Living things are sensitive to their environment. This means that they detect and respond to events in the world around them. Simple uni-cellular organisms such as Amoeba have limited sensitivity, while higher organisms such as mammals are more sensitive and can react to very small changes in light, sound, touch, taste, smell, temperature, etc.

In higher animals specific organs are developed for the purpose of detecting stimuli. The organisation of light sensitive cells into eyes of varying complexity from one species to another is an example.

Plants do not have sensory organs as such but there are clearly certain regions of their bodies such as the shoot tip that are sensitive to light, gravity, water and various chemicals.


Living things excrete. Excretion is the removal from the body of waste products which result from normal life processes. Waste products such as carbon dioxide must be removed. If they are allowed to accumulate they cause poisoning which slows down vital chemical reactions. When excretory organs such as kidneys are damaged, the organism quickly displays symptoms of poisoning and death is rapid unless treated.

Excretion should not be confused with egestion which is the removal from the body of substances with no food value that have passed unused through the digestive systems.


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