What Are Living Things?

A living thing is called an organism. Life on earth is represented by a great variety of different kinds of organisms. Organisms are often described as being simple or complex. Such organisms as amoebas and bacteria are called simple because they are small, and when compared with such complex organisms as trees or elephants, they are seemingly simple. However, no living organism, no matter how small, is in reality simple. The descriptions of simple and complex are useful only for making comparisons. The larger, complex organisms are distinguished by having specialized parts, such as hearts or leaves, that perform special jobs.

Simple organisms are probably similar to primitive living things that appeared early on the earth. During millions of years, the larger, complex organisms are thought to have developed from simple organisms through the process of evolution. The theory of evolution is one of the basic concepts of biology: it explains why there are so many different kinds of organisms.

Living things are divided into five groups, or kingdoms—the monera (bacteria and blue-green algae), protist (protozoans, slime molds, and algae), fungus (mushrooms, molds, and lichens), plant (mosses, ferns, cone-bearing plants, and flowering plants), and animal kingdoms. Organisms within each kingdom have certain basic similarities. Living things formerly were divided into two kingdoms—the plant kingdom and the animal kingdom.

Despite the vast differences between the simple and complex forms, all living organisms have certain qualities or properties in common. The basic unit that is fundamental to all living matter is the cell. Cells may differ in details of structure and function, but there are fundamental similarities in their composition and in the chemical reactions they perform.

Cells contain genes—molecules of DNA (deoxyribonucleic acid). It is DNA that determines the characteristics of an organism and directs all the chemical activities carried out by the cell, and ultimately by the whole organism. For example, its genes control what kinds of proteins a cell makes. Many proteins serve as enzymes—large molecules without which the chemical reactions could not take place.

Chemical activities, collectively called metabolism, take place in all living things, and all organisms need a source of energy to carry out their activities. Living organisms obtain energy from the chemical breakdown of food substances. Organisms such as plants and algae manufacture their own food, using the energy from sunlight, through the process of photosynthesis. Organisms that cannot make their own food, such as animals, must obtain it from their surroundings. The energy obtained from the breakdown of foods may be used immediately or may be stored in such chemical compounds as carbohydrates or fats for later use. Involved in using foodstuffs are such life processes as digestion, absorption, assimilation, and excretion.

Living things grow (increase in size) in an organized pattern. They also go through a process of development, during which the various parts of the organisms take shape, mature, and assume their adult functions. The simple forms of life, such as bacteria and amoebas, show comparatively little development. In more complex organisms, development is a complicated process that may take many years.

Living matter has the ability to reproduce itself so that the characteristics of each type of organism are carried on from one generation to the next. It is one of the fundamental principles of biology that life comes only from living things. Scientists believe that the original source of life was a mixture of organic compounds present on the earth billions of years ago.

Another characteristic of life is that living things are able to react or respond to conditions outside themselves. The stems of most plants, for example, turn toward a source of light. The ability to react to outside conditions is even more developed in complex animals than in plants. Living things also have the power of movement. In many organisms, however, this ability is quite limited.

Except possibly for some types of bacteria, every organism relies in some way upon some other form of life. Some plants—red clover, for example—can produce seeds only if their pollen is carried from one flower to another by insects. In some cases, a special type of relationship, called symbiosis, has developed between two entirely different kinds of organisms.

Protozoans, animals, and other organisms that cannot manufacture food must get their food from food-making organisms such as algae and plants. For example, an animal gets its food by eating either a plant or a plant-eating animal. When living things die, their tissues are broken down by decay through the action of such organisms as bacteria and fungi. The decaying organisms add elements to the soil that are needed by plants to grow.

Living things compete with each other. For example, plants vie for space, sunlight, and moisture; animals compete with each other for food and often prey on each other. This competition prevents the earth from becoming overcrowded with any one type of organism. The living things that cannot compete successfully become extinct, and disappear from the earth.