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Movement In and Out of Cells: Question 6

Syllabus 3.1

Multiple choice Core 1 mark

A technician makes two artificial "cells": each is a small bag made from partially permeable membrane, filled with exactly the same volume of a coloured dye solution. Bag X is made into a flat, wide shape, so it has a large membrane surface area. Bag Y is made into a compact, rounded shape, so it has a much smaller membrane surface area. Both bags are placed in separate beakers of plain water at the same temperature, and the dye solution inside each bag has the same starting concentration.

Which statement correctly compares the rate at which the dye diffuses out of the two bags?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify what is being compared

Both bags contain the same volume of dye solution at the same starting concentration, and both are in plain water at the same temperature. The only difference described is the shape of each bag, which gives bag X a larger membrane surface area than bag Y. This isolates surface area as the one variable that differs between the two bags.

Step 2: Apply the effect of surface area on diffusion rate

Diffusion happens as individual dye particles cross the membrane at random. A larger area of membrane gives more space across which particles can cross at any one moment, so more particles leave the bag per second. A larger membrane surface area therefore increases the rate of diffusion, all other factors (temperature, concentration gradient, distance) being equal. Bag X, with its larger surface area, will lose dye by diffusion faster than bag Y.

Why the other options are wrong

  • B: a smaller surface area gives fewer places for particles to cross at once, which slows the rate of diffusion. It does not speed it up. Concentrating particles into a smaller bag does not force them across the membrane any faster.
  • C: surface area is one of the recognised factors affecting the rate of diffusion (alongside concentration gradient, temperature and distance); it is not irrelevant.
  • D: the stem explicitly states both bags contain exactly the same volume of dye solution, so bag X’s larger area comes from its flatter shape, not from holding more solution. There is no difference in concentration gradient between the two bags.

Final answer

Bag X diffuses dye out faster than bag Y because its larger membrane surface area allows more dye particles to cross per unit time, option A.