Past Papers · SAQ
Cell Transport — Membrane Transport
2012B Q10
Exam questionDescribe transport mechanisms across cell membranes. Give an example of each.
CICMWrecks answer
Master answer
Transmembrane Transport Mechanisms
| Mechanism | Energy Expenditure | Electrochemical gradient | Example | ||||
|---|---|---|---|---|---|---|---|
| Diffusion | Passive diffusion | Molecule crosses a membrane to which it is permeable by diffusion | No | With | Carbon dioxide across vascular endothelium | ||
| Facilitated diffusion | Molecule crosses a membrane via a channel, without energy expenditure | No | With | Potassium across excitable cell membranes, via rectifier channels | |||
| Active transport | Primary | Primary active transport | Molecule crosses a membrane via a channel, with energy expenditure (ATP) | Yes | Against | 3Na+ /2K+ ATPase | |
| Secondary | Symport | Molecule crosses membrane against its electrochemical gradient, with the energy being provided by the transport of another molecule (Co-transported) | Not directly | May be Passive or Active based on gradient of 2nd | sodium and an amino acid | ||
| Antiport | Molecule crosses membrane against its electrochemical gradient, with the energy being provided by the transport of another molecule (Anti-transported) | Not directly | May be Passive or Active based on gradient of 2nd | Na+ / H+ antiporter in proximal convoluted tubule | |||
| Ligand-gated ion channel | Binding of a ligand causes conformational change in membrane channel, allowing movement of ion across membrane | No | With | Nicotinic acetylcholine receptor. ACh as ligand, Na+ and K + as ions | |||
| Exocytosis | Substance packed in vesicle, moves to cell membrane, two membranes merge -> substance exits cell | Yes | Usually against | Exocytosis of ACh by presynaptic neuron | |||
| Endocytosis | Cell membrane extends to engulf a substance or object, which is then contained in the cell within a vesicle | Yes | Can be with, against or refer to a macroscopic object | Phagocytosis of bacteria by macrophage | |||
JC / Mooney 2019
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2012B Q10 | Describe transport mechanisms across cell membranes. Give an example of each. | 50% |