90
UNIT ONE
in specialized regions of certain chromosomes. The
nucleolus is the site of ribosome production. Once
ribosomes form, they migrate through the nuclear
pores to the cytoplasm. A cell may have more than one
nucleolus. The nuclei of cells that synthesize abundant
protein, such as those of glands, may have especially
large nucleoli.
2.
Chromatin.
Chromatin consists of loosely coiled F
bers
in the nuclear fl
uid. Chromatin F
bers are composed of
continuous DNA molecules wrapped around clusters
of proteins called histones, giving the appearance
of beads on a string (see F
g. 4.19). Chromatin is the
material that becomes organized and compacted to form
chromosomes.
When cell division begins, these F
bers
more tightly coil to form the rodlike chromosomes.
The DNA molecules contain genes, the information for
synthesis of proteins. The tightness in which chromatin
is folded locally varies along the chromosomes,
depending upon which genes are being accessed for
their information at a particular time. “Chromatin”
means colored substance, and “chromosome” means
colored body.
Table 3.2
summarizes the structures and functions of cell
parts.
PRACTICE
14
How are the nuclear contents separated from the cytoplasm?
15
What is the function of the nucleolus?
16
What is chromatin?
3.3
MOVEMENTS INTO AND OUT
OF THE CELL
The cell membrane is a barrier that controls which sub-
stances enter and leave the cell. Oxygen and nutrient mole-
cules enter through this membrane, whereas carbon dioxide
and other wastes leave through it. These movements involve
physical
(or passive) processes, such as diffusion, osmosis,
facilitated diffusion, and filtration, and
physiological
(or
active) processes, such as active transport, endocytosis, and
exocytosis. Understanding the mechanisms that transport
substances across the cell membrane is important for under-
standing many aspects of physiology.
Diffusion
Diffusion
(dı˘-fu
zhun) (also called simple diffusion) is the
tendency of atoms, molecules, and ions in a liquid or air
solution to move from areas of higher concentration to areas
of lower concentration, thus becoming more evenly distrib-
uted, or more
diffuse.
Diffusion occurs because atoms, mol-
ecules, and ions are in constant motion. Each particle travels
in a separate path along a straight line until it collides with
The nucleus contains a fluid (nucleoplasm) in which
other structures are suspended. These structures include the
following:
1.
Nucleolus.
A nucleolus (nu-kle
o-lus) (“little nucleus”)
is a small, dense body largely composed of RNA and
protein. It has no surrounding membrane and is formed
Mitochondrion
Nucleus
Rough
endoplasmic
reticulum
Cell membrane
Microfilaments
(a)
Ribosome
Microtubules
Vesicle
FIGURE 3.18
The cytoskeleton provides an inner scaF
olding.
(
a
) Microtubules and micro±
laments help maintain the shape of a cell by
forming an internal framework beneath the cell membrane and in the
cytoplasm. (
b
) A falsely colored electron micrograph of cells showing
the cytoskeletons (orange and yellow) (750×).
(b)
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