Cell (biology)
The
cell is the structural and functional unit of all
living organisms, and is sometimes called the "building block of life."
[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 embryos. It is also common to describe small molecules such as amino acids as "molecular building blocks". Some organisms, such as
bacteria, are
unicellular, consisting of a single cell. Other organisms, such as
humans, are
multicellular, (humans have an estimated 100 trillion or 10
14 cells; a typical cell size is 10 µm, a typical cell mass 1 nanogram). The largest known cell is an
ostrich egg.
The
cell theory, first developed in 1839 by
Schleiden and
Schwann, states that all
organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, and cells contain the
hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
The word
cell comes from the
Latin cellulae, a small room. The name was chosen by
Robert Hooke when he compared the
cork cells he saw to the small rooms monks lived in.
["... 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. . ." â€" Hooke describing his observations on a thin slice of cork. Robert Hooke]Properties of cells
Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.
|
Mouse cells grown in a culture dish. These cells grow in large clumps, but each individual cell is about 10 micrometres across. |
All cells share several abilities
[The Universal Features of Cells on Earth in Chapter 1 of the Alberts textbook (reference #1, above).]:
*Reproduction by
cell division (
binary fission,
mitosis or
meiosis).
*Use of
enzymes and other
proteins
coded for by
DNA genes and made via
messenger RNA intermediates and
ribosomes.
*
Metabolism, including taking in raw materials, building cell components, converting
energy,
molecules and releasing
by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from
metabolic pathways.
*Response to external and internal
stimuli such as changes in temperature,
pH or nutrient levels.
*Cell contents are contained within a
cell surface membrane that contains proteins and a
lipid bilayer.
Some prokaryotic cells contain important internal membrane-bound compartments, but eukaryotic cells have a highly specialized
endomembrane system characterized by regulated
traffic and
transport of
vesicles[A. Rose, S. J. Schraegle, E. A. Stahlberg and I. Meier (2005) "Coiled-coil protein composition of 22 proteomes--differences and common themes in subcellular infrastructure and traffic control" in BMC evolutionary biology Vulume 5 article 66. ]
Rose et al. suggest that coiled-coil alpha helical vesicle transport proteins are only found in eukaryotic organisms. |
The cells of eukaryotes (left) and prokaryotes (right). |
All cells, whether
prokaryotic or
eukaryotic, have a
membrane, which envelopes the cell, separates its interior from its environment, regulates what moves in and out, and maintains the
electric potential of the cell. Inside the membrane, a
salty
cytoplasm takes up most of the cell volume. 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 will list these primary components of the cell, then briefly describe their function.
Cell membrane: A cell's defining boundary
Main article:
Cell membraneThe cytoplasm of a cell is surrounded by a
plasma membrane. The plasma membrane in plants and prokaryotes 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
double layer of lipids (
hydrophobic fat-like molecules) and
hydrophilic phosphorous molecules. Hence the layer is called a
phospholipid bilayer. Embedded within this membrane is a variety of
protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-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 signalling molecules such as
hormones.
Cytoskeleton: A cell's scaffold
Main article:
CytoskeletonThe 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. Eukaryotic cytoskeleton is composed of
microfilaments,
intermediate filaments and
microtubules. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.
Genetic material
Two different kinds of genetic material exist: (DNA) and (RNA). Most organisms use DNA for their long-term information storage, but
some viruses (e.g.,
retroviruses) have RNA as their genetic material. The biological information contained in an organism is
encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g.,
mRNA) and
enzymatic functions (e.g.,
ribosomal RNA) in organisms that use
DNA for the genetic code itself.
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial
chromosome) in the
nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called
chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like
mitochondria and
chloroplasts (see
endosymbiotic theory).
A human cell has genetic material in the 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. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins.
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.
Organelles
Main article:
OrganelleThe human body contains many different
organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called
organelles, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are found only in eukaryotes.
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 occur. The nucleus is spheroid in shape 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 mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm. | | ; Ribosomes (the protein production machine) : Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes â€" sometimes hundreds or even thousands â€" can be found throughout a cell. |
Mitochondria and Chloroplasts (the power generators) : Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell, they play a critical role in generating energy in the eukaryotic cell, a process involving a number of complex metabolic pathways. Chloroplasts are larger than mitochondria, and convert solar energy into a chemical energy ("food") via photosynthesis. Like mitochondria, chloroplasts have their own genome. Chloroplasts are found only in photosynthetic eukaryotes, like plants and algae. There is a number of plant organelles that are modified chloroplasts; they are broadly called plastids, and are often involved in storage.; Endoplasmic reticulum and Golgi apparatus (macromolecule managers) : The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface, and the smooth ER, which lacks them. Translation of the mRNA for those proteins that will either stay in the ER or be exported from the cell occurs at the ribosomes attached to the rough ER. The smooth ER is important in lipid synthesis, detoxification and as a calcium reservoir. The Golgi apparatus, sometimes called a Golgi body or Golgi complex is the central delivery system for the cell and is a site for protein processing, packaging, and transport. Both organelles consist largely of heavily-folded membranes. | Lysosomes and Peroxisomes (the cellular digestive system) : Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally-occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.| ; 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 mitotic appratus. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells. | | Vacuoles : Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba have contractile vacuoles, which are able to pump water out of the cell if there is too much water. | | |
There are two types of cells, eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multi-cellular organisms.
Prokaryotic cells
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called
flagella and
pili â€" proteins attached to the cell surface; a
cell envelope consisting of a capsule, a
cell wall, and a
plasma membrane; and a
cytoplasmic region that contains the
cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
*The
plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
*Most prokaryotes have a
cell wall (some exceptions are
Mycoplasma (a bacterium) and
Thermoplasma (an archaeon)). It consists of
peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" (
cytolysis) from
osmotic pressure against a
hypotonic environment. A cell wall is also present in some eukaryotes like
fungi, but has a different chemical composition.
*A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium
Borrelia burgdorferi, which causes
Lyme disease). Even without a real
nucleus, the DNA is condensed in a
nucleoid. Prokaryotes can carry extrachromosomal DNA elements called
plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.
Eukaryotic cells
Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a
cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true 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
chromosomes, which are associated with
histone proteins. All chromosomal DNA is stored in the
cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic
organelles also contain some DNA.
*Eukaryotes can move using
cilia or
flagella. The flagella are more complex than those of prokaryotes.
Cell growth and metabolism
Main articles: Cell growth,
Cell metabolismBetween 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, wherein 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 a less chemically-complex sugar molecule called
glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (
ATP), a form of energy, via two different pathways.
The first pathway,
glycolysis, requires no oxygen and is referred to as
anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.The second pathway, called the Krebs cycle, or
citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
 |
An overview of protein synthesis.
Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly-synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active. |
Creation of new cells
Main article:
Cell divisionCell 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.
Prokaryotic cells divide by
binary fission.
Eukaryotic 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
gametes in multicellular organisms, fusing to form new diploid cells.
DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.
Protein synthesis
Main article:
Protein biosynthesisCells 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 complimentary RNA strand. This RNA stand 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 3D protein molecule.
Main article:
Origin of lifeThe origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.
Origin of the first cell
If life is viewed from the point of view of
replicators, that is
DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile
DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of
biological complexity. If freely-floating DNA molecules that code for
enzymes are not enclosed into cells, the enzymes that benefit a given DNA molecule (for example, by producing nucleotides) will automatically benefit the neighbouring DNA molecules. This might be viewed as "
parasitism by default." Therefore the
selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: All molecules in a given neighbourhood are almost equally advantaged.
If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it.(Note. This is more a metaphor given for simplicity than complete accuracy since the earliest molecules of life, probably up to the stage of cellular life, were most likely
RNA molecules that acted as both replicators and enzymes: see
RNA world hypothesis. However, the core of the reasoning is the same.)
Biochemically, cell-like spheroids formed by
proteinoids are observed by heating
amino acids with
phosphoric acid as a catalyst. They bear much of the basic features provided by
cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth.
Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through
evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The
Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in ones mind as a
hydrophilic head on one end, and a
hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is being able to link together to form a
bilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble [
1] can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the
natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first
prokaryotes,
eukaryotes, and multi-cellular organisms could be achieved. This theory is expanded upon in the book,
The Cell: Evolution of the First Organism by
Joseph Panno Ph.D.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a
symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the
mitochondria and the
chloroplasts are what remains of ancient symbiotic oxygen-breathing
bacteria and
cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral
archaean prokaryote cell – a theory termed the
endosymbiotic theory.
There is still considerable debate on if organelles like the
hydrogenosome predated the origin of
mitochondria, or viceversa: see the
hydrogen hypothesis for the origin of eukaryotic cells.
*1632â€"1723:
Antony van Leeuwenhoek teaches himself to grind
lenses, builds a
microscope and draws
protozoa, such as
Vorticella from rain water, and
bacteria from his own mouth.
*1665:
Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.
*1839:
Theodor Schwann and
Matthias Jakob Schleiden elucidate 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.
*The belief that life forms are able to occur spontaneously (
generatio spontanea) is contradicted by
Louis Pasteur (1822â€"1895) (although
Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
*
Rudolph Virchow states that cells always emerge from
cell divisions (
omnis cellula ex cellula).
*1931:
Ernst Ruska builds first
transmission electron microscope (TEM) at the
University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously-unresolvable organelles.
*1953:
Watson and
Crick made their first announcement on the double-
helix structure for DNA on February 28.
*1981:
Lynn Margulis published
Symbiosis in Cell Evolution detailing the
endosymbiotic theory.
*
A549 cell*
Cariology is the study of the
cell nucleus.
*
Cell culture*
Cell types
*
Cellular component*
Cellular memory*
Cytorrhysis*
Cytotoxicity*
Life cycle of a cell*
Plant cell*
Plasmolysis*
Stem cell*
Syncytium*
The cell like a city.
*
Cells Alive!*
Journal of Cell Biology*
A simplified version of this article*
A comparison of the generational and exponential growth of cell populations*
High-resolution images of brain cells*
The Biology Project > Cell Biology*
Cell Biology for school and university with graphicsOnline textbooks
Molecular Biology of the Cell fourth edition, edited by Bruce Alberts (2002) published by Garland Science.
Molecular Cell Biology fourth edition, edited by Harvey Lodish (2000) published by W. H. Freeman and Company.
The Cell - A Molecular Approach second edition, by Geoffrey M. Cooper (2000) published by Sinauer Associates.
