61
CHAPTER TWO
Chemical Basis of Life
bon dioxide moves from cells into surrounding body fl
uids
and blood, most of it reacts with water to form a weak acid
(carbonic acid, H
2
CO
3
). This acid ionizes, releasing hydrogen
ions (H
+
) and bicarbonate ions (HCO
3
), which blood car-
ries to the respiratory organs. There, the chemical reactions
reverse, and carbon dioxide gas is produced and is eventu-
ally exhaled.
NO (nitric oxide) and CO (carbon monoxide) are two small chemicals
that can harm health, yet are also important to normal physiology.
NO is found in smog, cigarettes, and acid rain. CO is a colorless, odor-
less, gas that is deadly when it leaks from home heating systems or
exhaust pipes in closed garages. However, NO and CO are important
biological messenger molecules. NO is involved in digestion, mem-
ory, immunity, respiration, and circulation. CO functions in the spleen,
which recycles old red blood cells, and in the parts of the brain that
control memory, smell, and vital functions.
Inorganic Salts
Inorganic salts are abundant in body fl
uids. They are the
sources of many necessary ions, including ions of sodium
(Na
+
), chloride (Cl
), potassium (K
+
), calcium (Ca
+2
),
magnesium (Mg
+2
), phosphate (PO
4
–2
), carbonate (CO
3
–2
),
bicarbonate (HCO
3
), and sulfate (SO
4
–2
). These ions play
important roles in metabolism, helping to maintain proper
water concentrations in body fl
uids, pH, blood clotting,
bone development, energy transfer in cells, and muscle
and nerve functions. The body regularly gains and loses
these electrolytes, but they must be present in certain
concentrations, both inside and outside cells, to main-
tain homeostasis. Such a condition is called
electrolyte
balance.
Disrupted electrolyte balance occurs in certain
diseases, and restoring it is a medical priority.
Table 2.6
summarizes the functions of some of the inorganic compo-
nents of cells.
PRACTICE
17
What are the general diF
erences between an organic molecule
and an inorganic molecule?
18
What is the diF
erence between an electrolyte and a
nonelectrolyte?
19
De±
ne electrolyte balance.
Organic Substances
Important groups of organic chemicals in cells include carbo-
hydrates, lipids, proteins, and nucleic acids.
Carbohydrates
Carbohydrates
(kar
bo-hi
dra
¯tz) provide much of the energy
that cells require. They also supply materials to build certain
cell structures, and they often are stored as reserve energy
supplies.
as ether or alcohol. Organic compounds that dissolve in
water usually do not release ions and are therefore called
nonelectrolytes.
Inorganic Substances
Common inorganic substances in cells include water, oxy-
gen, carbon dioxide, and inorganic salts.
Water
Water (H
2
O) is the most abundant compound in living mate-
rial and accounts for about two-thirds of the weight of an
adult human. It is the major component of blood and other
body fl
uids, including fl
uids in cells.
When substances dissolve in water, the polar water
molecules separate molecules of the substance, or even
break them up into ions. These liberated particles are much
more likely to react. Consequently, most metabolic reac-
tions occur in water.
Water is also important in transporting chemicals in the
body. Blood, mostly water, carries oxygen, sugars, salts,
vitamins, and other vital substances from organs of the
digestive and respiratory systems to cells. Blood also car-
ries waste materials, such as carbon dioxide and urea, from
cells to the lungs and kidneys, respectively, which remove
them from the blood and release them outside the body.
Water absorbs and transports heat. Blood carries heat
released from muscle cells during exercise from deeper
parts of the body to the surface. At the same time, skin cells
release water in the form of perspiration that can carry heat
away by evaporation.
Oxygen
Molecules of oxygen gas (O
2
) enter the internal environment
through the respiratory organs and are transported through-
out the body by the blood, especially by red blood cells. In
cells, organelles use oxygen to release energy from nutrient
molecules. The energy then drives the cell’s metabolic activi-
ties. A continuing supply of oxygen is necessary for cell sur-
vival and, ultimately, for the survival of the person.
Carbon Dioxide
Carbon dioxide (CO
2
) is a simple, carbon-containing inor-
ganic compound. It is produced as a waste product when
energy is released during certain metabolic reactions. As car-
The word “organic” has several meanings. Technically, it refers to
carbon-containing chemical compounds (except carbon dioxide),
as opposed to inorganic compounds that do not contain carbon.
“Organic” may also refer to a substance obtained from an organism
or, even more generally, indicate a fundamental characteristic. In
agriculture, “organic” refers to growing crops without the use of syn-
thetic chemicals.
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