Organisms Respond To Changes In Their Environment - Survival And Response.

- Survival and Response -

There are different types of responses in plants and organisms. 

1. Taxes 
2. Kineses
3. Tropisms

The sequence of events can involve a chemical control or nerve cells, which can be summarised as: 

stimulus ⇒ receptors ⇒ coordinator ⇒ effector ⇒ response. 

Taxes 

This is a simple response whose direction is determined by direction of the stimulus.

So think of earthworms and bacteria. These organisms often move as a cause of environmental change. They either move to a favourable or unfavourable stimulus.

Positive taxis = Movement TOWARDS the stimulus.

Negative taxis = Movement AWAY from the stimulus.

Using the examples given;

Earthworms move away from light // Negative photo-taxis // thus increasing their chance of survival. Their body moves them to an environment which has a much better suited condition for them to live. Soil = moist, dark conditions.

Bacteria ( some species ) will move towards regions of high concentration of glucose // Positive chemo-taxis // thus increasing their chance of survival.

Kinesis

This is a when an organisms changes its speed at which it moves and rate at which it changes its direction.

Example -  

A woodlice crosses sharp dividing line between favourable (moist)  and unfavourable (dry) environment the RATE OF TURNING INCREASES // in other words the organism moving towards favourable environment would be a QUICK RETURN once they MOVE FROM a unfavourable environment. // Increases chance of survival dry environment means the woodlice would dry out. 

A woodlice crosses divide and travels a considerable amount of distance in the unfavourable (dry) environment, the return to favourable environment would be SLOW. often moving in a straight line before turning. 

Tropism 

This is the growth of part of the plant in response to a directional stimulus.

There are two types of responses -

1. Positive response. 
2. Negative response.

The response to the stimulus is to ensure that the survival value is increased.

Positive phototropism & Negative gravitropism - Plant shoots growing towards the light // favourable position to capture light for photosynthesis. 

Negative phototropism & Positive gravitropism - Plant roots growing towards the soil // better able to absorb water and minerals.