Structure of the Plasma Membrane
The plasma membrane is described by the fluid mosaic model. It consists of a phospholipid bilayer — two sheets of phospholipid molecules with their hydrophilic heads facing outward and hydrophobic tails facing inward. Embedded within the bilayer are proteins: some span the full width (integral proteins) and others attach to the surface (peripheral proteins).
Types of Membrane Transport
Comparison of the four main transport mechanisms across the cell membrane.
| Type | Direction | Energy | Examples |
|---|---|---|---|
| Simple diffusion | High → low concentration | None (passive) | O₂, CO₂, lipid-soluble substances |
| Facilitated diffusion | High → low concentration | None (passive) | Glucose, ions via channel proteins |
| Osmosis | High → low water potential | None (passive) | Water across aquaporins |
| Active transport | Low → high concentration | ATP required | Na⁺/K⁺ pump, glucose absorption in gut |
Osmotic direction
The Sodium–Potassium Pump
The Na⁺/K⁺ ATPase is a classic example of active transport. For every ATP hydrolysed, it pumps 3 Na⁺ out of the cell and 2 K⁺ in. This creates an electrochemical gradient essential for nerve impulse transmission and muscle contraction.
HSC Exam Focus
Always state BOTH the direction of movement AND the energy requirement. 'Glucose enters the intestinal epithelial cell by active transport against its concentration gradient, using ATP.' Exam marks are lost when students omit the energy source.
Real-Life Application
Oral rehydration salts (ORS) given during diarrhoea contain glucose and Na⁺ to activate the Na⁺–glucose co-transporter in the gut. This pulls water into the body by osmosis — saving millions of lives annually.
InstaTest
InstaTest: Membrane Transport
4-question MCQ sprint on diffusion, osmosis, and active transport.