This is a work in progress... please bear with me, come back and watch it grow? Send constructive suggestions?
What you see below might be seen as an outline for what will eventually be spread across multiple pages.
As I mentioned in the introduction to locomotion, a leopard must move to eat an impala... and an impala must move if it wishes to avoid being eaten.
But both predator and prey must know about one another before chasing or running away become issues, and that "knowing about each other" is an example of the sort of thing that we are talking about under the heading of "Response".
Not only do organisms need to perceive and respond to external stimuli, but there are internal stimuli, too. When I was first starting this page, I wrote here: "At the moment, my stomach is reminding me that since I got up four hours ago I have eaten nothing, so I hope you will forgive me for posting this page a little early?" I promised to come back to it, which I did, hence the material that follows.
The basic idea of response is, I hope, pretty easy to grasp?
But in your study of biology, I think you may find organisms' response mechanisms a source of wonder and delight. It is easy, especially in biology, to become fascinated by "the trees", and lose sight of "the forest". There are organisms with quite amazing powers of sight, smell, etc, etc. The more you study biology, the more amazing examples you will come across.
While enjoying the variety of ways to sense and respond to things, try also to remember that a successful organism is trying to maintain homeostasis... which is merely Greek for "same state". When we go out on a cold afternoon, our body temperature is meant to be about 37 degrees Celsius. If we had the sense to put on a warm coat, our body won't have to work too hard to keep us warm. If our systems find that heat is leaking away from us rapidly, they will "fire up" our "furnace", to counteract that leakage of heat.
It is easy to think that only animals "respond". Not so. Look at the leaves of a big tree on a sunny morning. You'll find they are arranged so that they are at right angles to the light falling on them. Look at them again in the afternoon. The sun is in a different part of the sky, and the leaves are still arranged so that the light falls on them at a right angle. The tree's leaves have responded to the movement of the light.
Homeostasis is a broad term... and if you remember to watch how organisms strive to achieve homeostasis, you will learn a lot about how they work. Seeking homeostasis is achieved most obviously with the organism's response mechanisms, but you could say that meeting all the other seven Great Challenges of Life is also the process of maintaining homeostasis.
One other observation about maintaining homeostasis. I'm repeating something I hinted at before. Environments change... a leopard appears in the distance, or a human goes from a warm building to a cold outdoors. It is when the environment changes that the mechanisms for maintaining homeostasis get their toughest challenges.
Different organisms can survive different environmental ranges. A polar bear would be unhappy in Costa Rica, and a hummingbird would be unhappy in Canada in winter. Each can maintain its homeostasis within certain limits. One of the wonders of biology is the variety if different combinations of environmental challenges which different organisms can thrive in.
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