44 Chapter 3
K+^ K+^ Na+ Na+^ + (^)
Na (^)
K+ (^)
2 3 K+^
(^1 4)
P (^)
Na+
Na+ (^) ADP P (^)
Na+^ ATP^ K+^ K^ +^
Figure 3- 5 The protein nature of the sodium-potassium pump of the cell membrane of muscle and nerve cells.
(^) ®
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Cytoplasm of the Cell
The liquid portion of a cell is called protoplasm. The pro-
toplasm outside the nucleus is called cytoplasm; the pro-
toplasm inside the nucleus is called nucleoplasm. The main
constituent of cytoplasm is water. This water, how-ever,
has many different kinds of chemical compounds
distributed among the water molecules. Some of these
compounds are nucleic acids like transfer ribonucleic acid
(RNA) and messenger RNA, enzymes, hormones, and
various other chemicals involved in the functioning of the
cell. Some of these compounds are in solution in the water,
whereas others are in a colloidal suspension. In both
solutions and colloids, substances are uniformly distributed
throughout the water medium. In a solu-tion, however,
individual atoms or ions are distributed throughout the
medium. In a colloid, clumps of atoms rather than
individual atoms are distributed throughout the medium.
The factor that determines whether a substance will go
into solution or a colloidal suspension in water is the
electronic interaction between the molecules of the sub-
stance and the molecules of water. Because the oxygen
atom in H 2 O has a stronger attraction for the electrons in
the H{O bond than the hydrogen atom (it shares the
electrons unequally), the oxygen atom is slightly nega-tive
and the two hydrogen atoms are slightly positive. Refer to
Figure 2-5 in Chapter 2. A molecule with such an unequal
electron distribution of bonding electrons is said to be
polar. Because of this polarity of the water molecule, other
polar compounds, like ionically bonded compounds such as
salt (sodium chloride), are readily soluble in water and go
into solution. The polarity of the water molecules lessens
the electrostatic forces holding ionically bonded molecules
together so that they disso-ciate into individual ions and
dissolve in the water.
Other compounds such as covalently bonded mole-
cules are made up of atoms that have equal attraction for
the bonding electrons that hold them together. Thus, the
bonding electrons are not attracted to one atom of the bond
more than the other. Compounds with such un-polarized
bonds are called nonpolar and do not dissolve readily in
water. The organic compounds with the C-H bonds are
nonpolar and thus go into a colloidal suspen-sion in the
watery medium of the cytoplasm. Proteins, carbohydrates,
fats, and nucleic acids are colloidally sus-pended in the
cytoplasm, whereas the mineral salts like sodium,
potassium, calcium, chlorine, and phosphorous are in
solution.
Some cellular components, such as storage granules
and fat droplets, are neither dissolved nor suspended in the
cytoplasm. These compounds are products of cellu-lar
functions that have collected at certain specific sites within
the cytoplasm. The cytoplasm will also contain structures
called vacuoles. A vacuole is an area within the
cytoplasm that is surrounded by a vacuolar membrane. This
membrane has the same structure as the cell mem-brane. A
vacuole is generally filled with a watery mixture but can
also contain stored food (food vacuole) or waste products
of the cell (waste vacuole).
The Nucleus
The nucleus is the most prominent structure in the cell. It
is clearly visible with a light compound microscope. It is a
fluid-containing structure that is separated from the
cytoplasm by the nuclear membrane, sometimes
referred to as the nuclear envelope. The nucleus is the
control center of the cell. Cells whose nuclei have been
removed lose their functions. Cells with a nucleus
transplanted from a different cell take on the characteristics
of the cell from which the nucleus was taken.