Life ProcessesWhat are Life Processes?

Even when you sit still in class — even when you are fast asleep — your body is busy with maintenance work. The processes that together do this maintenance job are called life processes.

Maintenance prevents damage and break-down, and that needs energy. The energy comes from outside the body, as food, and the process of bringing a source of energy from outside to the inside is called nutrition. A growing body also needs extra raw material from outside. Life on Earth is built from carbon-based molecules, so most food is carbon-based too, and different organisms use different kinds of nutrition depending on how complex their carbon sources are.

Food comes in all sorts of forms, so the body has to break it down or build it up, and finally turn it into one uniform source of energy for its molecular movements, and into the molecules it needs to grow. That takes a chain of chemical reactions. Oxidising–reducing reactions are among the most common ways to break molecules down, and many organisms use oxygen from outside the body for them. Taking in oxygen and using it to break down food for the cell's needs is called respiration.

Two more jobs follow. Food and oxygen are taken in at one place but needed everywhere, so the body needs transportation. And the reactions that release energy leave by-products that are useless or even harmful, so these must be thrown out of the body — excretion.

Match each life process to the job it does.

These four — nutrition, respiration, transportation and excretion — are the life processes of this chapter. One question is still open: why does a big body need special organs and a transport system, when a single cell manages without them?

One Cell or a Million?

A single-celled organism needs no special organs for taking in food, exchanging gases or removing waste: its whole surface touches the surroundings, and substances simply diffuse in and out. But what happens as a body gets bigger?

In this model, oxygen diffuses in from outside and reaches only about two cells deep. Green cells get enough oxygen; grey cells are starving.

In a multicellular organism, most cells are not in direct contact with the surroundings, so simple diffusion cannot meet the needs of all of them. Such bodies have specialised tissues for each job: tissues that take in food and oxygen, a transport system that carries them to every cell, and a specialised excretory tissue, to which the transport system carries the waste. Let us take these processes one at a time, starting with nutrition.