of phenotypes.
• Heterozygous × homozygous recessive gives a 1 : 1 ratio.
• Pedigree diagrams show inheritance through a family. If two unaffected parents have an affected child, the condition must be recessive and both parents are heterozygous.

Supplement Test crosses, codominance and sex linkage

• Test cross: cross the individual with the dominant phenotype (unknown genotype) with a homozygous recessive individual. All dominant offspring suggests homozygous dominant; any recessive offspring (roughly 1 : 1) shows it is heterozygous.
• Codominance: a situation in which both alleles in a heterozygous organism contribute to the phenotype.
• ABO blood groups: alleles I^A and I^B are codominant; I^o is recessive to both. Group A = I^AI^A or I^AI^o; group B = I^BI^B or I^BI^o; group AB = I^AI^B; group O = I^oI^o.
• Sex-linked characteristic: a feature in which the gene responsible is located on a sex chromosome, making it more common in one sex than the other.
• Red-green colour blindness is carried on the X chromosome. Write alleles as X^N and Xⁿ. Males (XⁿY) need only one recessive allele to be affected because the Y carries no copy; females need two, so a heterozygous female (X^NXⁿ) is a carrier.

Variation

Variation: differences between individuals of the same species.

• Genetic variation is inherited; environmental variation is caused by factors such as diet, light, temperature or accidents and is not passed on.

Mutation

• Mutation: genetic change.
• Gene mutation: a random change in the base sequence of DNA.
• Mutation is the way in which new alleles are formed.
• The rate of mutation is increased by ionising radiation (e.g. X-rays, gamma rays) and some chemicals (e.g. chemicals in tobacco smoke).

Sources of genetic variation

Remember “MMRR”: new alleles come from mutation, meiosis produces genetically different gametes, and which individuals mate and which gametes fuse are both matters of chance.

Adaptive features

• Adaptive feature: an inherited feature that helps an organism to survive and reproduce in its environment.
• Supplement Adaptive features are the inherited functional features of an organism that increase its fitness.
• Supplement Fitness: the probability of an organism surviving and reproducing in the environment in which it is found.

Supplement Plants in extreme water conditions

Natural selection

Supplement Adaptation: the process, resulting from natural selection, by which populations become more suited to their environment over many generations.

Supplement Antibiotic resistance in bacteria

• A random mutation gives one bacterium an allele for resistance.
• When antibiotic is used, non-resistant bacteria are killed; the resistant bacterium survives.
• It reproduces rapidly by binary fission, passing the resistance allele on.
• The population becomes mostly resistant, e.g. MRSA.
• Reduce the risk: prescribe antibiotics only when needed (not for viral infections) and always finish the course.

Selective breeding (artificial selection)

1. Choose individuals with the desired feature (e.g. high milk yield, disease resistance, large fruit).
2. Cross (breed) these individuals together.
3. From the offspring, select those showing the feature most strongly.
4. Repeat over many generations.

Energy flow

• The Sun is the principal source of energy input to biological systems.
• Energy flows through living organisms: light energy is converted to chemical energy by photosynthesis, passed along food chains, and is eventually lost to the environment as heat. Energy is not recycled, unlike nutrients.

• Consumers are classed as primary, secondary, tertiary and quaternary according to their position. Arrows point in the direction of energy flow (from the eaten to the eater).
• Removing or adding a species changes a food web: e.g. if a predator is removed, its prey may increase, then overgraze the producers.

Ecological pyramids

Supplement Efficiency of energy transfer

• Transfer between trophic levels is inefficient: only about 10% of energy is passed on.
• Energy is lost through respiration (released as heat), movement, excretion (urine, faeces) and parts that are not eaten (bones, roots, fur).
• This limits most food chains to fewer than five trophic levels.
• It is more energy-efficient for humans to eat crop plants than to eat livestock fed on crops, because a trophic level (and its energy losses) is removed, so more people can be fed per unit of land.

The carbon cycle

Deforestation and combustion both raise atmospheric CO₂: fewer trees remove CO₂ by photosynthesis, and burning or decaying wood adds CO₂.

Supplement The nitrogen cycle

• Nitrogen fixation: nitrogen-fixing bacteria (free-living in soil, or in root nodules of legumes such as peas and clover) convert N₂ gas into ammonium/nitrogen compounds. Lightning also fixes nitrogen.
• Absorption: plants take up nitrate ions by active transport through root hairs and use them to make amino acids and then proteins.
• Feeding: animals obtain proteins by eating, digest them to amino acids and build their own proteins.
• Deamination: excess amino acids are broken down in the liver to urea, which is excreted.
• Decomposition: decomposers break down proteins in dead organisms and urea into ammonium ions.
• Nitrification: nitrifying bacteria convert ammonium ions to nitrite and then to nitrate ions (needs oxygen, e.g. well-drained soil).
• Denitrification: denitrifying bacteria convert nitrate back to N₂ gas (in waterlogged, anaerobic soil).

Populations

Factors affecting the rate of population growth: food supply, competition, predation and disease.

Sigmoid population growth curve (e.g. yeast in a flask)

Supplement Be ready to explain each phase in terms of birth rate versus death rate and the limiting factors acting at that time.

Why bacteria are useful

• They reproduce rapidly, so large populations are grown quickly.
• They can make complex molecules (e.g. proteins such as insulin, antibiotics).
• Supplement There are few ethical concerns over their manipulation and growth.
• Supplement They contain plasmids (small circular loops of DNA), which are easily cut and used as vectors to carry genes.
• Supplement The genetic code is shared by all organisms, so bacteria can read a human gene and make the human protein.

Uses of microorganisms and enzymes

Word equation for anaerobic respiration in yeast: glucose → alcohol (ethanol) + carbon dioxide.

Supplement Industrial fermenters

Fermenters are large vessels used to grow microorganisms on a large scale to make products such as insulin (from GM bacteria), penicillin (from the fungus Penicillium) and mycoprotein (a fungal protein food).

Genetic modification

Genetic modification: changing the genetic material of an organism by removing, changing or inserting individual genes.

Examples

• Insertion of the human insulin gene into bacteria to produce human insulin for people with diabetes.
• Insertion of genes into crop plants to give resistance to herbicides (weedkiller can be sprayed without killing the crop).
• Insertion of genes into crop plants to give resistance to insect pests (e.g. plants that make a toxin harmful to caterpillars).
• Insertion of genes to give crops additional vitamins (e.g. rice producing beta-carotene, a source of vitamin A).

Supplement Making human insulin with bacteria

Supplement GM crops: evaluating the issues

Food supply

Modern technology has greatly increased food production through:

• Agricultural machinery to farm larger areas more efficiently.
• Chemical fertilisers to improve yields by supplying mineral ions (nitrate, phosphate, potassium).
• Insecticides to kill insect pests and herbicides to kill weeds that compete with crops.
• Selective breeding to improve crop and livestock yields.

Negative impacts of large-scale farming

Supplement Famine and food shortage have social, economic and environmental causes: increasing human population, unequal distribution of food, poverty, war, drought, flooding, climate change, pests, and poor transport or storage.

Habitat destruction

Reasons

• Increased area needed for food crops, livestock and housing.
• Extraction of natural resources (e.g. mining, logging).
• Freshwater and marine pollution.

Humans alter food webs by removing species or introducing new ones, which can cause populations of other species to rise or fall.

Undesirable effects of deforestation

• Extinction and loss of biodiversity as habitats are destroyed.
• Loss of soil: without roots to hold it, soil is eroded by rain; nutrients are leached out.
• Flooding: less water is absorbed and transpired, so runoff increases.
• Increase of CO₂ in the atmosphere: less photosynthesis, and burning or decomposing trees release CO₂.

Pollution

• Plastics are non-biodegradable, so they persist. In water they entangle or are swallowed by animals (e.g. turtles, seabirds), and break into microplastics that enter food chains. On land they litter habitats and release toxic chemicals when burned.
• Untreated sewage and excess fertiliser entering water cause eutrophication.
• Supplement Female contraceptive hormones in sewage enter rivers and can cause feminisation of male fish (reduced fertility) and may contribute to reduced sperm counts in men.

Supplement Eutrophication

The enhanced greenhouse effect

• Carbon dioxide (from combustion and deforestation) and methane (from cattle, rice paddies, landfill) are greenhouse gases.
• They absorb and re-radiate infrared (heat) energy that would otherwise escape from the Earth, warming the atmosphere.
• Increasing levels cause climate change: rising sea levels, melting ice, more extreme weather, changes in species distribution.

Conservation

• Sustainable resource: one which is produced as rapidly as it is removed from the environment so that it does not run out.
• Some resources, such as fossil fuels, cannot be replaced; others, such as forests and fish stocks, can be conserved.

Endangered species

Species become endangered or extinct because of climate change, habitat destruction, hunting, overharvesting, pollution and introduced species.

Conservation methods: monitoring and protecting species and habitats, education, captive breeding programmes and seed banks.

Supplement Deeper conservation points

• Sustainable development: development providing for the needs of an increasing human population without harming the environment; it requires cooperation between local, national and international groups.
• A population that becomes very small has reduced genetic variation, so it is less able to adapt to change (e.g. new disease) and is at greater risk of extinction.
• Captive breeding may use artificial insemination and in vitro fertilisation (IVF), and may move animals between zoos to increase genetic variation.
• Reasons for conservation: reduce extinction, protect vulnerable environments, and maintain ecosystem functions such as nutrient cycling and resource provision (food, drugs, fuel and genes).