Life

For other uses, seeLife (disambiguation).

Life Temporal range:3770–0 Ma Had'n Archean Proterozoic Pha. Archean–present(possibleHadeanorigin)
Stegosaurusin museum
Scientific classification
DomainsandSupergroups
Life on Earth: Non-cellular life Viruses Viroids Cellular life DomainBacteria DomainArchaea DomainEukaryota Archaeplastida Plantae SAR Excavata Amoebozoa Opisthokonta Holomycota Fungi Holozoa Animalia

Lifeis a concept inbiology.It is about what separates a living thing fromdeadmatter.

Most life onEarthis powered by solar energy: the only known exceptions are the chemo-synthetic bacteria living around thehydrothermal ventson the ocean floor. All life on Earth is based on thechemistryofcarboncompounds, involving long-chainmoleculessuch asproteinsandnucleic acid.Withwater,which all life needs, the long molecules are wrapped insidemembranesascells.This may or may not be true of all possible forms of life in theUniverse:it is true of all life on Earth today.

Summary

Living things, or organisms, can be explained asopen systems.They are always changing, because they exchangematerialsandinformationwith theirenvironment.They undergometabolism,maintainhomeostasis,possess a capacity togrow,respond tostimuliandreproduce.

Throughnatural selection,they adapt to their environment in successive generations. More complex living organisms can communicate by various means.Many life forms can be found onEarth.The properties common to these organisms—plants,animals,fungi,protists,archaea,andbacteria—are a carbon and water-basedcellularform with complexorganizationandheritablegeneticinformation.

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 thatinteractin an area),ecosystem(all of the organisms in an area and the non-living surroundings), andbiosphere(all parts of the Earth that have life).

At present, theEarthis 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 arenucleic acidsthat have been found in certain types ofmeteorites.

Definitions

One explanation of life is called thecell 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:

• grows,
• takes infood,uses thefoodforenergy,and passeswasteproducts (seemetabolism),
• moves:it must either move itself, or have movement inside itself,
• reproduces,eithersexually(with another living thing) orasexually,by creatingcopiesof itself,
• reactsto its surroundings,
• functions

However, not all living things fit every point on this list.

• Mulescannot reproduce, and neither can worker ants.
• Virusesand spores are not actively alive (metabolising) until the conditions are right.

They do, however, fit the biochemical definitions: they are made of the same kind of chemicals.

Thethermodynamicdefinition of life is any system which can keep itsentropylevels below maximum (usually throughadaptationandmutations).

A modern approach

A modern definition was given byHumberto MaturanaandFrancisco Varelain 1980,to which they gave the nameautopoiesis:

1. The production of their own components
2. The correct assembly of these components
3. Continuous repair and maintenance of their own existence.

Roth commented that "In short, organisms are self-reproducing and self-maintaining, or 'autopoietic', systems".This approach makes use ofmolecular biologyideas andsystems scienceideas.

What life needs

Chemistry

See also:Biochemistry

Life on Earth is made fromorganic compounds—molecules that contain carbon. Four types of long-chain molecules (macromolecules) are important:carbohydrates,lipids,proteins,andnucleic acids.

• Simple carbohydrates (sugars) are used forenergy,or as a building block. Complex carbohydrates, like starch and cellulose, can keep energy for a long time. They are also used to make a strong structure, like aplant stem.
• Lipids can beinsulationto keep a living thing warm, such as fat on apenguin,or to stop water from passing in or out, such as waterprooffeathers.Two layers of phospholid (a kind of lipid) make up allcell membranes.Some kinds of lipids arehormones,which send messages from one cell to another.
• Proteins, long chains ofamino acids,have many purposes. They fold into complex shapes because their amino acids interact. Proteins are involved in many chemical reactions, to make them go faster.
• Nucleic acids, includingDNAandRNA,are long chains ofnucleotides.There are only four kinds of nucleotides in each chain, but they are the instructions for life, like a language. Each three nucleotides tell the cell to make one amino acid. One part of a nucleic acid is thecodefor one protein molecule.

Almost all living things need thechemical elementscarbon,hydrogen,oxygen,nitrogen,sulfur,andphosphorus,to build these macromolecules.Living things also need small amounts of other elements, calledtrace elements.Water is a very important part of all living things. For example, humans are about two-thirds water. Water is asolventthat lets molecules mix and react with other molecules.

Energy sources

All living things need energy to survive, move, grow, and reproduce. Some can get energy from theenvironmentwithout help from other living things: these are called producers, orautotrophs.Plants, algae, and some bacteria, a group of producers calledphotoautotrophs,use the sun's light for energy. When producers use light to make and store organic compounds, this is calledphotosynthesis.Some other producers, calledchemoautotrophs,get energy from chemicals that come out of the ocean floor inhydrothermal vents.Other living things get their energy fromorganic compounds:these are called consumers, orheterotrophs.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 isaerobic 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, usinganaerobic respirationorfermentation.

Cells

See the main article:Cell (biology)

All living things have cells. Every cell has a cell membrane on the outside, and a jelly-like material that fills the inside, calledcytoplasm.The membrane is important because it separates the chemicals inside and outside. Some molecules can pass through the membrane, but others cannot. Living cells havegenes,made of DNA. Genes say to the cell what to do, like a language. One DNA molecule, with many genes, is called achromosome.Cells cancopy themselvesto make two new cells.

There are two main kinds of cells:prokaryoticandeukaryotic.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 acell nucleus.The DNA is inside the nucleus, and a membrane is around the nucleus. Eukaryotic cells also have other parts, calledorganelles.Some of these other organelles also have membranes.

Types of life

See the main article:Taxonomy

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 aredomain,kingdom,phylum,class,order,family,genus,andspecies.There are many ideas for the meaning ofspecies.One idea, called thebiological 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 commonancestor.This can now be done by comparing theirDNA.Originally, it was done bycomparing their anatomy.

The three domains of life areBacteria,Archaea,andEukarya.Bacteria and archaea are prokaryotic and haveonly one cell.Bacteria range in size from 0.15 cubicmicrometres(Mycoplasma) to 200,000,000 cubic micrometres (Thiomargarita namibiensis). Bacteria have shapes which are useful in classification, such as round, long and thin, andspiral.Some bacteria causediseases.Bacteria in ourintestinesare part of ourgut flora.They break down some of our food. Both bacteria and archaea may live where larger forms of life cannot. Bacteria have a molecule calledpeptidoglycanin theircell wall,butarchaeado not. Archaea have a molecule calledisoprenein their cell membrane, but bacteria do not.

Eukarya are living things with eukaryotic cells, and they can have one cell ormany cells.Most eukaryotes usesexual reproductionto make new copies of themselves. In sexual reproduction, two sex cells, one from each parent, join to make a new living thing.

Plantsare eukaryotes that use the Sun's light for energy. They includealgae,which live in water, and land plants. All land plants have two forms during theirlife cycle,calledalternation of generations.One form isdiploid,where the cells have two copies of their chromosomes, and the other form ishaploid,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 arevascular plantsandbryophytes.Vascular plants have long tissues that stretch from end to end of the plant. These tissues carry water and food. Most plants haverootsandleaves.

Animalsare eukaryotes with many cells, which have no rigid cell walls. All animals are consumers: they survive by eating otherorganicmaterial. Almost all animals haveneurons,a signalling system. They usually havemuscles,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 oppositesexto make offspring. Sex cells from the male and femalecan meetinside or outside the body.

Fungiare eukaryotes which may have one cell, likeyeasts,or many cells, likemushrooms.They aresaprophytes.Fungi break down living or dead material, so they aredecomposers.Only fungi, and a few bacteria, can break downligninandcellulose,two parts ofwood.Some fungi aremycorrhiza.They live under ground and give nutrients to plants, like nitrogen and phosphorus.Eukaryotes that are not plants, animals, or fungi are calledprotists.Most protists live in water.

Evolution

See the main article:Evolution

Over thousands or millions of years, living things can change, through the process of evolution. One kind of evolution is when a species changes over time, such as giraffes growing longer necks. Most of the time, the species becomes better suited to its environment, a process calledadaptation.Evolution can also cause one group of living things to split into two groups. This is calledspeciationif it makes a new species. An example is mockingbirds on the Galapagos Islands—one species of mockingbird lives on each island, but all the species split from a shared ancestor species. Groups that are bigger than species can also split from a shared ancestor—for example, reptiles and mammals. A group of living things and their shared ancestor is called aclade.

Living things can evolve to be quite different from their ancestors. As a result, parts of the body can also change. The same bone structure became the hands of humans, the hooves of horses, and the wings of birds. Different body parts that evolved from the same thing are calledhomologous.

Extinctionis when all members of a species die. About 99.9% of all species that have ever lived are extinct. Extinction can happen at any time, but it is more common in certain time periods calledextinction events.The most recent was65 million years ago,when thedinosaurswent extinct.

Origin of life

See the main article:Origin of life

By comparingfossilsand DNA, we know that all life on Earth today had a shared ancestor, called thelast universal common ancestor(LUCA). Other living things may have been alive at the same time as the LUCA, but they died out. A study from 2018 suggests that the LUCA is about 4.5 billion (4,500,000,000) years old, nearly as old as the Earth.The oldest fossil evidence of life is about 3.5 billion years old.

How did non-living material become alive? This is a difficult question. The first step must have been the creation of organic compounds. In 1953, theMiller–Urey experimentmade inorganic compounds into organic compounds, such as amino acids, using heat and energy.

Life needs a source of energy for chemical reactions. On the early Earth, the atmosphere did not have oxygen.Oxidationusing theKrebs cycle,which is common today, was not possible. The Krebs cycle may have acted backwards, doingreductioninstead of oxidation, and the cycle may have made larger molecules. To make life, molecules needed to make copies of themselves. DNA and RNA make copies of themselves, but only if there is acatalyst—a compound which speeds up the chemical reaction. One guess is that RNA itself served as a catalyst. At some time, the molecules were surrounded by membranes, which made cells.

Gallery of images of life

• 
  Atreeis an example of aplant.
• 
  Fishare examples ofmarinelife.
• 
  An adult citrus root weevil is an example of aninsect.
• 
  Salmonellatyphimuriumis an example ofbacteria.
• 
  Chromalveolatesare a group ofprotista.
• 
  Amanita muscaria(Fly agaric) is an example offungi.
• 
  The fungoldfrogis an example of anamphibian.
• 
  Ablue jayis an example of abird.
• 
  People are an example ofmammals.

Clickable

Life on Earth

(view•discuss)

0.2 mya
Humans

180 mya
Flowers

200 mya
Mammals

240 mya
Dinosaurs

3500 mya
Oxygen

4000 mya
Microbes

4410 mya
Water

4540 mya
Earth

Related pages

• Artificial life
• Biology
• Birth
• Death
• Earliest known life forms
• Evolution
• Tree of life