Co-ordination and Response » Stimulus, Response and Homeostasis

Responding to changes in the environment

Your surroundings are always changing. It gets hotter, brighter, louder or colder. Living things must notice these changes and react to them, or they would not survive for long. You met this idea in the lesson Response to Environment and Excretion, where sensitivity was one of the life processes. In this lesson we look at how the reaction actually happens.

Organisms respond to changes in different ways. Animals have receptors that detect the change, and parts of the body that move or change to react. Plants have no muscles, so they respond mainly by growing in a certain direction. A shoot growing towards a window is a response, and so is a hand pulled back from a hot pan.

Key terms:

• Receptor: a cell or organ that detects a stimulus.
• Co-ordinator: a part of the body that receives information from receptors and decides what to do about it.
• Effector: a part of the body that carries out the response, such as a muscle or a gland.
• Homeostasis: the maintenance of a constant internal environment.

The pathway of a co-ordinated response

A co-ordinated response means the different parts of the body work together in the right order. It needs three main things: a stimulus, a receptor and an effector. The information passes along a pathway:

Remember, a stimulus is a change in the environment that is detected.

stimulus → receptor → co-ordinator → effector → response

Look at the difference between the last two words. The effector is the part of the body that acts. The response is the thing that happens. Examiners like to check that you can tell them apart.

Plants use the same idea. Cells in a shoot detect light coming from one side, which is the stimulus. Cells in the stem then grow faster on one side. They are the effector, and the response is that the shoot bends towards the light. Plants do not have a brain or a nervous system, so their co-ordination works differently from ours. You will see how in the lessons Geotropism and Phototropism and Auxin and the Phototropic Response.

Homeostasis: keeping conditions steady

Responses do not only help you react to danger. They also keep the inside of your body steady. Homeostasis is the maintenance of a constant internal environment. Your cells work best when conditions such as temperature and water content stay within narrow limits. If they drift too far, your cells stop working properly.

You need to know two examples:

Homeostasis uses the same pathway

Homeostasis is made up of co-ordinated responses. When a condition in the body moves away from its normal level, that change is the stimulus. Receptors detect it, a co-ordinator processes the information and effectors act to bring the level back. When the level returns to normal, the correcting response is reduced. In this way the body keeps the condition steady.

For example, if the body gets too warm, receptors detect the change in temperature and effectors act to cool you down. If you get too cold, effectors act to warm you up. The details of how are for later lessons. For now, remember that homeostasis is a response that puts a condition back to normal.

Exam-style question

(a) Define the term homeostasis. [1 mark]

(b) State two examples of homeostasis in the human body. [2 marks]

(c) A person stands on a very cold surface and quickly lifts their foot. Put these words in the correct order to show the pathway of the response: effector, stimulus, co-ordinator, receptor. [2 marks]

(d) Name the effector in part (c). [1 mark]