How do we know that some things are alive? It might seem obvious in the case of animals that move about, or plants that unfurl colourful flowers. But some flowerless plants are so tough and grow so slowly that they can be mistaken for rocks. Some animals lay tiny eggs that survive for hundreds of years without change. Are these truly living? What exactly is life?
THE MEANING OF LIFE
Living things, called organisms, carry out key activities known as life processes. One is growth. Almost all living things, from the tiniest micro-organisms to the greatest trees and whales, get larger at some stage as they build new body parts, and the body parts they already have grown larger. Once they are fully grown, they must maintain their bodies and replace old, worn out parts with new ones. This continuing maintenance is also a feature of life.
Another key feature of life is the reproduction or breeding. All types of living things produce more of their own kind. This happens every 20 minutes with some of the micro-organisms, and every hundred years with some strange, slow-growing plants. Reproduction to make offspring is followed by the growth of the offspring, which then reproduce to make their own offspring, and so on.
INS AND OUTS
Another life process involves taking in substances called raw materials to be used as building blocks for maintenance, growth, and reproduction. Animals get their raw materials in nutrients from the food they eat. Most plants obtain theirs as simple mineral substances taken in by their roots from the soil. As new supplies of raw materials come from a living thing, unwanted materials and wastes leave. Getting rid of wastes is another important life process.
To power all these life processes, living things need energy. Animals take energy from their food, which they break down or digest inside their bodies. Plants obtain their energy from the Sun. They catch the light energy and use it to build up food substances, through a process called photosynthesis. Taking in energy and using it for life processes is itself a key process of life.
INSIDE A LIVING THING
A living thing can be thought of as a bag of continually changing chemical substances. Even a very simple microorganism contains many thousands of different chemical substances, and these are always on the move. Nutrients come in and wastes go out. Raw materials are put together to make new parts for growth. Tiny parts of the micro-organism wear away or break and need to be replaced. The same happens on a much larger scale in plants and animals, including human beings.
The way in which these chemical substances come together, build up, break down and change, is called metabolism. The study of all these chemicals, life processes and metabolic changes, and how they make an organism work inside, is known as physiology.
THE NEED FOR ENERGY
All living things are made of tiny building blocks called cells. Some life forms are just one cell, like most micro-organisms. Others are built up of billions of cells, like ourselves. Most of the changes in metabolism take place inside cells. One of the most important metabolic changes in cell respiration.
Cell respiration is different from bodily respiration, which is also called breathing. Animals use bodily respiration to take oxygen from the air into their bodies. Cell respiration uses this oxygen inside cells. Both animals and plants use cell respiration. It adds the oxygen to a substance called glucose sugar, which contains lots of energy. Adding the oxygen makes the glucose sugar break apart, releasing the energy. This energy is then available to power hundreds of other metabolic changes. The end results of cell respiration are water and a gas called carbon dioxide.
Animals take oxygen from the air that they have breathed in. The carbon dioxide in the air is removed from the body when they breathe out. Plants do not make obvious breathing movements. But they do also take in oxygen from the air, carry out cell respiration, and release carbon dioxide. The oxygen simply floats or passes through tiny holes in the leaves, to the cells inside. The carbon dioxide passes out the opposite way.
Plants and animals obtain their glucose sugar in different ways. Animals get it by breaking down or digesting food. Plants get it by building up simple mineral nutrients taken in from soil, using light energy from the Sun. Plants get their food during a process called photosynthesis. This uses up carbon dioxide and creates oxygen. These two gases again pass in and out of the plant through tiny holes in the leaves.
A BASIC LIFE PROCESS
The life process of cell respiration happens all the time, in nearly all living things—including the billions of cells in our own bodies. The glucose sugar that is broken down in cell respiration is carried to the cells in our body parts by the blood, and so it is also known as blood sugar.
To keep up the supply of glucose sugar, the amount of it in the blood is controlled by another substance, called insulin. This is one of many chemical messengers in the body called hormones. Dozens of different hormones control various metabolic changes and life processes inside living things. Some people cannot make enough insulin in their bodies. Lack of insulin causes the condition called diabetes. This is one example of what is called a problem of metabolism.
SO MUCH TO FIND OUT
Nearly all kinds of living things carry out cell respiration. It is one of the most basic of all metabolic changes and life processes. It is not the only process going on, though. Like a tiny part in a huge machine, it is just one of thousands of chemical changes happening every second in every living thing.