Page 78 - Fundamentals of anatomy physiology

In many cases, the steps in the synthetic pathway differ from
those in the decomposition pathway, and separate enzymes
are often involved.
It takes activation energy to start a chemical reaction, but ingredients. In contrast, carbon and hydrogen always form the
once it has begun, the reaction as a whole may absorb or re-
lease energy as it proceeds to completion. If the amount of
e­ nergy released is greater than the activation energy needed to
start the reaction, there will be a net release of energy. Reac-
tions that release energy are said to be exergonic (exo-, out-
side + ergon, work). Exergonic reactions are relatively common
in the body. They generate the heat that maintains your body
temperature.
If more energy is required to begin the reaction than is re- properties establish the conditions necessary for life. Most of
leased as it proceeds, the reaction as a whole will absorb en-
ergy. Such reactions are called endergonic (endo-, inside). The
synthesis of molecules such as fats and proteins result from
endergonic reactions.
Checkpoint
11. What is an enzyme?
1 2. Why are enzymes needed in our cells?
See the blue Answers tab at the back of the book.
2-5 Inorganic compounds
lack carbon, and organic compounds
contain carbon
Learning Outcome Distinguish between organic compounds
and inorganic compounds.
The human body is very complex, but it contains relatively
few elements (Table 2–1, p. 54). Just knowing the identity
and quantity of each element in the body will not help you
understand the body any more than only memorizing the al-
phabet will help you understand this textbook. Just as 26 let-
ters can be combined to form thousands of different words
in this book, only about 26 elements combine to form thou-
sands of different chemical compounds in our bodies. As we
saw in Chapter 1, these compounds make up the living cells
that form the framework of the body and carry on all its life
processes. Learning about the major classes of chemical com-
pounds will help you to understand the structure and function
of the human body.
We now turn our attention to nutrients and metabo-
lites. Nutrients are the substances from food that are neces-
sary for normal physiological functions. Nutrients include
carbohydrates, proteins, fats, vitamins, minerals, and water.
­Metabolites (me-TAB-o. -lı.ts; metabole, change) are substances
that are involved in, or a byproduct of, metabolism. We
can broadly categorize nutrients and metabolites as either
Chapter 2 The Chemical Level of Organization 65
inorganic or organic. Inorganic compounds generally do not
contain carbon and hydrogen atoms as their primary structural
2basis for organic compounds.
The most important inorganic compounds in the body
are (1) carbon dioxide, a byproduct of cell metabolism; (2)
oxygen, an atmospheric gas required in important metabolic
reactions; (3) water, which accounts for most of our body
weight; and (4) inorganic acids, bases, and salts—compounds
held ­together partially or completely by ionic bonds. In the
next section, we focus on water, its properties, and how those
the other inorganic molecules and compounds in the body
exist in association with water, the primary component of our
body fluids. Both carbon dioxide and oxygen, for example, are
gas molecules that are transported in body fluids. Also, all the
inorganic acids, bases, and salts we will discuss are dissolved
in body fluids.
Checkpoint
1 3. Compare organic compounds to inorganic compounds.
See the blue Answers tab at the back of the book.
2-6 Physiological systems
depend on water
Learning Outcome Explain how the chemical properties of water
make life possible.
Water (H2O) is the most important substance in the body. It
makes up to two-thirds of total body weight. A change in the
body’s water content can be fatal because virtually all physi-
ological systems will be affected.
Although water is familiar to everyone, it has some highly
unusual properties. These properties are due to the hydrogen
bonding between nearby water molecules.
1. Solubility. A remarkable number of inorganic and organic
molecules are soluble, meaning they will dissolve or break
up in water. The individual particles become distributed
within the water, and the result is a solution—a uniform
mixture of two or more substances. The liquid in which
other atoms, ions, or molecules are distributed is called
the solvent. The dissolved substances are the solutes. In
aqueous solutions, water is the solvent. The solvent proper-
ties of water are so important that we will consider them
further in the next section.
2. Reactivity. In our bodies, chemical reactions take place in
water, but water molecules also take part in some reac-
tions. Hydrolysis and dehydration synthesis are two exam-
ples noted earlier in the chapter.