What are life processes?

"How do you know something is alive?" A dog that runs or a cow chewing the cud is clearly alive. A sleeping person or a tree that doesn't seem to be growing is alive too. Movement we can see isn't the whole answer.

Living things are highly organised: cells, tissues and molecules arranged just so. Left alone, that order slowly breaks down. So a living body must keep repairing and maintaining itself all the time, and that needs energy and raw materials from outside. Inside every cell, molecules are always moving even when we can't see it.

The processes that keep this maintenance going are the life processes:

A single-celled organism such as Amoeba is in touch with its surroundings all over its surface. Food and oxygen simply diffuse in, and wastes diffuse out.

In a large body, most cells are deep inside, far from the air and the food. Diffusion alone would be far too slow to reach them. So big organisms have special organs: lungs to take in oxygen, a gut to take in food, a transport system (blood) to carry things to every cell, and kidneys to remove waste. The rest of this chapter looks at each one.