The Chemistry of Life
molecules together by forming a bridge between the
negative oxygen atom of one water molecule and the
positive hydrogen atoms of another water molecule.
Hydrogen- bonds also help bind various parts of one mol-
ecule into a three-dimensional shape such as a protein
molecule like an enzyme.
Elements or molecules furnishing electrons during a
reaction are called electron donors (e.g., sodium); those
that gain electrons during the process are called electron
acceptors (e.g., chlorine when salt is formed). Some very
special molecules will gain electrons only to lose them to
some other molecule in a very short time; these are
designated as electron carriers. These molecules are dis-
cussed in Chapter 4 and are very important in making the
cellular energy molecule A TP.
Bonds contain energy, the ability to do work. This
results from the interaction of the electrons and the nu-clei
of the bonded atoms. If we measure the amount of energy
present between two atoms, we discover that the amount
varies as the distance between the atoms changes. When
atoms are close to one another, the paths of their electrons
overlap. The natural repulsion of these negatively charged
electrons tends to drive the two at-oms apart. Thus, the
amount of energy necessary to keep them together is quite
high. This type of bond contains a high degree of energy. If
we break these bonds, as in the breakdown of a glucose
(C 6 H 12 O 6 ) molecule inside a cell, electron carriers in the
cell will use the energy of the released electrons to put
together an ATP molecule. ATP is the high-energy fuel
molecule that the cell needs to function. This high-energy
molecule that is used in the cell is called adenosine
triphosphate. This molecule is constantly being created and
broken down to release its energy to do the cell’s work. It is
abbreviated as ATP. It is created by adding a phosphate to
adenosine diphos-phate. When it is broken down (ATP S
ADP 1 PO 4 ), it -releases the energy contained in the
phosphate bond. We shall discuss this in further detail in
Chapter 4.
COMMON SUBSTANCES IN
LIVING SYSTEMS
There are 10 common substances found in living sys-tems.
They are water, carbon dioxide gas, molecular oxygen,
ammonia, mineral salts, carbohydrates, lipids, proteins,
nucleic acids, and adenosine triphosphate.
Water
Water is the most abundant substance in living cells,
approximately 60% to 80%; plasma, which is the liquid
portion of blood, is 91% water. Water is a small, simple
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molecule composed of two hydrogen atoms covalently
bonded to one oxygen atom. Because the oxygen atom
attracts electrons more strongly than do the hydrogen
atoms, water molecules are polar with a partial posi-tive
charge by the hydrogen atoms and a partial nega-tive
charge by the oxygen atom (Figure 2-5). This unique
feature of the water molecule (unequal sharing of elec-
trons) determines why ionic bonded molecules disso-ciate
in water. Negatively charged ions (e.g., chloride) are
attracted to the positively charged hydrogen atoms, and
positively charged ions (e.g., sodium) are attracted to the
negatively charged oxygen atoms. Thus, the ionic bonded
molecule salt dissociates in water.
Water has a number of roles in cells. It takes part in
some reactions, such as photosynthesis in plant cells, which
supplies our earth with molecular oxygen, and respiration
in both plant and animal cells, which produces energy.Photosynthesis:
6CO 2 1 12H 2 O S C 6 H 12 O 6 1 6O 2 1 6H 2 O
Respiration:
C 6 H 12 O 6 1 6O 2 S 6CO 2 6H 2 O 1
energy in the form of ATP
Digestion of food requires water to break down larger
molecules. This is called hydrolysis. Water serves as a
medium or solvent for other reactions, and water is1 1
Hydrogen (H) Oxygen (O) Hydrogen (H)Oxygen part^ +^ Hydrogen^
parts^Partial negative Partial positive (^)
charge at oxygen charge at (^)
end of molecule hydrogen end (^)
of molecule (^)
(^) +
®^
Water molecule (H 2 O)
Learning^
(^)
or (^) H
Cengage^
H O H O H
©^Figure 2- 5 The uniqueness of the water molecule.