SN5
SHORT NOTE
5M
★ High-yield
Write a short note on facilitated diffusion.
Definition
Facilitated diffusion = transport of a solute down its concentration gradient with the help of a carrier protein, without expenditure of energy.
Mechanism
Solute binds carrier on one side → Carrier undergoes conformational change → Solute released on the other side → Carrier reverts to original shape
Characteristics
| Feature | Simple diffusion | Facilitated diffusion |
|---|---|---|
| Carrier | Not required | Required |
| Gradient | Downhill | Downhill |
| Energy | Nil | Nil |
| Saturation (Tmax) | Absent | Present |
| Specificity | Absent | Present |
| Competition | Absent | Present |
| Rate vs concentration | Linear | Rises then plateaus |
Examples
| Transporter | Site | Substance |
|---|---|---|
| GLUT-1 | RBC, blood–brain barrier | Glucose |
| GLUT-2 | Liver, pancreatic β cell, basolateral gut | Glucose (high capacity, low affinity) |
| GLUT-3 | Neurones | Glucose |
| GLUT-4 | Skeletal muscle, adipose tissue | Glucose — insulin-dependent |
| Aquaporins | Renal tubule, RBC | Water |
| Urea transporters | Renal medulla | Urea |
Applied Aspects
- Type 2 diabetes mellitus — impaired GLUT-4 translocation → muscle and fat cannot take up glucose despite hyperglycaemia
- Insulin acts by moving GLUT-4 from intracellular vesicles to the cell membrane
- Exercise also recruits GLUT-4 — independently of insulin; the basis of exercise therapy in diabetes
- Nephrogenic diabetes insipidus — defective aquaporin-2 response to ADH
All questions in General Physiology & Body Fluids
- Q1Describe the structure of the cell membrane. Classify the mechanisms of transport across it and describe passive transport in detail. ★
- Q2Define active transport. Classify it and describe the sodium–potassium pump with its functions. Add a note on secondary active transport. ★
- Q3Enumerate the body fluid compartments with their volumes and ionic composition. Describe the methods used to measure them. ★
- Q4Define resting membrane potential. Describe its ionic basis and the factors responsible for it. Add a note on the Nernst equation. ★
- Q5Define homeostasis. Describe the components of a control system and explain negative and positive feedback with examples. ★
- SN1Define osmosis and osmotic pressure. Explain tonicity with examples. ★
- SN2Write a short note on the Gibbs–Donnan equilibrium.
- SN3Define oedema. Describe the Starling forces and enumerate the mechanisms by which oedema is produced. ★
- SN4Classify cell junctions and describe their functions.
- SN5Write a short note on facilitated diffusion. ★
- SN6Enumerate the second messenger systems and describe the cyclic AMP pathway. ★
- SN7Define apoptosis. Differentiate it from necrosis and describe its pathways.