OxfordAQA International GCSE Biology 3.2 Bioenergetics Exam Questions

About OxfordAQA International GCSE Biology 3.2 Bioenergetics

Bioenergetics is the largest section of OxfordAQA International GCSE Biology. It follows energy and materials through organisms: photosynthesis and limiting factors, water and sugar transport in plants, the human circulatory system and blood, digestion and enzymes, the breathing system and aerobic and anaerobic respiration. It links back to cells and exchange surfaces in 3.1 Organisation and forward to ecology. Some parts, such as transpiration, stents, blood groups and ventilators, are only in the separate Biology course. Examiners like limiting factor graphs, explaining enzyme denaturation, comparing arteries, veins and capillaries, and explaining oxygen debt after exercise. Three required practicals sit here.

What you need to know

• Photosynthesis Carbon dioxide + water → glucose + oxygen, with light absorbed by chlorophyll in chloroplasts; 6CO2 + 6H2O → C6H12O6 + 6O2. Low temperature, shortage of carbon dioxide and shortage of light can each limit the rate. Glucose is used for respiration, stored as starch, converted to fats, oils and cellulose, or made into proteins using nitrate ions.
• Transport in plants Water and mineral ions enter through root hairs; carbon dioxide enters leaves through stomata, which are opened and closed by guard cells. Xylem carries water and ions upward in the transpiration stream; phloem carries dissolved sugars to growing regions and storage organs (translocation). Transpiration is faster in hot, dry, windy conditions.
• Heart and blood vessels The heart pumps blood round a double circulation; it has two atria and two ventricles, and valves keep flow one way. A group of pacemaker cells in the right atrium controls resting heart rate. Arteries have thick, muscular, elastic walls; veins have thinner walls and valves; capillaries are narrow and thin walled for exchange. Learn the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries.
• Blood Plasma carries cells, carbon dioxide, digested food and urea. Red blood cells have no nucleus and contain haemoglobin, which forms oxyhaemoglobin in the lungs. White blood cells defend against microorganisms. Platelets help clotting, where fibrinogen is converted to fibrin, which traps blood cells. Separate Biology adds blood groups O, A, B and AB, transplants and rejection.
• Enzymes and digestion Enzymes are protein catalysts; high temperatures change the shape of the active site (denature) and each enzyme has an optimum pH. Amylase turns starch into sugars, protease turns proteins into amino acids, lipase turns lipids into fatty acids and glycerol. Stomach acid gives acidic conditions; bile from the liver, stored in the gall bladder, neutralises acid and emulsifies fats.
• Breathing Breathing in: intercostal muscles and diaphragm contract, thorax volume increases, pressure falls, air enters. Breathing out: the muscles relax, volume decreases, pressure rises, air leaves. Alveoli give a large surface area with a rich capillary supply.
• Respiration Aerobic: glucose + oxygen → carbon dioxide + water, mostly in mitochondria. Anaerobic in muscles: glucose → lactic acid, releasing much less energy and creating an oxygen debt. In plants and some microorganisms anaerobic respiration gives ethanol and carbon dioxide. During exercise heart rate, breathing rate and depth rise and glycogen is converted to glucose.
• Required practicals Investigate how variables affect the rate of photosynthesis; investigate the effects of exercise on the human body; and, in separate Biology, investigate how temperature and pH affect the rate of digestion.

Common mistakes in 3.2 Bioenergetics

• Saying enzymes are killed at high temperature. Enzymes are not alive; they are denatured because the active site changes shape.
• Saying plants respire only at night. Plants respire all the time; photosynthesis only happens when light is available.
• Saying veins have valves to stop blood flowing backwards because pressure is high. Pressure in veins is low, which is why valves are needed.
• Stating that the lungs take in oxygen by breathing alone. Breathing (ventilation) moves air; gas exchange happens by diffusion across the alveoli.
• Writing that anaerobic respiration in humans produces carbon dioxide. In human muscle the only product is lactic acid.

Try three 3.2 Bioenergetics exam-style questions