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.
Sample question
In the fluid-mosaic model of the cell membrane, what is the primary structural role of cholesterol molecules embedded within the phospholipid bilayer?
- They act as enzymes to catalyze membrane-bound reactions
- They facilitate the active transport of large polar molecules
- They regulate membrane fluidity by restricting lateral movement of phospholipids
- They serve as primary receptors for cell signaling pathways
See the answer
They regulate membrane fluidity by restricting lateral movement of phospholipids
Cholesterol molecules are hydrophobic and sit between the fatty acid tails of phospholipids. By interacting with these tails, they increase the packing density and reduce the fluidity of the membrane, especially at higher temperatures, preventing it from becoming too permeable.