FREE ESSAY ON CELLS

CELLS

Cells are composed of oxygen, hydrogen, carbon, and nitrogen, which contain most important
organic compounds in a cell are proteins, nucleic acids, lipids, and carbohydrates,Water
makes up 60 to 65 percent of the cell.Some cells are complete organisms, as the
unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are
components of multicellular organisms,Cells range in size from the smallest bacteria like
mycoplasmas, which are 0.1 micrometer in diameter, to a egg yolks of ostriches, which are
about 8 cm in diameter.tehj shape and althought they may look different they insides are
all the same, all cells have a surrounding membrane and an internal, water-rich substance
called the cytoplasmCells are of two distinctly different types, prokaryotes and
eukaryotes; thus, the living world is divided into two broad categories (see
Classification). The DNA of prokaryotes is a single molecule in direct contact with the
cell cytoplasm, whereas the DNA of eukaryotes is much greater in amount and diversity and
is contained within a nucleus separated from the cell cytoplasm by a membranous nuclear
envelope. Many eukaryotic cells are further divided into compartments by internal
membranes in addition to the nuclear envelope, whereas prokaryotic cells never contain
completely internal membranes. The prokaryotes include the mycoplasmas, bacteria, and
cyanobacteria (formerly known as blue-green algae). The eukaryotes comprise all plant and
animal cells. In general, plant cells differ from animal cells in that they have a rigid
cell wall exterior to the plasma membrane; a large vacuole, or fluid-filled pouch; and
chloroplasts that convert light energy to chemical energy for the synthesis of glucose.
Structure and Function 
Cells are composed primarily of oxygen, hydrogen, carbon, and nitrogen, the elements that
make up the majority of organic compounds. The most important organic compounds in a cell
are proteins, nucleic acids, lipids, and polysaccharides (carbohydrates). The solid
structures of the cell are complex combinations of these large molecules. Water makes up
60 to 65 percent of the cell, because water is a favorable environment for biochemical
reactions.
All cells are dynamic at some stage of their life cycle, in the sense that they use
energy to perform a variety of cell functions: movement, growth, maintenance and repair
of cell structure, reproduction of the cell, and manufacture of specialized cell products
such as enzymes and hormones. These functions are also the result of interactions of
organic molecules.
Plasma Membrane 
The plasma membrane (PM), a continuous double layer of phospholipid molecules 75 to 100
angstroms thick, constitutes the boundary between the cell and its external environment.
In addition to lipids, the PM has protein components (polypeptides) that are associated
with either the outer or inner surfaces of its layers or are buried within them. The
structure as a whole is selectively permeable, or semipermeable; that is, it permits the
exchange of water and selected atoms and molecules between the cell exterior and
interior. This is vital to the cell because while the PM helps maintain high local
concentrations of organic molecules within the cell, it also allows interaction between
the cell and its external environment.
The PM mediates such interactions in various ways. The exchange of mineral ions and small
nutrient molecules is controlled by PM proteins that act as pumps, carriers, and
channels. The PM also participates in the exchange of larger molecules through
phagocytosis, the engulfing of large food particles; endocytosis, the intake of fluids
and membrane components; and exocytosis, the expulsion of cell products or cell waste.
(The PM of some cells, such as those of the human intestine, is convoluted to enhance the
surface area for these exchanges.) In addition, the PM contains receptors that
selectively receive nerve and hormone signals and transmit them to the interior of the
cell. Finally, direct cell-to-cell interactions can occur through specialized regions of
the PM known as junctions. Organs such as the skin and the small intestine consist of
cells held together by tight junctions and local thickenings, or desmosomes, which
constitute another type of junction. Cells can communicate electrically through a third
type of junction, called a gap junction, that consists of tiny protein tunnels between
two cells, through which tiny message molecules and ions may be passed. When the PMs of
two cells are continuous, an actual bridge of cytoplasm forms between them; in plants
these bridges are called plasmodesmata.
Cell Walls 
Exterior to the PM of most plant cells and bacteria is a cell wall, a cell product made
largely of complex polysaccharides. In higher plants the polysaccharide is cellulose. The
presence of a cell wall makes these cells rigid and sturdy, but it poses special problems
for the transport of substances into and out of the cell.
Cytoplasm 
The cytoplasm is the water-rich matrix within a cell that contains and surrounds the
other cellular contents. It is more like a viscous gel than a watery solution, but it
liquefies when shaken or stirred. Such gel-to-sol transitions are thought by some cell
biologists to play a role in the movement of a cell's components from place to place
within the cell. Rapid movement of cell components is called either streaming or
cyclosis, depending on whether it occurs linearly or circularly.
Through an electron microscope the cytoplasmic gel appears as a three-dimensional lattice
of slender, protein-rich strands in a continuous water-rich phase. Because the
latticework is reminiscent of the internal structure of spongy bone, which is composed of
many struts, or trabeculae, it is called the microtrabecular lattice (MTL). The MTL
appears to interconnect and support the other solid structures of the cell. The
composition and function of the MTL are as yet still unknown, but it is thought to
control the spatial arrangement of cell components within the cytoplasm.
Cytoskeleton 
The so-called cytoskeleton influences the shape of the cell in much the same way tent
poles determine the shape of a tent. Without the cytoskeleton a cell tends to become
spherical. The cytoskeleton probably gives direction to the movement of components within
the cytoplasm as well and participates in movement of the cell itself. The cytoskeleton
is composed of three main filament types: the microtubules, microfilaments, and
intermediate-sized filaments that are supported and distributed within the MTL.
Microtubules are long rigid cylinders that act as the bones of the cell. They also may
act as tracks along which intracellular components are transported. The walls of the
cylinders are composed of two proteins, alpha- and beta-tubulin. Microfilaments are
composed of actin, a major protein of muscle. They often occur in long bundles called
stress fibers and may act as the muscles of the cell. The intermediate-sized filaments
are a heterogeneous class of proteins whose function is largely unknown.
Nucleus 
The membrane-bounded structures contained within the cytoplasm of eukaryotes are referred
to as organelles. The nucleus is the most easily recognizable of these. DNA, combined
with protein, is organized inside the nucleus into structural units called chromosomes,
which usually occur in identical pairs. The DNA in each chromosome is a single, very
long, highly coiled molecule subdivided into functional subunits called genes. Genes
contain the coded instructions for the assembly of polypeptides and larger proteins.
Together the chromosomes contain all the information needed to build an identical
functioning copy of the cell.
The nucleus is surrounded by an envelope of two concentric membranes. Interaction between
the nuclear contents and the surrounding cytoplasm is permitted through holes, called
nuclear pores, in this envelope. The nucleus also contains a specialized region, the
nucleolus, where nucleoprotein particles are assembled. These particles migrate through
the nuclear pores into the cytoplasm, where they are modified to become ribosomes.
Ribosomes 
Ribosomes are the factories where the instructions encoded in the DNA of the nucleus are
translated to make proteins. The instructions are carried from the DNA to the ribosomes
by long nucleic-acid molecules called messenger ribonucleic acids (RNAs).
Endoplasmic Reticulum (Er) 
Among the other membranous structures within the eukaryotic cell are extensive membrane
systems that make up the smooth and the rough endoplasmic reticulum (SER and RER). The
SER often takes the form of branching tubes. (In skeletal muscle it acts as a reservoir
for calcium ions and is called the sacroplasmic reticulum.) The RER is made up of
sheetlike flattened sacs, which often are stacked one on top of the other; the term rough
refers to the numerous ribosomes that dot the cytoplasmic surfaces of the sacs. The RER
is one of the sites of protein synthesis in the cytoplasm. Proteins are synthesized on
the cytoplasmic surface and pass through the membrane to become sequestered within the
sacs. These packaged proteins are destined for secretion to the outside of the cell.
Other proteins, synthesized on ribosomes that are not attached to membranes, are not
secreted and remain as structural proteins or metabolic enzymes.
Golgi Apparatus 
Similar in appearance to and perhaps continuous with the ER is a region of smooth,
stacked membranous sacs known as the Golgi apparatus. Cell biologists think that the
apparatus modifies proteins, after they are synthesized and packaged on the RER, by
linking them with sugars or other molecules.
Lysosomes 
Lysosomes are membrane-bounded bags, or vesicles, containing digestive enzymes. Their
normal function is digestion of complex nutrients and broken-down organelles. In disease
fighting, the lysosomes of white blood cells aid in the digestion of engulfed bacteria
and other foreign or toxic materials.
Mitochondria and Chloroplasts (Plastids) 
Mitochondria are the powerhouses of the animal cell, where the products of the enzymatic
breakdown, or metabolism, of nutrients such as glucose are converted into energy in the
form of the molecule adenosine triphosphate (ATP). This process uses up oxygen and is
called aerobic respiration. Plants possess, in addition to mitochondria, similar
organelles called chloroplasts. Each chloroplast contains the green pigment chlorophyll,
which is used to convert light energy from the sun into ATP. This process is called
photosynthesis.
Cilia and Flagella 
Some cells have flexible, whiplike external appendages called cilia and flagella, which
are used for locomotion and for capturing food. Cilia are 3 to 10 micrometers long and
are found on protozoa as well as in human oviducts and respiratory tracts. In the
respiratory tract they sweep large particles up the trachea and prevent them from passing
into the lungs. Flagella, which may be ten times as long, are found on some protozoa and
unicellular plants, and they are used for locomotion by the sperm of higher organisms.
Eukaryotic cilia and flagella are composed of microtubules covered by a membrane sheath.
Prokaryotic flagella are more slender and are composed of the protein flagellin. They
propel the cell by rotating like the propeller of a ship rather than by a whipping
motion.
Centrioles and Basal Bodies 
All animal and some plant cells contain a pair of centrioles, which are cylindrical
structures composed of short microtubules. They are surrounded by a cloud of fuzzy
material, the exact function of which is unknown. Centrioles control the arrangement of
microtubules in the cell cytoskeleton. Basal bodies, which are similar, are structures
that anchor cilia and flagella within the cytoplasm, just inside the plasma membrane.
Centrioles and basal bodies both contain DNA and apparently can duplicate themselves
independently of duplication of the entire cell.
Division, Reproduction, and Differentiation 
All cells are the products of the division of preexisting cells. Simple cell division, or
asexual reproduction, normally results in the production of two identical daughter cells,
each containing a set of chromosomes identical with those of the parent cell. Before the
onset of division, a cell grows to roughly twice its original size. In doing so it
duplicates its DNA, so that each chromosome is doubled. During division the duplicate
sets are physically separated, following longitudinal splitting of each double
chromosome, and are transported into opposite sides of the cell. The cell then constricts
around its equator and pinches in two. In cells that contain chromosomes, the separation
of chromosomes during division (mitosis) requires an oblong scaffold of parallel
microtubules, along which the chromosomes are moved. This scaffold, called the spindle,
forms at the beginning of mitosis under the direction of the centrioles.
Sexual Reproduction 
Sexual reproduction is the mingling of the DNA of two different organisms of the same
species to produce a cell, or cells, with a new combination of genes. When this occurs
between single-celled organisms, it is called conjugation. In multicellular organisms,
sexual reproduction requires the production of male and female germ cells (sperm and
eggs) by a process called meiosis. During this process a cell divides twice; but its
chromosomes are duplicated only once. Thus, four germ cells are produced, each containing
half the normal number of chromosomes. In the male organism the germ cells develop into
sperm; in the female they develop into eggs. A sperm and an egg then unite
(fertilization) to form a new cell, called a zygote, that has a complete set of
chromosomes and has received half its genetic information from each parent, thus making
it a new individual.
Differentiation 
Differentiation is the process by which a cell daughter becomes different from its parent
in appearance or function, or both, even though both parent and daughter cell contain
identical genetic information. The appearance and function of identical daughter cells
are initially specified by two kinds of information inherited by each in equal measure
from the parent: cytoplasmic and nuclear information. Alterations in either kind of
information will result in daughter cells being unlike their parents. Cytoplasmic
information consists chiefly of cell organelles (especially centrioles) and messenger
RNAs ready for translation into proteins, whereas nuclear information is contained in the
genetic code. Changes in cytoplasmic information generally are the result of unequal
divisions that produce an asymmetrical distribution of cytoplasmic organelles and
messenger RNAs between daughter cells. Changes in nuclear information involve restriction
of the use of some portion of the cell's genes, because genes can be turned on or off by
the cell in response to cellular environmental signals. The behavior of a cell at any
given time in differentiation largely depends on which subset of genes is turned on.
Differentiation primarily occurs through activation and deactivation of genes in a
programmed succession to produce orchestrated changes in cell characteristics. During
differentiation certain genes often are irreversibly turned off, and the change becomes
permanent. This limits the variety of ways in which a cell can respond to an
environmental signal, as well as the variety of signals to which it can respond, and the
cell is channeled toward its ultimate differentiated fate. This process is called
determination. Thus, a human nerve cell cannot transform into a human muscle cell even
though they each contain identical genetic information. Aging of cells is sometimes
viewed as a continuation of their differentiation, with death seen as the final
determination.
Origin of Life and Evolution of Cells 
Scientists have formulated many theories about the origin of life and how it evolved into
the various forms known today. These ideas are deduced from the evidence of the fossil
record, from laboratory simulations of conditions on the primeval earth, and from
consideration of the structure and function of cells.
The earth was created more than 3 billion years ago, although more than 2 billion years
probably passed before life as it is now known developed. Scientists believe that the
atmosphere of the young earth was mostly water vapor, methane, and ammonia, with very
little gaseous oxygen. Laboratory simulations have shown that all major classes of
organic molecules could have been generated from this atmosphere by the energy of the sun
or by lightning and that the lack of oxygen would prevent newly formed organic molecules
from being broken down by oxidation. Rain would have carried these molecules into lakes
and oceans to form a primordial soup.
When the concentration of organic molecules in this soup became high enough, molecules
would have begun to form stable aggregates. For example, lipids might coalesce into
droplets the way cooking oil does in water, thus generating simple membranes and trapping
other organic molecules in the interior of the droplet. Randomly formed aggregations that
could harness energy to grow and reproduce themselves would eventually far outnumber
other combinations. DNA may have been an essential component of the self-reproducing
aggregates; it and RNA are the only organic molecules able to duplicate themselves. These
supramolecular aggregations would have been extremely lifelike and with some refinements
would have resembled primitive prokaryotes. This concept of the origin of life, however,
does not explain the development of the genetic code and the precise interdependence
between the code and protein synthesis.
The relative absence of oxygen from the atmosphere of the young earth meant that no ozone
layer existed to screen out ultraviolet radiation and no oxygen was available for aerobic
respiration. Therefore, the first cells were probably photosynthetic and used ultraviolet
light. Because photosynthesis generates oxygen, the oxygen content of the atmosphere
gradually increased. As a result, cells that could use this oxygen to generate energy,
and photosynthetic cells that could use light other than ultraviolet, eventually became
predominant.
Eukaryotes may have evolved from prokaryotes. This idea comes from speculation about the
origin of mitochondria and chloroplasts. These organelles may be the degenerate
descendants of aerobic and photosynthetic prokaryotes that were engulfed by larger
prokaryotes but remained alive within them (endosymbiosis). Over the years the host cell
became dependent on the endosymbionts for energy (ATP), while they in turn became
dependent on the host for most other cell functions. The fact that mitochondria and
chloroplasts are surrounded by two membranes, as if they had originally entered the cell
by phagocytosis, supports this theory. In addition, these organelles contain their own
DNA and ribosomes, which resemble the DNA and ribosomes of bacteria more than those of
eukaryotes. It is possible that other eukaryotic organelles originated similarly.
History 
Cells were first described in 1665 by the English scientist Robert Hooke, who studied the
dead cells of cork with a crude microscope. Living cells were first described in detail
in the 1670s by the Dutch scientist Anton van Leeuwenhoek. These early descriptions were
not improved on until the early 19th century, when better-quality microscope lenses were
developed. In 1839 the German botanist Matthias Schleiden and the German zoologist
Theodor Schwann formulated the basic cell theory of today. Struck by the underlying
similarity between plant and animal cells, they stated that all living organisms consist
of cells and cell products. Thus, a whole organism could be understood through the study
of its cellular parts. In 1858 the German pathologist Rudolf Virchow's theory, that all
cells come from preexisting cells, led to the development of ideas about cell division
and cell differentiation.
The development in the late 19th century of techniques for staining cell parts enabled
scientists to detect tiny cell structures that were not actually seen in detail until the
advent of the electron microscope in the 1940s. The development of various advanced
optical techniques in the 20th century also increased the detection power of the light
microscope for observations of living cells.
The study of cells (cytology) is not limited to describing structures (morphology). A
central concept in modern cytology is that each structure has a function that may be
understood as a series of biochemical reactions. The understanding of these functions has
been greatly aided by the development of cell fractionation techniques, using an
ultracentrifuge to separate specific intracellular structures from the rest of the cell.
Another technique is tissue culture, by which specific kinds of cells can be isolated and
grown for stuDY.