89
CHAPTER THREE
Cells
skin layer, the epidermis. Here they form a strong inner
scaffolding that helps the cells attach and form a barrier.
11.
Other structures.
In addition to organelles, cytoplasm
contains chemicals called
inclusions.
These usually are
in a cell temporarily. Inclusions include stored nutrients,
such as glycogen and lipids, and pigments, such as
melanin in the skin.
PRACTICE
8
What are the functions of the endoplasmic reticulum?
9
Describe how the Golgi apparatus functions.
10
Why are mitochondria called the “powerhouses” of cells?
11
How do lysosomes function?
12
Describe the functions of microF
laments and microtubules.
13
Distinguish between organelles and inclusions.
Cell Nucleus
A nucleus is a relatively large, usually spherical, structure
that contains the genetic material (DNA) that directs the
activities of the cell. The extremely long molecules of DNA
are complexed with proteins to form dense, string-like chro-
matin F
bers.
The nucleus is enclosed in a double-layered
nuclear
envelope,
which consists of an inner and an outer lipid bilayer
membrane. These two membranes have a narrow space
between them, but are joined at places that surround open-
ings called
nuclear pores.
These pores are not bare holes, but
channels whose walls consist of more than 100 different types
of proteins. Nuclear pores allow certain dissolved substances
to move between the nucleus and the cytoplasm
(f g. 3.19)
.
Molecules of messenger RNA that carry genetic information
exit the nucleus through nuclear pores.
MicroF laments are tiny rods of the protein
actin that typically form meshworks or bundles and
provide certain cellular movements. In muscle cells,
for example, microF laments constitute
myof
brils,
which shorten or contract these cells. In other cells,
microF laments associated with the inner surface of the
cell membrane aid cell motility
(f
g. 3.17)
.
Microtubules are long, slender tubes with diameters
two or three times greater than those of microF
laments.
They are composed of the globular protein tubulin.
Microtubules are usually somewhat rigid, which helps
maintain the shape of the cell
(f
g. 3.18)
. In cilia and
fl agella, microtubule interactions provide movement
(see F
gs. 3.15 and 3.16). Microtubules also move
organelles and other cellular structures. ±or instance,
microtubules form centrioles, and also provide conduits
for organelles, like the tracks of a roller coaster.
All cells have microtubules and microF
laments, but
some specialized cells have a third type of cytoskeletal
component, intermediate F
laments. These are composed
of any of several types of proteins and take the general
form of dimers (protein pairs) entwined into nested,
coiled rods. Intermediate F
laments are abundant in the
actively dividing cells in the deepest part of the outer
FIGURE 3.16
±lagella form the tails of these human sperm cells
(1,400×).
Microtubules
Microfilaments
FIGURE 3.17
A falsely colored transmission electron micrograph of
microF
laments and microtubules in the cytoplasm (35,000×).
In a group of inherited disorders called epidermolysis bullosa, the
skin blisters easily as tissue layers separate due to abnormal inter-
mediate F
laments. A British documentary, called
The Boy Whose Skin
Fell Of
,
traces the life of a person with a severe form of the disease.
Experimental stem cell therapy and gene therapy have had encour-
aging results in treating epidermolysis bullosa.
previous page 119 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 121 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off