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How are substances moved into and out of cells by diffusion, osmosis and active transport?

Explain how substances are transported into and out of cells by diffusion, osmosis and active transport, investigate osmosis in potatoes, and calculate the percentage change in mass.

A focused answer to Edexcel GCSE Biology 1.15 to 1.17, covering diffusion, osmosis and active transport, the osmosis core practical with potatoes, and how to calculate the percentage change in mass.

Generated by Claude Opus 4.810 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Diffusion
  3. Osmosis
  4. Active transport
  5. The osmosis core practical
  6. Percentage change in mass
  7. Try this

What this dot point is asking

Edexcel statements 1.15 to 1.17 want you to explain the three transport processes (diffusion, osmosis and active transport), carry out the core practical on osmosis in potatoes (statement 1.16), and calculate the percentage change in mass (statement 1.17). The processes are easy to confuse, so precise definitions matter.

Diffusion

Diffusion supplies oxygen and glucose to cells and removes carbon dioxide and other wastes. Its rate increases with a steeper concentration gradient, a higher temperature, a larger surface area and a shorter diffusion distance.

Osmosis

A partially permeable membrane lets small water molecules through but not larger solute molecules such as sugar. Water always moves towards the more concentrated solution. In an animal cell, taking in too much water by osmosis can make it burst; losing too much makes it shrink. A plant cell does not burst because its cellulose cell wall resists the pressure, becoming firm (turgid).

Active transport

The osmosis core practical

In the core practical (statement 1.16) you cut potato cylinders of equal size, measure each one's mass, and leave them in sugar (or salt) solutions of different concentrations for a set time. After drying off surface liquid, you reweigh each cylinder. Cylinders in dilute solutions gain mass (water enters by osmosis); cylinders in concentrated solutions lose mass (water leaves). At one concentration the mass does not change, because the solution has the same water concentration as the potato cells.

To make it a fair test, keep the potato cylinders the same length and surface area, the volume of solution, the temperature and the time the same, changing only the concentration. Using percentage change rather than raw change in mass allows a fair comparison even if the cylinders started at slightly different masses.

Percentage change in mass

Try this

Q1. State two factors that increase the rate of diffusion. [2 marks]

  • Cue. Any two of: a steeper concentration gradient, a higher temperature, a larger surface area, a shorter diffusion distance.

Q2. Explain why active transport requires energy but diffusion does not. [2 marks]

  • Cue. Active transport moves substances against the gradient (low to high), which cannot happen passively, so energy from respiration is needed; diffusion moves substances down the gradient, which happens on its own.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20194 marksPotato cylinders were placed in sugar solutions of different concentrations. Explain why a potato cylinder placed in a concentrated sugar solution decreases in mass.
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A 4-mark explain question rewards a clear osmosis chain referring to water concentration.

  1. The concentrated sugar solution has a lower water concentration than the cell contents (the potato cells are more dilute).
  2. Water moves out of the potato cells, by osmosis, across the partially permeable cell membranes, from the higher water concentration inside to the lower water concentration outside.
  3. The cells lose water, so the mass of the potato cylinder decreases.

Markers reward the words osmosis, partially permeable membrane, water concentration gradient and the direction out of the cells. Talking about sugar molecules moving, rather than water, is a common error.

Edexcel 20222 marksA potato cylinder had a mass of 5.0 g before and 4.2 g after soaking. Calculate the percentage change in mass.
Show worked answer →

A 2-mark calculation rewards the change, the formula and the sign.

Change in mass =4.2−5.0=−0.8 g= 4.2 - 5.0 = -0.8\ g. Percentage change == (change ÷\div starting mass) ×100=(−0.8÷5.0)×100=−16%\times 100 = (-0.8 \div 5.0) \times 100 = -16\%.

Markers reward the correct value and a minus sign (or the word decrease), because the cylinder lost mass. Dividing by the final mass instead of the starting mass, or dropping the sign, loses a mark.

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