HOME

TheInfoList




In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, an organism () is any organic,
living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** extant taxon, Living species, one that is not extinct *Personal life, the course of an individual human ...

living
system that functions as an individual entity. All organisms are composed of cells (
cell theory In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of Cell (biology), cells, that they are the basic structural/organizational unit of all organisms, and that all cells ...
). Organisms are classified by
taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
into groups such as
multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biolo ...
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s,
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s, and
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
; or
unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Or ...
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s such as
protist A protist () is any (that is, an organism whose contain a ) that is not an , , or . While it is likely that protists share a (the ), the exclusion of other eukaryotes means that protists do not form a natural group, or . Therefore, some pro ...
s,
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
, and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
. All types of organisms are capable of
reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

reproduction
, growth and development,
maintenance Maintenance may refer to: Biological science * Maintenance of an organism * Maintenance respiration Non-technical maintenance * Alimony, also called ''maintenance'' in British English * Champerty and maintenance, two related legal doctrines ...
, and some degree of response to
stimuli A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity generally. For example, "The press provides stimulation of political discourse." ...
.
Beetle Beetles are a group of insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known a ...

Beetle
s,
squid Squid are cephalopod A cephalopod is any member of the mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is ...

squid
s,
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s,
mushroom A mushroom or toadstool is the fleshy, spore )'', growing on a thinning, thinned hybrid black poplar ''(populus, Populus x canadensis)''. The last stage of the moss#Life cycle, moss lifecycle is shown, where the sporophytes are visible befor ...

mushroom
s, and
vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large group of plants ( 300,000 ...
s are examples of multicellular organisms that differentiate specialized tissues and
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
during
development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking#Development, Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material i ...
. A
unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Or ...
organism may be either a
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...
or a
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
. Prokaryotes are represented by two separate domains –
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
. Eukaryotic organisms are characterized by the presence of a membrane-bound
cell nucleus In cell biology, the nucleus (pl. ''nuclei''; from Latin or , meaning ''kernel'' or ''seed'') is a biological membrane#Function, membrane-bound organelle found in eukaryote, eukaryotic cell (biology), cells. Eukaryotes usually have a single n ...

cell nucleus
and contain additional membrane-bound compartments called
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s (such as
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
in animals and plants and
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
s in plants and
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
, all generally considered to be derived from
endosymbiotic An endosymbiont or endobiont is any organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synony ...

endosymbiotic
bacteria). Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes. Estimates on the number of Earth's current
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
range from 2 million to 1 trillion, of which over 1.7 million have been documented. More than 99% of all species, amounting to over five billion species, that ever lived are estimated to be
extinct Extinction is the termination of a kind of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...

extinct
. In 2016, a set of 355
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s from the
last universal common ancestor The last universal common ancestor or last universal cellular ancestor (LUCA), also called the last universal ancestor (LUA), is the most recent population of organisms from which all organisms now living on Earth have a common descent Common ...
(LUCA) of all organisms was identified.


Etymology

The term "organism" (from
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
ὀργανισμός, ''organismos'', from ὄργανον, ''organon'', i.e. "instrument, implement, tool, organ of sense or apprehension") first appeared in the English language in 1703 and took on its current definition by 1834 (
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary A historical dictionary or dictionary on historical principles is a dictionary which deals not only with the latterday meanings of words but also the historica ...
). It is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings, going back at least to
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about r ...

Immanuel Kant
's 1790 ''
Critique of Judgment The ''Critique of Judgment'' (), also translated as the ''Critique of the Power of Judgment'', is a 1790 book by the German philosopher Immanuel Kant Immanuel Kant (, ; ; 22 April 1724 – 12 February 1804) was a German Philosophy, philo ...
''.


Definitions

An organism may be defined as an assembly of
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
s functioning as a more or less stable whole that exhibits the . Dictionary definitions can be broad, using phrases such as "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction". Many definitions exclude
virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

virus
es and possible man-made non-organic life forms, as viruses are dependent on the biochemical machinery of a host cell for reproduction. A
superorganism A superorganism or supraorganism is a group of synergetically interacting organisms of the same species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a ...
is an organism consisting of many individuals working together as a single functional or
social unit The term "level of analysis" is used in the social science Social science is the branch A branch ( or , ) or tree branch (sometimes referred to in botany Botany, also called , plant biology or phytology, is the science of plant ...
. There has been controversy about the best way to define the organism and indeed about whether or not such a definition is necessary. Several contributions are responses to the suggestion that the category of "organism" may well not be adequate in biology.


Viruses

Virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

Virus
es are not typically considered to be organisms because they are incapable of autonomous
reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

reproduction
, growth or
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a phys ...

metabolism
. Although some organisms are also incapable of independent survival and live as obligatory intracellular
parasites Parasitism is a symbiotic Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological or ...

parasites
, they are capable of independent metabolism and procreation. Although viruses have a few
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s and molecules characteristic of living organisms, they have no metabolism of their own; they cannot synthesize and organize the organic compounds from which they are formed. Naturally, this rules out autonomous reproduction: they can only be passively replicated by the machinery of the
host cell In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
. In this sense, they are similar to inanimate matter. While viruses sustain no independent metabolism and thus are usually not classified as organisms, they do have their own
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s, and they do by mechanisms similar to the evolutionary mechanisms of organisms. Thus, an argument that viruses should be classed as living organisms is their ability to undergo evolution and replicate through self-assembly. However, some scientists argue that viruses neither evolve nor self-reproduce. Instead, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. This is not true for cells. If viruses did not exist, the direction of cellular evolution could be different, but cells would nevertheless be able to evolve. As for reproduction, viruses totally rely on hosts' machinery to replicate. The discovery of viruses with genes coding for energy metabolism and protein synthesis fuelled the debate about whether viruses are living organisms. The presence of these genes suggested that viruses were once able to metabolize. However, it was found later that the genes coding for energy and protein metabolism have a cellular origin. Most likely, these genes were acquired through
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
from viral hosts.


Chemistry

Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival. The same laws that govern non-living chemistry govern . It is generally the phenomena of entire organisms that determine their fitness to an environment and therefore the survival of their
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
-based genes. Organisms clearly owe their origin, metabolism, and many other internal functions to chemical phenomena, especially the chemistry of large organic molecules. Organisms are complex systems of
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
s that, through interaction and environment, play a wide variety of roles. Organisms are semi-closed chemical systems. Although they are individual units of life (as the definition requires), they are not closed to the environment around them. To operate they constantly take in and release energy.
Autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
s produce usable energy (in the form of organic compounds) using light from the sun or inorganic compounds while
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...
s take in organic compounds from the environment. The primary
chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo du ...
in these compounds is
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
. The
chemical properties A chemical property is any of a material's properties that becomes evident during, or after, a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substan ...
of this element such as its great affinity for bonding with other small atoms, including other carbon atoms, and its small size making it capable of forming multiple bonds, make it ideal as the basis of organic life. It is able to form small three-atom compounds (such as
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
), as well as large chains of many thousands of atoms that can store data (
nucleic acid Nucleic acids are biopolymer Biopolymers are natural polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecule ...

nucleic acid
s), hold cells together, and transmit information (protein).


Macromolecules

Compounds that make up organisms may be divided into
macromolecule macromolecule A macromolecule is a very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neu ...
s and other, smaller molecules. The four groups of macromolecule are
nucleic acid Nucleic acids are biopolymer Biopolymers are natural polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecule ...

nucleic acid
s,
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s,
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s and
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
s. Nucleic acids (specifically deoxyribonucleic acid, or DNA) store genetic data as a sequence of
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
s. The particular sequence of the four different types of nucleotides (
adenine Adenine (A, Ade) is a nucleobase 230px, Pyrimidine nucleobases are simple ring molecules. Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides Nucleos ...

adenine
,
cytosine Cytosine () (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. Al ...

cytosine
,
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical me ...

guanine
, and
thymine Thymine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical m ...

thymine
) dictate many characteristics that constitute the organism. The sequence is divided up into
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
s, each of which is a particular sequence of three nucleotides and corresponds to a particular
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
. Thus a sequence of DNA codes for a particular protein that, due to the chemical properties of the amino acids it is made from,
folds Benjamin Scott Folds (born September 12, 1966) is an American singer-songwriter, musician, composer and record producer. Folds was the frontman and pianist of the alternative rock band Ben Folds Five from 1993 to 2000, and again in the early 2010s ...

folds
in a particular manner and so performs a particular function. These protein functions have been recognized: #
Enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

Enzyme
s, which catalyze the reactions of metabolism # Structural proteins, such as
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular protein 300px, 3-dimensional structure of hemoglobin, a globular protein. Globular proteins or spheroproteins are spherical ("globe-like") protein ...

tubulin
, or
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
# Regulatory proteins, such as
transcription factor In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...
s or cyclins that regulate the cell cycle # Signaling molecules or their receptors such as some
hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...

hormone
s and their receptors # Defensive proteins, which can include everything from
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibodies
of the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
, to toxins (e.g., dendrotoxins of snakes), to proteins that include unusual amino acids like A bilayer of
phospholipid Phospholipids, also known as phosphatides, are a class of lipid In and , a lipid is a macro that is soluble in solvents. are typically s used to dissolve other naturally occurring hydrocarbon lipid s that do not (or do not easily) disso ...

phospholipid
s makes up the
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes (outer coverings of cells or organelles that all ...
of cells that constitutes a barrier, containing everything within a cell and preventing compounds from freely passing into, and out of, the cell. Due to the selective permeability of the phospholipid membrane, only specific compounds can pass through it.


Structure

All organisms consist of structural units called
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
; some contain a single cell (unicellular) and others contain many units (multicellular). Multicellular organisms are able to specialize cells to perform specific functions. A group of such cells is a
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
, and in animals these occur as four basic types, namely
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
,
nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, se ...
,
muscle tissue Muscle tissues are soft tissue of a tendon A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton i ...

muscle tissue
, and
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
. Several types of tissue work together in the form of an
organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cate ...
to produce a particular function (such as the pumping of the blood by the
heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...

heart
, or as a barrier to the environment as the
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have differ ...

skin
). This pattern continues to a higher level with several organs functioning as an
organ system An organ system is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertainment, and media * ''Network'' (1976 film), a 1976 American film * ''Network'' (2019 film), an Indian film * ...
such as the
reproductive system The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs tha ...
, and
digestive system The human digestive system consists of the human gastrointestinal tract, gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food in ...

digestive system
. Many multicellular organisms consist of several organ systems, which coordinate to allow for life.


Cell

The
cell theory In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of Cell (biology), cells, that they are the basic structural/organizational unit of all organisms, and that all cells ...
, first developed in 1839 by
Schleiden Schleiden is a town in North Rhine-Westphalia, Germany. It lies in the Eifel hills, in the district of Euskirchen (district), Euskirchen, and has 12,998 inhabitants as of 30 June 2017. Schleiden is connected by a tourist railway to Kall, Germany, K ...

Schleiden
and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells, and cells contain the necessary for regulating cell functions and for transmitting information to the next generation of cells. There are two types of cells, eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multicellular organisms. Prokaryotic cells lack a
nuclear membrane The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a c ...

nuclear membrane
so
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
is unbound within the cell; eukaryotic cells have nuclear membranes. All cells, whether prokaryotic or eukaryotic, have a
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes (outer coverings of cells or organelles that all ...

membrane
, which envelops the cell, separates its interior from its environment, regulates what moves in and out, and maintains the . Inside the membrane, a
salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure fo ...

salt
y
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
takes up most of the cell volume. All cells possess DNA, the hereditary material of
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s, and
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
, containing the information necessary to various
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s such as
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s, the cell's primary machinery. There are also other kinds of
biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a Nobel Prize in Chemistry, Nobel Prize i ...
s in cells. All cells share several similar characteristics of:The Universal Features of Cells on Earth
in Chapter 1 of
Molecular Biology of the Cell
' fourth edition, edited by Bruce Alberts (2002) published by Garland Science.
* Reproduction by
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
(
binary fission Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineeri ...

binary fission
,
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
or
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
). * Use of enzymes and other proteins by DNA genes and made via
messenger RNA In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...
intermediates and
ribosome Ribosomes ( ), also called Palade granules, are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (mRNA translation). Ribosomes link amino acids ...

ribosome
s. * Metabolism, including taking in raw materials, building cell components, converting energy,
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
s and releasing
by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing Manufacturing is the creation or production Production may be: Economics and business * Production (economics) * Production, the act of manufac ...
s. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from
metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell (biology), cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence ...
s. * Response to external and internal signal transduction, stimuli such as changes in temperature, pH or nutrient levels. * Cell contents are contained within a cell membrane, cell surface membrane that contains proteins and a lipid bilayer.


Evolutionary history


Last universal common ancestor

The last universal common ancestor (LUCA) is the most recent organism from which all organisms now living on Earth Common descent, descend. Thus it is the most recent common ancestor of all current life on Earth. The LUCA is estimated to have lived some Timeline of evolution, 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era). The earliest evidence for life, life on Earth is graphite found to be Biogenic substance, biogenic in 3.7 billion-year-old Metasediment, metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct, it is likely that more than a billion species of life exist on Earth currently, with the highest estimates and projections reaching one trillion species. Information about the early development of life includes input from many different fields, including geology and planetary science. These sciences provide information about the history of the Earth and the changes produced by life. However, a great deal of information about the early Earth has been destroyed by geological processes over the course of time. All organisms are descended from a common ancestor or ancestral gene pool. Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds have wings, even those that do not fly. There is strong evidence from genetics that all organisms have a common ancestor. For example, every living cell makes use of
nucleic acid Nucleic acids are biopolymer Biopolymers are natural polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecule ...

nucleic acid
s as its genetic material, and uses the same twenty
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s as the building blocks for
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s. All organisms use the same genetic code (with some extremely rare and minor deviations) to translation (genetics), translate nucleic acid sequences into proteins. The universality of these traits strongly suggests common ancestry, because the selection of many of these traits seems arbitrary. Horizontal gene transfer makes it more difficult to study the last universal ancestor. However, the universal use of the same genetic code, same nucleotides, and same amino acids makes the existence of such an ancestor overwhelmingly likely.


Phylogeny


Location of the root

The most commonly accepted location of the root of the Tree of life (biology), tree of life is between a monophyletic domain (biology), domain Bacteria and a clade formed by Archaea and Eukaryota of what is referred to as the "traditional tree of life" based on several molecular studies. A very small minority of studies have concluded differently, namely that the root is in the domain Bacteria, either in the phylum Firmicutes or that the phylum Chloroflexi (phylum), Chloroflexi is basal to a clade with Archaea and Eukaryotes and the rest of Bacteria as proposed by Thomas Cavalier-Smith. Research published in 2016, by William F. Martin, by genetically analyzing 6.1 million protein-coding genes from sequenced prokaryotic genomes of various phylogenetic trees, identified 355 protein clusters from amongst 286,514 protein clusters that were probably common to the LUCA. The results "depict LUCA as anaerobic organism, anaerobic, CO2-fixing, H2-dependent with a Wood–Ljungdahl pathway (the reductive Acetyl-CoA, acetyl-coenzyme A pathway), N2-fixing and thermophilic. LUCA's biochemistry was replete with FeS clusters and radical reaction mechanisms. Its cofactors reveal dependence upon transition metals, Flavin mononucleotide, flavins, S-adenosyl methionine, coenzyme A, ferredoxin, molybdopterin, corrins and selenium. Its genetic code required nucleoside modifications and S-adenosylmethionine-dependent methylations." The results depict methanogenic clostridium, clostria as a basal clade in the 355 lineages examined, and suggest that the LUCA inhabited an anaerobic hydrothermal vent setting in a geochemically active environment rich in H2, CO2, and iron. However, the identification of these genes as being present in LUCA was criticized, suggesting that many of the proteins assumed to be present in LUCA represent later horizontal gene transfers between archaea and bacteria.


Reproduction

Sexual reproduction is widespread among current eukaryotes, and was likely present in the last common ancestor. This is suggested by the finding of a core set of genes for meiosis in the descendants of lineages that diverged early from the eukaryotic evolutionary tree. and Malik et al. It is further supported by evidence that eukaryotes previously regarded as "ancient asexuals", such as ''Amoeba'', were likely sexual in the past, and that most present day asexual amoeboid lineages likely arose recently and independently. In prokaryotes, Transformation (genetics)#Natural transformation, natural bacterial transformation involves the transfer of DNA from one bacterium to another and integration of the donor DNA into the recipient chromosome by recombination. Natural bacterial transformation is considered to be a primitive sexual process and occurs in both bacteria and archaea, although it has been studied mainly in bacteria. Transformation is clearly a bacterial adaptation and not an accidental occurrence, because it depends on numerous gene products that specifically interact with each other to enter a state of natural competence to perform this complex process. Transformation is a common mode of DNA transfer among prokaryotes.


Horizontal gene transfer

The ancestry of living organisms has traditionally been reconstructed from morphology, but is increasingly supplemented with phylogenetics – the reconstruction of phylogenies by the comparison of genetic (DNA) sequence.
Sequence comparisons suggest recent horizontal gene transfer, horizontal transfer of many genes among diverse
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
including across the boundaries of phylogenetics, phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes.
Biologist Peter Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research", therefore "biologists (should) use the metaphor of a mosaic to describe the different histories combined in individual genomes and use (the) metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes."


Future of life (cloning and synthetic organisms)

Modern biotechnology is challenging traditional concepts of organisms and species. Cloning is the process of creating a new multicellular organism, genetically identical to another, with the potential of creating entirely new species of organisms. Cloning is the subject of ethics of cloning, much ethical debate. In 2008, the J. Craig Venter Institute assembled a synthetic Bacterial genome size, bacterial genome, ''Mycoplasma genitalium'', by using recombination in yeast of 25 overlapping DNA fragments in a single step. The use of yeast recombination greatly simplifies the assembly of large DNA molecules from both synthetic and natural fragments. Other companies, such as Synthetic Genomics, have already been formed to take advantage of the many commercial uses of custom designed genomes.


See also

* Earliest known life forms


References


External links


BBCNews: 27 September 2000, When slime is not so thick
Citat: "It means that some of the lowliest creatures in the plant and animal kingdoms, such as slime and amoeba, may not be as primitive as once thought" *

*** [https://web.archive.org/web/20090601140743/http://science.psu.edu/iceworms/iceworms.html The Eberly College of Science: Methane Ice Worms discovered on Gulf of Mexico Sea Floor] download Publication-quality photos *
Artikel, 2000: Methane Ice Worms: Hesiocaeca methanicola. Colonizing Fossil Fuel Reserves
*

''Hesiocaeca methanicola'' In 1997, Charles Fisher, professor of biology at Penn State, discovered this remarkable creature living on mounds of methane ice under half a mile of ocean on the floor of the Gulf of Mexico.
BBCNews, 18 December 2002, 'Space bugs' grown in lab
Citat: "''Bacillus simplex'' and ''Staphylococcus pasteuri''...''Engyodontium album'' The strains cultured by Dr Wainwright seemed to be resistant to the effects of UV – one quality required for survival in space"
BBCNews, 19 June 2003, Ancient organism challenges cell evolution
Citat: "It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both"

* [http://www.personal.psu.edu/users/j/s/jsf165/Bio110.html Jacob Feldman: Stramenopila]
NCBI Taxonomy entry: root


Citat: "Number of kingdoms has not been resolved...Bacteria present a problem with their diversity...Protista present a problem with their diversity...",
Species 2000 Indexing the world's known species
Species 2000 has the objective of enumerating all known species of plants, animals, fungi and microbes on Earth as the baseline dataset for studies of global biodiversity. It will also provide a simple access point enabling users to link from here to other data systems for all groups of organisms, using direct species-links.

* [http://tolweb.org/tree/phylogeny.html The Tree of Life] * [https://www.scribd.com/doc/1016/Life-from-birth-to-death/ Frequent questions from kids about life and their answers] {{Authority control Organisms, ceb:organismo