Movement Across Membranes
Why Substances Need to Move
Cells constantly need to import raw materials (oxygen, glucose, mineral ions) and export waste products (carbon dioxide, urea). The cell membrane is partially permeable — it allows some substances to pass through freely and restricts others. Three processes control this movement.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration — i.e., down a concentration gradient. It is a passive process — no energy (ATP) is required.
Examples in biology:
- Oxygen diffuses from alveoli (high O₂) into capillary blood (low O₂) in the lungs
- Carbon dioxide diffuses from respiring cells (high CO₂) into the blood (low CO₂)
- Glucose diffuses from the small intestine into the blood after digestion
Factors affecting the rate of diffusion:
| Factor | Effect on rate |
|---|---|
| Concentration gradient | Steeper gradient → faster rate |
| Temperature | Higher temperature → faster particle movement → faster rate |
| Surface area | Larger surface area → more molecules can cross simultaneously → faster |
| Diffusion distance | Shorter distance → faster rate |
Surface area : volume ratio: Small cells have a large SA:V ratio → efficient diffusion. As cells grow, SA:V decreases and diffusion becomes less efficient — this limits cell size.
Osmosis
Osmosis is a special case of diffusion: the net movement of water molecules across a partially permeable membrane from a region of lower solute concentration (more dilute / higher water potential) to a region of higher solute concentration (more concentrated / lower water potential).
Key point: Water moves to dilute the concentrated solution — it moves toward lower water potential.
Effects on cells:
- Animal cell in pure water (hypotonic): water enters by osmosis → cell swells and bursts (lysis)
- Animal cell in concentrated solution (hypertonic): water leaves by osmosis → cell shrinks (crenation)
- Plant cell in pure water: water enters → vacuole swells → cell becomes turgid (rigid); cell wall prevents bursting
- Plant cell in concentrated solution: water leaves → vacuole shrinks → cell becomes flaccid; further water loss causes plasmolysis (membrane pulls away from wall)
Why turgidity matters in plants: Turgor pressure provides mechanical support — plants wilt when they lose turgor.
Active Transport
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration — against the concentration gradient. This requires energy (ATP) and carrier proteins in the membrane.
Examples:
- Absorption of glucose and amino acids from the small intestine into the blood (even when blood concentration is already higher than in the gut)
- Uptake of mineral ions (nitrates, phosphates) by plant root hair cells from dilute soil water
- Sodium–potassium pump in nerve cells
Key contrast: Active transport moves substances AGAINST the gradient; diffusion and osmosis move WITH the gradient. Only active transport uses energy.
Summary Table
| Process | Substance | Direction | Energy? | Carrier protein? |
|---|---|---|---|---|
| Diffusion | Any small molecule | High → low conc. | No | Not required |
| Osmosis | Water only | Low → high solute conc. | No | Not required |
| Active transport | Ions, glucose, amino acids | Low → high conc. | Yes (ATP) | Yes |
Surface Area Adaptations
Many biological structures are adapted to maximise diffusion:
- Alveoli in lungs — large surface area, thin walls, rich blood supply, moist surface
- Villi and microvilli in the small intestine — increase surface area for absorption
- Root hair cells — long extensions into soil increase surface area for water and mineral ion uptake
AI-generated · claude-opus-4-7 · v3-wjec-combined-science