Glossary: Movement of Substances Across a Plasma Membrane

This chapter's terms in context

Chapter 3 turns on five terms that describe how substances cross the plasma membrane, and SPM Biology treats the precise wording of each definition as a mark-earning skill in its own right, not just background knowledge. Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, until the concentration is uniform; it needs no energy from the cell because it is driven by the random kinetic movement of particles. Osmosis is a special case of diffusion restricted to water molecules moving across a selectively permeable membrane, from a region of higher water potential to a region of lower water potential. Students frequently write osmosis definitions using 'concentration' instead of 'water potential', which loses marks because osmosis is defined in terms of water potential specifically, not solute concentration in general.

Active transport is fundamentally different from both of the above: it is the movement of molecules or ions across a plasma membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy released from respiration and carrier proteins embedded in the membrane. Because it moves substances against the gradient, active transport requires ATP, and a cell carrying it out, such as a root hair cell absorbing mineral ions from dilute soil water, contains an unusually high number of mitochondria to supply that energy.

Hypotonic and hypertonic solutions describe the water potential of a solution relative to the cell placed inside it, and this comparative framing is exactly where students go wrong. A hypotonic solution has a higher water potential than the cell's cytoplasm, so water moves into the cell by osmosis, causing an animal cell to swell and potentially burst (haemolysis) or a plant cell to become turgid, supported by its cell wall. A hypertonic solution has a lower water potential than the cytoplasm, so water leaves the cell by osmosis, causing an animal cell to shrink (crenation) or a plant cell to become plasmolysed, with its cell membrane pulling away from the cell wall. Because 'hypotonic' and 'hypertonic' are always defined relative to a specific cell rather than as absolute properties of a solution, the same solution can be hypotonic to one cell and hypertonic to another, which is precisely the kind of comparative reasoning SPM structured questions are designed to test.

A third term worth knowing alongside these two, isotonic, describes a solution with the same water potential as the cell, so there is no net movement of water in either direction and the cell's size stays unchanged, a comparison point examiners sometimes insert into a table question alongside hypotonic and hypertonic to test whether a candidate truly understands the full range rather than only the two extremes. These five terms, diffusion, osmosis, active transport, hypotonic and hypertonic, together explain almost every membrane-transport example in the syllabus, from gaseous exchange in the alveolus to mineral ion uptake in a root hair cell, so a firm grip on their exact definitions pays off well beyond this one chapter.

Paper 3 practical questions on this chapter typically involve visking tubing, potato or onion tissue placed in solutions of different concentrations, and a candidate is expected to predict, and then explain, a change in mass or appearance using the correct term for the correct direction of water movement. A frequent scoring pattern rewards candidates who state the direction of net water movement explicitly (into or out of the cell), name the process correctly as osmosis rather than diffusion, and then explain the result in terms of water potential rather than vaguely mentioning 'concentration'. Because active transport does not feature in most osmosis practicals, students sometimes default to explaining every membrane-related result with active transport out of habit; a quick check, did the process need energy, and did it move a substance against its gradient, usually resolves whether osmosis, diffusion or active transport is the term a given result actually requires.