The plasma membrane is a fragile, transparent, bag-like structure which
surrounds the cell contents and
separates them from the outside environment. This membrane has a core of
two phospholipid layers (with
the water soluble phosphate head facing to the outside of the cell and,
in the bottom layer, into the cell),
interspersed with cholesterol molecules to lend it rigidity and stability.
Also throughout are proteins, which
are responsible for most of the specialized functions of the membrane (channel
proteins). Some of the
proteins are enzymes; many are mounted on the outside to serve as receptors,
or binding sites, for
hormones or other chemical messengers. But most of the portions are involved
in transport. They form
pores through which water and small water soluble molecules or ions can
move through. Others more
actively move larger substances, or ions and such against the concentration
gradient. Some of the proteins
are glycoprotein: these proteins determine your blood type, act as receptor
that certain bacteria, viruses
or toxins can bind to, or work in cell to cell interactions. Microvilli
are tiny finger-like projections of the
plasma membrane to give more surface area for absorption. Membrane junctions
vary in structure and
role. In tight junctions, the plasma membranes are fused together, much
like a zipper. These junctions
connect cells into leak-proof sheets which stop substances from passing
through the spaces between cells
(blood vessels, intestines). Desmosomes are adhesion junctions that prevent
mechanically stressed cells
from tearing apart (skin). The membranes are bound together by proteins,
the second with root-like
structures. Gap junctions allow direct passage of chemical molecules, such
as ions, from one cell to the
next (heart, embryonic cells). These are mainly used for communication
The cytoplasm is made up of the intracellular material that is outside
the nucleus. It is the site of most
cellular activity, and contains three parts: the cytosol, the organelles,
and the inclusions. Cytosol it the
semi-transparent fluid which suspends the other two. Dissolved into the
cytosol (made up mostly of water),
are various nutrients and other solutes. The organelles are the cellular
machinery which do all of the cells
work. Inclusions are not "work" units, but rather storage units for nutrients
and such. They include fat
droplets, glycogen granules, pigments, and water-containing vacuoles common
in mucus
Mitochondria are usually depicted as tiny, thread-like ot sausage shaped
organelles. In the living cell,
however, they are far more animated; squirming, lengthening, and changing
shape almost constantly. The
walls of the mitochondria consist of a double membrane, equal to two plasma
membranes placed side by
side. The outer wall is smooth, but the inner is riddled with shelf-like
protrusions called cristae. Enzymes in
the mitochondria carry out cellular respiration; that is, break down food
using oxygen to produce adenosine
triphosphate (atp). Thus, the mitochondria are often called the "powerhouses"
of the cell. Cells such as liver
or muscle cells use a great deal of energy, and so contain a large number
of mitochondria. By comparison,
some cells, such as an egg cell, have very few
Ribosomes are small, round, dark organelles made up of proteins and ribosomal
ribonucleic acid (rna). They
are the sight of protein synthesis. They are both suspended in the cytoplasm,
and attached to the rough
endoplasmic reticulum
The endoplasmic reticulum is a system of fluid-filled cisterns that twist
their way about the cell. These
conduits account for about half of the cell's membranes. They serve as
a circulatory within the cell. The
rough endoplasmic reticulum is made such by the presence to ribosomes,
and move about the proteins the
ribosomes make. It also manufactures the building materials of cellular
membranes, lipids. The other type
of endoplasmic reticulum is smooth. It plays no role in protein synthesis.
It's function is cholesterol synthesis
and breakdown, fat metabolism, and detoxification of drugs. These smooth
endoplasmic reticulum are
plentiful in liver cells, and cells that produce steroid-based hormones,
such as the testes in males
The Golgi apparatus appears as a stack of flattened membrane sacks, associated
with numerous, small
vesicles. Generally located in the vicinity of the nucleus, it serves as
a traffic director of cellular proteins.
Protein synthesis is completed here, and the proteins, hormones (protein
and steroid -based alike), and
enzymes are packages and sent out of the cells from the Golgi. As the substances
are "tagged" for release,
the sacks swell and pinch off to form secretory vesicles, which move to
the cell's membrane and fuse with
it, releasing the vesicle's contents. Some of the vesicles do not go to
the plasma membrane. Some deliver
their contents (enzymes) to digest a substance, thus becoming lysosomes.
Lysosomes are especially
abundant in white blood cells, which digest bacteria and other potentially
harmful substances. The lysosomal
membrane us usually quite stable, but when the cell is injured or deprived
of oxygen, they self-digest,
becoming "suicide sacks"
Peroxisomes are membrane sacks which contain powerful oxidase enzymes.
These enzymes aid in the
digestion of fats and to detoxify a number of poisons, such as alcohol
and formaldehyde. By removing and
transferring hydrogen atoms from them to oxygen, the cell produces hydrogen
peroxide. Although
hydrogen peroxide is a normal byproduct of cellular functions, it can cause
damage if too much
accumulates. The peroxisome enzymes convert the hydrogen peroxide into
water and oxygen gas. Though
the perixisomes look like lysomes, their membranes appear to have formed
directly from the budding of
the rough endoplasmic reticulum
The cytoskeleton is an elaborate network of (from largest to smallest)
microtubules, intermediate
filaments, and microfilaments. They support, hold, and change the cells
shape and structure
The two sets of paired centrioles are rod-shaped organelles that lie close
to the nucleus. They are made up
of microtubules, and migrate to apposing side of the nucleus during mitosis
to pull apart the chromosomes,
forming mitotic spindles
The nucleus is the "control center" of the cell. It contains the deoxyribonucleic
acid (dna), which codes for
all life. More specifically, it contains the blueprints for proteins. Most
often, the nucleus is elliptical or
spherical, but conforms to the shape of the cell. In an elongated cell,
for instance, the nucleus is also
stretched. The nucleus has three distinct regions: The nuclear membrane
is a double-membrane barrier.
Between the two layers is a fluid filled space, or "moat". At various places,
the two membranes join and
fuse, forming the nuclear pores. Passage through this membrane is somewhat
freer than other membranes
through it large pores. The dna, however, cannot leave. Within the membrane,
a jelly-like fluid called
nucleoplasm suspends the nucleoli and the chromatin The nucleus contains
one or more roundish bodies
called nucleoli. These are the sites where ribosomes are born. When a cell
is not dividing, the dna forms a
lose network of bumpy threads called chromatin. When the cell divides,
the chromatin forms denser,
rod-like bodies called chromosomes