OCR GCSE Biology A B2.2 The Challenges of Size Exam Questions

About OCR GCSE Biology A B2.2 The Challenges of Size

B2.2 explains why large multicellular organisms need specialised exchange surfaces and transport systems. On the OCR Gateway Biology A course it completes Topic B2 Scaling up, assessed in Paper 1 (Foundation) or Paper 3 (Higher). You study surface area to volume ratio, the human circulatory system and transport in plants, linking back to diffusion and osmosis in B2.1 and forward to health and cardiovascular disease in B6.3. Examiners like to test surface area to volume calculations, the double circulatory system, adaptations of arteries, veins and capillaries, and the factors affecting transpiration measured with a potometer.

What you need to know

• Surface area to volume ratio As organisms get bigger, their surface area to volume ratio decreases, so diffusion across the body surface is too slow. Large organisms need exchange surfaces such as alveoli and villi, plus transport systems.
• The heart and double circulation Humans have a double circulatory system: blood passes through the heart twice per circuit. The right side pumps deoxygenated blood to the lungs and the left side pumps oxygenated blood to the body; the left ventricle wall is thicker to generate higher pressure.
• Blood vessels Arteries have thick muscular and elastic walls to withstand high pressure. Veins have thinner walls, a wide lumen and valves to stop backflow. Capillaries have walls one cell thick for fast diffusion.
• Components of blood Red blood cells carry oxygen using haemoglobin, have no nucleus and are biconcave. White blood cells defend against pathogens, platelets help clotting and plasma carries dissolved substances such as glucose, carbon dioxide and urea.
• Xylem and phloem Xylem vessels are dead, hollow, lignified tubes that carry water and mineral ions upwards. Phloem is living tissue that transports dissolved sugars up and down the plant, a process called translocation.
• Transpiration Transpiration is the loss of water vapour from leaves through stomata, which pulls water up the xylem. Its rate increases with higher light intensity, temperature and air movement and with lower humidity, and can be measured using a potometer.

Common mistakes in B2.2 The Challenges of Size

• Saying larger organisms have a smaller surface area. They have a larger surface area but a smaller surface area to volume ratio.
• Stating that arteries carry oxygenated blood and veins carry deoxygenated blood in every case. Arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood.
• Writing that phloem carries water. Phloem carries dissolved sugars; xylem carries water and mineral ions.
• Saying capillaries have thin walls 'so blood can flow through'. The one-cell-thick walls give a short diffusion distance for exchange.
• Assuming a potometer measures water lost. It measures water uptake, which is roughly equal to the rate of transpiration.

Try three B2.2 The Challenges of Size exam-style questions