The Cell Membrane and Transport
The Fluid Mosaic Model
Membrane Structure
The cell membrane is described by the Fluid Mosaic Model (Singer & Nicolson, 1972). It is a phospholipid bilayer — two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward. Embedded in this bilayer are:
- Integral proteins (span the membrane, form channels/carriers)
- Peripheral proteins (attached to surface, often signalling roles)
- Cholesterol (maintains fluidity across temperature changes)
- Glycoproteins/glycolipids (cell recognition, immune response)
Transport Mechanisms
Passive transport (no energy/ATP needed):
- Simple diffusion: small nonpolar molecules (O₂, CO₂, ethanol) move along concentration gradient
- Osmosis: water moves through aquaporins from low to high solute concentration
- Facilitated diffusion: polar/charged molecules via channel or carrier proteins, still down gradient
Active transport (requires ATP):
- Pumps move substances against concentration gradient
- Example: Na⁺/K⁺-ATPase pumps 3 Na⁺ out and 2 K⁺ in per ATP
Bulk transport: Endocytosis (in), exocytosis (out) — large molecules via vesicles
Osmosis and Cell Volume
In a hypotonic solution (low solute), water enters the cell → may cause lysis (in animal cells) or turgidity (in plant cells). In a hypertonic solution, water leaves → cell shrinks (crenation in animals, plasmolysis in plants). Isotonic solutions cause no net water movement.
Transport Summary
| Type | Energy? | Direction | Example |
|---|---|---|---|
| Simple diffusion | No | High→Low conc. | O₂ into cells |
| Facilitated diffusion | No | High→Low conc. | Glucose via GLUT transporters |
| Osmosis | No | Low→High solute | Water through aquaporins |
| Active transport | Yes (ATP) | Low→High conc. | Na⁺/K⁺ pump |
| Endocytosis | Yes (ATP) | Into cell | Phagocytosis of bacteria |
| Exocytosis | Yes (ATP) | Out of cell | Neurotransmitter release |
A red blood cell placed in distilled water will:
Distilled water is hypotonic — water flows into the cell by osmosis, causing swelling and eventual lysis.
Correct answer: Swell and potentially burst (lysis)
The Na⁺/K⁺ pump moves:
The Na⁺/K⁺-ATPase uses one ATP to export 3 sodium ions and import 2 potassium ions, maintaining the electrochemical gradient essential for nerve and muscle function.
Correct answer: 3 Na⁺ out and 2 K⁺ in per ATP
Explain why oxygen can cross the cell membrane by simple diffusion but glucose cannot.
Membrane permeability depends on molecule size and polarity.
Correct answer: Oxygen (O₂) is a small, nonpolar molecule that can dissolve in the hydrophobic lipid bilayer and diffuse through. Glucose is large and polar — it cannot dissolve in the bilayer and requires specific carrier proteins (GLUT transporters) for facilitated diffusion.