HUMAN ANATOMY & PHYSIOLOGY
Copyright W
iley, 2020
953
CHAPTER 25
The food we eat is our only source of energy for running, walking, and
even
eathing. Many molecules needed to maintain cells and tissues
can be made from simpler precursors by the body’s metabolic reactions;
others—the essential amino acids, essential fatty acids, vitamins, and
minerals—must be obtained from our food. As you learned in Chapter 24,
ca
ohydrates, lipids, and proteins in food are digested by enzymes
and abso
ed in the gastrointestinal tract. The products of digestion
that reach body cells are monosaccharides, fatty acids, glycerol,
monoglycerides, and amino acids. Some minerals and many vitamins
are part of enzyme systems that catalyze the
eakdown and synthesis
of ca
ohydrates, lipids, and proteins. Food molecules abso
ed by the
gastrointestinal (GI) tract have three main fates:
1. Most food molecules are used to supply energy for sustaining life pro-
cesses, such as active transport, DNA replication, protein synthesis,
muscle contraction, maintenance of body temperature, and mitosis.
2. Some food molecules serve as building blocks for the synthesis of
more complex structural or functional molecules, such as muscle
proteins, hormones, and enzymes.
3. Other food molecules are stored for future use. For example, glycogen
is stored in liver cells, and triglycerides are stored in adipose cells.
In this chapter we discuss how metabolic reactions harvest the
chemical energy stored in foods; how each group of food molecules
contributes to the body’s growth, repair, and energy needs; how energy
alance is maintained in the body; and how body temperature is
egulated. Finally, we explore some aspects of nutrition to discover why
you should opt for fish instead of a burger the next time you eat out.
Q Did you ever wonder how fasting and starvation affect the
ody?
Metabolism and Nutrition
Metabolism, Nutrition, and Homeostasis
Metabolic reactions contribute to homeostasis by harvesting chemical energy from consumed nutrients
for use in the body’s growth, repair, and normal functioning.
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954 CHAPTER 25 Metabolism and Nutrition
for less than a minute before being used. Thus, ATP is not a long-term
storage form of cu
ency, like gold in a vault, but rather convenient
cash for moment-to-moment transactions.
Recall from Chapter 2 that a molecule of ATP consists of an adenine
molecule, a ribose molecule, and three phosphate groups bonded to
one another (see Figure XXXXXXXXXXFigure 25.1 shows how ATP links anabolic
and catabolic reactions. When the terminal phosphate group is split
off ATP, adenosine diphosphate (ADP) and a phosphate group (sym-
olized as P ) are formed. Some of the energy released is used to drive
anabolic reactions such as the formation of glycogen from glucose. In
addition, energy from complex molecules is used in catabolic reactions
to combine ADP and a phosphate group to resynthesize ATP:
ADP + P + energy ATP
About 40% of the energy released in catabolism is used for cellular
functions; the rest is converted to heat, some of which helps maintain
normal body temperature. Excess heat is lost to the environment.
Compared with machines, which typically convert only 10–20% of
energy into work, the 40% eff iciency of the body’s metabolism is
impressive. Still, the body has a continuous need to take in and process
external sources of energy so that cells can synthesize enough ATP to
sustain life.
Checkpoint
1. What is metabolism? Distinguish between anabolism and
catabolism, and give examples of each.
2. How does ATP link anabolism and catabolism?
25.1 Metabolic Reactions
OBJECTIVES
• Define metabolism.
• Explain the role of ATP in anabolism and catabolism.
Metabolism (me-TAB-ō-lizm; metabol- = change) refers to all of the
chemical reactions that occur in the body. There are two types of
metabolism: catabolism and anabolism. Those chemical reactions
that
eak down complex organic molecules into simpler ones are
collectively known as catabolism (ka-TAB-ō-lizm; cata- = downward).
Overall, catabolic (decomposition) reactions are exergonic; they pro-
duce more energy than they consume, releasing the chemical energy
stored in organic molecules. Important sets of catabolic reactions
occur in glycolysis, the Krebs cycle, and the electron transport chain,
each of which will be discussed later in the chapter.
Chemical reactions that combine simple molecules and monomers
to form the body’s complex structural and functional components are
collectively known as anabolism (a-NAB-ō-lizm; ana- = upward).
Examples of anabolic reactions are the formation of peptide bonds
etween amino acids during protein synthesis, the building of fatty
acids into phospholipids that form the plasma mem
ane bilayer, and
the linkage of glucose monomers to form glycogen. Anabolic reactions
are endergonic; they consume more energy than they produce.
Metabolism is an energy-balancing act between catabolic
(decomposition) reactions and anabolic (synthesis) reactions. The
molecule that participates most oft en in energy exchanges in living cells
is ATP (adenosine triphosphate), which couples energy-releasing
catabolic reactions to energy-requiring anabolic reactions.
The metabolic reactions that occur depend on which enzymes
are active in a particular cell at a particular time, or even in a par -
ti cular part of the cell. Catabolic reactions can be occu
ing in the
mitochondria of a cell at the same time as anabolic reactions are
taking place in the endoplasmic reticulum.
A molecule synthesized in an anabolic reaction has a limited life-
time. With few exceptions, it will eventually be
oken down and its com-
ponent atoms recycled into other molecules or excreted from the body.
Recycling of biological molecules occurs continuously in living tissues,
more rapidly in some than in others. Individual cells may be refu
ished
molecule by molecule, or a whole tissue may be rebuilt cell by cell.
Coupling of Catabolism and Anabolism
y ATP
The chemical reactions of living systems depend on the eff icient
transfer of manageable amounts of energy from one molecule to
another. The molecule that most oft en performs this task is ATP, the
“energy cu
ency” of a living cell. Like money, it is readily available to
“buy” cellular activities; it is spent and earned over and over. A typical
cell has about a billion molecules of ATP, each of which typically lasts
FIGURE 25.1 Role of ATP in linking anabolic and catabolic
eactions. When complex molecules and polymers are split apart
(catabolism, at left ), some of the energy is transfe
ed to form ATP and
the rest is given off as heat. When simple molecules and monomers are
combined to form complex molecules (anabolism, at right), ATP provides the
energy for synthesis, and again some energy is given off as heat.
The coupling of energy-releasing and energy-requiring reactions is
achieved through ATP.
ATP
Simple molecules such as
glucose, amino acids,
glycerol, and fatty acids
Complex molecules such
as glycogen, proteins, and
triglycerides
PADP +
Heat
eleased
Anabolic reactions
transfer energy from
ATP to complex
molecules
Catabolic reactions
transfer energy from
complex molecules
to ATP
Heat
eleased
Q In a pancreatic cell that produces digestive enzymes, does
anabolism or catabolism predominate?
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25.2 Energy Transfer 955
25.2 Energy Transfe
OBJECTIVES
• Describe oxidation–reduction reactions.
• Explain the role of ATP in metabolism.
Various catabolic reactions transfer energy into the “high-energy”
phosphate bonds of ATP. Although the amount of energy in these bonds
is not exceptionally large, it can be released quickly and easily. Before
discussing metabolic pathways, it is important to understand how this
transfer of energy occurs. Two important aspects of energy transfer are
oxidation–reduction reactions and mechanisms of ATP generation.
Oxidation–Reduction Reactions
Oxidation (ok′-si-DA- -shun) is the removal of electrons from an atom or
molecule; the result is a decrease in the potential energy of the atom or
molecule. Because most biological oxidation reactions involve the loss of
hydrogen atoms, they are called dehydrogenation reactions. An example
of an oxidation reaction is the conversion of lactic acid into pyruvic acid:
Pyruvic acidLactic acid
COOH
C O
CH3
H
COOH
|
‖C || OH
|
|
|
CH3
Oxidation
)HRemove 2 H ( H++ −
In the preceding reaction, 2H (H+ + H−) means that two neutral
hydrogen atoms (2H) are removed as one hydrogen ion (H+) plus one
hydride ion (H−).
Reduction (rē-DUK-shun) is the opposite of oxidation; it is the
addition of electrons to a molecule. Reduction results in an increase in
the potential energy of the molecule. An example of a reduction reaction
is the conversion of pyruvic acid into lactic acid:
dicacitcaLdicacivuryP
H
COOH
C OH
CH3
COOH
C O
CH3
Reduction
Add 2 H (H H )
|
||
|
‖
|
| +
+ −
When a substance is oxidized, the liberated hydrogen atoms do not
emain free in the cell but are transfe
ed immediately by coenzymes to
another compound. Two coenzymes are commonly used by animal
cells to ca
y hydrogen atoms: nicotinamide adenine dinucleotide
(NAD), a derivative of the B vitamin niacin, and flavin adenine dinu-
cleotide (FAD), a derivative of vitamin B2 (riboflavin). The oxidation and
eduction states of NAD+ and FAD can be represented as follows:
ReduceddezidixO
decudeRdezidixO
FADH2FAD
H )2 H (H
NAD NADH H
H )2 H (H
++
++ +
− +
+
+
−
−
H )2 H (H
H )2 H (H
++
− +
+
+
−
−
When NAD+ is reduced to NADH + H+, the NAD+ gains a hydride
ion (H−), neutralizing its charge, and the H+ is released into the sur-
ounding solution. When NADH is oxidized to NAD+, the loss of the
hydride ion results in one less hydrogen atom and an additional
positive charge. FAD is reduced to FADH2 when it gains a hydrogen ion
and a hydride ion, and FADH2 is oxidized to FAD when it loses the same
two ions.
Oxidation and reduction reactions are always coupled; each time
one substance is oxidized, another is simultaneously reduced. Such
paired reactions are called oxidation–reduction or redox reactions.
For example, when lactic acid is oxidized to form pyruvic acid, the two
hydrogen atoms removed in the reaction are used to reduce NAD+.
This coupled redox reaction may be written as follows:
Lactic acid
Reduced
Pyruvic acid
Oxidized
NAD+
Oxidized
NADH + H+
Reduced
An important point to remember about oxidation–reduction
eactions is that oxidation is usually an exergonic (energy-releasing)
eaction. Cells use multistep biochemical reactions to release energy
from energy-rich, highly reduced compounds (with many hydrogen
atoms) to lower-energy, highly oxidized compounds (with many oxygen
atoms or multiple bonds). For example, when a cell oxidizes a molecule
of glucose (C6H12O6), the energy in the glucose molecule is removed in a
stepwise manner. Ultimately, some of the energy is captured by trans-
fe
ing it to ATP, which then serves as an energy source for energy-
equiring reactions within the cell. Compounds with many hydrogen
atoms such as glucose contain more chemical potential energy than
oxidized compounds. For this reason, glucose is a valuable nutrient.
Mechanisms of ATP Generation
Some of the energy released during oxidation reactions is captured
within a cell when ATP is formed. Briefly, a phosphate group P is
added to ADP, with an input of energy, to form ATP. The two high-
energy phosphate bonds that can be used to transfer energy are
indicated by “squiggles” (∼):
Adenosine — P ∼ P + P + energy
ADP
Adenosine — P ∼ P ∼ P
ATP
The high-energy phosphate bond that attaches the third phos-
phate group contains the energy stored in this reaction. The addition
of a phosphate group to a molecule, called phosphorylation (fos′-
for-i-LĀ-shun), increases its potential energy. Organisms use three
mechanisms of phosphorylation to generate ATP:
1. Substrate-level phosphorylation generates ATP by transfe
ing a
high-energy phosphate group from an intermediate phosphory-
lated metabolic compound—a substrate—directly to ADP. In hu-
man cells, this process occurs in the cytosol.
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956 CHAPTER 25 Metabolism and Nutrition
acids that can be used for lipogenesis (lip-ō-JEN-e-sis), the syn-
thesis of triglycerides. Triglycerides then are deposited in adipose
tissue, which has virtually unlimited storage capacity.
Glucose Movement into Cells
Before glucose can be used by body cells, it must first pass through the
plasma mem
ane and enter the cytosol. Glucose absorption in the gas-
trointestinal tract (and kidney tubules) is accomplished via secondary
active transport (Na+–glucose symporters). Glucose entry into most
other body cells occurs via GluT molecules, a family of transporters that
ing glucose into cells via facilitated diff usion (see