The cell (from Latin cella, meaning "small room"
Cells consist of cytoplasm enclosed within a Cell membrane, 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 microscope, with dimensions between 1 and 100 .
The number of cells in plants and animals varies from species to species, it has been estimated that Homo sapiens contain somewhere around 40 trillion (4×1013) cells.
Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery.
Cell types
Cells are of two types:
eukaryotic, which contain a
Cell nucleus, and
prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.
Prokaryotic cells
include
bacteria and
archaea, two of the three domains of life. Prokaryotic cells were the first form of
life on Earth, characterized by having vital biological processes including
cell signaling. They are simpler and smaller than eukaryotic cells, and lack a
cell nucleus, and other membrane-bound
. The
DNA of a prokaryotic cell consists of a single 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.
[ Microbiology : Principles and Explorations By Jacquelyn G. Black]
A prokaryotic cell has three regions:
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Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall 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 Mycoplasma (bacteria) and Thermoplasma (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 peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a Hypotonicity environment. Some eukaryotic cells ( and fungus cells) also have a cell wall.
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Inside the cell is the cytoplasm that contains the genome (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 spirochete bacteria, including members of the genus Borrelia 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 DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
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On the outside, flagella and Pilus 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
Plants,
animals,
fungi,
,
protozoa, and
algae are all
Eukaryote. 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
cell nucleus,
an organelle that houses the cell's
DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:
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The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
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The eukaryotic DNA is organized in one or more linear molecules, called , which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane.
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Many eukaryotic cells are cilium with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, 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."
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Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants.
[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.
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Subcellular components
All cells, whether
prokaryotic or
eukaryotic, have a
cell 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. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for
hemoglobin) possess
DNA, the hereditary material of
, and
RNA, containing the information necessary to
gene expression 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
cell membrane, 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
cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a
lipid bilayer, which are
amphiphilic (partly
hydrophobic and partly
hydrophilic). Hence, the layer is called a 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.
The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (
molecule or
ion) 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
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
, 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,
cell polarity and cytokinesis.
The subunit protein of microfilaments is a small, monomeric protein called
actin. The subunit of microtubules is a dimeric molecule called
tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include
vimentin,
desmin,
lamin (lamins A, B and C),
keratin (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M).
Genetic material
Two different kinds of genetic material exist:
DNA (DNA) and
RNA (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is
Genetic code in its DNA sequence.
RNA is used for information transport (e.g.,
mRNA) and
enzyme functions (e.g.,
ribosome RNA).
Transfer 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 nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different,
A human cell has genetic material contained in the cell nucleus (the genome) 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 sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes,
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 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 Cell nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.
Eukaryotic
-
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. 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.
-
Mitochondria and Chloroplasts: generate energy for the cell. Mitochondrion are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.
-
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.
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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 bacteria. 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.
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Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the . They are also found in some fungi and algae cells.
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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 ribosome is a large complex of RNA and protein molecules.
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
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 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.
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 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.
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.
Prokaryote cells divide by
binary fission, while
Eukaryote cells usually undergo a process of nuclear division, called
mitosis, followed by division of the cell, called
cytokinesis. A
diploid cell may also undergo
meiosis to produce haploid cells, usually four.
Haploid cells serve as
in multicellular organisms, fusing to form new diploid cells.
DNA replication, or the process of duplicating a cell's genome,
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.
DNA repair
In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out
DNA repair 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
mutation.
Escherichia coli bacteria are a well-studied example of a cellular organism with diverse well-defined
DNA repair 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 (
photolyase).
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 agent, and
anabolism, 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
monosaccharides such as
glucose. 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
amino acid 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 messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called 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
flagella and
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.
Multicellularity
Cell specialization
Multicellular organisms are
that consist of more than one cell, in contrast to single-celled organisms.
In complex multicellular organisms, cells specialize into different that are adapted to particular functions. In mammals, major cell types include skin cells, muscle cells, , , , stem cells, and others. Cell types differ both in appearance and function, yet are Genetics identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the they contain.
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 during cell division).
Origin of multicellularity
Multicellularity has evolved independently at least 25 times,
including in some prokaryotes, like
cyanobacteria,
myxobacteria,
actinomycetes,
Magnetoglobus multicellularis or
Methanosarcina. 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 (
Chloroplastida), once or twice for
, once for
brown algae, and perhaps several times for
fungi,
Mycetozoa, and
red algae.
Multicellularity may have evolved from colonies of interdependent organisms, from
cellularization, or from organisms in
Symbiosis.
The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.
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
Abiogenesis, 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
early Earth. 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.
RNA 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.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a
symbiosis of prokaryotic cells. DNA-bearing organelles like the
mitochondria and the
chloroplasts are descended from ancient symbiotic oxygen-breathing
proteobacteria and
cyanobacteria, respectively, which were endosymbiosed by an ancestral
prokaryote.
There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, 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 Vorticella from rain water, and Bacterium from his own mouth.
-
1665: Robert Hooke discovered cells in cork, then in living plant tissue using an early compound microscope. He coined the term cell (from Latin cella, meaning "small room"
[" … 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: Theodor Schwann 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: Rudolf Virchow stated that new cells come from pre-existing cells by cell division ( omnis cellula ex cellula).
-
1859: The belief that life forms can occur spontaneously ( Abiogenesis) was contradicted by Louis Pasteur (1822–1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
-
1931: Ernst Ruska 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 Francis Crick made their first announcement on the double helix structure of DNA.
-
1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.
See also
Notes
Further reading
External links
