IGCSE Biology 0610: Active Transport, Diffusion and a Respiratory Poison

A marine biologist studies gill cells taken from a fish. The gill cells continuously accumulate potassium ions from the seawater around them, building up a much higher concentration of potassium ions inside the cells than in the seawater outside. Protein carriers in the cell membrane are responsible for this uptake.

To find out how this uptake works, the biologist adds a poison called sodium cyanide to the seawater. Cyanide blocks respiration inside the mitochondria of a cell but does not damage the cell membrane or the protein carriers themselves. She then compares two separate measurements, made before and after the cyanide is added: the rate at which potassium ions continue to accumulate inside the gill cells, and the rate at which oxygen continues to enter the gill cells from the seawater.

(a) State what is meant by active transport. [2]

(b) Predict and explain the effect of adding the cyanide poison on the uptake of potassium ions into the gill cells. [3]

(c) Predict and explain the effect of adding the cyanide poison on the entry of oxygen into the gill cells, contrasting your answer with part (b). [2]

Part (a): Defining active transport

Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient. It requires protein carriers embedded in the membrane, and it needs energy, which is released by respiration inside the cell.

Part (b): Predicting the effect on potassium ion uptake

Before the cyanide is added, the gill cells actively transport potassium ions from the seawater into the cell, against the concentration gradient, using protein carriers powered by energy from respiration. Cyanide blocks respiration inside the mitochondria, so the cell can no longer release this energy, even though the protein carriers and the cell membrane are left undamaged. Since the protein carriers need a continuous supply of energy to keep changing shape and moving potassium ions against the gradient, cutting off that energy supply means the carriers can no longer do their job. The uptake of potassium ions into the gill cells will therefore slow down and eventually stop once respiration is blocked, because the cell has no other way of powering this movement against the concentration gradient.

Part (c): Predicting the effect on oxygen entry, and contrasting with (b)

Diffusion, unlike active transport, does not need any energy from the cell. It happens simply because particles move at random, producing a net movement down a concentration gradient. Oxygen enters the gill cells by diffusion, moving from its higher concentration in the surrounding seawater to its lower concentration inside the cells. Because this movement does not depend on energy from respiration, blocking respiration with cyanide has no direct effect on it: oxygen will continue to enter the gill cells at essentially the same rate as before the poison was added. This is the key contrast with part (b): potassium ion uptake stops because active transport depends on energy from respiration, while oxygen entry by diffusion continues because it needs no such energy at all.

Final answers

(a) Active transport: movement of particles against their concentration gradient, across a cell membrane, using protein carriers and energy from respiration.
(b) Potassium ion uptake will slow down and stop, because cyanide blocks respiration, cutting off the energy the protein carriers need to move ions against their concentration gradient.
(c) Oxygen entry will continue unaffected, because diffusion needs no energy from respiration, unlike the active transport of potassium ions in (b).