Cell Transport Processes in GCSE Biology

Diffusion

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient. It is a passive process, meaning it does not require energy.

Osmosis

Osmosis is a type of diffusion. It is the net movement of water molecules from a region of high water potential (dilute solution) to a region of low water potential (concentrated solution) across a partially permeable membrane.

Active Transport

Active transport is the movement of substances against a concentration gradient, from an area of low concentration to high concentration, using energy from respiration (ATP) and carrier proteins in the membrane.

Surface Area to Volume Ratio (SA:V)

A larger surface area relative to volume increases the rate of exchange. Smaller organisms or cells tend to have higher SA:V ratios, which allows more efficient diffusion.

Distance

The shorter the distance the substance has to travel, the faster the rate of movement. Thin membranes speed up diffusion and osmosis (e.g. alveoli walls, capillary walls).

Temperature

Higher temperatures increase the kinetic energy of molecules, making them move faster and increasing the rate of diffusion and osmosis. Active transport also increases with temperature up to a point because enzymes work faster.

Concentration Gradient

A steeper concentration gradient (larger difference between high and low concentrations) speeds up diffusion and osmosis. Active transport works regardless of the gradient, but it still depends on how much ATP is available.

Oxygen Diffusion Example

Oxygen diffusing from the alveoli into the blood.

Carbon Dioxide Diffusion Example

Carbon dioxide diffusing out of cells into the bloodstream.

Glucose Diffusion Example

Glucose diffusing into cells from the small intestine.

Water Movement in Root Hair Cells

Water moving into root hair cells from the soil.

Animal Cells Water Movement

Water moving into or out of animal cells depending on the surrounding solution.

Plant Cell Turgidity

In pure water, water enters and the cell becomes turgid (swollen but doesn't burst due to the cell wall).

Plant Cell Flaccidity

In a concentrated solution, water leaves the cell and the cell becomes flaccid or even plasmolysed (membrane pulls away from cell wall).

Animal Cell Lysis

In animal cells, too much water causes lysis (bursting); loss of water causes crenation (shrinking).

Mineral Ions Active Transport Example

Mineral ions (e.g. nitrates) absorbed by plant root hair cells from low concentrations in soil.

Glucose Reabsorption Example

Glucose reabsorbed into the blood from the kidney tubules.

Sodium and Potassium Ions Movement

Sodium and potassium ions moved in and out of nerve cells.

Passive Processes

Diffusion and osmosis are passive (no energy needed); active transport requires energy.

Efficiency Factors

The efficiency of these processes depends on surface area, distance, temperature, and concentration gradient.

Cell Adaptations

Adaptations like thin membranes, large surface areas, and specialised transport proteins help cells carry out efficient transport.