Human Physiology, 14th edition (2016)

56 Chapter 3

3.2 Cytoplasm and Its Organelles

Many of the functions of a cell are performed by structures

called organelles. Among these are the lysosomes, which

contain digestive enzymes, and the mitochondria, where

most of the cellular energy is produced. Other organ-

elles participate in the synthesis and secretion of cellular

products.

cilium. Centrosomes are also involved in the process of pulling
duplicated chromosomes apart, as discussed in section 3.5.
Sperm cells are the only cells in the body that have flagella.
The flagellum is a single, whiplike structure that propels the
sperm through its environment. Like the motile cilia, a flagel-
lum is composed of microtubules with a “9  1  2” arrangement.
The subject of sperm motility by means of flagella is consid-
ered with the reproductive system in chapter 20.

Microvilli

In areas of the body that are specialized for rapid diffusion,
the surface area of the cell membranes may be increased by
numerous folds called microvilli. The rapid passage of the
products of digestion across the epithelial membranes in the
intestine, for example, is aided by these structural adaptations.
The surface area of the apical membranes (the part facing the
lumen) in the intestine is increased by the numerous tiny fin-
gerlike projections ( fig.  3.6 ). Similar micro-villi are found in
the epithelia of the kidney tubules, which must reabsorb vari-
ous molecules that are filtered out of the blood.

Figure 3.5 Cilia, as seen with the electron microscope. ( a ) Scanning electron micrograph of cilia on the epithelium lining
the trachea; ( b ) transmission electron micrograph of a cross section of cilia, showing the “9  1  2” arrangement of microtubules within
each cilium.

Cilia

(a) 10 μm (b) 0.15 μm

Figure 3.6 Microvilli in the small intestine. Microvilli

are seen in this colorized electron micrograph, which shows two

adjacent cells joined together by junctional complexes.

Junctional

complexes

Lumen

Microvilli

| CHECKPOINTS

1a. Describe the structure of the plasma membrane.

1b. Describe the structure and function of cilia, flagella,

and microvilli.

2a. Describe the different ways that cells can engulf

materials in the extracellular fluid.

2b. Explain the process of exocytosis.