All living organisms have an external environment that consists of air, water or solid around them. These environments can sometimes change which can cause a lot of stress to the organism. The organism must change its behaviour or physiology to minimise the change.
STIMULUS: a change in the environment that causes a response
RESPONSE: a response is a change in behaviour or physiology as a result of a change
in the environment
Internal conditions of living organisms must be maintained as cells undergo metabolic reactions which release products which may be unwanted or toxic. These substances diffuse out of the cells into the tissue fluid. Eg: CO2 build up will change the pH of the tissue fluid and result in enzymes denaturing.