Living Cells and Life Processes

In and Out of Cells

Water is the main component of all cells. We find water in the cytoplasm and in cell sap. This water contains many dissolved substances and these substances plus the water enter and leave the cells through the cell membrane.  The cell membrane allows certain particles through it but it blocks the passage of others. Because of this nature it is described as a partially (or selectively) permeable membrane. Particles enter and leave cells by three processes:

1. Diffusion
2. Osmosis
3. Active transport

Diffusion

Particles in liquids and gases have kinetic energy, therefore they move about at speed in all directions. These particles move in a random motion. Where there is an area of high concentration some of these particles collide into one another, lose energy and slow down. Others will escape from the area of high concentration to an area of low concentration elsewhere. Very few particles travel the opposite way. The result is a concentration gradient with particles diffusing from an area of high concentration to an area of low concentration. Diffusion occurs in gases and with any substance in a solution.

Therefore we can say that the definition of diffusion is as follows:

Diffusion: The movement of particles from an area of high concentration to an area of low concentration until they spread out evenly.

There are two important rules to remember.

1. The larger the particles the slower the rate of diffusion.
2. The greater the difference in concentration the greater the rate of diffusion. This difference is known as the concentration gradient.
   

Example of diffusion in plant cells

In order to carry out photosynthesis a plant requires carbon dioxide. On the underside of leaves there are small holes known as stomata, carbon dioxide diffuse into the leaves via these. Leaves produce oxygen and water vapour and these in turn diffuse out via the stomata.

Example of diffusion in an animal cell

The cells in our body need glucose and oxygen for respiration. Both these are carried in the blood. When blood reaches the cells the molecules of glucose and oxygen diffuse out of the blood and into the cells.

As cells use up the glucose and oxygen they produce waste chemicals and carbon dioxide. If these were to build up in the cells they would poison them, therefore they diffuse out of the cells into the blood.

Another example of diffusion in animal cells is the exchange of carbon dioxide and oxygen between the alveoli in the lungs and the blood. See gaseous exchange.