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referred to as the universal solvent. The chemistry of life is
dominated by the chemistry of water. Chemical reac-tions
occur in cells between individual atoms, ions, or molecules,
not between large aggregations of these par-ticles. It is as
these particles move about in the water that they come in
contact with other particles and chemical reactions occur.
In addition, water is a basis for the trans-port of materials
such as hormones and enzymes in the plasma of blood.
Water also absorbs and releases high levels of heat
before its temperature changes, thus helping control normal
body temperature. Vigorous exercise liberates heat from
contracting muscle cells. This excess heat is absorbed by
the water in the cells and then released. Water is part of
amniotic fluid and protects the devel-oping fetus. It is also
part of the cerebrospinal fluid and protects the brain and
spinal cord by functioning as a shock absorber. Finally,
water is the base for all body -lubricants such as mucus in
the digestive tract and -synovial fluid in joints.
Carbon Dioxide
The small carbon dioxide (CO 2 ) molecule contains one
carbon atom covalently bonded to two oxygen atoms. It is
produced as a waste product of cellular respiration and
must be eliminated quickly from the body through
expiration via the respiratory system and the cardiovas-
cular system. It is also necessary for photosynthesis in plant
cells to convert the radiant energy of the sun into usable
chemical energy such as glucose for both plant and animal
cells. It is also a source of the element car-bon, found in all
organic compounds of living systems. If carbon dioxide is
allowed to accumulate within cells, it becomes toxic by
forming carbonic acid as it reacts with water. Hence, we
exhale it quickly from the lungs.
Molecular Oxygen
Molecular oxygen (O 2 ), formed when two oxygen atoms
are covalently bonded together, is required by all organisms
that breathe air. It is necessary to convert chemical energy
(food), such as the energy found in a glucose (C- 6 H 12 O 6 )
molecule, into another form of chemical energy, ATP, that
can be used by cells to do work. Because O 2 is a product of
photosynthesis, it becomes obvious how dependent we
animals are on plants for our survival. Without plants there
would be no molecular oxygen in our atmosphere, and
without O 2 there would be no life on our planet as we know
it. The level of O 2 in our atmosphere is maintained at a
nearly constant level (about 21% of the gas in the
atmosphere is oxygen) by the many different kinds of
plants found on our earth.
Chapter 2Ammonia
The ammonia (NH 3 ) molecule comes from the decom-
position of proteins via the digestive process and the
conversion of amino acids in cellular respiration to ATP
molecules. Note that an important element in ammonia is
nitrogen. Nitrogen is an essential element in amino acids,
which are the building blocks of proteins. Because even a
small amount of ammonia is injurious to cells, the human
body must quickly dispose of this material. Through en-
zymes, the liver converts the toxic ammonia to a harmless
substance called urea. Because urea is soluble in water, the
blood then carries the urea to the kidneys to be filtered and
eliminated from the body as urine. Because many plants are
able to use NH 3 or the products of bacterial action on NH 3
as a nitrogen source for protein synthesis, ammonia is a
common constituent of fertilizers.Mineral Salts/Electrolytes
Mineral salts/electrolytes are composed of small ions.
They are essential for the survival and functioning of the
body’s cells. They function in numerous ways as parts of
enzymes or as portions of the cellular environment
necessary for enzyme or protein action. Calcium (Ca^1 ) is
necessary for muscle contraction and nervous trans-mission
as well as building strong bones. It is the fifth most
abundant element in the body. Phosphate (PO 42 ) is
necessary- to produce the high-energy molecule ATP.
Chloride (Cl^2 ) is necessary for nervous transmission.
Sodium (Na^1 ) and potassium (K^1 ) are also necessary for
muscle cell contraction and nervous transmission.Carbohydrates
Carbohydrates (kar-boh-HIGH-draytz) are made up of the
atoms of carbon, hydrogen, and oxygen in a 1:2:1 ratio- (e.g.,
glucose or C 6 H 12 O 6 ). The smallest carbohy-drates are the
simple sugars that cannot be made to react with water to
produce a simpler form. Sugars are gener-ally chains of either
five or six carbon atoms. Important five-carbon sugars are
ribose and deoxyribose, which are parts of the RNA and
DNA nucleic acid molecules. Impor-tant six-carbon sugars
are glucose and fructose (the suffix ose denotes a sugar)
(Figure 2-6). Note the repetition of the H-C-OH unit in the
molecule. This is typical of sug-ars. Starch, glycogen
(animal starch), cellulose (the ma-terial of plant cell walls that
forms fiber in our diets), chitin (KYE-tin) (the exoskeleton of
arthropods such as insects and lobsters), as well as many other
complex car-bohydrates, are formed by bonding together a
number of glucose molecules. Besides glucose, there are other
six-carbon sugars. Combinations of these with glucose