Overview:
Different forms of classification of living organisms have been proposed to account for their biological diversity. The earliest classification called for just two different types: plants and animals. As more organisms were discovered that were neither plants nor animals, other classifications were proposed.
How Do Plants and Animals Differ?
The main way that plants and animals differ is that plants are able to make their own food by photosynthesis and similar processes, while animals are dependent upon plants and other animals for food. Most plants can use the energy in sunlight directly to convert carbon dioxide and water into sugars, as well as absorbing nutrients from the soil. Animals, on the other hand, cannot use the energy in sunlight directly. Although they require carbon compounds in order to grow, the only way they can get them is to ingest them from plants or other animals.
What Are Protists?
Protists are one-celled organisms that share some characteristics of plants and some of animals. They are related by molecular structure. Many of them live in freshwater or saltwater environments, and include familiar types such as plankton , protozoa, and amoeba. Some of them capture food particles, or feed on one another. Other types of protists, such as some forms of algae, also live in aquatic environments, but they produce their own food from sunlight. They contain chlorophyll, and live close to the surface of the water. Some of them form colonies of organisms, but none of the cells are specialized, so they are still considered as separate one-celled organisms.
What Are Monera?
Monera are also one-celled, but they are even simpler organisms than the protists. Their cells do not have true nuclei or organelles. Some of them contain chlorophyll and make their own food, but others do not. Bacteria are the most familiar type of monera, some rod-shaped, and others spherical. Some bacteria move as rapidly as other one-celled organisms, but others attach themselves and do not move at all. In addition, some live by a different type of photosynthesis, producing energy from sunlight but using different chemicals than do plants.
What Are Fungi?
Fungi were once considered plants, but they do not produce energy by photosynthesis. They absorb nutrients from decaying organisms, but they are not animals. Since they are multicellular , they are not protists or monera. Some fungi produce antibiotics, but other parasitic forms, cause disease as they grow in the host. As scientists discover more features and differences in molecular structure in other organisms, they have proposed other forms of classification of species. For example, there are forms of monera than do not resemble others genetically, and there are some biologists who consider bacteria separate from other monera.
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