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Cell membrane
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Cell membrane: Every cell is enclosed in a
membrane. The membrane is a double layer of
lipids (lipid bilayer) but is made quite complex by
the presence of numerous proteins that are
important to cell activity. These proteins include
receptors, pores, and enzymes. The membrane is
responsible for the controlled entry and exit of
ions like sodium (Na) potassium (K), calcium
(Ca++).
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Plasma Membrane
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According to the accepted current theory, known as the fluid mosaic model, the
plasma membrane is composed of a double layer (bilayer) of lipids, oily
substances found in all cells (see Figure 1). Most of the lipids in the bilayer can
be more precisely described as phospholipids, that is, lipids that feature a
phosphate group at one end of each molecule. Phospholipids are
characteristically hydrophilic ("water-loving") at their phosphate ends and
hydrophobic ("water-fearing") along their lipid tail regions. In each layer of a
plasma membrane, the hydrophobic lipid tails are oriented inwards and the
hydrophilic phosphate groups are aligned so they face outwards, either toward
the aqueous cytosol of the cell or the outside environment. Phospholipids tend to
spontaneously aggregate by this mechanism whenever they are exposed to
water. Within the phospholipid bilayer of the plasma membrane, many diverse
proteins are embedded, while other proteins simply adhere to the surfaces of the
bilayer. Some of these proteins, primarily those that are at least partially exposed
on the external side of the membrane, have carbohydrates attached to their outer
surfaces and are, therefore, referred to as glycoproteins. The positioning of
proteins along the plasma membrane is related in part to the organization of the
filaments that comprise the cytoskeleton, which help anchor them in place. The
arrangement of proteins also involves the hydrophobic and hydrophilic regions
found on the surfaces of the proteins: hydrophobic regions associate with the
hydrophobic interior of the plasma membrane and hydrophilic regions extend
past the surface of the membrane into either the inside of the cell or the outer
environment.
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Vacuole
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A vacuole is a membrane-bound
sac that plays roles in intracellular
digestion and the release of cellular
waste products. In animal cells,
vacuoles are generally small.
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The Nucleus and Nucleolus
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The nucleus is the most obvious
organelle in any eukaryotic cell. It is
a membrane-bound organelle and is
surrounded by a double membrane.
It communicates with the
surrounding cytosol via numerous
nuclear pores.
Within the nucleus is the DNA
responsible for providing the cell
with its unique characteristics. The
DNA is similar in every cell of the
body, but depending on the specific
cell type, some genes may be
turned on or off - that's why a liver
cell is different from a muscle cell.
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Mitochondria
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Mitochondria are membranebound organelles, and like the
nucleus have a double membrane.
The outer membrane is fairly
smooth. But the inner membrane
is highly convoluted, forming folds
called cristae. The cristae greatly
increase the innerimembrane's
surface area. It is on these cristae
that food (sugar) is combined with
oxygen to produce ATP - the
primary energy source for the cell.
They are the power centers of the
cell. They are about the size of
bacteria but may have different
shapes depending on the cell type.
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Lysosomes
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Lysosomes:Lysosomes (common
in animal cells but rare in plant
cells) contain hydrolytic
enzymes necessary for
intracellular digestion. In white
blood cells that eat bacteria,
lysosome contents are carefully
released into the vacuole
around the bacteria and serve
to kill and digest those bacteria.
Uncontrolled release of
lysosome contents into the
cytoplasm can also cause cell
death(necrosis).
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Golgi Apparatus
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The Golgi apparatus is a
membrane-bound structure with a
single membrane. It is actually a
stack of membrane-bound vesicles
that are important in packaging
macromolecules for transport
elsewhere in the cell. The stack of
larger vesicles is surrounded by
numerous smaller vesicles
containing those packaged
macromolecules. The enzymatic or
hormonal contents of lysosomes,
peroxisomes and secretory vesicles
are packaged in membrane-bound
vesicles at the periphery of the
Golgi apparatus.
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Secretory vesicle
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Cell secretions - e.g. hormones,
neurotransmitters - are
packaged in secretory vesicles
at the Golgi apparatus. The
secretory vesicles are then
transported to the cell surface
for release.
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Endoplasmic Reticulum
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When viewed by electron microscopy,
some areas of the endoplasmic
reticulum look "smooth" (smooth ER)
and some appear "rough" (rough ER).
The rough ER appears rough due to
the presence of ribosomes on the
membrane surface. Smooth and
Rough ER also have different
functions. Smooth ER is important in
the synthesis of lipids and membrane
proteins. Rough ER is important in
the synthesis of other proteins.
Information coded in DNA sequences
in the nucleus is transcribed as
messenger RNA. Messenger RNA
exits the nucleus through small pores
to enter the cytoplasm. At the
ribosomes on the rough ER, the
messenger RNA is translated into
proteins. These proteins are then
transferred to the Golgi in "transport
vesicles" where they are further
processed and packaged into
lysosomes, peroxisomes, or
secretory vesicles.
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Ribosomes
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Protein synthesis requires the
assistance of two other kinds of
RNA molecules in addition to
rRNA. Messenger RNA (mRNA)
provides the template of
instructions from the cellular DNA
for building a specific protein.
Transfer RNA (tRNA) brings the
protein building blocks, amino
acids, to the ribosome. There are
three adjacent tRNA binding sites
on a ribosome: the aminoacyl
binding site for a tRNA molecule
attached to the next amino acid in
the protein , the peptidyl binding
site for the central tRNA molecule
containing the growing peptide
chain, and an exit binding site to
discharge used tRNA molecules
from the ribosome.
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The Centrosome and the
Centrioles
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Animal cell centrosome:It is also
called the "microtubule organizing
center", is an area in the cell where
microtubles are produced. Within an
animal cell centrosome there is a
pair of small organelles, the
centrioles, each made up of a ring
of nine groups of microtubules.
There are three fused microtubules
in each group. The two centrioles
are arranged such that one is
perpendicular to the other.
During animal cell division, the
centrosome divides and the
centrioles replicate (make new
copies). The result is two
centrosomes, each with its own pair
of centrioles. The two centrosomes
move to opposite ends of the
nucleus, and from each centrosome,
microtubules grow into a "spindle"
which is responsible for separating
replicated chromosomes into the
two daughter cells.
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Microtubules
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Microtubules are biopolymers that are composed of subunits made from
an abundant globular cytoplasmic protein known as tubulin, as
illustrated in Figure 1. Each subunit of the microtubule is made of two
slightly different but closely related simpler units called alpha-tubulin
and beta-tubulin that are bound very tightly together to form
heterodimers. In a microtubule, the subunits are organized in such a
way that they all point the same direction to form 13 parallel
protofilaments. This organization gives the structure polarity, with only
the alpha-tubulin proteins exposed at one end and only beta-tubulin
proteins at the other.
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Cytoskeleton
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As its name implies, the
cytoskeleton helps to maintain cell
shape. But the primary importance
of the cytoskeleton is in cell motility.
The internal movement of cell
organelles, as well as cell
locomotion and muscle fiber
contraction could not take place
without the cytoskeleton.
The cytoskeleton is an organized
network of three primary protein
filaments: microtubules, actin
filaments, and intermediate fibers.
The complexity of the cytoskeleton
can be seen in the abundant F-actin
stress fibers (green) in the
endothelial cell shown above.
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Microfilaments
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Illustrated in Figure 2 is a fluorescence
digital image of an Indian Muntjac deer
skin fibroblast cell stained with
fluorescent probes targeting the nucleus
(blue) and the actin cytoskeletal network
(green). Individually, microfilaments are
relatively flexible. In the cells of living
organisms, however, the actin filaments
are usually organized into larger, much
stronger structures by various accessory
proteins. The exact structural form that a
group of microfilaments assumes
depends on their primary function and the
particular proteins that bind them together.
For instance, in the core of surface
protrusions called microspikes,
microfilaments are organized into tight
parallel bundles by the bundling protein
fimbrin. Bundles of the filaments are less
tightly packed together, however, when
they are bound by alpha-actinin or are
associated with fibroblast stress fibers
(the parallel green fibers in Figure 2).
Notably, the microfilament connections
created by some cross-linking proteins
result in a web-like network or gel form
rather than filament bundles.
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Intermediate Filaments
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Intermediate filaments are a very broad class of fibrous proteins that play
an important role as both structural and functional elements of the
cytoskeleton. Ranging in size from 8 to 12 nanometers (in diameter; see
Figure 1), intermediate filaments function as tension-bearing elements to
help maintain cell shape and rigidity, and serve to anchor in place
several organelles, including the nucleus and desmosomes. Intermediate
filaments are also involved in formation of the nuclear lamina, a net-like
meshwork array that lines the inner nuclear membrane and governs the
shape of the nucleus.
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Peroxisomes
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Peroxisomes contain a variety of
enzymes, which primarily function
together to rid the cell of toxic
substances, and in particular,
hydrogen peroxide (a common
byproduct of cellular metabolism).
These organelles contain enzymes
that convert the hydrogen peroxide
to water, rendering the potentially
toxic substance safe for release back
into the cell. Some types of
peroxisomes, such as those in liver
cells, detoxify alcohol and other
harmful compounds by transferring
hydrogen from the poisons to
molecules of oxygen (a process
termed oxidation). Others are more
important for their ability to initiate
the production of phospholipids,
which are typically used in the
formation of membranes.
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