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Cell Structure
Introductory article
Article Contents
Nancy J Lane, University of Cambridge, Cambridge, UK
. Prokaryotic and Eukaryotic Cells
Cells are living entities made up of a central nuclear region which contains the hereditary
material, surrounded on all sides by cytoplasm, which, encompassed by a delimiting
membrane, contains all the structures required for biological processes, such as making
protein and extracting utilizable energy from food. These events may occur in separate
compartments, the organelles, which include the endoplasmic reticulum, the Golgi
apparatus, the mitochondria, the chloroplasts and the lysosomes; there is also an internal
cytoskeleton.
. The Nucleus and Genetic Information
. Cell Division: Mitosis and Meiosis
. Cell Cycles
. The Cell Surface and Plasma Membrane
. Internal Membranes and Subcellular Compartments:
Organelles
. Cytoskeleton
. Multicellularity
. Intercellular Junctions: Communication and Adhesion
Prokaryotic and Eukaryotic Cells
Cells are the subunits of all living systems, both plant and
animal, and are of two major types: prokaryotic and
eukaryotic. Prokaryotic cells are relatively small (1–5 mm
in diameter) and simple, and are those that make up singlecelled microorganisms or bacteria. They are so-called
because each of them lacks a definitive nucleus (or karyon),
having only a central area in the cell cytosol where the
genetic material, the deoxyribonucleic acid (DNA), is
found. This lies naked, without associated protein, usually
in a circular configuration. As these cells possess no distinct
membrane-bound nucleus, they are prokaryotic. Eukaryotic (with a karyon) cells, in contrast, are the constituents
of all multicellular organisms. They possess a distinct
nucleus, bounded by a double nuclear envelope, wherein
lies the genetic material in the form of the DNA-containing
chromosomes. This DNA is in the form of linear strings of
genes, and is combined with histone protein to make up
chromatin. Such cells are much larger than prokaryotic
cells, ranging in diameter from 10 mm up to many
millimetres (of which egg cells are the largest), although
averaging about 30–60 mm in diameter. The reason for this
disparity in size is in large part due to the fact that
prokaryotic cells lack organelles, or internal compartmentalization. Eukaryotic cells, on the other hand, have
extensive subcellular compartments in the form of different
kinds of ‘organelles’ (Figure 1).
The prokaryotic cell type is sufficiently small for simple
diffusion alone to allow for the efficient transport and
exchange of materials within its substance; no compartments are required. The cells contain a cytosol, or ground
cytoplasm, which contains all the enzymes (catalysts) and
building blocks (metabolites) to synthesize cell components; prokaryotic cells are bounded by a plasma (cell)
membrane, and then sometimes also by a second, outer cell
membrane. They also possess an outer cell wall, which
provides support and strength to maintain the integrity of
the cell during environmental stress.
. Differentiation
. Communication
The Nucleus and Genetic Information
Eukaryotic nuclei contain the genetic material, DNA, in
the form of discrete chromosomes; these are present in
variable numbers, depending on the species. All cells in a
given individual contain the same amount of DNA, save
for the germline cells. The DNA is associated with histone
protein in the form of nucleosomes, which look like beads
on a string in the unfolded state between cell divisions.
Prokaryotic cell
Recycling membrane
Mitochondrion
Nucleolus
Nucleus
Lysosome
Exocytosis
Ribosomes
Endoplasmic
reticulum
Secretory
granule
Golgi
Figure 1 Cross-sectional diagram of a generalized eukaryotic cell,
revealing the central DNA-containing nucleus with peripheral cytoplasm,
in which can be found many organelles, such as mitochondria, lysosomes,
cisternae of rough endoplasmic reticulum, saccules of the Golgi apparatus,
secretory granules being released by exocytosis, and endosomes forming
by endocytosis. The mitochondria are thought to have arisen by the
endocytotic uptake of prokaryotic cells.
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Cell Structure
These coil and fold up, or are ‘condensed’, into sausageshaped structures in dividing eukaryotic cells that can be
seen with the light microscope. This is not the case in
prokaryotic cells where the DNA remains ‘naked’ and does
not condense into visible structures during cell division.
Cell Division: Mitosis and Meiosis
Mitosis
Cell turnover and programmed cell death (apoptosis)
occur as normal events in biological systems. Cell
replacement occurs by cell division.
All living cells have the capacity to divide into two
identical daughter cells, by a process called binary fission (a
mitotic-like process) (in prokaryotic cells) or mitosis (in
eukaryotic cells). In mitosis, nuclear chromosomes replicate into two genetically identical sister chromatids which
initially remain together. The act of replication occurs
during ‘S’ (synthesis) phase which punctuates interphase,
the period between successive acts of mitosis. Replication
of chromosomes is actually replication of the DNA double
helix which occurs at a ‘replication fork’, involving a host
of different enzymes. After replication in the ‘S’ phase,
there is a gap (‘G’) phase before the cell enters prophase,
the first stage of mitosis. The sister chromatids then move
on to the metaphase plate, of the so-called ‘spindle’ of
microtubules, fully divide and separate into two distinct
chromosomes, which are moved in opposite directions to
two poles during anaphase and telophase, the last stages of
mitosis. In animal cells, after division of the cytoplasm into
two, by pinching together (cytokinesis), two identical
daughter cells result, and each reverts back into the
interphase, or ‘resting’, state. In plant cells, a cell
membrane is laid down by a ‘phragmoplast’, which
gradually extends out to the cell edges, after which a cell
wall of cellulose is formed.
Meiosis
Eukaryotic germline cells, contained in the sexual organs
of animals and plants, the testis (or anther) and ovary (or
ovule), undergo reciprocal genetic exchange, or gene
recombination, during a process called meiosis, in which
chromosomal crossing-over occurs. This takes place when
diploid germ cells (with two sets of chromosomes) are being
transformed into haploid cells (with just one set of
chromosomes), and is a way of ensuring genetic variability
in the germ cells (and hence in the next generation). Most
organisms, plant and animal, are diploid. Meiotic division
takes place when the germ cells (eggs and sperm or ovary
and pollen) are being produced. Every germ cell must
become haploid by undergoing a ‘reduction’ division, so
that when the haploid (x1n (where n 5 number of sets of
2
chromosomes)) sperm cell fertilizes the haploid (x1n) egg, a
new, diploid (x2n) individual again results. This enables
parental germ cell fusion (haploid egg fertilized by a
haploid sperm) to form a diploid zygote, which gives rise to
a genetically unique diploid individual, different from both
parents (but with a genetic contribution from each), which
is the offspring, or filial (F1) generation. Genetic exchange
also occurs in prokaryotic cells, but it is a horizontal, not
vertical, recombination, and only parts of the genome (the
total DNA of any organism) are exchanged. This exchange
is triggered by a vector (carrier) which may be a plasmid, an
extra bit of DNA in the form of an F1 factor, or even part of
a virus which has infected the cell. Hence ‘genetic
engineering’ results from genes, attached to vectors, being
integrated into a different, recipient cell, and subsequently
expressing the proteins for which the integrated genes code,
which would not normally be synthesized in the recipient
cell.
Cell Cycles
The differences in overall nuclear arrangements in prokaryotic and eukaryotic cells mean that there are significant
differences in the cell cycle of the two kinds of cells – that is,
in the timing and the way in which their chromosomal
DNA is replicated and two new cells form from the
original. The cell cycle is much more protracted in
eukaryotic than prokaryotic cells. The period between cell
divisions is called interphase, and, far from being an actual
resting stage, it is then that the synthetic activity of the cell
occurs, i.e. the making of ribonucleic acid (RNA) and
protein, as well as the replication of the DNA.
In interphase the nuclear DNA makes messenger RNA
(mRNA) molecules, which leave the nucleus and go to the
cytoplasm, to tell the ribosomes which proteins to
synthesize. These are, therefore, informational molecules
and the particular mRNA made will depend on the cell
type, which will in turn depend on the tissue in question.
Not all genes (a gene 5 the DNA producing the message to
make a particular protein) will be active at all times.
Different genes are ‘turned on’ at different stages in
development. There are also distinctions in the modifications of mRNA made in eukaryotic compared with
prokaryotic cells. The way ‘gene action’ or the expression
of genes (the making of mRNA from DNA and then
making protein from the mRNA) is controlled, and the
recognition systems for targeting of molecules between and
within cells to activate such expression, also differ in the
two cell types. Different, too, is the structure of the
cytoplasmic ribosomes, or the tiny RNP particles
(RNP 5 RNA plus protein) in the two kinds of cell.
Ribosomes are distinguishable by cell biologists by their
sedimentation characteristics in sucrose density gradients.
They are, as a result, called 70S (prokaryotic) or 80S
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Cell Structure
(eukaryotic) ribosomes, reflecting the number of Svedberg
(S) units they exhibit. These are the protein-making
machinery of all cells and have roughly the same function
in both cell types, in spite of their differing ‘S’ values.
The Cell Surface and Plasma Membrane
The cell surface and the cell (plasma) membrane also differ
between eukaryotic and prokaryotic cells. As mentioned
earlier, the cell surface of some prokaryotic cells is covered
with a double membrane, as well as an external cell wall of
crosslinked polysaccharides. A mucous ‘coat’ often also
lies around this outer wall. Of eukaryotic cells, those of
plants also have an external cell wall, which is primarily
composed of cellulose fibrils to give strength and rigidity.
Cells of animals lack a cell wall; instead, they possess a
glycocalyx of carbohydrate-rich transmembrane molecules that straddle the outer plasma membrane and enable
adjacent cells to stick to one another, albeit at a set
distance, 15–20 nm. This spacing permits the recognition,
by circulating chemical signals, of membrane-associated
cellular receptor or adhesion molecules.
The plasma membrane is a lipid bilayer in which are
inserted a number of proteins. These are arranged in a
variety of patterns that are related to their function, and
have the capacity to move translaterally like ‘icebergs’ in
the bilayer sea of lipid. The membrane is therefore said to
have a ‘fluid’ character. It is not possible to resolve the
details of the molecular structure of a membrane even with
the high powered electron microscope, which can achieve a
resolution of 1–2 nm. As molecular subunits are too small
to be resolved as separate entities, models of the plasma
membrane have been designed over the years to try to
explain its molecular structure. The first model was devised
by Davson and Danielli in the early part of the twentieth
century and was called the Davson–Danielli lipid bilayer
model. With the advent of the electron microscope, J.
David Robertson, around 1960, looked at the ultrastructure of membranes and proposed a new model which he
called the ‘unit’ membrane model; this suggested that all
membranes shared a single, unifying structure. After the
development of a technique called freeze-fracturing, it was
possible to observe the three-dimensional arrangements of
putative protein particles in the bilayer ‘sea of lipid’. These
particles were called intramembranous particles. They are
thought to be composed mostly of protein and can be
arranged in a variety of patterns, linear or patch-like. Since
the word ‘mosaic’ reflects the fact that various microdomains of membrane, even from different parts of the
same eukaryotic cell, may have different patterns, as is
found in the mosaic floors of ancient Roman villas, the
term ‘fluid-mosaic’ model was put forward for membranes
by Singer and Nicolson in 1982. This is now generally
accepted as the most reasonable of the models.
The bilayer structure of the membrane permits the
lateral motion of lipids and proteins, but not normally flipflop movement, i.e. shifting from inside to outside or vice
versa; this can occur only rarely, in the case of lipids with
the aid of ‘flippase’ enzymes. The membrane therefore is
asymmetrical, with certain proteins facing the extracellular
space or the outside environment, and others the internal
cytoplasm. These tend to have special properties relating to
the function they have, say, of recognition or anchoring,
and may be peripheral or extrinsic, as distinct from integral
or intrinsic, to the lipid bilayer. The anchors observed at
the cytoplasmic surface tend to be the cytoskeletal
elements, the microfilaments, intermediate filaments or
the microtubules, as well as G proteins and other
components of the second messenger signalling system.
The external face has projecting carbohydrate moieties,
which produce the ‘glycocalyx’ of the plasma membrane.
Here, too, reside receptor molecules that can be recognized
by circulating hormones, antibodies or other molecules.
‘Channels’ are also found straddling the plasma
membrane. These may allow the inward or outward
passage of ions such as Na 1 , K 1 , Cl 2 or Ca2 1 , and
leakage or pumping of ions across the membrane may
establish transmembrane resting potentials. These may be
temporarily destroyed when action potentials occur in
‘excitable’ membranes, such as those of nerve or muscle
cells, with the transmission of a nervous impulse.
Internal Membranes and Subcellular
Compartments: Organelles
The plasma membrane of all cells surrounds the cytosol or
cytoplasm (cyto 5 cell) and separates each cell from the
external environment. The internal compartmentalization
of eukaryotic cells is also executed by membranes, which
have the same basic bilayer structure as the plasma
membrane. These divide up the large volume of cell
cytoplasm into separate compartments, in which are
concentrated specific substrates and enzymes for particular
cellular activities. In this way, different cellular functions
can occur at special sites with enhanced efficiency. These
compartments permit the vital functions of the cell to occur
at specialized settings, and so are like the organs in the body
of organisms. Since the cytoplasm surrounding the nucleus
in the cell is called the cell body, these diminutive
compartments have been termed ‘organelles’ (Figure 1).
Endoplasmic reticulum
One of the most striking cellular organelles is the
endoplasmic reticulum (ER), composed of numerous
flattened membranous cisternae lying throughout the
cytoplasm; this is in continuity with the double nuclear
envelope. It is often studded with ribosomes; if so, it is
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Cell Structure
termed rough endoplasmic reticulum (RER); the RER
makes proteins for export. If ribosomes are absent from the
cisternae, it is called smooth endoplasmic reticulum (SER);
SER is involved in lipid or steroid synthesis. The ribosomes
synthesize the proteins required for cell structure and
function by using information coding for the protein’s
structure from the linear DNA-like mRNA molecules.
This mRNA, which emerges from the nucleus via nuclear
pores, will not only be different for every protein but will be
peculiar to each individual, because it reflects the unique
genetic make-up, or DNA structure, of that individual.
The process of making mRNA from DNA is called
transcription and occurs in the nucleus. Transfer RNA
(tRNA) is also required during the process of protein
synthesis, to bring the subunits of proteins, amino acids,
into a linear array that is coded for by the mRNA. The
ribosomal RNA (rRNA) is contained within the ribosomes, which act as the factory or sites in which the
requisite components assemble together to synthesize the
linear protein molecule. This process is termed translation
and takes place in the cytoplasm. In eukaryotic cells this
new protein molecule subsequently folds up within the
cisternae of the RER, into which it has been inserted during
the process of its synthesis by the ribosomes, to form the
mature, functional form of the protein, destined for export.
The protein enters the ER cisternae by a process commonly
referred to as the ‘signal hypothesis’, whereby a terminal
signal sequence of the protein being synthesized tells the
cell that the protein should be inserted, via a pore through
the RER membrane, into the ER cisternal space there. The
linear protein finally folds up, inside the ER cisternae, into
a three-dimensional functional form that is maintained,
mainly by weak secondary forces, in the most thermodynamically stable configuration. Proteins can also be
synthesized for use by the cell, without entering the ER
cisternae at all. This is always the case in prokaryotic cells,
which lack ER.
Golgi apparatus
The proteins synthesized in the ER are transported via
membranous vesicles, to the so-called Golgi apparatus.
This is a series of stacked saccules which often appear as
‘dictyosomes’, or scale-like bodies, under the light microscope. The structure was first described as a network or
reticulum in 1898 under the light microscope, by the Italian
cytologist, Camillo Golgi, in nerve cells. Subsequent
observers referred to these entities as Golgi’s bodies, Golgi
complexes or the Golgi apparatus, by which terms it has
been variously known ever since. Only with the advent of
the electron microscope could its structure be definitely
established, and it then became clear that Golgi bodies
were stacks of flattened saccules with associated vesicles
and granules. They are like stacks of saucers with two
distinct faces, a ‘forming’ and a ‘maturing’ face. These
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saccules receive the proteins from the RER at the forming
face and then modify them by adding other components,
such as carbohydrate moieties, and condensing them into
mature dense granules. These become the secretory
granules, which are pinched off, in a budding process,
from the ends of the maturing face of the saccule stacks.
These secretory products then move, probably along
microtubules, to the surface of the cell, the plasma
membrane, where they fuse their membrane with that of
the surface, to undergo exocytosis. At this time the
secretory product is released to the outside, and the
secretory granular membrane becomes incorporated into
that of the plasma membrane (see Figure 1).
Endosomes
The reverse of this process, endocytosis, takes place when
materials are taken into the cell. Exogenous materials
become surrounded by an area of plasma membrane,
which phagocytoses them by taking them into the
cytoplasm by first forming a pit round them, and then
budding it off internally. This membrane often includes a
protein called clathrin, which makes the initial pit appear
‘coated’. This ‘coat’ remains with the internalized membrane, now called the endosome, for a period of time before
being recycled back to the plasma membrane.
Lysosomes
The endosomes are moved to the Golgi area, where their
contents may be utilized or degraded by lysosomes. These
are organelles synthesized by the maturing Golgi saccules,
in an area sometimes referred to as the GERL. This region
has a complex of enzymes peculiar to its activities,
including the lytic enzymes which become incorporated
into the forming, primary lysosomes. Once a primary
lysosome has fused with an endosome and degraded its
contents as far as is possible, it becomes a secondary
lysosome. Lysosomes have been called ‘suicide bags’
because their enzymatic contents, contained within the
organelle membrane, are capable of degrading proteins,
lipids, carbohydrates and nucleic acids. However, if, as
sometimes happens, its enzymes are incapable of digesting
all the endosomal contents, the lysosome is then called a
residual body, as it now contains the indigestible residues
of the ingested endosome. The molecules produced by the
catabolic breakdown of the endosomal contents move out,
through the lysosomes’ peripheral semipermeable membrane, into the ground cytoplasm to be reutilized. These
secondary lysosomes tend to be found in the Golgi area of
the cells, where they accumulate over time because they are
incapable of themselves being extruded from the cell.
Ageing cells, therefore, tend to have greater numbers of
residual bodies. Collections of small vesicles enclosed
within a membrane, termed multivesicular bodies, also
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Cell Structure
tend to congregate in the Golgi and lysosomal area; these
may be a form of endosome.
Peroxisomes
Peroxisomes, another kind of organelle, may be found,
especially in liver and kidney; these appear rather similar to
lysosomes but they contain a special spectrum of enzymes –
d-amino acid oxidase, catalase and peroxidase. Like
lysosomes, they are surrounded by a semipermeable
membrane, but they possess a characteristic crystalline
internum which is not to be found in lysosomes.
reaction, occurs in the stroma or internal space of the
chloroplast. This is the pathway by which sugars are made
from carbon dioxide and water. Like the mitochondria,
these organelles also possess circular DNA and 70S
ribosomes in their stromal space, and are thought to have
evolved from symbiotic photosynthetic prokaryotes. The
‘light’ reaction occurs in the thylakoid membranes,
whereby photosystems I and II of the chlorophyll reactive
centres capture the energy of sunlight and harness it to the
production of reduced coenzymes and a proton gradient.
Protons flowing down the gradient lead to the production
of the energy-rich ATP via CF0 (a proton channel) and CF1
(ATP synthase) in the chloroplast thylakoid membrane.
Mitochondria
Of comparable size, or perhaps a bit larger (about 2–3 mm)
than the lysosomes, are the mitochondria. These are the
organelles concerned with respiration. They possess both
the components of the Krebs (or tricarboxylic acid) cycle
and the proton pumps and ATP synthase, the enzyme that
makes adenosine triphosphate (ATP), the source of
cellular energy. The Krebs cycle in the mitochondrial
matrix further breaks down the glucose, which has been
initially partly degraded by the cytoplasmic glycolytic
pathway, into smaller molecules. These drive the reduction
of certain coenzymes which feed into an electron transport
chain that forces protons to become concentrated in the
space between inner and outer mitochondrial membranes.
Mitochondria have two encompassing membranes, the
inner of which is thrown into a series of folds called cristae.
These cristae are studded with ‘stalked particles’, the stalk
of which is called F0 and the head F1. The F0 is the proton
channel across the inner mitochondrial membrane down
which the hydrogen ions (protons) flow to drive the enzyme
reaction which generates the energy-rich ATP, which is
produced by the enzyme ATP synthase of the F1 head. In
terms of its evolutionary development, the outer mitochondrial membrane is thought to have arisen from a
primitive endocytotic event, when a prokaryotic cell was
internalized by a eukaryotic cell and remained as a
symbiont. Support for this contention comes from the fact
that mitochondria are delimited by two membranes, and
contain 70S ribosomes and circular, naked DNA, as do
prokaryotic cells, within their matrix, or inner space. Their
DNA codes for some, but not all, of their own component
proteins, so mitochondria are only semiautonomous.
Chloroplasts
Chloroplasts are organelles that are found only in the cells
of plants. They are the basis of photosynthesis and are
rather larger than mitochondria. They, too, have two outer
membranes, as well as a third set of folded internal
membranes, which make up the thylakoids, or stacked,
chlorophyll-rich lamellae. The Calvin pathway, or ‘dark’
Cytoskeleton
Cells maintain their shape by a complex of internal
structures jointly referred to as the cytoskeleton. This is
composed of three major entities, the microtubules, the
intermediate filaments and the microfilaments. These all
feature linear cable-like structures, which are composed of
alignments of subunits or monomers. They can be
disassembled into their component monomers at any time
and hence represent a very flexible system. This is
particularly apparent at cell division, when the tubular
microtubules polymerize from individual monomers of
tubulin and form the spindle, which is the structure to
which the chromosomes become attached at cell division
(mitosis). This is the time when each chromosome divides
into two genetically identical daughter chromosomes, with
one moving to each pole of the cell. The microtubules of the
spindle disassemble after division has occurred, and are
recycled back into the cytoplasmic microtubules and
tubulin monomers of interphase. This process occurs only
in eukaryotic cells, as prokaryotic cells do not have
chromosomes which condense, nor do they possess a
spindle.
Microtubules, as the name suggests, are tiny tubules, 23–
25 nm in diameter. Their basic subunit is a dumbbellshaped heterodimer of a- and b-tubulin. These polymerize
into protofilaments and there are usually 13 of these
forming the wall of the microtubule around a hollow core.
They not only comprise the spindle, but also form the 9 1 2
arrays in the motile cilia and flagella, and are the cables
along which proteins are transported from the nerve cell
body of neurons to the terminal synapse, through the
cytoplasmic axonal process. They appear to be associated
with ‘motor’ molecules such as dynein, kinesin or myosin,
which empower the transport of, for example, vesicles
along the microtubules of the axonal process.
Similarly, microfilaments are constructed of subunits of
globular actin, called G-actin, which are linked into
polymers called filamentous, or F-actin. This is the
functional form which makes up the microfilaments, 4–
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Cell Structure
6 nm in diameter, which lie as twin cables, intertwining in a
helical array. These act as the devices to maintain the shape
of cellular projections such as the microvilli of intestinal
cells (Figure 2), which increase the cellular surface area
dramatically in a highly advantageous way for absorption
of nutritional molecules. They are also involved in cellular
motility and, in muscle fibres, make up a major subcomponent, actin, which lies alongside myosin, the motor
molecule. In striated, or skeletal, muscle, as well as cardiac
muscle, this is a highly ordered, close-packed array of the
two molecules; this packing is less regular in smooth
muscle. The two sets of filaments, actin and myosin, slide
past each other during muscle contraction, the myosin
heads ‘walking’ along the actin fibrils. This activity
requires ATP as well as calcium (Ca2 1 ) ions; the latter
are stored in cisternae of ER, ready for instant release when
a muscular contraction is required. The Ca2 1 binds to
another molecule, troponin, which shifts tropomyosin
molecules, which in turn allow the myosin heads to interact
with the actin, thereby activating the sliding filament
Nucleus
Cytoskeleton
Intercellular junction
Microvilli
Multicellularity
Prokaryotic cells are always unicellular, but eukaryotic
cells are the subunits of all multicellular organisms – plants
and animals. These cells are able to form multicellular
systems because of their capacity to become differentiated
or specialized. This is possible because of their compartmentalization into organelles. Cells become specialized for
different tasks depending on the arrangement of their
internal compartments. The differentiated state requires a
reorganization or redistribution of certain of the major
organelles, which will be different for each kind of
specialized cell. Muscle cells require much energy, so have
many ATP-generating mitochondria, while secretory cells,
such as the pancreas, have extensive Golgi bodies, to help
produce the requisite secretory products. This division of
labour means that certain organs, made up of a certain
kind of specialized cell, can be specially programmed for a
particular function. The brain serves as a good example, as
it is a collection of highly differentiated neurons or nerve
cells (surrounded by supportive glial cells), which synthesize and exchange chemical signals that permit nervous
functioning, such as the capacity of animals to respond to
their environment.
Intercellular Junctions: Communication
and Adhesion
(a)
(b)
Figure 2 The cytoskeleton of cells helps them to maintain different
shapes: for example, (a) a nerve cell with an elongated axonal process; (b)
an epithelial cell, in which the cytoskeletal components may help form the
microvilli and the cell–cell junctions that hold adjacent cells together and
maintain cells in sheets or layers.
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motion. Without ATP, the muscle filaments cannot slide
past each other, as the myosin heads cannot release and
reattach to the actin, and ‘rigor mortis’ ensues.
The intermediate filaments may differ chemically in
different cell types but they all share certain similar
structural characteristics. They are composed of varying
numbers of fibrous subunits, which associate side by side
into rope-like structures that possess enormous tensile
strength. They are found around the inner nuclear
envelope, as so-called ‘nuclear lamins’, as well as throughout the cell cytoplasm, anchoring on to the plasma
membrane at specific junctures, the spot desmosomes,
thereby acting as joints or junctions to hold adjacent cells
together. These filaments can accumulate as keratin in
dead cells, persisting to form the hair and nails.
Cells in a multicellular organism are linked together within
organs by a variety of different intercellular junctions.
These include: (1) adhering (adherens) junctions, such as
the desmosomes, intermediate junctions and septate
junctions (unique to the tissues of invertebrates); (2) the
occluding junctions, such as the tight junctions; and (3) the
communicating junctions, such as the gap junctions, which
couple cells together. These are all variations on a theme of
plasma membrane modification and serve to maintain the
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Cell Structure
integrity of tissues by holding adjacent cells together.
Desmosomes are associated with underlying intermediate
filaments, and the intermediate and tight junctions with
actin microfilaments (Figure 2). Septate junctions, characterized by ladder-like septa straddling the intercellular
cleft, seem to have associations with both actin and
microtubules.
Gap junctions appear to have no internal cytoskeletal
attachments but they are responsible for cell–cell communication, rather than merely holding adjacent cells
together. They are formed by transmembrane subunits,
connexons, which become precisely aligned in adjacent
cells. Each connexon has a central pore, or channel, which
is spatially associated with one of the adjacent cell’s
connexons, so that ions and small molecules can be
exchanged between cells via the channels. This allows the
cells to communicate, as they can pass regulatory
molecular morphogens from one cell to the other. The
cells are then said to be ‘coupled’. They can become
‘uncoupled’ by conditions such as high pH or calcium
concentration, which cause the channels to become closed
down. This appears to occur by a configurational change in
the six hexameric subunits that make up each connexon.
Plant cells, which possess thick cellulose cell walls, can
only exchange information via plasmodesmata, which run
between the cytoplasm of adjacent cells. These are tiny
cytoplasmic passageways through the cellulose walls that
carry a minute strand of ER from one cell to the next,
through which molecules may move.
Differentiation
All new individuals arise from fertilized eggs, or zygotes,
which then divide many times thereafter to produce
embryos, then adults. The egg cell cytoplasm contains an
organized system of determinants which assign cells to
different developmental pathways. These cytoplasmic
differences require the zygote nuclear gene expression.
This selective activation and expression of specific genes
leads to cell and tissue differentiation. The details of the
embryonic pattern are filled in by mechanisms which sort
cells into particular pathways of differentiation by their
relative position in the developing organism. Groups of
cells generate positional information, perhaps due to
gradients of diffusible substances within boundaries in
tissues, leading to ‘pattern’ formation that is seen, for
example, in limb formation. Genetic studies have led to the
discovery of homeotic (‘HOX’) genes (master regulatory
genes), which determine the mainstream developmental
pathways of the different appendages (e.g. arms versus
legs) or units of the body plan of organisms. Developmental cell biology is rapidly emerging as the branch of the
subject producing some of the most exciting revelations
about the way the complex pattern of differentiated tissues
becomes organized.
Communication
The developing multicellular organism becomes divided
into groups of cells cooperating as tissues. These must
selectively adhere to one another to remain as organs, and
their component cells must also be able to communicate
with each other if division of labour is to occur effectively.
This they do by a number of devices, of which one of the
most important is the nervous system, where signals are
sent by chemical neurotransmitters from one neuron to
other cells. Hormones, protein or steroid, also have
profound chemical effects on developmental processes,
including the acquisition of secondary sexual characteristics, by activating different genes in the DNA. Circulating
hormones may also trigger secondary signalling pathways.
Local hormones play important roles in adjacent cellular
activity, as may other chemicals that operate by chemotaxis or via chemical attractants. Signalling via cell surface
molecules also occurs in the immune system, which is based
on cellular signals and responses that activate special B and
T cells in the lymphoid system, using either humoral or cellmediated mechanisms. This is important in enabling the
organism to survive infection. Growth factors and cellular
adhesion molecules may also affect the patterns of
differentiation of cells in the formation, and then the
maintenance, of organs. This may involve the assembly of
the cell–cell junctions, described earlier, which keep the
adjacent cells associated together in particular arrangements.
The interplay of chemical signals between the cells of the
same or different organs produces an orchestration of
cellular activity resulting in a successful, functional
organism. The healthy organism, therefore, is dependent
on the effective functioning of the cells that make up its
organs.
Further Reading
Alberts B, Bray D, Lewis J et al. (1994) Molecular Biology of the Cell, 3rd
edn. New York: Garland.
Alberts B, Bray D, Johnson A et al. (1998) Essential Cell Biology: An
Introduction to the Molecular Biology of the Cell. New York: Garland.
Darnell J, Lodish H and Baltimore D (1990) Molecular Cell Biology.
New York: Scientific American.
Fawcett D (1981) An Atlas of Fine Structure: The Cell. New York:
Saunders.
Holtzman E and Novikoff AB (1984) Cells and Organelles. New York:
Saunders.
Judson HF (1996) The Eighth Day of Creation: Makers of the Revolution
in Biology. Cold Spring Harbor: Cold Spring Harbor Laboratory
Press.
Lewin B (1983) Genes. New York: Wiley.
Rensberger B (1996) Life Itself: Explaining the Realm of the Living Cell.
Oxford: Oxford University Press.
Smith CA and Wood EJ (1996) Cell Biology. London: Chapman and
Hall.
Stryer L (1981) Biochemistry. New York: Freeman.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
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