Movement of substances for N(A)-Level Science (Biology): diffusion, osmosis and active transport, the factors that change their rate, and the effect of osmosis on plant and animal cells

What this module covers

Movement of substances explains how things get into and out of cells. It is the basis of gas exchange, the absorption of food, water uptake by roots and almost every other transport process in N(A)-Level Science (Biology). You need to define and compare diffusion, osmosis and active transport, know the factors that change the rate of each, and predict what osmosis does to plant and animal cells.

This overview links every dot point in the module. Work through them in order, then test yourself with the questions at the end. See the full set at /sg-n-level/biology/syllabus.

Diffusion: moving down the gradient

Start with the simplest process. The page on diffusion defines it as the net movement of particles from a region of high concentration to one of low concentration, down the gradient, and explains why it matters for gas exchange and absorption. It also covers the factors that change the rate: the concentration gradient, the temperature, the surface area and the diffusion distance. No energy is needed because the random movement of particles drives it.

Osmosis: the special case of water

Water moves by its own rule. The page on osmosis defines osmosis as the movement of water molecules across a partially permeable membrane, from a higher to a lower water potential, and works through the effects on cells. A plant cell in pure water becomes turgid; in a concentrated solution it becomes flaccid and finally plasmolysed. An animal cell, with no cell wall, swells and bursts in pure water and shrinks in a concentrated solution. These contrasting outcomes are a common exam question.

Active transport: working against the gradient

Some substances must move the wrong way. The page on active transport defines it as the movement of particles against the concentration gradient, from low to high, using energy from respiration. The main example is the uptake of mineral ions by root hair cells, even when the soil has a lower concentration of those ions than the cell. The energy requirement is the feature that sets it apart from diffusion.

How this module is examined

At N(A) level there is no standalone pure Biology paper. Biology is taken as part of a combined Science subject, either Science (Physics, Biology) syllabus 5106 or Science (Chemistry, Biology) syllabus 5107, so this module is examined within whichever combination your school offers.

• Define precisely. Each process has a one-sentence definition that must mention the direction of movement and, for osmosis, the partially permeable membrane and water potential.
• Predict cell behaviour. Osmosis questions reward stating the direction of water movement first, then the effect (turgid, flaccid, plasmolysed, swollen, shrunken or burst).
• Apply the ideas. Many questions describe a real situation, such as oxygen entering blood or ions entering a root, and ask you to name and explain the process.

Check your knowledge

A mix of definition, prediction and application questions. Attempt them, then check against the solutions.

1. Define osmosis. (2 marks)
2. State two ways in which active transport differs from diffusion. (2 marks)
3. A red blood cell is placed in pure water. Describe and explain what happens to it. (3 marks)
4. State three factors that increase the rate of diffusion. (3 marks)