McDougal Littell Science Cells and Heredity 

Table of Contents
Page 9 (continued)

Living things are different from nonliving things.

You know life when you see it. Perhaps your class takes a field trip to a local state park to collect water samples. You are surrounded by trees. There is a stream, with rocks covered with moss and green algae. There are fish and frogs; there are birds and insects. You are surrounded by life. But how would you define it?

One way to answer the question is to think about what makes a living thing different from a nonliving thing. You might ask if a thing uses energy. Or maybe you would observe it to see if it moves. You could investigate whether it consumes food and water. These are characteristics of living things, or organisms. Any individual form of life that uses energy to carry out its activities is an organism. All organisms get water and other materials from the environment.

Page 10

Characteristics of Life

Living things have these characteristics:

  • organization
  • the ability to develop and grow
  • the ability to respond to the environment
  • the ability to reproduce

An organism's body must be organized in a way that enables it to meet its needs. Some organisms, like bacteria, are very simple. A more complex organism, such as the kingfisher shown in the photograph below, is organized so that different parts of its body perform different jobs, called functions. For example, a kingfisher has wings for flying, a heart for pumping blood, and eyes for seeing.

Another characteristic of organisms is that they grow and, in most cases, develop into adult forms. Some organisms change a great deal in size and appearance throughout their lifetimes, whereas others grow and change very little. Organisms also respond to the world outside them. Think of how the pupils of your eyes get smaller in bright light. Finally, organisms can reproduce, producing new organisms that are similar to themselves.

Arrow
CHECK YOUR READiNG

What four characteristics are common to all living things?

Needs of Life

Organisms cannot carry out the activities that characterize life without a few necessities: energy, materials, and living space. What does it mean to need energy? You know that if you want to run a race, you need energy. But did you know that your body also needs energy to sleep or to breathe or even to think? All organisms require a steady supply of energy to stay alive. Where does this energy come from, and how does an organism get it?

The energy used by almost all forms of life on Earth comes from the Sun. Some organisms, like plants and some bacteria, are able to capture this energy directly. Your body, like the bodies of other animals, uses food as a source of energy. The food animals eat comes from plants or from organisms that eat plants. Food also provides the materials necessary for growth and reproduction. These materials include substances such as carbon dioxide, nitrogen, oxygen, and water. Finally, all organisms need space to live and grow. If any one of these requirements is missing, an organism will die.

Bird

APPLY Identify three living things in this photograph. How do they meet their needs?

Food is a source of energy and materials.

Water provides materials and living space.