Cell (biology)

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)

Cell
prokaryotic cell (right)
Identifiers
MeSHD002477
THH1.00.01.0.00001
FMA686465
Anatomical terminology]

The cell is the basic structural and functional unit of all

motility
.

Cells are broadly categorized into two types:

fungi. Eukaryotic cells contain organelles including mitochondria, which provide energy for cell functions; chloroplasts, which create sugars by photosynthesis, in plants; and ribosomes
, which synthesise proteins.

Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks 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 living organisms, and that all cells come from pre-existing cells.

Number of cells

The number of cells in plants and animals varies from species to species; it has been estimated that the human body contains around 37 trillion (3.72×1013) cells,[1] and more recent studies put this number at around 30 trillion (~36 trillion cells in the male, ~28 trillion in the female).[2] The human brain accounts for around 80 billion of these cells.[3]

Cell types

Cells are broadly categorized into two types:

single-celled organisms, whereas eukaryotes can be either single-celled or multicellular.[4]

Prokaryotic cells

prokaryotic
cell

circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter.[5][page needed
]

A prokaryotic cell has three regions:

Eukaryotic cells

Structure of a typical animal cell
Structure of a typical plant cell

eukaryotic. 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 organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus,[6] an organelle that houses the cell's DNA
. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Some of the other differences are:

Comparison of features of prokaryotic and eukaryotic cells
Prokaryotes Eukaryotes
Typical organisms
bacteria, archaea
protists, algae, fungi, plants, animals
Typical size ~ 1–5 μm[10] ~ 10–100 μm[10]
Type of nucleus
nucleoid region
; no true nucleus
true nucleus with double membrane
DNA
circular
(usually)
linear molecules (chromosomes) with histone proteins
RNA/protein synthesis coupled in the cytoplasm
RNA synthesis in the nucleus
protein synthesis
in the cytoplasm
Ribosomes
30S
40S
Cytoplasmic structure very few structures highly structured by endomembranes and a cytoskeleton
Cell movement flagella made of flagellin flagella and
lamellipodia and filopodia containing actin
Mitochondria none one to several thousand
Chloroplasts none in algae and plants
Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells
Cell division
binary fission
(simple division)
mitosis (fission or budding)
meiosis
Chromosomes single chromosome more than one chromosome
Membranes cell membrane Cell membrane and membrane-bound organelles

Subcellular components

All cells, whether

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

Cell membrane

Detailed diagram of lipid bilayer of cell membrane

The

phospholipid bilayer, or sometimes a fluid mosaic membrane. 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.[6] 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.[11] Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.[12]

Cytoskeleton

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

NF–L, NF–M
).

Genetic material

Deoxyribonucleic acid (DNA)

Two different kinds of genetic material exist:

mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation
.

Prokaryotic genetic material is organized in a simple

endosymbiotic theory
).

A

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,[6]
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 transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.

Organelles

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).[6] 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

lysosomes) can be numerous (hundreds to thousands). The cytosol
is the gelatinous fluid that fills the cell and surrounds the organelles.

Eukaryotic

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.
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.[6] The smooth ER plays a role in calcium sequestration and release and also helps in synthesis of lipid.
  • 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.
  • 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.[6]
  • Centrosome: the cytoskeleton organizer: The
    mitotic spindle
    . A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled spaces 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. Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients.

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules.[6] 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).[14]
  • 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. And these pigments also 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.[15]

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 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 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.

Prokaryotic

Capsule

A gelatinous

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.[16]: 87 

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

Fimbriae

A

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
.

Cellular processes

.

Replication

Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in

Haploid cells serve as gametes
in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome,[6] always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before

meiosis II.[17] Replication, like all cellular activities, requires specialized proteins for carrying out the job.[6]

DNA repair

Cells of all organisms contain enzyme systems that scan their DNA for DNA damage and carry out repair processes when damage 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).[18]

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: 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.

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),[6] 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
.

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

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 messenger RNA (mRNA), which is free to migrate through the cell. 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. 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

cilia
.

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.[19] 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.[20][19]

Navigation, control and communication

In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to

chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners.[21][22][23]

Multicellularity

Cell specialization/differentiation

Staining of a Caenorhabditis elegans highlights the nuclei of its cells.

Multicellular organisms are

single-celled organisms.[24]

In complex multicellular organisms, cells specialize into different

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.

Most distinct cell types arise from a single

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
).

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,

fungi, slime molds, and red algae.[27] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships
.

The first evidence of multicellularity is from

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in

Origins

The origin of cells has to do with the

history of life
on Earth.

Origin of the first cell

Stromatolites are left behind by cyanobacteria, also called 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

the earliest self-replicating molecule, as it can both store genetic information and catalyze chemical reactions.[29]

Cells emerged around 4 billion years ago.[30][31] 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.[32][33]

Origin of eukaryotic cells

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.[34]

syngamy), peroxisomes, and a dormant cyst with a cell wall of chitin and/or cellulose.[37][38] In turn, 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.[39][40] The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added chloroplasts, derived from cyanobacteria.[34]

History of research

Robert Hooke's drawing of cells in cork, 1665

In 1665,

Matthias Schleiden and Theodor Schwann both also 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 not only fundamental to plants, but animals as well.[42]

See also

References

  1. S2CID 16247166
    .
  2. .
  3. .
  4. ^ "Differences Between Prokaryotic Cell and Eukaryotic Cell". BYJU'S. Archived from the original on 2021-10-09. Retrieved 2021-09-18.
  5. .
  6. ^
    NCBI. 30 March 2004. Archived from the original
    on 2009-12-08. Retrieved 3 May 2013.
  7. ^ European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi Archived 2013-05-06 at the Wayback Machine, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012
  8. PMID 18365235
    . 1432-119X.
  9. .
  10. ^ a b Campbell Biology – Concepts and Connections. Pearson Education. 2009. p. 320.
  11. ^ a b "Why is the plasma membrane called a selectively permeable membrane? – Biology Q&A". BYJUS. Archived from the original on 2021-09-18. Retrieved 2021-09-18.
  12. .
  13. .
  14. (PDF) from the original on 2021-01-21. Retrieved 2020-09-01.
  15. .
  16. from the original on April 14, 2021. Retrieved November 9, 2020.
  17. ^ Campbell Biology – Concepts and Connections. Pearson Education. 2009. p. 138.
  18. ^ Snustad, D. Peter; Simmons, Michael J. Principles of Genetics (5th ed.). DNA repair mechanisms, pp. 364–368.
  19. ^
    PMID 17589565
    .
  20. .
  21. ^ Willingham, Emily. "Cells Solve an English Hedge Maze with the Same Skills They Use to Traverse the Body". Scientific American. Archived from the original on 4 September 2020. Retrieved 7 September 2020.
  22. ^ "How cells can find their way through the human body". phys.org. Archived from the original on 3 September 2020. Retrieved 7 September 2020.
  23. from the original on 2020-09-12. Retrieved 2020-09-13.
  24. .
  25. ^ (PDF) on 2016-03-04. Retrieved 2013-12-23.
  26. (PDF) from the original on 2016-07-29. Retrieved 2013-12-23.
  27. ISSN 1093-4391. Archived from the original
    (PDF, 0.2 MB) on March 8, 2012.
  28. .
  29. .
  30. from the original on 8 September 2017. Retrieved 2 March 2017.
  31. .
  32. .
  33. ^ "First cells may have emerged because building blocks of proteins stabilized membranes". ScienceDaily. Archived from the original on 2021-09-18. Retrieved 2021-09-18.
  34. ^ from the original on 24 March 2019. Retrieved 27 August 2017.
  35. .
  36. .
  37. .
  38. .
  39. .
  40. .
  41. ^ Hooke, Robert (1665). "Observation 18". Micrographia.
  42. .
  43. ^ .
  44. from the original on 7 August 2021. Retrieved 5 August 2021.
  • ^ Hooke, Robert (1665). Micrographia: ... London: Royal Society of London. p. 113. ... 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: Robert Hooke Archived 1997-06-06 at the Wayback Machine
  • ^ Schwann, Theodor (1839). Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen. Berlin: Sander.
  • .
  • .
  • Further reading

    External links