Cell
The cell is the structural and functional unit of
all
living
organisms, and is sometimes called the "building block
of life."[1]
Some organisms, such as
bacteria, are
unicellular (consist of a single cell). Other organisms,
such as
humans, are
multicellular. (Humans have an estimated 100 trillion or
1014 cells; a typical cell size is 10 µm;
a typical cell mass is 1 nanogram.)
The largest known cell is an
ostrich
egg.
Drawing of the structure of
cork as it appeared under the microscope to
Robert Hooke from
Micrographia which is the origin of the word
" cell".
The
cell theory, first developed in 1839 by
Matthias Jakob Schleiden and
Theodor Schwann, states that all
organisms are composed of one or more cells. All cells
come from preexisting cells. Vital functions of an organism
occur within cells, and all 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 cellula, 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.[2]
Overview
Properties of cells
Each cell is at least some what 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:[3]
Some
prokaryotic cells contain important internal
membrane-bound compartments,[4]
but
eukaryotic cells have a highly specialized
endomembrane system characterized by regulated
traffic and
transport of
vesicles.[5]
Anatomy of cells
There are two types of cells: eukaryotic and prokaryotic.
Prokaryotic cells are usually singletons, while eukaryotic
cells are usually found in multicellular 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 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 — 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 plants (cellulose) and
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.
Table 2: Comparison of structures between
animal and plant cells
|
Typical animal cell |
Typical plant cell |
| Organelles |
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| Additional structures |
|
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Subcellular components
The cells of eukaryotes (left) and prokaryotes
(right).
All cells, whether
prokaryotic or
eukaryotic, have a
membrane that envelops the cell, separates its interior
from its environment, regulates what moves in and out
(selectively permeable), 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
-
The 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
phosphorus molecules. Hence, the layer is called a
phospholipid bilayer. It may also be called a fluid
mosaic membrane. 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
-
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
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. The prokaryotic
cytoskeleton is less well-studied but is involved in the
maintenance of cell shape, polarity and cytokinesis.[6]
Genetic material
Two different kinds of genetic material exist:
deoxyribonucleic acid (DNA) and
ribonucleic acid (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
The 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.
|
 |
- 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, 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.
|
 |
- The ER contains many Ribosomes (the protein
production machine)
- The
ribosome is a large complex composed of many
molecules, in prokaryotes only exist floating
freely in the cytosol, whereas in eukaryotes
they can be either free or bound to membranes.
|
- Lysosomes and Peroxisomes (of the eukaryotic
cell)
- The cell could not house such destructive
enzymes if they were not contained in a
membrane-bound system. These organelles are
often called a "suicide bag" because of their
ability to detonate and destroy the cell.
|
- 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
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.
Cell functions
Cell 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
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
-
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.
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
-
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
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.
Origins of cells
-
The 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 one's mind as a
hydrophilic head on one end, and a
hydrophobic tail on the other. Phospholipids
also possess an important characteristic, that is
having the function 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 about whether
organelles like the
hydrogenosome predated the origin of
mitochondria, or viceversa: see the
hydrogen hypothesis for the origin of eukaryotic
cells.
History
See also
References
-
^
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".
- ^
a
b
"... 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.
Robert Hooke
-
^
The Universal Features of Cells on Earth in
Chapter 1 of the Alberts textbook (reference #1,
above).
- ^
L.M., Mashburn-Warren;
Whiteley, M. (2006). "Special
delivery: vesicle trafficking in prokaryotes.".
Mol Microbiol 61 (4): 839-46.
-
^ 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.
Entrez PubMed
16288662
Rose et al. suggest that
coiled-coil alpha helical vesicle transport
proteins are only found in eukaryotic organisms.
- ^
Michie K, Löwe
J (2006). "Dynamic filaments of the bacterial
cytoskeleton". Annu Rev Biochem 75:
467-92.
PMID 16756499.
External links
Online 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.
-
Landmark Papers in Cell Biology Gall JG,
McIntosh JR, eds. Landmark Papers in Cell
Biology. Bethesda, MD and Cold Spring Harbor,
NY: The American Society for Cell Biology and
Cold Spring Harbor Laboratory Press; 2001.
Commentaries and links to original research
papers published in the ASCB Image & Video
Library Landmark Papers Collection in September
2006.
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The content of this section is licensed under the
GNU Free Documentation License
(local copy). It
uses material from the
Wikipedia article "Cell"
modified April 4, 2007 with previous authors
listed in its
history.
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