1.4 Movement of substances

Concentration gradient

The difference in concentration between two regions

Diffusion

The net movement of particles (atoms, molecules or ions) from a region where they are of higher concentration to a region where the are of lower concentration, that is, down a concentration gradient.

Factors that affect the rate of diffusion

• Concentration gradient

• Diffusion distance

• Surface area-to-volume ratio

How does diffusion distance affect rate of diffusion?

The shorter the diffusion distance, the less time is needed for the substance to travel, hence the rate of diffusion is higher.

How does surface area to volume ratio affect the rate of diffusion?

The greater the surface area to volume ratio of a cell, the higher the rate at which substances move in and out of it.

Facilitated diffusion

Diffusion of substances through cell membranes using specific protein channels or carriers from regions where they are at higher concentrations to regions where they are at lower concentrations.

Osmosis

The movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Water potential

A measure of the tendency of water to move from one place to another.

What happens to a plant cell in a solution with higher water potential?

1. The cell sap has a lower water potential than surrounding solution

2. There is a net movement of water into the cell by osmosis

3. The cell expands and becomes turgid

4. The vacuole increases in size and pushes the cytoplasm against the cell wall. The cell does not lyse as it is protected by the inelastic cell wall.