Why is Membrane Transport Important?
Cells must control what enters and exits. The phospholipid bilayer is selectively permeable — small nonpolar molecules pass freely, but ions and large molecules need help.
Learning Goals:
- Distinguish passive from active transport.
- Describe four transport mechanisms.
- Predict the direction of water movement using water potential.
Comparison Table
| Feature | Simple Diffusion | Facilitated Diffusion | Osmosis | Active Transport |
|---|---|---|---|---|
| Energy | None | None | None | ATP required |
| Direction | High → Low conc. | High → Low conc. | High → Low | Low → High conc. |
| Proteins | None | Channel/carrier | Aquaporins | Carrier pumps |
| Molecules | O₂, CO₂, steroids | Glucose, ions | Water only | Na⁺, K⁺, Ca²⁺ |
Osmosis & Water Potential
Osmosis is the net movement of water from a region of higher water potential () to lower water potential through a partially permeable membrane.
| Solution | Relative [solute] | Water movement | Cell effect |
|---|---|---|---|
| Hypotonic | Lower outside | Water enters cell | Animal: lyses. Plant: turgid |
| Isotonic | Equal | No net movement | Normal |
| Hypertonic | Higher outside | Water leaves cell | Animal: crenates. Plant: plasmolysed |
Worked Examples
Example 1: Red blood cell in distilled water
Distilled water is hypotonic. Water enters by osmosis → cell swells → lysis (bursts). Plant cells don't burst because of the rigid cell wall — they become turgid.
Example 2: Why do Na⁺/K⁺ pumps use ATP?
They transport ions against their concentration gradient (3 Na⁺ out, 2 K⁺ in). This requires energy from ATP hydrolysis.
Common Mistakes
- "Osmosis is diffusion of all molecules" — Osmosis specifically refers to water movement through a partially permeable membrane.
- Confusing hypotonic and hypertonic — Hypotonic = lower solute concentration relative to the cell.
- Forgetting facilitated diffusion is still passive — No ATP is used; it goes down the concentration gradient.
Exam Tips
- Define transport type by three criteria: energy source, direction, protein involvement.
- For osmosis questions, always state the water potential gradient direction.
- Active transport example: Na⁺/K⁺ pump, gut absorption of glucose.
Related Topics
- Action Potentials — Na⁺/K⁺ pump maintains resting potential.
- Enzyme Kinetics — Carrier proteins share kinetic properties with enzymes.