Transport in Cells (B1.3)
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to lower concentration (down a concentration gradient). Passive — no energy required.
Factors that increase rate of diffusion:
- Steeper concentration gradient
- Higher temperature (more kinetic energy)
- Larger surface area
- Thinner diffusion distance
Examples: O₂ into red blood cells; CO₂ out of respiring cells; glucose from gut into blood.
Osmosis
Osmosis is the movement of water molecules through a partially permeable membrane from a region of higher water potential (dilute solution) to lower water potential (concentrated solution). Also passive.
| Situation | Effect on plant cell | Effect on animal cell |
|---|---|---|
| In dilute solution (high water potential) | Water enters → turgid | Water enters → may burst (lysis) |
| In concentrated solution (low water potential) | Water leaves → plasmolysed | Water leaves → crenated |
Active transport
Movement of substances against the concentration gradient (low → high). Requires ATP from respiration. Uses carrier proteins in the membrane.
Examples:
- Mineral ion absorption from soil into root hair cells (soil [ions] < cell [ions])
- Glucose absorption from gut lumen into blood after digestion
Surface area to volume ratio (SA:V)
SA:V decreases as organism grows larger. Small cells have enough surface area to exchange materials by diffusion alone. Large multicellular organisms need specialised exchange surfaces and transport systems.
Worked calculation: cube side 2 cm → SA = 24 cm², V = 8 cm³, SA:V = 3:1. Cube side 4 cm → SA = 96 cm², V = 64 cm³, SA:V = 1.5:1. Larger = lower SA:V.
Adaptations to increase SA: villi/microvilli (gut), alveoli (lungs), root hair cells, gill filaments.
Common exam errors
- Omitting "partially permeable membrane" from osmosis definition — loses a mark.
- Saying osmosis needs energy — it's passive.
- Confusing active transport (against gradient, needs ATP) with diffusion (down gradient, no ATP).
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