A-Level Biology: Transport Across Membranes

Membrane transport is one of A-Level Biology's most reliably examined topics, because it combines a structural model with four distinct transport mechanisms and one flagship example — sodium-glucose co-transport — that the specification names explicitly. This quiz covers the full set.

The structural questions test the fluid-mosaic model beyond its name: why "fluid" (lateral movement of components) and why "mosaic" (scattered protein arrangement), cholesterol's role as a fluidity buffer across temperatures, glycoproteins as recognition sites and receptors, and intrinsic proteins tested on their span and their hydrophobic interactions with the fatty-acid core.

The mechanism questions work through the transport ladder: simple diffusion and the factors from Fick's law that raise its rate (surface area, gradient, lipid solubility — with membrane thickness as the inverse factor), facilitated diffusion as passive but protein-limited — including the plateau that appears when all carriers are occupied, a classic curve-interpretation point — osmosis reasoned in water-potential terms with a turgid plant cell scenario, and active transport with its ATP requirement and conformational changes.

The co-transport question addresses the ileum mechanism the specification highlights: the sodium gradient as the driving force and the sodium-potassium pump as the gradient's maintainer — the two-step logic that distinguishes complete answers.

Every question is self-contained, and each explanation names the mechanism and its energetics precisely — the vocabulary that separates "moves across the membrane" answers from mark-earning ones. Pairs directly with the Cell Structure quiz in this collection.

• Justify the fluid-mosaic model's name and each component's role
• Apply Fick's law factors to the rate of simple diffusion
• Explain facilitated diffusion's protein-limited plateau
• Reason about osmosis in water-potential terms in plant and animal cells
• Describe sodium-glucose co-transport and the pump that powers it

Topic scope follows the transport across cell membranes content of the DfE A level biology subject content as delivered in current exam-board specifications: the fluid-mosaic model, diffusion, osmosis, active transport and co-transport in the ileum.