Life
From Halbeeg, the open encyclopedia · Af-Soomaali
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Life (also called a living thing or organism) is any natural entity that possesses the characteristics of life: internal chemical change (metabolism), growth, response to stimuli or movement, and the ability to reproduce. Living things consist of a single cell or multiple cells working together.
The science that studies living organisms is biology. Scientists classify living things into broad groups called domains or kingdoms, based on cell structure and the history of evolutionary relationships.
General characteristics
Most definitions of life rest on several conditions: having a cellular structure, using energy and food materials, maintaining internal stability (homeostasis), growth and development, response to environment, reproduction, and capacity for change across generations (evolution). When one of these conditions is absent—such as in viruses, which cannot generate energy independently—scientists disagree on whether to classify it as living or not.
The cell and hereditary information
The cell is the smallest unit of life that can function independently. Living things store their hereditary information in nucleic acids (DNA, and in some cases RNA), which is passed on during reproduction. Small variations in that information and natural selection form the basis of the diversity of living things.
Classification
A common system of classification divides living things into those without a nucleus (bacteria and archaea) and those with a nucleus (eukaryotes), which include animals, plants, fungi, and various microorganisms. Modern classification uses comparison of hereditary material, so names and groups are updated when new research emerges.
Environment and relationships
Living things cannot exist in isolation; they depend on environment, water, energy, and other organisms. The relationships among them—such as feeding, symbiosis (mutualistic association), competition, and parasitism—form ecological systems. Environmental change, such as fire and water availability, affects the numbers and needs of living things.