SN4
SHORT NOTE
5M
Classify cell junctions and describe their functions.
Definition
Cell junctions = specialised regions of contact between adjacent cells, or between a cell and the extracellular matrix.
Classification
| Type | Other name | Function |
|---|---|---|
| Occluding | Tight junction (zonula occludens) | Seals the gap |
| Adhering | Desmosome, adherens junction, hemidesmosome | Mechanical strength |
| Communicating | Gap junction | Direct cell-to-cell transfer |
Tight Junction (zonula Occludens)
- Proteins — occludin, claudin
- Forms a continuous seal around the apex of epithelial cells
- Functions:
– Prevents paracellular leak → forces transcellular transport
– Maintains cell polarity — separates apical from basolateral membrane
- Sites — intestinal epithelium, renal tubule, blood–brain barrier, blood–testis barrier
Adhering Junctions
| Junction | Protein | Attached to |
|---|---|---|
| Desmosome (macula adherens) | Cadherins (desmoglein) | Intermediate filaments (keratin) |
| Adherens junction | E-cadherin | Actin filaments |
| Hemidesmosome | Integrins | Basement membrane |
Gap Junction (nexus)
- Protein — connexin; 6 connexins form a connexon
- Pore diameter about 1.5 nm → allows ions, glucose, cAMP (< 1000 Da); not proteins
- Provides low-resistance electrical coupling
- Sites — cardiac muscle (intercalated disc), smooth muscle, liver, lens epithelium
- Allows the myocardium to behave as a functional syncytium
Applied Aspects
- Pemphigus vulgaris — autoantibodies against desmoglein → desmosome breakdown → intraepidermal blisters
- Bullous pemphigoid — hemidesmosome antibodies → subepidermal blisters
- Loss of E-cadherin → tumour invasion and metastasis
- Cholera and other toxins disrupt tight junctions → leaky gut
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.