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The cell (from cella, meaning "small room") is the basic structural, functional, and biological unit of all known . A cell is the smallest unit of . Cells are often called the "building blocks of life". The study of cells is called , cellular biology, or cytology.

Cells consist of enclosed within a , which contains many such as and . Cell Movements and the Shaping of the Vertebrate Body in Chapter 21 of Molecular Biology of the Cell fourth edition, edited by Bruce Alberts (2002) published by Garland Science.
The Alberts text discusses how the "cellular building blocks" move to shape developing . It is also common to describe small molecules such as as " molecular building blocks".
Most plant and animal cells are only visible under a , with dimensions between 1 and 100 .

(2020). 9780132508827, Pearson Prentice Hall. .
Organisms can be classified as (consisting of a single cell such as ) or (including and ). Most unicellular organisms are classed as .

The number of cells in plants and animals varies from species to species, it has been estimated that contain somewhere around 40 trillion (4×1013) cells. The accounts for around 80 billion of these cells.

Cells were discovered by in 1665, who named them for their resemblance to cells inhabited by Christian monks in a .

(2009). 9780470483374, John Wiley & Sons.
(1990). 9788184243697, Allied Publishers.
, first developed in 1839 by Matthias Jakob Schleiden and , states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells.
(1997). 9780134234762, Prentice Hall. .
Cells emerged on Earth at least 3.5 billion years ago.


Cell types
Cells are of two types: , which contain a , and , which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.


Prokaryotic cells
include and , two of the three domains of life. Prokaryotic cells were the first form of on Earth, characterized by having vital biological processes including . They are simpler and smaller than eukaryotic cells, and lack a , and other membrane-bound . The of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the . The nuclear region in the cytoplasm is called the . Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. Microbiology : Principles and Explorations By Jacquelyn G. Black

A prokaryotic cell has three regions:

  • Enclosing the cell is the – generally consisting of a covered by a which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as (bacteria) and (archaea) which only possess the cell membrane layer. 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 in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting () from due to a environment. Some eukaryotic cells ( and cells) also have a cell wall.
  • Inside the cell is the that contains the (DNA), ribosomes and various sorts of inclusions. 30 March 2004. The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called , which are usually circular. Linear bacterial plasmids have been identified in several species of bacteria, including members of the genus notably Borrelia burgdorferi, which causes Lyme disease.European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012 Though not forming a nucleus, the is condensed in a . Plasmids encode additional genes, such as antibiotic resistance genes.
  • On the outside, and project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.


Eukaryotic cells
, , , , , and are all . These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound (compartments) in which specific activities take place. Most important among these is a , an organelle that houses the cell's . This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:
  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
  • The eukaryotic DNA is organized in one or more linear molecules, called , which are associated with proteins. All chromosomal DNA is stored in the , separated from the cytoplasm by a membrane. Some eukaryotic organelles such as also contain some DNA.
  • Many eukaryotic cells are with primary cilia. Primary cilia play important roles in chemosensation, , and thermosensation. Each cilium may thus 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."
  • Motile eukaryotes can move using or . Motile cells are absent in and .PH Raven, Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York Eukaryotic flagella are more complex than those of prokaryotes.

+Comparison of features of prokaryotic and eukaryotic cells


Subcellular components
All cells, whether or , have a that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the takes up most of the cell's volume. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for ) possess , the hereditary material of , and , containing the information necessary to various such as , the cell's primary machinery. There are also other kinds of in cells. This article lists these primary cellular components, then briefly describes their function.


Membrane
The , or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a . This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a , which are (partly and partly ). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the the universal secretory portal in cells and a variety of molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is semi-permeable, and selectively permeable, in that it can either let a substance ( or ) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as .


Cytoskeleton
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during , the uptake of external materials by a cell, and , the separation of daughter cells after ; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of , intermediate filaments and . There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called . The subunit of microtubules is a dimeric molecule called . Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include , , (lamins A, B and C), (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M).


Genetic material
Two different kinds of genetic material exist: (DNA) and (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is in its DNA sequence. RNA is used for information transport (e.g., ) and functions (e.g., RNA). (tRNA) molecules are used to add amino acids during protein translation.

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

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

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


Organelles
Organelles are parts of the cell which 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). Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the and ) are typically solitary, while others (such as , , and ) can be numerous (hundreds to thousands). The is the gelatinous fluid that fills the cell and surrounds the organelles.


Eukaryotic
  • Cell nucleus: A cell's information center, the is the most conspicuous organelle found in a cell. It houses the cell's , and is the place where almost all replication and synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the . 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, is transcribed, or copied into a special , called (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the .
  • Mitochondria and Chloroplasts: generate energy for the cell. are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using to release energy stored in cellular nutrients (typically pertaining to ) to generate ATP. Mitochondria multiply by , like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through .

  • 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. The smooth ER plays a role in calcium sequestration and release.
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the such as and that are synthesized by the cell.
  • Lysosomes and Peroxisomes: contain (acid ). They digest excess or worn-out , food particles, and engulfed or . have enzymes that rid the cell of toxic . The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.
  • Centrosome: the cytoskeleton organiser: The produces the of a cell – a key component of the . It directs the transport through the ER and the . Centrosomes are composed of two , which separate during and help in the formation of the . A single centrosome is present in the . They are also found in some fungi and algae cells.
  • Vacuoles: sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.


Eukaryotic and prokaryotic
  • Ribosomes: The is a large complex of and molecules. 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).


Structures outside the cell membrane
Many cells also 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 semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.


Cell wall
Many types of prokaryotic and eukaryotic cells have a . 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 , fungi cell walls are made up of and bacteria cell walls are made up of .


Prokaryotic

Capsule
A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be as in , or as Bacillus anthracis or as in . Capsules are not marked by normal staining protocols and can be detected by India ink or ; which allows for higher contrast between the cells for observation.
(1996). 9780080984735, Newnes. .


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. A different type of flagellum is found in archaea and a different type is found in eukaryotes.


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


Cellular processes

Replication
Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. cells divide by , while cells usually undergo a process of nuclear division, called , followed by division of the cell, called . A cell may also undergo to produce haploid cells, usually four. cells serve as in multicellular organisms, fusing to form new diploid cells.

, or the process of duplicating a cell's genome, always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the .

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before . DNA replication does not occur when the cells divide the second time, in . Replication, like all cellular activities, requires specialized proteins for carrying out the job.


DNA repair
In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out when damages are detected.D. Peter Snustad, Michael J. Simmons, Principles of Genetics – 5th Ed. (DNA repair mechanisms) pp. 364-368 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 damages that could lead to . bacteria are a well-studied example of a cellular organism with diverse well-defined processes. These include: (1) nucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) recombinational repair and (5) light-dependent repair ().


Growth and metabolism
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: , in which the cell breaks down complex molecules to produce energy and , and , in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called such as . Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a molecule that possesses readily available energy, through two different pathways.


Protein synthesis
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 building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

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 (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called located in the , 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 (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.


Motility
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include and .

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. 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.

(2020). 9780815340720, Garland Science.


Multicellularity

Cell specialization
Multicellular organisms are that consist of more than one cell, in contrast to single-celled organisms.
(2020). 9780321554185, Pearson Benjamin Cummings.

In complex multicellular organisms, cells specialize into different that are adapted to particular functions. In mammals, major cell types include , , , , , , and others. Cell types differ both in appearance and function, yet are identical. Cells are able to be of the same but of different cell type due to the differential expression of the they contain.

Most distinct cell types arise from a single cell, called a , 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 during ).


Origin of multicellularity
Multicellularity has evolved independently at least 25 times, including in some prokaryotes, like , , , Magnetoglobus multicellularis or . However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants. It evolved repeatedly for plants (), once or twice for , once for , and perhaps several times for , , and . Multicellularity may have evolved from colonies of interdependent organisms, from , or from organisms in .

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

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


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


Origin of the first cell
There are several theories about the origin of small molecules that led to life on the . They may have been carried to Earth on meteorites (see Murchison meteorite), created at deep-sea vents, or synthesized by lightning in a reducing atmosphere (see Miller–Urey experiment). There is little experimental data defining what the first self-replicating forms were. is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see RNA world hypothesis), but some other entity with the potential to self-replicate could have preceded RNA, such as clay or peptide nucleic acid.

Cells emerged at least 3.5 billion years ago.

(2020). 9780071122610, McGraw-Hill Education. .
The current belief is that these cells were . The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form.


Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a of prokaryotic cells. DNA-bearing organelles like the and the are descended from ancient symbiotic oxygen-breathing and , respectively, which were endosymbiosed by an ancestral prokaryote.

There is still considerable debate about whether organelles like the predated the origin of , or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.


History of research
  • 1632–1723: Antonie van Leeuwenhoek taught himself to make lenses, constructed basic optical microscopes and drew protozoa, such as from rain water, and from his own mouth.
  • 1665: discovered cells in cork, then in living plant tissue using an early compound microscope. He coined the term cell (from cella, meaning "small room") in his book (1665)." … I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular … these pores, or cells, … were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this … " – Hooke describing his observations on a thin slice of cork. See also:
[http://www.ucmp.berkeley.edu/history/hooke.html Robert Hooke]
     
  • 1839: and Matthias Jakob Schleiden elucidated the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
  • 1855: stated that new cells come from pre-existing cells by ( omnis cellula ex cellula).
  • 1859: The belief that life forms can occur spontaneously ( ) was contradicted by (1822–1895) (although had performed an experiment in 1668 that suggested the same conclusion).
  • 1931: built the first transmission electron microscope (TEM) at the University of Berlin. By 1935, he had built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
  • 1953: Based on Rosalind Franklin's work, Watson and made their first announcement on the structure of DNA.
  • 1981: published Symbiosis in Cell Evolution detailing the endosymbiotic theory.


See also


Notes

Further reading


External links

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