The animal kingdom is made up of a diverse array of organisms. Originally, all organisms were divided into a two-kingdom system that placed them into either the plant kingdom or the animal kingdom. This classification system was first proposed by Carolus Linnaeus. Later on, ecologist Robert H. Whittaker developed a five-kingdom system that placed prokaryotes in the kingdom Monera, and organisms with eukaryotic cells were divided into four other kingdoms (Protista, Plantae, Fungi, and Animalia). In time, the prokaryotes of the kingdom Monera were separated into the kingdom Archaea and the kingdom Bacteria.
The kingdom Animalia consists of multicellular, eukaryotic organisms. The eukaryotic cells that make up these individuals have a membrane-bound nucleus and other membrane-bound organelles. Animal cells differ from those of plants and fungi because animal cells do not contain a cell wall. All animals are heterotrophs, meaning they feed on other organisms or the waste products of those organisms. Members of the kingdom Animalia vary widely, from the tiniest of insects, to the fish in the sea, to land creatures great and small. The majority of animals increase their numbers by means of sexual reproduction.
Plants include organisms such as flowers, trees, grasses, vines, green algae, and mosses. They are multicellular eukaryotes and have the ability to carry out photosynthesis. Plant cells contain chloroplasts, and they have cell walls which are rigid and made of cellulose. Plants are distributed throughout the world. They have coevolved with animals, and many animals pollinate the flowers of plants in exchange for food, such as nectar or pollen. Other animals consume plants as a source of food. Cultivation of plants by humans is a part of agriculture. The scientific study of plants is known as botany.
The kingdom Fungi is made up of a large group of eukaryotic organisms. These eukaryotes include microorganisms such as molds and yeasts, as well as mushrooms. The original classification of fungi by Linnaeus placed this group within the kingdom Plantae. Fungi are heterotrophic organisms that are mostly immobile. Many live by decomposing organic matter and absorbing small organic molecules, thus playing an important role in nutrient cycling and exchange. They reproduce by sexual and asexual means. Fungi are found throughout the world and can grow in a wide range of habitats. They have been used in the production of antibiotics, most notably penicillin.
Protists are a diverse group of eukaryotic microorganisms. This group includes mostly unicellular organisms but also has multicellular organisms that are without specialized tissues. Their simple cellular organization distinguishes them from other eukaryotes, such as plants, animals, and fungi. Protists live in almost any environment that contains water. Many protists are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean. These photosynthetic protists are called algae. Heterotrophic species of protists are called protozoa. They have variable means of obtaining nutrition. Some flagellates are filter feeding and other protozoa engulf bacteria and digest them internally. Some protists are significant pathogens of both animals and plants, such as those that cause malaria.
This kingdom consists of single-celled prokaryotic microorganisms. Prokaryotic cells have no nucleus or other bound organelles within them. Archaea occur in various shapes, such as spheres, rods, and other forms. Some species form aggregates or filaments of cells. Archaea possess genes and have many metabolic pathways that are closely related to those of eukaryotes. They use a great variety of sources of energy, such as ammonia and sugars. Archaea that are salt-tolerant use sunlight as an energy source, while other species of archaea are able to fix carbon. Members of the kingdom Archaea reproduce asexually using binary fission, budding, or fragmentation. They are found in a wide range of habitats that include oceans, marshlands, soils, and even the human colon. They are particularly numerous in the oceans.
Bacteria make up a large group of single-cell, prokaryotic microorganisms. Bacterial cells do not contain a nucleus and rarely contain membrane-bound organelles. Many species of bacteria exist solely as single cells, while other species associate in characteristic patterns. They come in a wide range of shapes, ranging from spheres to rods and spirals. Bacteria often adhere to surfaces and form dense masses called biofilms. The energy metabolism of bacteria can be based on phototrophy, which uses light through photosynthesis, or it can be based on chemotrophy, the use of chemical substances for energy. The majority of bacteria are only able to take in raw materials in the form of relatively small molecules, which enter the cell by diffusion or through molecular channels in cell membranes. Bacteria are found in all habitats on Earth. They can grow in soil, radioactive waste, acidic environments like hot springs, water, as well as in organic matter and even inside the bodies of plants and animals. Pathogens are bacteria that form a parasitic connection with other organisms. These are a major source of human disease and death. Bacteria, in combination with yeasts and molds, have been used for thousands of years in the preparation of fermented foods such as cheese, vinegar, wine, and yogurt. The study of bacteria is known as bacteriology, a branch of microbiology.
The division of the Earth's organisms into separate kingdoms is one way to classify the diversity of life into a scheme that is useful and reflective of evolutionary history. Advancing molecular and cellular evidence has made it necessary to reconsider previous classification outlines and to occasionally modify the kingdom system.