CCEA A Level Biology 2.1 Transport and Exchange Mechanisms Exam Questions

About CCEA A Level Biology 2.1 Transport and Exchange Mechanisms

Section 2.1 is the largest part of Unit AS 2: Organisms and Biodiversity. It has four parts: the principles of exchange and transport, gaseous exchange, transport in plants and transpiration, and the circulatory system in mammals. You need Fick's law, alveolar structure and breathing, respirometers, root tissues and the apoplast and symplast pathways, cohesion-tension, xerophytes and hydrophytes, blood vessels, the cardiac cycle and its control, blood clotting, haemoglobin, the Bohr effect, myoglobin and altitude. Examiners often set graph interpretation on oxygen dissociation curves and potometer data, and essays on the heart or water movement.

What you need to know

• Size and exchange Large organisms have a small surface area to volume ratio, so need specialised exchange surfaces and mass transport. Efficient surfaces are large, thin and have steep concentration gradients (Fick's law).
• Gas exchange Alveoli have squamous epithelium, surfactant, a moist lining and a rich blood supply. Ventilation uses intercostal muscles and the diaphragm. Smoking damages cilia and causes bronchitis, emphysema and lung cancer. A respirometer with KOH measures oxygen uptake.
• Water uptake Root hairs take in water by osmosis and ions by active transport. Water crosses the cortex by the apoplast and symplast pathways; the Casparian strip in the endodermis forces water into the symplast.
• Transpiration and xylem Water evaporates mainly through stomata, creating tension that pulls a continuous column up the xylem by cohesion and adhesion. Rate is affected by light, air currents, temperature, humidity and soil water. Xerophytes have sunken stomata, hairs, rolled leaves and thick cuticles.
• Circulation and vessels Mammals have a double circulation. Arteries have thick elastic and muscle layers; veins have thin walls, a large lumen and valves; capillaries are one squamous endothelial cell thick. Atheroma can lead to atherosclerosis.
• The heart The cardiac cycle is diastole, atrial systole and ventricular systole. The SA node starts excitation, the AV node delays it, then the bundle of His and Purkinje fibres spread it through the ventricles from the apex up. ECGs and angiography diagnose heart disease.
• Blood and haemoglobin Clotting involves platelets, thromboplastin, prothrombin to thrombin (with calcium ions and vitamin K) and fibrinogen to fibrin. Haemoglobin carries up to four O2. The Bohr effect (more CO2, higher temperature, lower pH) shifts the curve right; myoglobin's curve lies to the left.

Common mistakes in 2.1 Transport and Exchange Mechanisms

• Saying water is pushed up the xylem by transpiration. Evaporation creates tension that pulls the cohesive column of water up.
• Saying the Bohr effect increases oxygen affinity. A right shift means lower affinity, so more oxygen is unloaded at respiring tissues.
• Saying the ventricles contract from the top down. Purkinje fibres carry excitation to the apex so ventricles contract from the base upwards, pushing blood into the arteries.
• Describing veins as having thicker walls than arteries. Arteries have thicker muscle and elastic layers to withstand high pressure.
• Forgetting that the AV node delays the impulse. The delay allows the atria to empty before ventricular systole.

Try three 2.1 Transport and Exchange Mechanisms exam-style questions