Animals, plants, fungi, algae, protozoans, and bacteria are living things. Living things are also called organisms. Scientists can tell living things and nonliving things apart because living things behave in ways that nonliving things do not. Scientists have discovered about 1.5 million different kinds of living things on Earth.

What Living Things Do

The most obvious clue to whether something is living is its ability to move. All living things can move, using their own energy. Even though plants stay in one spot, they move their leaves to get sunlight.

Living things are also sensitive. This means that they are able to feel. The simplest life forms can feel only when something touches them, or they have only a sense of hot and cold.

Living things take in certain chemicals and release other chemicals. Humans and other animals breathe in oxygen and breathe out carbon dioxide. Green plants take in carbon dioxide through their leaves and release oxygen into the air.

All living things need the nutrients and energy that food gives. Green plants make their own food with the help of sunlight. Animals eat plants and other animals to get energy.

Living things reproduce. This means that they create a new generation of life. People and animals have babies. Plants make seeds or spores that grow into new plants. Even the most basic life form, a single cell, reproduces. It does so by dividing into two new cells.

Living things grow. When many cells in a plant or an animal divide, the plant or animal becomes larger. Living things also grow by making new parts—for example, branches or teeth.

Finally, living things get rid of waste. When an animal or plant takes in nutrients, there is always a part that is not needed. This part is excreted, or released from the system.

Groups of Living Things

Scientists divide living things into groups. This process is called classification. The most basic groups are called kingdoms. There are five kingdoms of living things.

The first kingdom is called Monera. Monerans are single-celled organisms. They are too simple to be called plants or animals. They are so small that it takes the power of a microscope to see them. Bacteria, or germs, are monerans.

The second kingdom is called Protista. Most of these organisms are also single cells. Algae and protozoans are in this kingdom.

The third kingdom is Fungi. Most fungi have many cells arranged in thread-like groups. Mushrooms are fungi, and so are yeasts and molds.

The fourth kingdom is Plantae. Plants are also made of many cells. Most plants are green or have parts that are green.

The fifth and largest kingdom is Animalia. Like fungi and plants, animals have many cells. Unlike other living things, animals move around easily and quickly react to their surroundings.