Cell Structure and Genetic Control 57
cells. These structures aid in amoeboid movement, for exam-
ple, so that the cytoskeleton is also the cell’s “musculature.”
Microtubules, which are polymers of tubulin proteins, form a
“track” along which motor proteins move their cargo through
the cytoplasm (as described shortly). Additionally, microtu-
bules form the spindle apparatus that pulls chromosomes away
from each other in cell division. Microtubules also form the
central parts of cilia and flagella and contribute to the structure
and movements of these projections from the cells.
The cytoskeleton forms an amazingly complex “rail-
way” system in a cell, on which large organelles (such as the
nucleus), smaller membranous organelles (such as vesicles),
and large molecules (including certain proteins and messenger
RNA) travel to different and specific destinations. The molecu-
lar motors that move this cargo along their cytoskeletal tracks
are the proteins myosin (along filaments of actin) and kinesins
and dyneins (along microtubules). One end of these molecular
motors attaches to their cargo while the other end moves along
the microfilament or microtubule. For example, vesicles are
moved in an axon (nerve fiber) toward its terminal by kinesin,
while other vesicles can be transported in the opposite direc-
tion along the microtubule by dynein.
The cytoplasm of some cells contains stored chemicals in
aggregates called inclusions. Examples are glycogen granules
Cytoplasm and Cytoskeleton
The material within a cell (exclusive of that within the nucleus)
is known as cytoplasm. Cytoplasm contains structures called
organelles that are visible under the microscope, and the fluid-
like cytosol that surrounds the organelles. When viewed in a
microscope without special techniques, the cytoplasm appears
to be uniform and unstructured. However, the cytosol is not a
homogeneous solution; it is, rather, a highly organized struc-
ture in which protein fibers—in the form of microtubules and
microfilaments —are arranged in a complex latticework sur-
rounding the membrane-bound organelles. Using fluorescence
microscopy, these structures can be visualized with the aid
of antibodies against their protein components ( fig. 3.7 ). The
interconnected microfilaments and microtubules are believed
to provide structural organization for cytoplasmic enzymes and
support for various organelles.
The latticework of microfilaments and microtubules is said
to function as a cytoskeleton ( fig. 3.8 ). The structure of this
“skeleton” is not rigid; it is capable of quite rapid movement
and reorganization. Contractile proteins—including actin and
myosin, which are responsible for muscle contraction—are
associated with the microfilaments and micro-tubules in most
Figure 3.7 Immunofluorescence micrograph showing
microtubules. In these fibroblast cells, the microtubules are
green and the nuclei are blue.
Figure 3.8
The formation
of the
cytoskeleton by
microtubules.
Microtubules are
also important
in the motility
(movement)
of the cell and
movement of
materials within
the cell.
Plasma
membrane
Mitochondrion
Polysome
Microtubule
Endoplasmic
reticulum
Ribosome
Nuclear
envelope
LEARNING OUTCOMES
After studying this section, you should be able to:
- Describe the structure and function of the
cytoskeleton, lysosomes, peroxisomes,
mitochondria, and ribosomes. - Describe the structure and functions of the
endoplasmic reticulum and Golgi complex, and
explain how they interact.