The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 4

Endosomes
Endosomes are membrane-bound vesicles that have a very important role in the
endocytic pathway. They are involved with the movement of material internalized
from the plasma membrane to lysosomes for degradation, or they may recycle the
material back to the plasma membrane itself. Endosomes also transport material
from the Golgi body to lysosomes, and they provide an environment for material to
be sorted before it reaches the degradative lysosome. Endosomes are characterized as
early, late, and recycling endosomes differentiated by their role and specific markers
present on them.
Peroxisomes
Peromisomes are single-membrane-bound vesicles harboring various oxidase enzymes.
Primarily, they regulate fatty acid metabolism and remove toxic peroxides from the
body.
Golgi Body
The Golgi body is composed of membrane-bound cisternae, important organelles of
the cell, that deal with processing and secretion of proteins and lipids in vesicles.
Cytoskeleton and Intracellular Movement
The cell is crisscrossed with a network of cytoskeletal structures that provide it nec-
essary structural support while imparting resilience and capacity to bear external
stress. The three major types of cytoskeletal structures include microtubules, actin
filaments, and intermediate filaments, which fulfill different functions of the cell.
Microtubules are comparatively firm structures composed of tubulin, originating
from the centrosomes and extending into the cytosol. They exhibit dynamism
because they can elongate or shrink by the addition or loss of tubulin subunits. The
nucleation process is a guanosine triphosphate (GTP)-aided mechanism, and the
growing ends of the microtubules are stabilized by a few cytosolic structures. This
stability-enhancing attachment determines the cellular location of the microtubules.
These are further stabilized by specific microtubule-associated proteins (MAPs). The
microtubule motor proteins, kinesins, and dynesins constitute MAP and mediate the
movement along microtubules. Actin filaments are also similar to microtubules in
the dynamism exhibited; they too are rapidly formed and lost. Actin filaments are
present as bundles or networks rather than as filaments. They form a cortical ring
just underneath the plasma membrane and are key regulators of cellular shape and
movement, as well as playing a crucial role in cytokinesis and phagocytosis. The
movement along actin filaments is effected through myosin protein. Intermediate
filaments, long polymers of fibrous polypeptide providing the resilience and stress-
bearing property to the cell, exhibit different chemical composition in different tis-
sues. For example, they are present as keratin filaments in epithelial cells and as