Look out of your window and you will see both living things and non-living things. Houses, cars, walls, and fences are not alive. They are made by us from a variety of materials for different purposes. Once made, these things cannot develop or grow. Trees, birds, grass, cats, and worms are all living, breathing, growing organisms—and there are many more living things that you cannot see, such as bacteria and tiny insects.
The main aim of all living things is to reproduce—to make identical or near identical copies of themselves to make sure that their species continues. All living things possess genes or instructions for making the body. Even the simplest bacteria have about 2,000 genes. Vertebrates, such as humans, have as many as 100,000. Viruses cannot reproduce independently so cannot really be thought of as living things.
Every living thing—whether it is an insect, a whale or a tree—is made up of cells, and nothing less than a cell can truly be said to be alive. Some microscopic organisms, such as bacteria, are made up of single cells, but animals and plants have many millions of cells with different functions. The cells work together to allow the organism to survive.
TYPES OF LIFE
There is much disagreement about how to group living things, but the system used in this encyclopedia organizes all life into five kingdoms. These are prokaryotes (simple organisms without nuclei, such as bacteria), Protoctista (mostly single-celled organisms with nuclei), fungi, plants, and animals.
Fungi include mildews, molds, mushrooms, and yeasts. They live by absorbing their food, usually from the organic matter around them.
The plant kingdom contains at least 260,000 species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees and various other forms which live on Earth and in fresh and salt water. Plants use the Sun’s energy to make their own food, and this process is called photosynthesis.
The animal kingdom includes at least two million species of many-celled creatures. It includes creatures from tiny mites and ticks to jellyfish, squid, birds, whales, and people. All animals live by actively getting food and digesting it inside their bodies.
Every living thing has its own life cycle—its life begins, it grows, develops and reproduces and it dies. A fruit fly’s entire life cycle takes place in a week. A large tree, such as a redwood, may live for hundreds of years. But both the fly and the redwood, like all living things, need food, water, and oxygen in order to survive and grow.
Living things constantly have to battle against threats to their well-being. A plant may die if it has no water or if the weather suddenly becomes very cold. Animals cannot live for long without food or water. Many creatures depend on other creatures for their survival. An insect may eat a plant. The insect, in turn, may be gobbled up by a bird and a cat may kill the bird. Life is fragile.