Movement of Substances into and out of Cells

Diffusion

Diffusion is a method of transporting materials in and out of cells and it is driven by a concentration gradient.
It is a passive process, which means it does not require energy from the cell.
It involves the movement of particles from an area of higher concentration to an area of lower concentration until they are evenly distributed.
Oxygen and carbon dioxide are often transported by diffusion.

Osmosis

Osmosis is a special type of diffusion involving the movement of water molecules across a selectively permeable membrane.
Like diffusion, it is a passive process and doesn’t require energy, instead it relies on differences in water concentration.
Water moves from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration) through the cell membrane.
Osmosis is critical for the absorption of water in plant roots and the absorption and release of water in all cells.

Active Transport

In contrast to diffusion and osmosis, active transport does require energy to move substances across cell membranes.
This energy is usually sourced from ATP, the cell’s energy storage molecule.
Active transport mechanisms move substances from an area of lower concentration to an area of higher concentration, working against the concentration gradient.
Active transport is necessary when the cell needs substances that are not in abundance in the environment, such as glucose or ions.

Factors Affecting the Rate of Transport

The rate of substance transport into and out of cells is influenced by a number of factors: temperature, surface area, length of diffusion paths and concentration gradients.
Higher temperatures increase the rate because the particles have more energy and move more quickly.
Greater surface area of the cell membrane also increases the rate as there is more area for substances to move across.
Shorter diffusion paths increase the rate because the substances have less distance to travel.
A steeper concentration gradient increases the rate because the difference between concentrations inside and outside the cell is greater, driving faster movement.