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This article is about the basic unit of lifeforms. For the branch of biology that studies them, see Cell biology.

Cell
Onion (Allium cepa) root cells in different phases of the cell cycle (drawn by E. B. Wilson, 1900)
A eukaryotic cell (left) and prokaryotic cell (right)
Identifiers
MeSHD002477
THH1.00.01.0.00001
FMA686465
Anatomical terminology

The cell is the basic structural and functional unit of all forms of life or organisms. The term comes from the Latin word cellula meaning 'small room'. A biological cell consists of cytoplasm enclosed within a membrane. Most cells are only visible under a microscope. All cells (except red blood cells ) are capable of replication, and protein synthesis, and some types are motile. Cells emerged on Earth about four billion years ago.

All organisms are grouped into prokaryotes, and eukaryotes. Prokaryotes are single-celled, and include archaea, and bacteria. Eukaryotes can be single-celled or multicellular, and include protists, some species of algae, and most types of fungi, plants, and animals. All multicellular organisms are made up of many different types of cell. The diploid cells that make up the body of a plant or animal are known as somatic cells, and in animals excludes the haploid gametes.

Prokaryotic cells lack the membrane-bound nucleus present in eukaryotic cells, and instead have a nucleoid region. In eukaryotic cells the nucleus is enclosed in the nuclear membrane. Eukaryotic cells contain other membrane-bound organelles such as mitochondria, which provide energy for cell functions, and chloroplasts, in plants that create sugars by photosynthesis. Other non-membrane-bound organelles may be proteinaceous such as the ribosomes present (though different) in both groups. A unique membrane-bound prokaryotic organelle the magnetosome has been discovered in magnetotactic bacteria.

Cells were discovered by Robert Hooke in 1665, who named them after their resemblance to cells in a monastery. Cell theory, developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all organisms, and that all cells come from pre-existing cells. The studies of cell biology (cytology), microbiology and its branches, and molecular biology are all combined in the field of cellular microbiology.

Cell types

See also: Cell type

Organisms are broadly grouped into eukaryotes, and prokaryotes. Eukaryotic cells possess a membrane-bound nucleus, and prokaryotic cells lack a nucleus but have a nucleoid region.[1] Prokaryotes are single-celled organisms, whereas eukaryotes can be either single-celled or multicellular.[2] Animals, plants, most fungi, and some algae species are multicellular organisms.[3][4]

Multicellular organisms are made up of many different types of cell known overall as somatic cells.[5] Typical plant cells include parenchyma cells including transfer cells, and collenchyma cells. Animal cells include all those that make up the four main tissue types of epithelium – a number of different epithelial cells; connective tissue such as osteoblasts in bone, and chondrocytes in cartilage; nervous tissue including different brain cells, and muscle tissue having different muscle cells.[1] The number of cells in these groups vary with species. Studies on the human have estimated a total body count at around 30 trillion cells (~36 trillion cells in the male, and ~28 trillion in the female).[6][7]

Prokaryotic cells

Main articles: Prokaryote, Bacteria, and Archaea
Structure of a typical bacterial cell, generally similar with the archaeal cell structure. The bacterial flagellum shown, differs from the archaellum in archaea

Prokaryotes include bacteria and archaea, two of the three domains of life.[8] Prokaryotic cells were likely the first form of life on Earth,[9][10] characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, lack a nucleus, and the other usually present membrane-bound organelles.[11] Certain membrane-bound prokaryotic organelles have been discovered. They include the magnetosome of magnetotactic bacteria,[12] and the anammoxosome of anammox bacteria.[13][14] Compartmentalization is a feature of eukaryotic cells but some bacteria, including Escherichia coli, and some species of Salmonella, also have organelle-like microcompartments.[15][16]

The DNA of most prokaryotic cells consists of a single circular chromosome that is in direct contact with the cytoplasm; however, some bacteria contain multiple circular or even linear chromosomes.[17][18] The nuclear region in the cytoplasm is called the nucleoid.[19] Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter.[20] The largest bacterium known, Thiomargarita magnifica, is visible to the naked eye with a length of 1 to 2 cm.[21]

Prokaryotic cells are enclosed in a cell envelope, that protects the interior from the exterior.[22] It generally consists of a plasma membrane covered by a cell wall which, for some bacteria, is covered by a third layer called a bacterial capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer.[23] The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria and acts as an additional barrier against exterior forces.[24] It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment.[25] Some eukaryotic cells (plant cells and fungal cells) have a cell wall.[26]

Cytoplasm fills the interior of cell and contains the genome (DNA), ribosomes and various inclusions.[27] The genetic material is extrachromosomal DNA freely found in the nucleoid in the cytoplasm. Elements called plasmids, which are usually circular may also be found in the cell, they encode additional genes, such as those of antibiotic resistance.[28] Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease.[29] Cell-surface appendages on some prokaryotes include bacterial flagella or archeaella, and pili; these structures are made of proteins that facilitate movement and communication between cells.[30]

Eukaryotic cells

Colored illustration showing the body of an animal cell surrounded by tissue. The nucleus, mitochondria, gogli, ER, and lysosomes are labelled.
Structure of a typical animal cell
Colored illustration showing the body of a plant cell. Among the labelled features are the nucleus, cell wall, chloroplasts, vacuole, mitochondrion, golgi body, and ribosomes.
Structure of a typical plant cell
Main article: Eukaryote

Plants, animals, fungi,[31] slime moulds,[32] protozoa[33] and algae[34] are all eukaryotes. The main distinguishing feature of a eukaryotic cell is the presence of a cell nucleus, a membrane-bound organelle which houses most of the organism's genome, and coordinates processes such as protein biosynthesis, and cell division.[35][36] Mitochondria contain some mitochondrial DNA.[37] Eukaryotic DNA is organized in multiple linear molecules, called chromosomes, that are coiled around histone proteins.[36][38] The nucleus gives the eukaryote its name, which means "true nut" or "true kernel", where "nut" means the nucleus.[39] These cells can be 2 to 1000 times larger in diameter than a typical prokaryote.[40]

Another defining eukaryotic feature is the presence of other membrane-bound organelles sometimes called cellular compartments, in which specific activities take place. Some of these compartments, such as mitochondria, were likely acquired from symbiotic interaction with prokaryotes.[41]

Most eukaryotic cells are ciliated with primary cilia.[42] Primary cilia play important roles in chemosensation and mechanosensation.[43] Each cilium may be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[44] Eukaryotic flagella are more complex than those of prokaryotes.[45]

Comparison of features of prokaryotic and eukaryotic cells
Prokaryotes Eukaryotes
Typical organisms bacteria, archaea protists, algae, fungi, plants, animals
Typical size ~ 1–5 μm[11] ~ 10–100 μm[11]
DNA In nucleoid region In nucleus with double membrane[46]
Chromosomes Single, usually circular, in some prokaryotes multiple and/or linear Multiple paired linear chromosomes with histone proteins
RNA/protein synthesis coupled in the cytoplasm[47] RNA synthesis in the nucleus
protein synthesis in the cytoplasm
Ribosomes 50S and 30S[48] 60S and 40S[48]
Cytoplasmic structure Microcompartments with proteins, cytoskeleton Endomembrane system, cytoskeleton
Cell movement flagella flagella and cilia; lamellipodia and filopodia
Mitochondria none[49] Zero[49] to several thousand[50]
Chloroplasts none in algae and plants
Organization single cells, colonies, biofilms single cells, colonies, multicellular organisms with specialized cells
Cell division binary fission (simple division) mitosis (fission or budding)
meiosis
Membranes cell membrane Cell membrane and membrane-bound organelles

Subcellular components

All cells, whether prokaryotic or eukaryotic, have a cell membrane, cytoplasm, ribosomes, and DNA.[51] The defining difference between the two classes is the presence of a membrane-bound nucleus housing the DNA (and other membrane-bound organelles) in the eukaryotic cell; in the prokaryotic cell the DNA is not membrane-bound and other membrane-bound organelles are not typically present.[51] The exception is the eukaryotic red blood cell that does not have a nucleus or other organelle.[52]

Cell membrane

Main article: Cell membrane
Diagram of the cell membrane detailing the lipid bilayer

The cell membrane, or plasma membrane, is a selectively permeable biological membrane that surrounds the cytoplasm of all cells.[53] In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a lipid bilayer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic), and is sometimes referred to as a fluid mosaic membrane.[54] Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.[27] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, to a limited extent or not at all.[55] Cell surface receptors embedded in the membrane allow cells to detect external signaling molecules such as hormones.[56]

Cytoskeleton

Main article: Cytoskeleton
Further information: Morphogenesis
A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green.

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.[27]

The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.[57]

Genetic material

Main articles: DNA and RNA
Deoxyribonucleic acid (DNA)

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence.[27] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.[58]

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different,[27] linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).[59]

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans, the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes,[27] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.[60]

Foreign genetic material (most commonly DNA) can be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses can insert their genetic material into the genome via transformation.[61]

Organelles

Main article: Organelle

Organelles are parts of the cell that are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function).[27] Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.[62]

There are several types of organelles in a cell. Some (such as the nucleus and Golgi apparatus) are typically solitary,[citation needed] while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). Most organelles vary in size and/or number based on the growth of the host cell.[63] The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles, forming 30%–50% of a cell volume.[64]

Eukaryotic

Human cancer cells, specifically HeLa cells, with DNA stained blue. The central and rightmost cell are in interphase, so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through mitosis and its chromosomes have condensed.
  • Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope, space between these two membrane is called perinuclear space. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm.[27]
  • Mitochondria and chloroplasts: generate energy for the cell. Mitochondria are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.[27] Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP (aerobic respiration).[65] Mitochondria and chloroplasts multiply by binary fission, like prokaryotes.[66] Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.[67]
Diagram of the endomembrane system
  • Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes.[27] The smooth ER plays a role in calcium sequestration and release, and helps in synthesis of lipid.[68]
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. It is typically organized as a stack of plate-like structures known as cisternae.[69]
  • Lysosomes and peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides, Lysosomes are optimally active in an acidic environment. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.[27][70]
  • Centrosome: the cytoskeleton organizer: The centrosome produces the microtubules of a cell—a key component of the cytoskeleton.[71] It directs the transport through the ER and the Golgi apparatus.[72] Centrosomes are composed of two centrioles which lie perpendicular to each other in which each has an organization like a cartwheel, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells.[71] They are also found in some fungi and algae cells.[73]
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are described as liquid filled spaces and are surrounded by a membrane.[74] Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water.[75] The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.[74] Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients.[76]

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules.[27] They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).[77]
  • Plastids: Plastid are membrane-bound organelle generally found in plant cells and euglenoids and contain specific pigments, thus affecting the colour of the plant and organism. These pigments helps in food storage and tapping of light energy. There are three types of plastids based upon the specific pigments. Chloroplasts contain chlorophyll and some carotenoid pigments which helps in the tapping of light energy during photosynthesis. Chromoplasts contain fat-soluble carotenoid pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage. Leucoplasts are non-pigmented plastids and helps in storage of nutrients.[78]

Structures outside the cell membrane

Many cells have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Further information: Cell wall

Many types of prokaryotic and eukaryotic cells have a carbohydrate-based cell wall. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan.[26]

Prokaryotic

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue, which allows for higher contrast between the cells for observation.[79]

Flagella

Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature.[80] Separate varieties of flagellum are found in archaea and in eukaryotes, having independently evolved their own structure, composition, and propulsion mechanism.[81]

Fimbriae

A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved in bacterial conjugation.[82]

Cell physiology

Main article: Cell physiology
Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis.

Replication

Main article: Cell division

Cell division is the process that involves a single cell's (called a mother cell) division into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.[83]

DNA replication, or the process of duplicating a cell's genome,[27] always happens when a cell divides through mitosis or binary fission.[83] This occurs during the S (synthesis) phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II.[84] Replication, like all cellular activities, requires specialized proteins.[27]

DNA repair

Main article: DNA repair

Cells of all organisms contain enzyme systems that scan their DNA for damage and carry out repair processes when it is detected. Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to mutation. E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: nucleotide excision repair, DNA mismatch repair, non-homologous end joining of double-strand breaks, recombinational repair and light-dependent repair (photoreactivation).[85]

Growth and metabolism

Main articles: Cell growth, Metabolism, and Photosynthesis

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.[86]

Complex sugars can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP),[27] a molecule that possesses readily available energy, through two different pathways. In plant cells, chloroplasts create sugars by photosynthesis, using the energy of light to join molecules of water and carbon dioxide.[87]

Protein synthesis

Main article: Protein biosynthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.[58]

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate into the cytoplasm. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome.[58] The new polypeptide then folds into a functional three-dimensional protein molecule.

Motility

Main article: Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.[81]

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.[88] The process is divided into three steps: protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.[89][88]

See also: Cybernetics § In biology

In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners.[90][91][92]

Multicellularity

Main article: Multicellular organism

Cell specialization/differentiation

Main article: Cellular differentiation
Staining of a nematode Caenorhabditis elegans highlights the nuclei of its cells.

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms.[93]

In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions.[94] In animals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain.[95]

Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division).[96]

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,[97] including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.[98] It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae.[99] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships.[100]

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.[97] Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon.[101]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.[97]

Cell biology (science)

This section is an excerpt from Cell biology.[edit]
Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells.[102][103] All organisms are made of cells. A cell is the basic unit of life that is responsible for the living and functioning of an organism.[104] Cell biology is the study of the structural and functional units of cells. Cell biology encompasses both prokaryotic and eukaryotic cells and has many subtopics which may include the study of cell metabolism, cell communication, cell cycle, biochemistry, and cell composition. The study of cells is performed using several microscopy techniques, cell culture, and cell fractionation. These have allowed for and are currently being used for discoveries and research pertaining to how cells function, ultimately giving insight into understanding larger organisms. Knowing the components of cells and how cells work is fundamental to all biological sciences while also being essential for research in biomedical fields such as cancer, and other diseases. Research in cell biology is interconnected to other fields such as genetics, molecular genetics, molecular biology, medical microbiology, immunology, and cytochemistry.

Clinical significance

Main article: Cytopathology

Cytopathology studies and diagnoses diseases on the cellular level. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to the pathology branch of histopathology, which studies whole tissues. Cytopathology is commonly used to investigate diseases involving a wide range of body sites, often to aid in the diagnosis of cancer, and in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology is the Pap smear, a screening test used to detect cervical cancer, and precancerous cervical lesions that may lead to cervical cancer.[105]

Origins

Main article: History of life

The origin of cells has to do with the origin of life, which began the history of life on Earth.

Origin of life

Further information: Abiogenesis and Evolution of cells
Stromatolites are left behind by cyanobacteria, known as blue-green algae. They are among the oldest fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.

Small molecules needed for life may have been carried to Earth on meteorites, created at deep-sea vents, or synthesized by lightning in a reducing atmosphere. There is little experimental data defining what the first self-replicating forms were. RNA may have been the earliest self-replicating molecule, as it can both store genetic information and catalyze chemical reactions.[106] This process required an enzyme to catalyze the RNA reactions, which may have been the early peptides that formed in hydrothermal vents.[107]

Cells emerged around 4 billion years ago.[108][109] The first cells were most likely heterotrophs. The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered vesicles in water, and could have preceded RNA.[110][111]

First eukaryotic cells

Main article: Eukaryogenesis
In the theory of symbiogenesis, a merger of an archaean and an aerobic bacterium created the eukaryotes, with aerobic mitochondria, some 2.2 billion years ago. A second merger, 1.6 billion years ago, added chloroplasts, creating the green plants.[112]

Eukaryotic cells were created some 2.2 billion years ago in a process called eukaryogenesis. This is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor.[112] However, the sequence of the steps involved has been disputed.[citation needed] It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along the way.[113][114]

This cell had a new level of complexity and capability, with a nucleus[115][113] and facultatively aerobic mitochondria.[112] It featured at least one centriole and cilium, sex (meiosis and syngamy), peroxisomes, and a dormant cyst with a cell wall of chitin and/or cellulose.[116][114] The last eukaryotic common ancestor gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.[117][118] The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added chloroplasts, derived from cyanobacteria.[112]

History of research

Main article: Cell theory § Discovery of cells
Robert Hooke's drawing of cells in cork, 1665

In 1665, Robert Hooke examined a thin slice of cork under his microscope, and saw a structure of small enclosures. He wrote "I could exceeding plainly perceive it to be all perforated and porous, much like a honeycomb, but that the pores of it were not regular".[119] To further support his theory, Matthias Schleiden and Theodor Schwann studied cells of both animal and plants. What they discovered were significant differences between the two types of cells. This put forth the idea that cells were fundamental to both plants and animals.[120]

Research methods

The studies of cell biology (cytology) is combined with microbiology and molecular biology in the field of cellular microbiology.[131] Research methods in all these fields continue to be developed and include:

  • Cell culture that allows for a large number of a specific cell type to be cultivated and studied. Cell culture can provide model systems for the study of ageing for example; the effects of drugs and toxins on the cells, and carcinogenesis.[132] Other uses include drug screening and drug development.
  • Fluorescence microscopy uses fluorescent markers such as GFP to label a specific cell component. A specific light wavelength is used to excite the fluorescent marker which can then be visualized.[132]
  • Phase-contrast microscopy uses the optical aspect of light to represent the solid, liquid, and gas-phase changes as brightness differences.[132]
  • Confocal microscopy combines fluorescence microscopy with imaging by focusing light and snap shooting instances to form a 3-D image.[132]
  • Transmission electron microscopy involves metal staining and the passing of electrons through the cells, which will be deflected upon interaction with metal. This ultimately forms an image of the components being studied.[132]
  • Cytometry – cells are placed in the machine which uses a beam to scatter them based on different aspects separating them based on size and content. Cells may be tagged with GFP-fluorescence and separated that way.[133]
  • Cell fractionation is a process that requires breaking up the cell using high temperature or sonification followed by centrifugation to separate the cell components for study.[132]

See also

References

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  2. ^ "Prokaryote structure". khanacademy. Retrieved 2025-08-16.
  3. ^ Knoll, Andrew H. (2011). "The Multiple Origins of Complex Multicellularity". Annual Review of Earth and Planetary Sciences. 39: 217–239. Bibcode:2011AREPS..39..217K. doi:10.1146/annurev.earth.031208.100209.
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