Eukaryotic cells also contain other membrane-bound organelles
within their cytoplasm (the region between the nucleus and the plasma
membrane). These subcellular structures vary tremendously in structure
and function.
Most eukaryotic cells have mitochondria, which contain the enzymes
and machinery for aerobic respiration and oxidative phosphorylation.
Thus, their main function is generation of adenosine triphosphate
(ATP), the primary currency of energy exchanges within the cell. This organelle
is bounded by a double membrane. The inner membrane, which
houses the electron transport chain and the enzymes necessary for ATP
synthesis, has numerous foldings called cristae, which protrude into the
matrix, or central space. Mitochondria contain their own DNAand ribosomes,
but most of their proteins are imported from the cytoplasm.
Chloroplasts contain the photosynthetic systems for utilizing the radiant
energy of sunlight and are found only in plants and algae. Photosynthesis is the process that converts light energy into the chemical bond
energy of ATP, which in turn can be used to convert carbon dioxide (CO
2)
and water (H
2O) into carbohydrates. Chloroplasts contain an internal system
of membranes called thylakoids, a circular chromosome, and their
own ribosomes. The flattened, vesicular thylakoids contain the chlorophyll
pigments, the enzymes, and other molecules needed to harness light
energy for conversion to chemical energy. Carbon fixation occurs in the
stroma, the space between the thylakoids and the inner membrane.
 |
| Figure 1-1 An animal cell. |
Prokaryotic cells lack internal membranes, but photosynthetic bacteria
contain invaginations of the plasma membrane called mesosomes.
Centrioles, located within the centrosome, are associated with the
cell’s polar regions, toward which the chromosomes migrate during cell
division, and are found only in animal cells. The endoplasmic reticulum
(ER) amplifies the surface area available for specialized biochemical reactions
and the synthesis of certain types of proteins. The Golgi complex
directs the transport of proteins and other biomolecules to specific locations
within the cell. Vacuoles serve as storage compartments for food,
water, or other molecules. Enzymes digest materials brought into the cell
within lysosomes.
Ribosomes function in the manufacture of proteins. The ribosomes
in prokaryotes are smaller than those found in the cytoplasm in eukaryotes,
but are similar in size and structure to those found in the mitochondria
and chloroplasts of eukaryotes. Eukaryotic ribosomes associated
with the ER give it a granular appearance, hence the name rough ER.
Motility is accomplished by different means in prokaryotic and eukaryotic
cells. Eukaryotic cells, such as amoebas and white blood cells,
creep along substrates as an undulating mass of constantly changing morphology.
This type of motion is achieved by a massive network of protein
fibers, the cytoskeleton. Motile bacteria are usually propelled by one or
more hairlike appendages called flagella that originate in the plasma membrane
and rotate like propellar shafts (Figure 1-2). These filaments are
constructed of the protein flagellin. Some eukaryotic cells also have flagella,
but they consist of bundles of microtubules made of tubulin, and
they originate from a basal body in the cytoplasm. Eukaryotic flagella
such as those in sperm tails bend back and forth in quasi-sinusoidal waves.
Eukaryotic cilia are structurally similar but are much shorter, more numerous,
and more rigid on the powerstroke. Some bacteria also have long
hollow tubes called pili or fimbriae composed of a protein called pilin.
These structures do not contribute to motility, but to the adhesiveness of
bacteria and the facilitation of conjugation.
One of the distinguishing features between plants and animals is that
plants and fungi have cell walls made of cellulose and chitin, respectively,
but animal cells do not. Almost all bacteria have a rigid cell wall surrounding
the plasma membrane, but it has a different structure than the
plant cell wall and is composed of peptidoglycan. Some bacteria also
have a polysaccharide capsule or a glycocalyx surrounding the cell wall.
 |
| Figure 1-2 A bacterial cell. |
These protect the bacteria from predatory cells and promote their attachment
to various objects and to each other. Most eukaryotic cells also have
a glycocalyx that covers the surface of the cell and promotes cell adhesions
in the formation of specific tissues. In addition, many types of animal
cells are surrounded by an extracellular matrix, which comprises a
variety of proteins that give specific tissues their characteristic properties.
Notes:
Mitochondria are nicknamed the “powerhouses” of
the cell because of their role in ATP production.
Proteins that are:
1. membrane bound
2. secreted
3. compartmentalized
are synthesized on the ER.
