Organism
An organism is an individual, living entity. Organisms range in size and complexity from tiny microorganisms, to large, multicellular plants and animals. Biologists classify organisms into five kingdoms on the basis of their cellular and sub-cellular organization, metabolism, reproduction, and behavior. These kingdoms are described below, in order of their earliest appearance in the fossil record of life.
Monera are prokaryotic microorganisms, meaning their genetic material is not contained within a bounded organelle called a nucleus. This group includes the simplest organisms, including viruses, which consist of little more than a protein shell containing nucleic acids. Viruses are incapable of reproduction unless they parasitize the metabolism of an unrelated host cell. Blue-green bacteria and true bacteria are two other groups of monerans.
Protista are a diverse group of microorganisms, including the simplest eukaryotic organisms, which have an organized nucleus, one or more flagellae, and usually mitochondria and plastids. Protozoans are the most representative group, but some flagell ated fungi and algae are also considered protists.
Fungi are a diverse group of non-flagellated, unicellular or multicellular organisms. They range in complexity from single-celled yeasts, through multicellular but microscopic fungi growing as a thread-like mycelium, to relatively complex fungi that develop large mushrooms as reproductive structures.
Plantae, or green plants, are photosynthetic organisms that absorb solar radiation using chlorophyll and other pigments, and utilize the energy to fix simple mineral nutrients into energy-rich biochemicals. Organisms in this group range from unicellular algae, through multicellular but non-vascular algae, liverworts, and mosses, to vascular plants such as ferns, conifers, and flowering plants.
Animalia, or multicellular animals, are heterotrophic organisms, meaning they must ingest their food in the form of biomass of plants or other animals. Animals are capable of movement, often in response to sensory stimuli. They range in size and complexity from small sponges and arthropods to large vertebrates weighing ton.
To varying degrees, all organisms are related, and they share certain elements of their physiology and other functions. Many distinctive organisms have a relatively ancient evolutionary lineage that extends far back into the geological past, while others have evolved more recently. Some organisms are enormously more complex in biological organization than others. However, the modern interpretation of life suggests that none of Earth's organisms are "higher" or more "primitive" than any others, and none have greater intrinsic value. Evolution is not interpreted as a deterministic progression of kinds of organisms that represent a logical, directed succession from simple types such as viruses and bacteria, to more complex ones such as birds and mammals (including humans). Earth's enormous biodiversity of living organisms utilizes many body and metabolic plans of varying complexity, but all species represent successful adaptations to the planet's habitable environments.
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