[Biology Form 4] Diffusion

Various substances pass through the plasma membrane of a cell. The simplest mode of transport for these substances is by diffusion.

Diffusion is defined as the movement of substances from an area of high concentration to an area of low concentration. When substances move in this manner, we say that they are moving along the concentration gradient.

This movement process is simple and passive, which means that the substances move easily from the region of high concentration to the one whose concentration is lower without requiring any energy.

When water moves from an area of high concentration to an area of low concentration across a semi-permeable membrane, its movement called osmosis.

Therefore, when a cell absorbs water by diffusion through its semi-permeable cell membrane, we say that water has entered the cell through osmosis.

Eg: Roots absorb water through osmosis because the soil solution around the roots has a higher water concentration than the root cells.

Certain substances need help in the diffusion process. These substances move into a cell by diffusion through the membrane but they are brought in by special carrier proteins. Such a diffusion process is known as facilitated diffusion.

An active diffusion takes place when a substance moves from an area of low concentration to an area of high concentration, with the use of energy. This movement goes against the concentration gradient.

An active diffusion process usually involves transporting important ions into and out of a cell, whose plasma membrane may have special protein pumps to engage in it.

Schematic representation of mixing of
two substances by diffusion

Self diffusion, exemplified with an isotopic tracer of
radioactive isotope ²²Na

Example of chemical (classical, Fick's, or Fickian) diffusion of
sodium chloride in water

Illustration of low (top) and
high (bottom) entropy.