Eukaryotic Animal Cell
by Yueshan Goh
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Eukaryotic Animal Cell
Nucleus: The nucleus is the most obvious organelle in any eukaryotic
cell. It is enclosed in a nuclear membrane and 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, and a
muscle cell is different from a fat cell. When a cell is dividing, the
nuclear chromatin (DNA and surrounding protein) condenses into
chromosomes that are easily seen by microscopy.
Nucleolus: The prominent structure in the nucleus is the
nucleolus. The nucleolus produces ribosomes, which move
out of the nucleus and take positions on the rough
endoplasmic reticulum where they are critical in protein
Cytosol: The cytosol is the "soup" within which all the other cell
organelles reside and where most of the cellular metabolism occurs.
Though mostly water, the cytosol is full of proteins that control cell
metabolism including signal transduction pathways, glycolysis,
intracellular receptors, and transcription factors.
Cytoplasm:This is a collective term for the
cytosol plus the organelles suspended within
Centrosomes: The centrosome, or MICROTUBULE ORGANIZING CENTER (MTOC), is an area in the cell
where microtubules are produced. Plant and animal cell centrosomes play similar roles in cell division,
and both include collections of microtubules, but the plant cell centrosome is simpler and does not
have centrioles. During animal cell division, the centrioles replicate (make new copies) and the
centrosome divides. 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
Centrioles: Each centriole is a ring of nine groups of fused microtubules.
There are three microtubules in each group. Microtubules (and
centrioles) are part of the cytoskeleton. In the complete animal cell
centrosome, the two centrioles are arranged such that one is
perpendicular to the other.
Cytoskeleton: 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
Actin filaments (microfilaments): Microfilaments are fine, thread-like protein
fibers, 3-6 nm in diameter. They are composed predominantly of a contractile
protein called actin, which is the most abundant cellular protein. Microfilaments'
association with the protein myosin is responsible for muscle contraction.
Microfilaments can also carry out cellular movements including gliding,
contraction, and cytokinesis.
Microtubules: Microtubules are cylindrical tubes, 20-25 nm in diameter. They are composed
of subunits of the protein tubulin--these subunits are termed alpha and beta. Microtubules
act as a scaffold to determine cell shape, and provide a set of "tracks" for cell organelles
and vesicles to move on. Microtubules also form the spindle fibers for separating
chromosomes during mitosis. When arranged in geometric patterns inside flagella and cilia,
they are used for locomotion.
Intermediate Filaments: Intermediate filaments are about 10 nm diameter and provide tensile strength for the cell.
Golgi Apparatus: 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.
Secretory Vesicle: 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.
Lysosomes: Lysosomes contain hydrolytic enzymes
necessary for intracellular digestion. They are common in
animal cells, but rare in plant cells. Hydrolytic enzymes of
plant cells are more often found in the vacuole.
Peroxisomes: Peroxisomes are membrane-bound packets of oxidative
enzymes. In animal cells, peroxisomes protect the cell from its own
production of toxic hydrogen peroxide. As an example, white blood cells
produce hydrogen peroxide to kill bacteria. The oxidative enzymes in
peroxisomes break down the hydrogen peroxide into water and oxygen.
Cell membrane: Every cell is enclosed in a membrane, a double layer of phospholipids (lipid bilayer). The
exposed heads of the bilayer are "hydrophilic" (water loving), meaning that they are compatible with water
both within the cytosol and outside of the cell. However, the hidden tails of the phosopholipids are
"hydrophobic" (water fearing), so the cell membrane acts as a protective barrier to the uncontrolled flow of
water. The membrane is made more complex by the presence of numerous proteins that are crucial to cell
activity. These proteins include receptors for odors, tastes and hormones, as well as pores responsible for
the controlled entry and exit of ions like sodium (Na+) potassium (K+), calcium (Ca++) and chloride (Cl-).
Mitochondria: Mitochondria provide the energy a cell needs to move, divide, produce secretory products,
contract - in short, 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. Mitochondria are membrane-bound organelles, and
like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane
is highly convoluted, forming folds (cristae) as seen in the cross-section, above. The cristae greatly
increase the inner membrane'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.
Vacuole: 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
Smooth endoplasmic reticulum: Throughout the eukaryotic cell, especially those
responsible for the production of hormones and other secretory products, is a vast
network of membrane-bound vesicles and tubules called the endoplasmic
reticulum, or ER for short. The ER is a continuation of the outer nuclear membrane
and its varied functions suggest the complexity of the eukaryotic cell. The smooth
endoplasmic reticulum is so named because it appears smooth by electron
microscopy. Smooth ER plays different functions depending on the specific cell type
including lipid and steroid hormone synthesis, breakdown of lipid-soluble toxins in
liver cells, and control of calcium release in muscle cell contraction.
Rough endoplasmic reticulum: Rough endoplasmic reticulum appears
"pebbled" by electron microscopy due to the presence of numerous
ribosomes on its surface. Proteins synthesized on these ribosomes
collect in the endoplasmic reticulum for transport throughout the
Ribosomes: Ribosomes are packets of RNA and protein that play a crucial role in
both prokaryotic and eukaryotic cells. They are the site of protein synthesis. Each
ribosome comprises two parts, a large subunit and a small subunit. Messenger
RNA from the cell nucleus is moved systematically along the ribosome where
transfer RNA adds individual amino acid molecules to the lengthening protein