Life Processes

What separates a living thing from a non-living one? The answer is a set of basic functions that go on all the time to keep an organism alive, even when it is resting. These are the life processes: nutrition, respiration, transportation and excretion. Together they take in energy and materials, use them, and remove the waste. They are the core of biology.

Nutrition is the process of taking in food and using it for energy, growth and repair. There are two main modes:

• Autotrophic nutrition: organisms make their own food. Green plants do this by photosynthesis, using sunlight, carbon dioxide and water (with chlorophyll) to make glucose and release oxygen.
• Heterotrophic nutrition: organisms take food from others. Animals, fungi and many microbes are heterotrophs.

In humans, food is broken down step by step in the digestive system. Teeth and enzymes in the mouth, stomach and small intestine do the breaking down. This allows the body to absorb the nutrients.

Respiration is the process of releasing energy from food inside the cells. The energy is stored in a molecule called ATP. There are two kinds:

• Aerobic respiration: uses oxygen, breaking glucose fully into carbon dioxide and water and releasing a lot of energy.
• Anaerobic respiration: happens without oxygen, releasing less energy. In our muscles during hard exercise it produces lactic acid (causing cramp). In yeast it produces alcohol.

In humans, breathing brings oxygen to the lungs. From the lungs, oxygen passes into the blood. Carbon dioxide passes the other way and is breathed out.

Materials must be carried around the body. This is transportation.

• In humans and animals, the circulatory system does the job: the heart pumps blood through blood vessels (arteries, veins and capillaries), carrying oxygen, food and waste.
• In plants, two tissues transport materials: xylem carries water and minerals up from the roots, and phloem carries food made in the leaves to the rest of the plant.

Before xylem can carry water upward, the plant must first draw it out of the soil. This is the job of the root hairs, tiny thread-like outgrowths on the surface of young roots. They greatly increase the surface in contact with the soil. The actual absorption of water and minerals happens at these root hairs, not at the general root surface. Each root hair is very delicate. When a seedling is pulled up or moved to a new spot, the root hairs are easily torn off. With them gone, the plant can no longer absorb water properly, so a transplanted seedling often wilts or fails to grow until new root hairs form.

The life processes produce harmful wastes, which must be removed. This removal is called excretion.

In humans, the main organs of excretion are the kidneys. They filter the blood and remove waste, chiefly urea, as urine. The lungs excrete carbon dioxide. The skin removes some waste through sweat. In plants, waste is removed more slowly. Some is stored in the plant. Some is released through the stomata. Some is shed when leaves fall.

For a body to work as one, its many parts must be coordinated: the eye that sees danger must make the legs run. Animals do this in two ways: a fast electrical system (the nervous system) and a slower chemical system (hormones).

The nervous system gives fast, precise control. It is built from special cells called neurons that carry electrical signals, and it has three main parts:

• Brain: the control centre that thinks, remembers and decides.
• Spinal cord: links the brain to the body and handles reflexes.
• Nerves: carry messages to and from every part of the body.

Sense organs detect stimuli. Neurons carry the message to the brain. The brain then sends instructions to the muscles, all in a fraction of a second.

Some responses are too urgent to wait for the brain to think. A reflex action is an automatic, very rapid response to a stimulus. For example, you instantly pull your hand away from something hot, or blink when something nears the eye.

Reflexes are handled by a short pathway called the reflex arc, which runs through the spinal cord without first going to the brain. This speed protects the body from harm before we are even aware of it.

The second system of control is chemical, using hormones, chemical messengers carried in the blood. They are released by endocrine glands (ductless glands) and act on distant organs.

Examples include:

• Insulin: from the pancreas, controls blood sugar.
• Adrenaline: from the adrenal glands, the "fight or flight" hormone.
• Thyroxine: from the thyroid, controls the rate of metabolism. Growth hormone controls growth.

Hormonal control is slower than nervous control but longer-lasting. It regulates processes like growth, metabolism and reproduction. Together the nervous and hormonal systems keep the body coordinated and in balance.