Eucaryotic Cell Organelles
The cells
of protozoa, higher plants and animals are highly structured. These
cells tend to be larger than the cells of bacteria, and have developed
specialized packaging and transport mechanisms that may be necessary
to support their larger size. Use the Interactive animation of plant
and animal cells to learn about their respective organelles.
: The nucleus is the most
obvious organelle in any eukaryotic cell. It is enclosed in a double
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.
: 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 synthesis.
: 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.
: This is
a collective term for the cytosol plus the organelles suspended within
the cytosol.
: 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 cells.
(animal cells only): 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.
: 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.
: 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 are membrane-bound
packets of oxidative enzymes. In plant cells, peroxisomes play a
variety of roles including converting fatty acids to sugar and assisting
chloroplasts in photorespiration. 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 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.
: 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 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.
: 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. Vacuoles tend to be large in plant cells and play several
roles: storing nutrients and waste products, helping increase cell
size during growth, and even acting much like lysosomes of animal
cells. The plant cell vacuole also regulates turgor pressure in the
cell. Water collects in cell vacuoles, pressing outward against the
cell wall and producing rigidity in the plant. Without sufficient
water, turgor pressure drops and the plant wilts.
(plant cells only): Plant
cells have a rigid, protective cell wall made up of polysaccharides.
In higher plant cells, that polysaccharide is usually cellulose.
The cell wall provides and maintains the shape of these cells and
serves as a protective barrier. Fluid collects in the plant cell
vacuole and pushes out against the cell wall. This turgor pressure
is responsible for the crispness of fresh vegetables.
(plant cells
only): Chloroplasts are specialized organelles
found in all higher plant cells. These organelles contain
the plant
cell's chlorophyll responsible for the plant's green color
and the ability to absorb energy from sunlight. This energy is used to
convert water plus atmospheric carbon dioxide into metabolizable sugars
by the biochemical process of photosynthesis. Chloroplasts
have a double outer membrane. Within the stroma are other
membrane
structures - the thylakoids. Thylakoids appear in stacks
called "grana" (singular
= granum).
: 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 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 cell.
: 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 chain.
: 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 (microfilaments)
- intermediate fibers
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