Organization of the Body
High heat capacity.
Water has a high heat capacity. In other
words, it absorbs and releases large amounts of heat before
changing appreciably in temperature itself. Tis property of
water prevents sudden changes in temperature caused by ex-
ternal factors, such as sun or wind exposure, or by internal
conditions that release heat rapidly, such as vigorous mus-
cle activity. As part of blood, water redistributes heat among
body tissues, ensuring temperature homeostasis.
High heat of vaporization.
When water evaporates, or va-
porizes, it changes from a liquid to a gas (water vapor). Tis
transformation requires that large amounts of heat be ab-
sorbed to break the hydrogen bonds that hold water mol-
ecules together. Tis property is extremely beneﬁcial when
we sweat. As perspiration (mostly water) evaporates from
our skin, large amounts of heat are removed from the body,
providing eﬃcient cooling.
Polar solvent properties.
Water is an unparalleled solvent.
Indeed, it is oFen called the
is “wet chemistry.” Biological molecules do not react chemi-
cally unless they are in solution, and virtually all chemical
reactions occurring in the body depend on water’s solvent
Because water molecules are polar, they orient them-
selves with their slightly negative ends toward the positive
ends of the solutes, and vice versa, ﬁrst attracting the sol-
ute molecules, and then surrounding them. Tis polarity
of water explains why ionic compounds and other small
reactive molecules (such as acids and bases)
water, their ions separating from each other and becom-
ing evenly scattered in the water, forming true solutions
Water also forms layers of water molecules, called
, around large charged molecules such as proteins,
shielding them from the eﬀects of other charged substances
in the vicinity and preventing them from settling out of so-
lution. Such protein-water mixtures are
Blood plasma and cerebrospinal ﬂuid (which surrounds the
brain and spinal cord) are examples of colloids.
Water is the body’s major transport medium because it is
such an excellent solvent. Nutrients, respiratory gases, and
metabolic wastes carried throughout the body are dissolved
in blood plasma, and many metabolic wastes are excreted
from the body in urine, another watery ﬂuid. Specialized
molecules that lubricate the body (e.g., mucus) also use water
as their dissolving medium.
Water is an important
in many chemi-
cal reactions. ±or example, foods are digested to their build-
ing blocks by adding a water molecule to each bond to be
broken. Such decomposition reactions are more speciﬁcally
ĭ-sis; “water splitting”).
Conversely, when large carbohydrate or protein molecules
are synthesized from smaller molecules, a water molecule is
removed for every bond formed, a reaction called
By forming a resilient cushion around certain
body organs, water helps protect them from physical trauma.
equilibrium eventually occurs unless additional reactants are
added or products are removed from the reaction site.
Smaller particles move faster than larger ones
(at the same temperature) and tend to collide more frequently
and more forcefully. Hence, the smaller the reacting particles,
the faster a chemical reaction goes at a given temperature and
Many chemical reactions in nonliving systems
can be speeded up simply by heating, but drastic increases in
body temperature are life threatening because important bio-
logical molecules are destroyed. Still, at normal body tempera-
tures, most chemical reactions would proceed far too slowly to
maintain life were it not for the presence of catalysts.
ah-lists) are substances that increase the rate of chemical
reactions without themselves becoming chemically changed
or part of the product. Biological catalysts are called
zīmz). Later in this chapter we describe how enzymes work.
Check Your Understanding
Which reaction type—synthesis, decomposition, or
exchange—occurs when fats are digested in your small
Why are many reactions that occur in living systems
irreversible for all intents and purposes?
What speciﬁc name is given to decomposition reactions in
which food fuels are broken down for energy?
For answers, see Appendix H.
is the study of the chemical composition and re-
actions of living matter. All chemicals in the body fall into one
of two major classes: organic or inorganic compounds.
contain carbon. All organic compounds are cova-
lently bonded molecules, and many are large.
All other chemicals in the body are considered
. Tese include water, salts, and many acids and
bases. Organic and inorganic compounds are equally essential
for life. ²rying to decide which is more valuable is like trying
to decide whether the ignition system or the engine is more es-
sential to run your car!
Explain the importance of water and salts to body homeostasis.
Deﬁne acid and base, and explain the concept of pH.
Water is the most abundant and important inorganic com-
pound in living material. It makes up 60–80% of the volume of
most living cells. What makes water so vital to life? Te answer
lies in several properties: