Past Papers · SAQ
Cell Membrane Structure & Transmembrane Transport
2014A Q16
Exam questionDescribe, with the aid of a diagram, the structure of the cell membrane, (40% marks) and transmembrane transport processes. (60% marks)
CICMWrecks answer
Master answer
Structure of Cell Membrane
Cell Membrane
- Made of
- Phospholipids
- Proteins
- Cholesterol – Found in eukarocytes ie cells with nuclei
- Cell membrane = 7.5nm thick semi permeable structure
Lipid Bilayer
- Fluid rather than solid
- Phospholipids have:
- eg phosphatidyl-choline & phosphatidyl-ethanol-amine
- Hydrophilic head
- Water soluble
- Exposed to aqueous exterior & interior
- Glycerol backbone
- Fatty acid tails –
- Hydrophobic
- Meet in middle of cell membrane
- Proteins can be either:
- Integral – ie pass through bilayer eg ion channels
- Peripheral = straddling
- make up 50% of cell membranes mass
Function of CM Proteins
- Structural
- Carriers for facilitated diffusion (i.e. down electrochemical gradient)
- Pumps for ion active transport
- Ion channels (diffusion down electro- or chemical gradient or both; e.g. K-“leak” channels)
- Receptors for chemical messengers (i.e. hormones , neurotransmitters , autacoids…)
- Enzymes
- Glycoproteins involved in AB processing or anticoagulation (e.g. the mucopolysaccharide glycocalyx of the endothelium which repels clotting factors + PLT’s → helps prevent blood from clotting in intact blood vessels)
Transmembrane Transport Processes
| 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 | |||
Mooney / JC 2019
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2014A Q16 | Describe, with the aid of a diagram, the structure of the cell membrane, (40% marks) and transmembrane transport processes. (60% marks) | 60% |