Cell Membrane and Transport
~8 min read
- Structure: Phospholipid bilayer with proteins and cholesterol embedded — fluid mosaic model.
- Property: Selectively permeable — allows certain substances and blocks others.
- Transport: Passive (diffusion, osmosis — no energy). Active (against gradient, uses ATP).
The cell membrane is the gatekeeper of the cell. CDS/OTA tests its structure (phospholipid bilayer), and the difference between osmosis, diffusion and active transport.
Structure of Cell Membrane
- Phospholipid bilayer — two layers of phospholipid molecules with hydrophilic heads outward and hydrophobic tails inward.
- Proteins and cholesterol are embedded in this bilayer — the fluid mosaic model (Singer & Nicolson, 1972).
- Selectively permeable — small/non-polar molecules (O₂, CO₂) pass freely; ions and large polar molecules need transport proteins.
- Flexibility allows the cell to engulf materials (phagocytosis, pinocytosis).
- In plants and bacteria, an additional outer cell wall provides shape and rigidity (cellulose in plants, peptidoglycan in bacteria).
Modes of Transport
| Type | Driving force | Energy? | Example |
|---|---|---|---|
| Simple diffusion | Concentration gradient | No | O₂, CO₂ across alveolar membrane |
| Facilitated diffusion | Gradient + carrier protein | No | Glucose into RBC via GLUT-1 |
| Osmosis | Water gradient across semi-permeable membrane | No | Water uptake by root hairs |
| Active transport | Against gradient — pump uses ATP | Yes (from respiration) | Na⁺/K⁺ pump in neurons |
| Endocytosis / Exocytosis | Membrane folds around material | Yes | WBC engulfing bacteria |
Osmosis and RBC Behaviour
- Hypotonic solution (e.g. distilled water): water enters RBC → swells → bursts (haemolysis).
- Hypertonic solution (concentrated salt): water leaves RBC → shrinks (crenation).
- Isotonic solution (normal saline, 0.9% NaCl): no net water movement → RBC stays the same.
- Reverse osmosis (RO): applied pressure > osmotic pressure forces water across a semi-permeable membrane, leaving impurities behind.
CDS/OTA PYQ Examples
Q: Osmosis is the movement of water molecules from its:
(a) higher to lower concentration through cell wall (b) lower to higher through fully permeable membrane (c) higher to lower through fully permeable membrane (d) higher to lower through semi-permeable membrane
Answer: (d) Higher to lower water concentration through a semi-permeable membrane. [CDS-I 2021]
Q: Which one of the following is active transport?
(a) Against gradient using energy from respiration (b) Against gradient without energy (c) Against gradient using energy from photosynthesis (d) Along gradient using respiration energy
Answer: (a) Against gradient using ATP from respiration. [CDS-II 2019]
Q: Which one of the following statements is correct about the cell membrane?
(a) Made up of cellulose (b) Permeable to all substances (c) Flexible — enables cell to engulf materials (d) Does not allow diffusion of gases
Answer: (c) Flexible membrane enables endocytosis. [CDS-I 2022]
Q: Which is a feature of cell membrane?
(a) Phospholipid bilayer with proteins and cholesterol embedded (b) Protein bilayer with lipid and cholesterol (c) Neutral lipid bilayer with proteins (d) Neutral lipid bilayer lacking proteins
Answer: (a) Phospholipid bilayer with embedded proteins and cholesterol. [CDS-II 2022]
Q: Desalination of seawater by reverse osmosis requires applied pressure:
(a) Larger than osmotic pressure (b) Smaller than osmotic pressure (c) Equal to osmotic pressure (d) Equal to atmospheric pressure
Answer: (a) Larger than osmotic pressure. [CDS-II 2017]
Q: If RBCs are placed in normal saline (isotonic solution), what happens?
(a) RBCs swell (b) RBCs swell and burst (c) RBCs shrink (d) No change in diameter
Answer: (d) No change — isotonic means no net water movement. [CDS-I 2023]
Drill Cell Membrane and Transport for CDS/OTA
CDS/OTA-pattern items on Cell Membrane and Transport with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is the cell membrane called 'selectively permeable'?
It allows certain molecules to pass freely (water, gases, small uncharged molecules) but restricts others (ions, large polar molecules) which need specific channels or pumps.
How does the Na⁺/K⁺ pump work?
It actively pumps 3 Na⁺ out and 2 K⁺ in per ATP consumed, maintaining the electrochemical gradient that drives nerve impulses.
Can plant cells burst in hypotonic solution like RBCs?
No — the rigid cell wall prevents bursting. The cell becomes turgid instead, which is the normal state in well-watered plants.