How Organisms Communicate

The Effect of Stimuli on the Behaviour of an Organism

1. Both animals and plants react to stimuli
    
2. The four main stimuli are light, sound, chemicals and touch.
    
3. The chemical stimuli may act on the sense of smell, the sense of taste or on both of these senses.

Emitters and Receptors

1. An emitter organ is responsible for producing a stimulus e.g. a light emitter organ.
    
2. A receptor is a group of scattered cells or an organ which receives a stimulus e.g. the ear of mammals.
    
3. Some organisms have a more sensitive receptor organ for a particular stimulus than for other types of stimuli. This is because they specialize in one particular sense e.g. smell in dogs.
    
4. The efficiency of one type of receptor organ varies between different organisms.

Communication Inside the Organism

1. Sensory cells are linked to muscle cells and other tissues by nerve cells.
    
2. In animals with a complicated body structure there is a control centre which links the sensory cells to the muscle cells.
    
3. As the bodies of different animals become more complex, so does the control centre - the brain.
    
4. The brain co-ordinates the body's response to stimuli.
    
5. The human central nervous system is composed of the brain and the spinal cord.
    
6. Reflex actions are not co-ordinated by the brain although, in most cases, the brain is immediately aware that the reflex action has taken place.

The Interdependence of Living Things

Social Behaviour

1. Some large groups of animals are only temporary.
2. Some permanent groups of animals do not show any social behaviour.
    
3. Being a member of a group is an advantage to an individual because it can feed in greater safety, it is less likely to be attacked and it is able to find a mate more easily.
    
4. A group of animals which shows social behaviour will be organized enough to show division of labour and co-operation between its members.
    
5. Members of these groups are direct relations of one another.

Special Relationships

1. Mutualism is an association between two organisms which benefits both of the organisms in the relationship.
    
2. Parasitism is an association between two organisms, one of which harms the other one in some way.

The Ecosystem

1. The producer organisms in an ecosystem are the green plants. They convert the sun's energy into chemical energy.
    
2. The herbivores are the primary consumers. They take into their bodies the energy which has been stored by the producers.
    
3. The secondary consumers are carnivores. They take into their bodies the energy which has been stored by the herbivores.
    
4. Green plants are autotrophs. They use the energy of the sunlight to make their food.
    
5. Animals are heterotrophs. They gain their energy by eating other organisms.
    
6. Energy flows from the sun to the autotrophs and then from the autotrophs to the heterotrophs.
    
7. A food chain ends with the top carnivore. A food chain is never very long because the amount of energy available for the consumers decreases along the food chain.
    
8. Since most animals have a varied diet, a food web will show more clearly how many different things an animal eats.
    
9. All green plants compete with each other for sunlight, water and minerals. All animals which eat the same thing are also in competition with each other.
    
10. Decomposer organisms and scavengers help to re-cycle essential chemicals through the food web.

© Paul Billiet, Shirley Burchill, Alan Damon and Deborah James 2008