Past Papers · SAQ
Cell Transport — Na/K Pump
2022A Q02
Exam questionExplain the mechanisms of transport of substances across cell membranes including appropriate examples (75% marks). Outline the structure and function of the Na+/K+-ATPase pump (25% marks).
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 | |||
Na+/K+ – ATPase pump
- Found in virtually all cells of the body – It is a heterodimer integral membrane protein that spans the entire membrane:
- α-subunit (larger subunit) – Binds Na+, ATP and PO43- intracellularly, and K+ and ouabain/glycosides extracellularly
- β-subunit (smaller subunit) – A glycoprotein
- Functions as an “electrogenic pump” as it extrudes 3 Na+ from the cell and takes in 2 K+ per ATP hydrolysed
- Process of pump function:
- Na+ and ATP bind to α-subunit intracellularly
- ATP is hydrolysed to ADP and the phosphate is transferred to an Asp-phosphorylation site
- Conformational change of the protein causes Na+ to be extruded into ECF
- K+ binds to the α-subunit extracellularly
- Dephosphorylation of the Asp-phosphorylation site causes the protein to return to its resting conformation, which then transports K+ intracellularly
- Pump function is inhibited by ouabain and digitalis glycosides
JC / Mooney / Bianca 2019
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2022A Q02 | Explain the mechanisms of transport of substances across cell membranes including appropriate examples (75% marks). Outline the structure and function of the Na+/K+-ATPase pump (25% marks). | 61% |