Mitochondria contains the
molecular machinery for the release of energy from the phosphate bonds of
adenosine triphosphate (ATP), which becomes available to power cellular functions. Mitochondria
consists mainly of
protein, but also contains lipid, DNA and RNA. Under the electron
microscope, thesespherical organelles have an outer membrane surrounding an
inner membrane that folds as cristae into a scaffolding for oxidative
phosphorylation and electron transport enzymes. The mitochondrial matrix
contains the enzymes of the citric acid cycle and fatty acid oxidation.
Mitochondrial DNA is
double-stranded and circular. Mitochndrial RNA are classified as ribosomal, messenger and
transfer. The metabolic activity of the cell is proportional to the
number of mitochondria and the number of cristae. Cells
with high metabolic activity have large amounts of well-developed mitochondria.
Ribosomes are small organelles composed of ribosomal RNA (rRNA) and 80
different proteins. Ribosomal subunits are assembled in nucleolus from rRNA and imported
proteins from the cytoplasm. Following assembly, the ribosomal subunits pass
to the cytoplasm
where they participate in protein synthesis. Some ribosomes are free in
the cytoplasm and can integrate with a polyribosomal structure; other ribosomes are attached to the
reticulum. Synthesised proteins may be designated for secretion,
storage, or membrane-integration.
Within the cytoplasm of cells is a configuration of connecting and
branching channels constituted by a continuous membrane known as the
endoplasmic reticulum (ER). ER are classified as rough ER when
ribosomes are attached to the cytosolic side of the membrane, and
classified as smooth
ER when ribosomes are absent. The main function of rough ER is the
segregation of proteins destined for export from the cell or for
intracellular use. Proteins are modified within the ER by the addition
of carbohydrate, removal of a signal sequenc, and other post-translational
Smooth ER appears more tubular than rough ER.
Smooth ER allows the performance of specialized functions: steroid sysnthesis, metabolism and detoxification of substances in
the liver, phospholipid synthesis, and excitation-contraction
The Golgi, a curved membrane stack,
finishes the post-transitional modifications, concentrates and packages
proteins for export or storage. Proteins made within the rough ER
bud off in vesicles and are transported to the Golgi appartus where the vesicles
fuse with the membrane and the components are further modified,
concentrated and packaged, before budding off as vesicles on the
Lysosomes are vesicles containing
more than 40 hydrolytic enzymes that can digest most macromolecules. These organelles are the sites of intracellular
digestion. Common lysosomal enzymes include acid phosphatase,
ribonuclease, deoxyribonuclease, proteases, sulfatases, and lipases. The
limiting membrane and the inactivity of the enzymes to function at the
cytosol pH prevents the
cell from digesting itself.
Primary lysosomes are small concentrated sacs of enzymes. Primary lysosomes fuse with a phagocytic vacuole to
become secondary lysosomes where digestion begins. As
the substances are digested the nutrients diffuse through the lysosomal
membrane to the cytosoplasm. Residual bodies are formed when indigestable
elements are retained. Lysosomes also participate in the turnover of cellular organelles.
These small organelles contain oxidative enzymes.
Peroxisomes contain amino oxidases, hydroxyacid oxidase and catalase.
Catalase protects the cell from hydrogen peroxide damage. Enzymes
involved in lipid metabolism are also found in peroxisomes.
Secretory granules are found in cells that store and release hormones, neurotransmitters or
digestive enzymes. These vesicles contain a concentrated form of the secretory product.
The cytoskeleton is a complex network of filaments, anchor proteins, and
protein motors that constitute a support and transport scaffold. This
network confers cellular shape and facilitates cellular
These variable length tubules have a dense wall and central hollow core. Microtubules are composed of
repeating heterodimers of alpha and beta tubulin.
Microtubules provide the necessary intracellular transportation system for the
movement of organelles and vesicles from one place to another. Molecular
motors such as dynien and kinesin transport packages via the
microtubules in an energy-requiring process.
Actin and myosin are microfilament proteins responsible for contraction
in muscle cells. Myosin in motile nonmuscle cells polymerizes to effect cell movement.
Microfilaments form thin sheaths beneath cell membranes associated with endocytosis, exocytosis and cell movement.
Intermediate filaments are
made of several proteins. These filaments include keratins, vinmentin,
desmin, glial fibrillary acidic protein, neurofilaments, and nuclear
These cylindric organelles participate in cell division as microtubule
organizing centers. Centrioles are composed of tubulin in a
characteristic nine microtubular triplets arrangement. A single pair
of centrioles is sited near
the Golgi complex in non-dividing cells. During cell division, a pair of
centrioles moves to opposite poles of the cell to become organizing
centers for the mitotic
CILIA AND FLAGELLA
Cilia and flagella have a microtubular core. Both cilia and flagella
have a core 9 + 2 arrangement of
microtubules covered by a cell membrane. This structure, axoneme, consists of 9 microtubular doublets surrounding an inner core
of two sheathed microtubules. Adjacent doublets are linked to one
another by protein bridges called nexins, and are connected to the central pair by
radial spokes. The tubules of each peripheral pair are called subfibers
A and B. At the base
of each cilia or flagella is a centriole-like structure called the basal
body. Cilia and flagella are motile structures. Movement is effected by
the sliding of adjacent doublets over one another by an energy-requiring
The nucleus is a spherical, sometimes elongated structure that contains
the genetic material, the DNA, of the cell. The highly organized
chromatin is enveloped by a nuclear membrane that contains
numerous pores for the transport of molecules to the cytoplasm. The nuclear envelope
consists of two parallel
unit membranes with an intervening perinuclear space.
Chromatin consists of coiled DNA bound to basic proteins called histones
and non-histone proteins. The degree of coiling
of the chromatin varies during cell activity. There are two types of chromatin:
heterochromatin and light euchromatin. The basic structure of the chromatin is the nucleosome, which consists
of a core of histones with 166 DNA base pairs wrapped around the core.
Nucleosomes have an intervening 48 base-pair linking segment.
This spherical structure within the nucleus is composed of DNA, RNA and
protein. The DNA present codes for rRNA, and are known as
nucleolar organizers. Proteins synthesized in the cytoplasm pass through the nuclear pores and become associated with the newly made rRNA
in the nucleolus. Thereafter, the ribosomal subunits migrate to the
The nuclear matrix, consisting of
proteins, metabolites and ions, is a
fibrillar nucleo-skeleton that forms a scaffold for the folded DNA.
A fibrous lamina lies
just under the nuclear envelope which dissociates and reforms during