Movement of Substances Into & Out of CellsEdexcel GCSE Biology: Revision notes

Section 1

Why Do Substances Need to Move Across Cell Membranes?

Cells need a constant supply of useful substances (oxygen, glucose, water) and must remove waste (carbon dioxide, urea). The cell membrane is partially permeable. Three processes move substances across membranes: diffusion, osmosis and active transport.

Key termscell membranepartially permeable

Section 2

Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient, requiring no energy.

Factors affecting rate: concentration gradient (steeper = faster), temperature (higher = faster), surface area (larger = faster).

Examples: oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli; digested food diffuses from the small intestine into the blood.

Key termsdiffusionconcentration gradient

Think of it like this

Think of perfume spreading through a room — particles spread from where concentrated until evenly spaced.

Section 3

Osmosis

Osmosis is the net movement of water molecules from a dilute solution to a concentrated solution, through a partially permeable membrane.

Solution type	Effect on cell
Dilute	Cell gains water; animal cells may burst; plant cells become turgid
Concentrated	Cell loses water; animal cells shrink; plant cells become flaccid/plasmolysed
Isotonic	No net movement

Key termsosmosisturgidplasmolysis

Exam tip

Specify direction using 'dilute' and 'concentrated', not just 'high to low'.

Section 4

Core Practical: Osmosis in Potatoes

Cut equal-sized potato cylinders, measure starting mass, place in different sucrose concentrations, leave a set time, blot dry and reweigh. Calculate percentage change in mass. Gain = dilute solution; loss = concentrated solution.

Key termsindependent variabledependent variablecontrol variable

Common mistake

Forgetting to keep cylinder size identical across tubes ruins the fair test.

Section 5

Calculating Percentage Change in Mass

Percentage change in mass = ((final mass − starting mass) ÷ starting mass) × 100. Positive = gain; negative = loss.

Key termspercentage change in mass

Example

5.0 g → 5.6 g: ((5.6−5.0)÷5.0)×100 = 12% gain.

Section 6

Active Transport

Active transport moves particles from lower to higher concentration (against the gradient), using energy from respiration.

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

Key termsactive transport

Exam tip

State explicitly 'energy from respiration' to gain the mark.

Must Know

Diffusion: high to low concentration, no energy.
Osmosis: water, dilute to concentrated, partially permeable membrane.
Active transport: against gradient, needs energy from respiration.
Percentage change in mass = ((final−start)÷start)×100.
Mass gain = more dilute external solution; mass loss = more concentrated.

That's the notes covered.

Carry on to the next subtopic.