The cell is the basic structural, functional and biological unit of all known living organisms. Cells are the
smallest unit of life that is classified as a living thing, and are often called the "building blocks of life".
Cells consist of a protoplasm enclosed within a membrane, which contains many biomolecules such
as proteins and nucleic acids.[1]Organisms can be classified as unicellular (consisting of a single cell;
including most bacteria) or multicellular (including plants andanimals). While the number of cells in plants
and animals varies from species to species, humans contain about 100 trillion (1014) cells.[2]Most plant
and animal cells are between 1 and 100 micrometres and therefore are visible only under the
microscope.[3]
The cell was discovered by Robert Hooke in 1665. The cell theory, first developed in 1839 by Matthias
Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells,
that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all
cells contain thehereditary information necessary for regulating cell functions and for transmitting
information to the next generation of cells.[4] Cells emerged on Earth at least 3.5 billion years ago.[5][6][7]
The word cell comes from the Latin cella, meaning "small room".[8] It was coined by Robert Hooke in his
book Micrographia (1665), in which he compared the cork cells he saw through his microscope to the
small rooms monks lived in.[9]
A eukaryote (/juːˈkæri.oʊt/ or /juːˈkæriət/) is an organism whose cells contain a nucleus and other
organelles enclosed within membranes. Eukaryotes are formally thetaxon Eukarya or Eukaryota. The
defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus,
or nuclear envelope, within which the genetic material is carried.[1][2][3] The presence of a nucleus gives
eukaryotes their name, which comes from the Greek ευ (eu, "good") and κάρυον (karyon, "nut" or
"kernel").[4] Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria or
the Golgi apparatus. In addition, plants and algae containchloroplasts. Many unicellular organisms are
eukaryotes, such as protozoa. All multicellular organisms are eukaryotes,
including animals, plants and fungi.
Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). There are
two types of division processes. In mitosis, one cell divides to produce two genetically identical cells.
In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each
chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes,
and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one
complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.
The domain Eukaryota appears to be monophyletic, and so makes up one of the three of domains of life.
The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features.
Eukaryotes represent a tiny minority of all living things;[5] even in a human body there are 10 times more
microbes than human cells.[6]However, due to their much larger size, their collective worldwide biomass is
estimated at about equal to that of prokaryotes.[7] Eukaryotes first developed approximately 1.6–2.1 billion
years ago.
The prokaryotes are a group of organisms whose cells lack a membrane-bound nucleus (karyon). The organisms
whose cells do have a nucleus are called eukaryotes. Most prokaryotes are unicellular organisms, although a few
such as myxobacteria have multicellular stages in their life cycles[1] or create large colonies like cyanobacteria. The
word prokaryote comes from the Greekπρό- (pro-) "before" and καρυόν (karyon) "nut or kernel".[2][3] Prokaryotes do
not have a nucleus, mitochondria, or any other membrane-bound organelles. In other words, all their intracellular
water-soluble components (proteins, DNA and metabolites) are located together in the same area enclosed by cell
membrane, rather than separated in different cellular compartments.
The division to prokaryotes and eukaryotes reflects two distinct levels of cellular organization rather than biological
classification of species. Prokaryotes include two major classification domains: the bacteria and thearchaea. Archaea
were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable
conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.