Movement of substances for O-Level Biology (SEAB 6093): diffusion, osmosis and active transport, the factors that affect their rate, and how surface area to volume ratio controls exchange in organisms

What this module covers

Movement of substances explains how things get into and out of cells, which is the basis of gas exchange, absorption, water uptake and almost every other transport process in O-Level Biology (SEAB 6093). You need to define and compare diffusion, osmosis and active transport, know the factors that change the rate of each, predict the effect of osmosis on plant and animal cells, and explain why surface area to volume ratio decides whether an organism needs special exchange surfaces.

This overview links every dot point in the module; work through them, then test yourself with the questions at the end. See the full set at /sg-o-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, temperature, surface area and 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 in cells defines osmosis as the diffusion 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 favourite 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. Examples include the uptake of mineral ions by root hair cells and the absorption of glucose in the gut even when the gut already has a lower concentration. The energy requirement is the feature that distinguishes it from diffusion.

Surface area to volume ratio

Finally, the principle that ties exchange to body size. The page on surface area to volume ratio explains why the ratio falls as an organism grows, why small organisms can exchange across their whole surface, and why large organisms need specialised exchange surfaces such as lungs, gills and villi. This idea links forward to gas exchange and to the structure of exchange surfaces throughout the syllabus.

How this module is examined

• 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 and then the effect (turgid, flaccid, plasmolysed, swollen, shrunken or burst).
• Apply the ratio. Surface area to volume questions reward linking a small ratio in large organisms to the need for specialised exchange surfaces.

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. Explain why a large organism needs specialised exchange surfaces but a single-celled organism does not. (3 marks)