Human Physiology, 14th edition (2016)

Cell Structure and Genetic Control 55

Cilia and Flagella

Cilia are tiny hairlike structures that project from the surface of a

cell into the extracellular fluid. Motile cilia (those able to move)

can beat like rowers in a boat, stroking in unison. Such motile

cilia are found in only particular locations in the human body,

where they project from the apical surface of epithelial cells

(the surface facing the lumen, or cavity) that are stationary and

line certain hollow organs. For example, ciliated epithelial cells

are found in the respiratory system and the female reproductive

tract. In the respiratory airways, the cilia transport strands of

mucus to the pharynx (throat), where the mucus can be swal-

lowed or expectorated. In the female reproductive tract, the beat-

ing of cilia on the epithelial lining of the uterine tube draws the

ovum (egg) into the tube and moves it toward the uterus.

Almost every cell in the body has a single, nonmotile pri-

mary cilium. The functions of the primary cilia in most organs

of the body are not presently understood, but primary cilia are

believed to serve sensory functions. For example they are mod-

ified to form part of the photoreceptors in the retina of the eyes

(chapter 10) and are believed to detect fluid movement within

the tubules of the kidneys (chapter 17).

Cilia are composed of microtubules (thin cylinders formed

from proteins) and are surrounded by a specialized part of the

plasma membrane. There are 9 pairs of microtubules arranged

around the circumference of the cilium; in motile cilia, there

is also a pair of microtubules in the center, producing an

arrangement described as “9  1 2” ( fig.  3.5 ). The nonmotile

primary cilium lacks the central pair of microtubules, and so is

described as having a “9  1  0” arrangement.

Within the cell cytoplasm at the base of each cilium is a

pair of structures called centrioles, composed of microtubules

and oriented at right angles to each other (see fig.  3.28 ). The

pair together is called a centrosome. The centriole that points

along the axis of the cilium is also known as the basal body,

and this structure is required to form the microtubules of the

as well as any other molecules that may be present in the extra-
cellular fluid.
Another type of endocytosis involves a smaller area of
plasma membrane, and it occurs only in response to specific mol-
ecules in the extracellular environment. Because the extracellu-
lar molecules must bind to very specific receptor proteins in the
plasma membrane, this process is known as receptor-mediated
endocytosis.
In receptor-mediated endocytosis, the interaction of spe-
cific molecules in the extracellular fluid with specific mem-
brane receptor proteins causes the membrane to invaginate,
fuse, and pinch off to form a vesicle ( fig. 3.4 ). Vesicles formed
in this way contain extracellular fluid and molecules that
could not have passed by other means into the cell. Choles-
terol attached to specific proteins, for example, is taken up
into artery cells by receptor-mediated endocytosis. This is in
part responsible for atherosclerosis (chapter 13, section 13.7).
Hepatitis, polio, and AIDS viruses also exploit the process of
receptor-mediated endocytosis to invade cells.

Exocytosis

Exocytosis is a process by which cellular products are secreted
into the extracellular environment. Proteins and other mol-
ecules produced within the cell that are destined for export
(secretion) are packaged within vesicles by an organelle known
as the Golgi complex. In the process of exocytosis, these secre-
tory vesicles fuse with the plasma membrane and release their
contents into the extracellular environment (see fig.  3.12 ).
Nerve endings, for example, release their chemical neurotrans-
mitters in this manner (chapter 7, section 7.3).
When the vesicle containing the secretory products of the
cell fuses with the plasma membrane during exocytosis, the
total surface area of the plasma membrane is increased. This
process replaces material that was lost from the plasma mem-
brane during endocytosis.

Figure 3.4 Electron micrograph showing endocytosis by a liver cell. The plasma membrane can be seen to invaginate
and create a vesicle that pinches off, containing extracellular material.

Plasma

membrane

Extracellular

Membrane

pouching

inward

Vesicle Cytoplasm

Vesicle within cell