Organisms increase their chance of survival by responding to changes in their environment. Tropisms as responses to directional stimuli that can maintain the roots and shoots of flowering plants in a favourable environment. Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment. A simple reflex arc involving three neurones. The importance of simple reflexes in avoiding damage to the body.

Organisms move away from things that make them more less likely to be able to reproduce, and toward things that make the more likely to reproduce. This means they will be moving away from things that will kill them and towards things that will protect them, help them to grow, get energy or find a mate.

These movements can happen in three different ways:

• Growing towards or away from something (tropism)
• Moving towards or away from something (taxis)
• Speeding up or slowing down movement and amount of direction changes (kinesis) to make an organism more likely to get out of or stay in an environment

Positive means towards and negative means away from.

In plant roots we see positive hydro-tropism (growth towards water) and positive gravi-tropism (growth towards gravity). In plant shoots we see negative gravi-tropism (growth away from gravity) and positive photo-tropism (growth towards light).

In wood lice we see kinesis in woodlice when they speed up and change direction more rapidly in arid conditions, but slow down and change direction less in humid conditions (because they need the water in the air so they don't dehydrate).

In mice we see positive thigmo-taxis, this is where they will move towards the edge of something.

These movements are all responses to stimuli, caused by a coordinator (such as the brain) which decides a response. The sequence goes stimulus, receptor, coordinator, effector, response.