Question
Describe the fluid mosaic model of the cell membrane. Explain the different mechanisms of transport across the membrane: passive (diffusion, osmosis, facilitated diffusion) and active transport.
Solution — Step by Step
The cell membrane is a phospholipid bilayer with proteins embedded in it. “Fluid” because the phospholipids and proteins can move laterally within the layer. “Mosaic” because the proteins are scattered like tiles in a mosaic.
Components:
- Phospholipids — hydrophilic heads face outward (toward water), hydrophobic tails face inward (away from water)
- Integral (intrinsic) proteins — span the entire membrane (transmembrane)
- Peripheral (extrinsic) proteins — attached to the surface only
- Cholesterol — in animal cells, regulates fluidity
- Glycoproteins and glycolipids — on the outer surface, involved in cell recognition
Passive transport moves substances down their concentration gradient (high → low concentration).
Simple diffusion — small, nonpolar molecules (O₂, CO₂, N₂) pass directly through the lipid bilayer.
Osmosis — diffusion of water through a selectively permeable membrane. Water moves from a region of lower solute concentration to higher solute concentration.
Facilitated diffusion — larger or polar molecules (glucose, ions) pass through channel proteins or carrier proteins. Still passive (down the gradient) but requires a protein to assist.
Active transport moves substances against their concentration gradient (low → high) using energy from ATP.
Sodium-potassium pump (Na⁺/K⁺ ATPase) — pumps 3 Na⁺ out and 2 K⁺ in per ATP molecule. Maintains the electrochemical gradient essential for nerve impulses.
Bulk transport:
- Endocytosis — cell engulfs material (phagocytosis for solids, pinocytosis for liquids)
- Exocytosis — cell expels material (secretion of enzymes, hormones)
flowchart TD
A[Membrane Transport] --> B[Passive - no ATP]
A --> C[Active - uses ATP]
B --> B1[Simple diffusion: O₂, CO₂]
B --> B2[Osmosis: water]
B --> B3[Facilitated: glucose via proteins]
C --> C1[Na+/K+ pump]
C --> C2[Endocytosis]
C --> C3[Exocytosis]Why This Works
The lipid bilayer is selectively permeable — small nonpolar molecules pass freely, but charged or large molecules need protein channels or carriers. Active transport exists because cells often need to maintain concentrations different from their surroundings (nerve cells maintain a resting potential).
Common Mistake
Students confuse facilitated diffusion with active transport. Both use membrane proteins, but facilitated diffusion is passive (moves DOWN the concentration gradient, no ATP). Active transport moves AGAINST the gradient and requires ATP. The direction of movement relative to the gradient is the key distinction.
Osmosis has three scenarios: hypotonic solution (cell swells/turgid), hypertonic (cell shrinks/plasmolysis), isotonic (no net water movement). Plant cells in hypotonic solution become turgid but don’t burst (cell wall prevents it). Animal cells burst (no cell wall).