CB1.3 — Transport in cells (Edexcel 1SC0)

Three processes move substances in and out of cells: diffusion, osmosis, and active transport.

Diffusion

Movement of particles from a region of higher concentration to a region of lower concentration — down the concentration gradient. No energy needed (passive). Examples: oxygen into cells; carbon dioxide out of cells; glucose from small intestine into blood.

Factors affecting rate:

• Concentration gradient (steeper → faster)
• Temperature (higher → more kinetic energy → faster)
• Surface area (larger → faster)
• Distance (shorter → faster)

Osmosis

The movement of water molecules across a partially permeable membrane from a region of higher water potential (dilute solution) to lower water potential (concentrated solution).

• If a plant cell is placed in a dilute solution → water enters → cell becomes turgid (rigid).
• If placed in a concentrated solution → water leaves → cell becomes plasmolysed (shrunken).
• If animal cell in hypotonic solution → cell swells and may burst (crenation in hypertonic).

Active transport

Movement of substances against the concentration gradient (low → high), requiring ATP energy from respiration. Uses carrier proteins in the membrane.

Examples: absorption of glucose from gut into blood; mineral ion uptake by root hair cells.

Surface area to volume ratio (SA:V)

Smaller cells have a higher SA:V ratio — more efficient exchange across the membrane. As organisms grow larger, SA:V decreases → need specialised exchange surfaces (lungs, villi, gills).

Required practical — osmosis with potato cylinders

Cut potato cylinders to the same length; place in solutions of different sucrose concentrations; measure percentage change in mass after 30 minutes. Plot % change in mass vs concentration; extrapolate to 0% change = isotonic concentration (same solute concentration as potato cells).

Exam tips

• Always say osmosis involves water movement only.
• State "partially permeable" not "semi-permeable".
• Use % change in mass (not absolute change) in the potato practical for fair comparison.