Living things, or organisms, can be explained as open systems. They are always changing, because they exchange materials and information with their environment. They undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli and reproduce.
Through natural selection, they adapt to their environment in successive generations. More complex living organisms can communicate through various means. Many life forms can be found on Earth. The properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon and water-based cellular form with complex organization and heritable genetic information.
The systems that make up life have many levels of organization. From smallest to biggest, they are: molecule, cell, tissue (group of cells with a common purpose), organ (part of the body with a purpose), organ system (group of organs that work together), organism, population (group of organisms of the same species), community (all of the organisms that interact in an area), ecosystem (all of the organisms in an area and the non-living surroundings), and biosphere (all parts of the Earth that have life).
At present, the Earth is the only planet humans have detailed information about. The question of whether life exists elsewhere in the Universe is open. There have been a number of claims of life elsewhere in the Universe. None of these have been confirmed so far. The best evidence of life outside of Earth is are nucleic acids that have been found in certain types of meteorites.
One explanation of life is called the cell theory. The cell theory has three basic points: all living things are made up of cells. The cell is the smallest living thing that can do all the things needed for life. All cells must come from pre-existing cells.
Something is often said to be alive if it:
However, not all living things fit every point on this list.
They do, however, fit the biochemical definitions: they are made of the same kind of chemicals.
The thermodynamic definition of life is any system which can keep its entropy levels below maximum (usually through adaptation and mutations).
A modern definition was given by Humberto Maturana and Francisco Varela in 1980, to which they gave the name "autopoiesis":
Roth commented that "In short, organisms are self-reproducing and self-maintaining, or 'autopoietic', systems". This approach makes use of molecular biology ideas and systems science ideas.
Life on Earth is made from organic compounds — molecules that contain carbon. Four types of long-chain molecules (macromolecules) are important: carbohydrates, lipids, proteins, and nucleic acids.
Almost all living things need the chemical elements carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus, to build these macromolecules. Living things also need small amounts of other elements, called "trace elements". Water is a very important part of all living things. For example, humans are about two-thirds water. Water is a solvent that lets molecules mix and react with other molecules.
All living things need energy to survive, move, grow, and reproduce. Some can get energy from the environment without help from other living things: these are called producers, or autotrophs. Plants, algae, and some bacteria, a group of producers called photoautotrophs, use the sun's light for energy. When producers use light to make and store organic compounds, this is called photosynthesis. Some other producers, called chemoautotrophs, get energy from chemicals that come out of the ocean floor in hydrothermal vents. Other living things get their energy from organic compounds: these are called consumers, or heterotrophs. Animals, fungi, most bacteria, and most protists are consumers. Consumers can eat other living things or dead material.
Both producers and consumers need to break down organic compounds to free energy. The best way to do this is aerobic respiration, which frees the most energy, but living things can only do aerobic respiration if they have oxygen (O2). They can also break down these compounds without oxygen, using anaerobic respiration or fermentation.
All living things have cells. Every cell has a cell membrane on the outside, and a jelly-like material that fills the inside, called cytoplasm. The membrane is important because it separates the chemicals inside and outside. Some molecules can pass through the membrane, but others cannot. Living cells have genes, made of DNA. Genes say to the cell what to do, like a language. One DNA molecule, with many genes, is called a chromosome. Cells can copy themselves to make two new cells.
There are two main kinds of cells: prokaryotic and eukaryotic. Prokaryotic cells have only a few parts. Their DNA is the shape of a circle, inside the cytoplasm, and they have no membranes inside the cell. Eukaryotic cells are more complex, and they have a cell nucleus. The DNA is inside the nucleus, and a membrane is around the nucleus. Eukaryotic cells also have other parts, called organelles. Some of these other organelles also have membranes.
Taxonomy is how lifeforms are put into groups. The smaller groups are more closely related, but the larger classes are more distantly related. The levels, or ranks, of taxonomy are domain, kingdom, phylum, class, order, family, genus, and species. There are many ideas for the meaning of species. One idea, called the biological species concept, is as follows. A species is a group of living things that can mate with each other, and whose children can make their own children.
Taxonomy aims to group together living things with a common ancestor. This can now be done by comparing their DNA. Originally, it was done by comparing their anatomy.
The three domains of life are Bacteria, Archaea, and Eukarya. Bacteria and archaea are prokaryotic and have only one cell. Bacteria range in size from 0.15 cubic micrometres ("Mycoplasma") to 200,000,000 cubic micrometres ("Thiomargarita namibiensis"). Bacteria have shapes which are useful in classification, such as round, long and thin, and spiral. Some bacteria cause diseases. Bacteria in our intestines are part of our gut flora. They break down some of our food. Both bacteria and archaea may live where larger forms of life cannot. Bacteria have a molecule called peptidoglycan in their cell wall, but archaea do not. Archaea have a molecule called isoprene in their cell membrane, but bacteria do not.
Eukarya are living things with eukaryotic cells, and they can have one cell or many cells. Most eukaryotes use sexual reproduction to make new copies of themselves. In sexual reproduction, two sex cells, one from each parent, join together to make a new living thing.
Plants are eukaryotes that use the Sun's light for energy. They include algae, which live in water, and land plants. All land plants have two forms during their life cycle, called alternation of generations. One form is diploid, where the cells have two copies of their chromosomes, and the other form is haploid, where the cells have one copy of their chromosomes. In land plants, both diploid and haploid forms have many cells. Two kinds of land plants are vascular plants and bryophytes. Vascular plants have long tissues that stretch from end to end of the plant. These tissues carry water and food. Most plants have roots and leaves.
Animals are eukaryotes with many cells, which have no rigid cell walls. All animals are consumers: they survive by eating other organic material. Almost all animals have neurons, a signalling system. They usually have muscles, which make the body move. Many animals have a head and legs. Most animals are either male or female. They need a mate of the opposite sex to make offspring. Sex cells from the male and female can meet inside or outside the body.