SN1
SHORT NOTE
5M
★ High-yield
Define osmosis and osmotic pressure. Explain tonicity with examples.
Definitions
Osmosis = movement of water across a semipermeable membrane from a region of lower to higher solute concentration.Osmotic pressure = the pressure that must be applied to prevent osmosis.
Units
| Term | Definition |
|---|---|
| Osmole | 1 gram-molecular weight of undissociated solute |
| Osmolarity | Osmoles per litre of solution |
| Osmolality | Osmoles per kg of water — used in the body, as it is temperature-independent |
- Plasma osmolality = 290–300 mOsm/kg
- Calculated: 2 × [Na+] + glucose/18 + urea/2.8 (mg/dL)
- NaCl dissociates into 2 particles → 1 mmol gives 2 mOsm
Effective VS Ineffective Osmoles
| Effective osmole | Ineffective osmole | |
|---|---|---|
| Crosses membrane? | No | Yes |
| Causes water shift? | Yes | No |
| Examples | Na+, mannitol, glucose (without insulin) | Urea, alcohol |
Tonicity
| Solution | Effect on RBC | Example |
|---|---|---|
| Isotonic | No change | 0.9% NaCl, 5% dextrose |
| Hypotonic | Cell swells, may haemolyse | 0.45% NaCl, water |
| Hypertonic | Cell shrinks (crenation) | 3% NaCl |
- Tonicity ≠ osmolarity — tonicity counts only effective osmoles
- Urea solution may be iso-osmotic but is hypotonic, since urea crosses the membrane
Applied Aspects
- Mannitol — effective osmole; used to reduce raised intracranial pressure and intraocular pressure
- Osmotic fragility test — ↑ in hereditary spherocytosis, ↓ in thalassaemia
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.