AS 2024 — CCEA AS Biology Practice Questions

Question 1

Compare the process of gas exchange in a fish gill with that in an insect tracheal system.

Accepted Answer

Both surfaces have a very large surface area — the many gill filaments and lamellae in a fish, and the extensive branching tracheoles in an insect — and in both the exchange surface is thin, giving a short diffusion distance. The fish maintains its concentration gradient by counter-current flow, with water and blood flowing in opposite directions so that a gradient exists along the whole length of the lamella, whereas the insect has no respiratory pigment and no blood transport of oxygen: oxygen diffuses directly from the tracheoles into the respiring tissues. Ventilation also differs — the fish maintains a continuous one-way flow of water over the gills, while an insect ventilates by abdominal pumping movements and closes its spiracles to limit water loss.

Question 2

Explain how tissue fluid is formed at the arterial end of a capillary and how most of it returns to the blood.

Accepted Answer

At the arterial end of a capillary the hydrostatic pressure of the blood is high, and this forces water and small dissolved solutes such as glucose and ions out through the capillary wall to form tissue fluid. Plasma proteins are too large to pass through, so they stay in the capillary. By the venous end the hydrostatic pressure has fallen because fluid has been lost, while the plasma proteins left behind give the blood a lower, more negative water potential than the tissue fluid. Water therefore moves back into the capillary by osmosis. The excess tissue fluid that does not return drains into the lymphatic vessels and is eventually returned to the blood at the subclavian vein.

Question 3

Explain how a phagocyte destroys a bacterium, and explain why this response is described as non-specific.

Accepted Answer

A phagocyte is attracted along a concentration gradient of chemicals released by the bacterium and by the products of the immune response, and attaches to antigens on the bacterial surface. The cell surface membrane then extends around the bacterium and engulfs it by endocytosis, enclosing it in a vesicle called a phagosome. Lysosomes move towards the phagosome and fuse with it, releasing hydrolytic enzymes such as lysozyme that hydrolyse the bacterial cell wall and contents. The soluble products are absorbed into the cytoplasm of the phagocyte and the antigens may then be displayed on its surface. The response is non-specific because the phagocyte uses the same mechanism and the same digestive enzymes on any pathogen it meets, rather than responding to one particular antigen.

AS 2024: CCEA Biology practice questions

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Compare the process of gas exchange in a fish gill with that in an insect tracheal system.

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