Cell Membrane

• Fluid mosaic model of plasma membrane — lipid bilayer with embedded proteins.
• Composition: phospholipids, cholesterol (eukaryotes), glycoproteins/glycolipids.
• Selective permeability — passive vs active transport across membrane.
• Specialised membranes — cell wall (plants), plasmodesmata, tight junctions (animal preview).

Level 1 — Structure and fluid mosaic model

Verbal: Singer–Nicolson fluid mosaic model: phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward. Proteins float ("mosaic") and lipids move laterally ("fluid") at physiological temperature.

Symbolic: Fluid mosaic: phospholipid bilayer; osmosis Ψ water potential; Na⁺/K⁺ pump ATP-driven; s = rθ N/A — membrane transport uses concentration gradients.

Components:

• Phospholipids — amphipathic, main fabric
• Integral proteins — transmembrane channels, carriers
• Peripheral proteins — inner/outer surface, enzymatic/scaffold
• Cholesterol — modulates fluidity (animal cells)
• Carbohydrate chains — cell recognition (glycocalyx)

Functions: Boundary, transport, endocytosis/exocytosis, cell signalling, adhesion.

Level 2 — Transport mechanisms

Osmosis: Diffusion of water across selectively permeable membrane — hypertonic, hypotonic, isotonic (NCERT egg/rabbit RBC examples).

Plant cell wall: Cellulose outside membrane — turgor pressure when turgid; plasmolysis when hypertonic external solution.

NEET link: Membrane potential, action potential (nerve) built on ion pumps — preview.

NCERT spotlight — Endocytosis and cell junctions

Phagocytosis (cell eating) and pinocytosis (cell drinking) import bulk material. Receptor-mediated endocytosis is selective — LDL uptake example.

Plant plasmodesmata: Cytoplasmic channels through cell walls connect adjacent cells — symplastic transport in plants.

Tight junctions and desmosomes in animals: Block leakage between epithelial cells; gap junctions allow communication — tissue-level organisation beyond single cell membrane.

A red blood cell is placed in (a) distilled water (b) 0.9% NaCl (c) 10% NaCl. Predict cell shape changes and explain using tonicity.

Step 1 — (a) Distilled water: hypotonic → water enters by osmosis → cell swells → may haemolyse (lyse).
Step 2 — (b) 0.9% NaCl: isotonic to cytoplasm → no net water movement → normal biconcave disc maintained.
Step 3 — (c) 10% NaCl: hypertonic → water leaves → cell shrinks (crenation).
Step 4 — Membrane freely permeable to water; NaCl impermeant at short times → osmotic gradient drives water.
Step 5 — Plant cell in (a): turgid; in (c): plasmolysis — membrane pulls from wall.

Applications — dialysis and drug delivery

Kidney dialysis membrane allows waste diffusion while retaining blood proteins — selective permeability clinical application. Liposomes (artificial phospholipid bilayers) encapsulate drugs for targeted delivery — mimics cell membrane structure. Anaesthesia affects nerve cell membrane ion channel function — pharmacology connection to membrane proteins.

Deep dive — membrane transport and cell volume regulation

Animal cells in hypotonic medium swell because water enters by osmosis until membrane tension limits further intake or lysis occurs — no cell wall to resist. Plant cells become turgid when vacuole fills, pressing membrane against wall — turgor maintains erect stems. Plasmolysis in hypertonic solution shrinks protoplast away from wall; deplasmolysis upon returning to hypotonic medium demonstrates reversible osmosis in onion peel lab. Facilitated diffusion of glucose via GLUT transporters is saturable — unlike simple diffusion, rate plateaus when all carriers engaged. Na-K pump maintains low intracellular Na+ and high K+ against gradients using ATP — essential for nerve resting potential. Endocytosis types: phagocytosis (solid particles), pinocytosis (fluid), receptor-mediated (LDL cholesterol uptake). Gap junctions in cardiac muscle synchronise contraction via ion flow between cells — membrane specialisations extend beyond single-cell boundary.

Review and practice drill

Review checklist: (1) Fluid mosaic phospholipid bilayer. (2) Passive versus active transport. (3) Osmosis water only. (4) Plasmolysis hypertonic plant cell. Practice: RBC crenation in hypertonic saline.

For board exams, reproduce labelled diagrams where NCERT provides them and define every technical term in one precise sentence before using it in longer answers. Link this topic to adjacent units in your revision map so multi-chapter questions feel familiar rather than surprising on exam day.

• Draw labelled fluid mosaic diagram.
• Distinguish simple and facilitated diffusion.
• What is plasmolysis?

Open the Practice tab for graded questions on Membrane.