Human Physiology, 14th edition (2016)

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:

3. Describe the structure and function of the
   cytoskeleton, lysosomes, peroxisomes,
   mitochondria, and ribosomes.


4. Describe the structure and functions of the
   endoplasmic reticulum and Golgi complex, and
   explain how they interact.