To be classified as a living thing, an object must have all six of the following characteristics:

1. It responds to the environment.
2. It grows and develops.
3. It produces offspring.
4. It maintains homeostasis.
5. It has complex chemistry.
6. It consists of cells.

Response to the Environment (irritability)

All living things detect changes in their environment and respond to them. What happens if you step on a rock? Nothing; the rock doesn’t respond because it isn’t alive. But what if you think you are stepping on a rock and actually step on a turtle shell? The turtle is likely to respond by moving—it may even snap at you!

Growth, development and adaptation

All living things grow and develop. For example, a plant seed may look like a lifeless pebble, but under the right conditions it will grow and develop into a plant. Animals also grow and develop. For example tadpoles go through many changes to become adult frogs. All living things also change through generations to become better adapted to the environment they live in.

Reproduction

All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproducing may be as simple as a single cell dividing to form two daughter cells. Generally, however, it is much more complicated. Nonetheless, whether a living thing is a huge whale or a microscopic bacterium, it is capable of reproduction.

Keeping things constant (homeostasis)

All living things are able to maintain a more-or-less constant internal environment. They keep things relatively stable on the inside regardless of the conditions around them. The process of maintaining a stable internal environment is called homeostasis. Human beings, for example, maintain a stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn’t freeze. Instead, by shivering and other means, it maintains a stable internal temperature.

Complex chemistry (metabolism)

All living things—even the simplest life forms—have complex chemistry. Living things consist of large, complex molecules, and they also undergo many complicated chemical changes to stay alive (metabolism). Complex chemistry is needed to carry out all the functions of life.

Cells

All forms of life are built of at least one cell. A cell is the basic unit of the structure and function of living things. Living things may appear very different from one another on the outside, but their cells are very similar.

Summary

• All living things detect changes in their environment and respond to them.
• All living things grow and develop.
• All living things are capable of reproduction, the process by which living things give rise to offspring.
• All living things are able to maintain a constant internal environment through homeostasis.
• All living things have complex chemistry.
• All forms of life are built of cells. A cell is the basic unit of the structure and function of living things.