In this topic you will learn about classifying living things.

Organisms in the animal kingdom have many cells. The cells have a true nucleus and no cell wall. Animals eat and digest other organisms. Most animals can move from place to place, and have tissues organized into organs and organ systems.

Plants are many-celled organisms. Many plant cells contain chlorophyll. In light they make food and oxygen. Plant cells have a true nucleus and the cells have walls. Roots or rootlike structures anchor the plant and absorb water.

Mosses were among the first plants to live on the land. They have no special tissues for transporting water or nutrients. Some mosses have a stalk and a capsule where spores, or reproductive cells, are found. They do not reproduce by producing seeds. Flowering plants reproduce by seeds that develop inside a protective chamber, called an ovary. Like ferns and cone-bearing plants, they have true roots, stems, and leaves. Cone-bearing plants carry their seeds in cones, rather than in pods or fruit.

Members of the fungus kingdom are mostly many-celled organisms, although some are one-celled organisms. These organisms do not have chlorophyll and cannot make their own food. They absorb and digest food. Their cells have walls and have a true nucleus. The fungus kingdom is grouped into divisions, rather than phyla. This kingdom includes microbes. A microbe is a living thing so small that it can only be seen with a microscope.

The fungus kingdom also includes larger organisms. Mold on bread is a microbe that is part of the fungus kingdom. Some molds are useful to humans. In 1928 Alexander Fleming discovered that a mold killed bacteria. This accidental discovery led to the development of the world's most widely used antibiotic, called penicillin. Mushrooms are also fungi. The part of the mushroom that rises above ground is the fruiting body. The growing, vegetative part lies underground in the form of a mass of dense, tangled threads.

A virus is a microscopic particle made of hereditary material and a protein coat, which can reproduce in a living cell. Viruses are not made up of cells. They do not have the basic characteristics of all kingdoms. Unlike true organisms, viruses do not grow, eat, or respond to stimuli. They are able to reproduce only inside a living cell, called a host cell. Once inside a host cell, a virus directs the cell to produce new virus particles from the cell material. The new viruses are released from the cell and can infect other cells. That is how some viruses can cause diseases.