The
cell is the basic structural and functional
unit of all known
living organisms. It is the smallest unit of life that is
classified as a living thing, and is often 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 most
bacteria, are
unicellular (consist 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 is
1
nanogram.) The largest known cell is
an unfertilized
ostrich egg
cell.
In 1835, before the final cell theory was developed,
Jan Evangelista Purkyně
observed small "granules" while looking at the plant tissue through
a microscope. 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, that all cells come from preexisting cells, that vital
functions of an organism occur within cells, and that 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, meaning, a small room. The descriptive term for
the smallest living biological structure was coined by
Robert Hooke in a book he published in 1665
when he compared the
cork cells he
saw through his microscope 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. . ." – Hooke
describing his observations on a thin slice of cork.
Robert Hooke
General principles
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.
All cells have several different abilities:
Some
prokaryotic cells contain important
internal membrane-bound compartments, but
eukaryotic cells have a specialized set of
internal membrane
compartments.
Anatomy of cells
There are two types of cells: eukaryotic and prokaryotic.
Prokaryotic cells are usually independent, while eukaryotic cells
are often found in multicellular organisms.
Prokaryotic cells
The
prokaryote cell is simpler, and
therefore smaller, than a eukaryote cell, lacking a
nucleus and most of the other
organelles of eukaryotes. There are two kinds of
prokaryotes:
bacteria and
archaea; these share a similar overall
structure.
A prokaryotic cell has three architectural regions:
- on the outside, flagella and pili project from the cell's surface. These are
structures (not present in all prokaryotes) made of proteins that
facilitate movement and communication between cells;
- enclosing the cell is the cell
envelope – generally consisting of a cell
wall covering a plasma membrane
though some bacteria also have a further covering layer called a
capsule. The envelope gives
rigidity to the cell and separates the interior of the cell from
its environment, serving as a protective filter. Though most
prokaryotes have a cell wall, there are exceptions such as
Mycoplasma (bacteria) and
Thermoplasma (archaea). 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 finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote
cells (plant cells and fungi cells) also have a cell wall;
- inside the cell is the cytoplasmic
region that contains the cell genome
(DNA) and ribosomes and various sorts of inclusions. A prokaryotic
chromosome is usually a circular molecule (an exception is that of
the bacterium Borrelia
burgdorferi, which causes Lyme disease). Though not
forming a nucleus, the DNA is condensed
in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called
plasmids, which are usually
circular. Plasmids enable additional functions, such as antibiotic resistance.
Eukaryotic cells
[[Image:Biological cell.svg|thumb|400px|Diagram of a typical
animal (
eukaryotic) cell, showing subcellular
components.
Organelles:
(1)
nucleolus
(2)
nucleus
(3)
ribosome
(4)
vesicle
(5)
rough endoplasmic
reticulum (ER)
(6)
Golgi apparatus
(7)
Cytoskeleton
(8)
smooth endoplasmic
reticulum
(9)
mitochondria
(10)
vacuole
(11)
cytoplasm
(12)
lysosome
(13)
centrioles within
centrosome]]
Eukaryotic cells are about 15 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 such as mitochondria also contain some DNA.
- Many eukaryotic cells are ciliated with
primary cilia. Primary cilia play important roles in
chemosensation, mechanosensation,
and thermosensation. Cilia may thus be "viewed as sensory cellular
antennae that coordinate a large
number of cellular signaling pathways, sometimes coupling the
signaling to ciliary motility or alternatively to cell division and
differentiation."
- Eukaryotes can move using motile 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 |
|
|
Subcellular components
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 cell membrane or
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 signaling 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.
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.
Transfer RNA (tRNA) molecules
are used to add specific amino acids during the process of protein
translation.
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 23 pairs of linear DNA molecules
called
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, it codes for 13
proteins involved in mitochondrial energy production as well as
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
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.
There are several types of organelles within an animal cell. Some
(such as the
nucleus and
golgi apparatus) are typically solitary,
while others (such as
mitochondria,
peroxisomes and
lysosomes) can be numerous (hundreds to
thousands). The
cytosol is the gelatinous
fluid that fills the cell and surrounds the organelles.
- 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. Mitochondria play a critical
role in generating energy in the eukaryotic cell. Mitochondria
generate the cell's energy by the process of oxidative phosphorylation,
utilizing oxygen to release energy stored in
cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by
splitting in two.
- Organelles that are modified chloroplasts are broadly called
plastids, and are involved in energy storage
through the process of photosynthesis, which utilizes solar energy
to generate carbohydrates and oxygen from carbon dioxide and
water.
- Mitochondria and chloroplasts each
contain their own genome, which is separate and distinct from the
nuclear genome of a cell. Both of these organelles contain this DNA
in circular plasmids, much like prokaryotic cells, strongly
supporting the evolutionary theory of endosymbiosis; since these organelles contain
their own genomes and have other similarities to prokaryotes, they
are thought to have developed through a symbiotic relationship
after being engulfed by a primitive cell.
- Ribosomes
- The ribosome is a large complex of
RNA and protein
molecules. They each consist of two subunits, and act as an
assembly line where mRNA 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).
- 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 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. The nucleolus is a specialized region within the
nucleus where ribosome subunits are assembled. In prokaryotes, DNA
processing takes place in the cytoplasm.
|
 Diagram of a cell nucleus
|
|
- Endoplasmic reticulum – eukaryotes only
- 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
|
- Golgi apparatus – eukaryotes only
- 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. It is particularly important in
the processing of proteins for secretion.
The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells.
Vesicle that enter the Golgi
apparatus are processed in a cis to trans direction, meaning they
coalesce on the cis side of the apparatus and after processing
pinch off on the opposite (trans) side to form a new vesicle in the
animal cell.
|
 Diagram of an endomembrane
system
|
|
- Lysosomes and Peroxisomes – eukaryotes only
- 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. The cell could not
house these 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. The vacuoles of eukaryotic cells are usually larger
in those of plants than animals.
|
Structures outside the cell wall
Capsule
A gelatinous capsule is present in some bacteria outside the cell
wall. The capsule may be
polysaccharide as in
pneumococci,
meningococci or
polypeptide as
bacillus anthracis or
hyaluronic acid as in
streptococci.Capsules not marked by ordinary
stain and can detected by
special stain. The capsule
is
antigenic. The capsule has
antiphagocytic function so it determines the
virulence of many bacteria. It also plays a role in attachment of
the organism to mucous membranes.
Flagella
Flagella are the organelles of cellular
mobility. They arise from cytoplasm and extrude through the cell
wall. They are long and thick thread-like appendages, protein in
nature. Are most commonly found in bacteria cells but are found in
animal cells as well.
Fimbriae (pili)
They are short and thin hair like filaments, formed of protein
called pilin (antigenic).
Fimbriae are
responsible for attachment of bacteria to specific receptors of
human cell (adherence). There are special types of pili called (sex
pili) involved in the process of conjunction.
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.
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.
Cell movement or motility
Cells can move during many processes: such as wound healing, the
immune response and
cancer
metastasis. For wound healing to occur, white blood cells and
cells that ingest bacteria move to the wound site to kill the
microorganisms that cause infection.
At the same time fibroblasts (connective tissue cells) move there
to remodel damaged structures. In the case of tumor development,
cells from a primary tumor move away and spread to other parts of
the body. Cell motility involves many receptors, crosslinking,
bundling, binding, adhesion, motor and other proteins. The process
is divided into three steps – protrusion of the leading edge of the
cell, adhesion of the leading edge and de-adhesion at the cell body
and rear, and cytoskeletal contraction to pull the cell forward.
Each of these steps is driven by physical forces generated by
unique segments of the cytoskeleton.
Evolution
The origin of cells has to do with the origin of life, which began
the
history of life on
Earth.
Origin of the first cell
There are three leading hypotheses for the source of small
molecules that would make up life in an early Earth.
One is that they came
from meteorites (see Murchison meteorite
). Another is that they were created at
deep-sea vents. A third is that
they were synthesized by lightning in a reducing atmosphere
(
see Miller–Urey
experiment); although it is not sure Earth had such an
atmosphere. There is essentially no experimental data to tell what
the first self-replicate forms were.
RNA is
generally assumed to be the earliest self-replicating molecule, as
it is capable of both storing genetic information and catalyze
chemical reactions (
see RNA
world hypothesis). But some other entity with the
potential to self-replicate could have preceded RNA, like
clay or
peptide nucleic
acid.
Cells emerged at least 3.0–3.3 billion years ago. The current
belief is that these cells were
heterotrophs. An important characteristic of
cells is the
cell membrane, composed
of a bilayer of
lipids. The early cell
membranes were probably more simple and permeable than modern ones,
with only a single fatty acid chain per lipid. Lipids are known to
spontaneously form bilayered
vesicle in water, and could have preceded
RNA. But the first cell membranes could also have been produced by
catalytic RNA, or even have required structural proteins before
they could form.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a
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
proteobacteria 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.
Sex, as the stereotyped choreography of meiosis and syngamy that
persists in nearly all extant eukaryotes, may have played a role in
the transition from prokaryotes to eukaryotes. An 'origin of sex as
vaccination' theory suggests that the eukaryote genome accreted
from prokaryan parasite genomes in numerous rounds of lateral gene
transfer. Sex-as-syngamy (fusion sex) arose when infected hosts
began swapping nuclearized genomes containing co-evolved,
vertically transmitted symbionts that conveyed protection against
horizontal infection by more virulent symbionts.
History
See also
References
- The Universal Features of Cells on Earth in
Chapter 1 of the Alberts textbook (reference #1, above).
- Alberts B, Johnson A, Lewis J. et al. Molecular Biology of the
Cell, 4e. Garland Science. 2002
- Ananthakrishnan R, Ehrlicher A. The Forces Behind Cell
Movement. Int J Biol Sci 2007; 3:303–317.
http://www.biolsci.org/v03p0303.htm
External links
Textbooks
. By 1935, he has built an EM with twice the
resolution of a light microscope, revealing previously-unresolvable
organelles.
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