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2.1: Characteristics of Living Organisms

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • List the defining characteristics of living things.
  • List the needs of all living things.


  • cell
  • embryo
  • homeostasis
  • organism

Characteristics of Life

How do you define a living thing? What do mushrooms, daisies, cats, and bacteria have in common? All of these are living things, or organisms. It might seem hard to think of similarities among such different organisms, but they actually have many things in common. Living things are similar to each other because all living things evolved from the same common ancestor that lived billions of years ago. See http://vimeo.com/15407847 for a powerful introduction to life.

All living organisms:

  1. Need energy to carry out life processes.
  2. Are composed of one or more cells.
  3. Respond to their environment.
  4. Grow and reproduce.
  5. Maintain a stable internal environment (homeostasis).

Living Things Need Resources and Energy

Why do you eat everyday? To get energy. The work you do each day, from walking to writing and thinking, is fueled by energy. But you are not the only one. In order to grow and reproduce, all living things need energy. But where does this energy come from?

The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat. Here is how animals, plants and fungi obtain their energy:

  • All animals must eat plants or other animals in order to obtain energy and building materials.
  • Plants don’t eat. Instead, they use energy from the sun to make their "food" through the process of photosynthesis.
  • Mushrooms and other fungi obtain energy from other organisms. That’s why you often see fungi growing on a fallen tree; the rotting tree is their source of energy (Figure below).

Since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy of living things initially comes from the sun.

Bracket fungi and lichens on a rotting log in Cranberry Glades Park near Marlinton, West VIrginia. Fungi obtain energy from breaking down dead organisms, such as this rotting log.

Living Things Are Made of Cells

If you zoom in very close on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells (Figure below). Cells are the smallest unit of living things. Most cells are so small that they are usually visible only through a microscope. Some organisms, like bacteria, plankton that live in the ocean, or the paramecium shown in Figure below are made of just one cell. Other organisms have millions of cells. On the other hand, eggs are some of the biggest cells around. A chicken egg is just one huge cell.

The Inner Life of the Cell can be viewed at http://www.youtube.com/watch?v=Mszlckmc4Hw (5:28).

All cells share at least some structures. Although the cells of different organisms are built differently, they all function much the same way. Every cell must get energy from food, be able to grow and reproduce, and respond to its environment.

Reptilian blood cell showing the characteristic nucleus. A few smaller white blood cells are visible. This image has been magnified 1000 times its real size.

This paramecium is a single-celled organism.

Living Things Respond to their Environment

All living things are able to react to something important or interesting in their external environment. For example, living things respond to changes in light, heat, sound, and chemical and mechanical contact. Organisms have means for receiving information, such as eyes, ears, and taste buds.

Living Things Grow and Reproduce

All living things reproduce to make the next generation. Organisms that do not reproduce will go extinct. As a result, there are no species that do not reproduce (Figure below).

Like all living things, cats reproduce themselves and make a new generation of cats. When animals and plants reproduce they make tiny undeveloped versions of themselves called embryos, which grow up and develop into adults. A kitten is a partly developed cat.

Living Things Maintain Stable Internal Conditions

When you are cold, what does your body do to keep warm? You shiver to warm up your body. When you are too warm, you sweat to release heat. When any living thing gets thrown off balance, its body or cells help them return to normal. In other words, living things have the ability to keep a stable internal environment. Maintaining a balance inside the body or cells of organisms is known as homeostasis. Like us, many animals have evolved behaviors that control their internal temperature. A lizard may stretch out on a sunny rock to increase its internal temperature, and a bird may fluff its feathers to stay warm (Figure below).

A bird fluffs his feathers to stay warm (keep from losing energy) and to maintain homeostasis.

Lesson Summary

  • All living things grow, reproduce, and maintain a stable internal environment.
  • All organisms are made of cells.
  • All living things need energy and resources to survive.

Review Questions


1. Define the word organism.

2. What are three characteristics of living things?

Apply Concepts

3. What are a few ways organisms can get the energy they require?

4. What is a cell?

Think Critically

5. Think about fire. Can fire be considered a living thing? Why or why not?

Points to Consider

  • DNA is considered the “instructions” for the cell. What do you think this means?
  • What kinds of chemicals do you think are necessary for life?
  • Do you expect that the same chemicals can be in non-living and living things?

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