Nutrients Are Metabolized in a Number of Ways
LEARNING OBJECTIVES
1.
Outline
the pathways of glycolysis, the Krebs
cycle, and oxidative phosphorylation.
2.
Compare
the metabolism of carbohydrates,
proteins, and lipids.
3.
Explain
how carbohydrates are made by gluco-
neogenesis.
M
etabolism
(me-TAB-o-lizm)
refers
to
all
the chemical reactions in the body. Most of
the chemical reactions are catalyzed (sped
up) by enzymes (see Chapter 2). Some en-
zymes work together with coenzymes, which are organic
molecules that temporarily carry atoms (or their com-
ponents) during a reaction. Two important examples
are the electron carriers nicotinamide adenine dinu-
cleotide (NAD+) (referred to as NADH when carrying
electrons) and flavin adenine dinucleotide (FAD) (re-
ferred to as FADH2
when carrying electrons).
Metabolic reactions include synthesis reactions (an-
abolic reactions, or
a n a b o lism )
and decomposition re-
actions (catabolic reactions, or
c a ta b o lism ).
In anabolic
reactions, simple organic molecules are combined to
make more complex ones; the process usually requires
energy, generated by splitting ATP into ADP and phos-
phate. Conversely, in catabolic reactions, complex or-
ganic molecules are broken down into simpler ones; this
process usually releases energy, which is stored in the
form of ATP, a molecule created from ADP and phos-
phate (Figure 14.11). Of the energy released in cata-
bolic reactions, only 40% is captured as ATP. The rest of
the energy is released as heat, which warms your body.
To provide ATP for anabolic reactions, your body
breaks down some of the carbohydrates, fats, and pro-
teins absorbed by the digestive system. Most cells make
ATP from glucose. Some cells, such as neurons and red
blood cells, typically use only glucose to make ATP. Most
other cells can use lipids (fats) and amino acids (pro-
teins) when glucose is unavailable.
Carbohydrates Are Converted
to Glucose
Let’s take a look at the catabolism of glucose as an example
of the basic process of making ATP. During digestion, car-
R o le o f A T P in lin k in g c a t a b o l i c a n d a n a b o l i c
r e a c t i o n s •
F ig u r e 1 4 . 1 1
When complex molecules are split (catabolism), some energy
is transferred to make ATP. Conversely, when simple molecules
combine to form complex ones (anabolism), energy is usually
supplied by splitting ATP.
Heat
released
Simple molecules such as
glucose, amino acids,
glycerol, and fatty acids
Catabolic reactions
Anabolic reactions
transfer energy from
transfer energy from
complex molecules
V
J
ATP to complex
to ATP
'
ADP +
P
molecules
Complex molecules such
as glycogen, proteins, and
triglycerides
A
Heat
released
bohydrates are broken down into many monosaccharides
(for example, glucose, fructose, galactose), all of which are
absorbed by the small intestine. Shortly after absorption,
the monosaccharides are converted to glucose. The fate of
glucose depends on the needs of your body’s cells:
If cells require ATP, then glucose
is broken down via
oxidation
.
Excess glucose is stored as gly-
cogen in liver and muscle cells.
If
the
glycogen
stores
are
full, liver cells convert excess
glucose into triglycerides for
storage
in
adipose
tissue.
Triglycerides can be converted
back to glucose when needed.
Let’s see how your cells break down glucose to make ATP.
oxid ation
(ok-si-DA-
shun) The removal
of electrons from
a molecule or, less
commonly, the addi-
tion of oxygen to a
molecule that results
in a decrease in the
energy content of the
molecule.
Cellular Respiration Creates ATP
Like a fire combines fuel and oxygen to release energy,
your cells “burn glucose” by combining it with oxygen in
a process called cellular respiration. The overall process
can be summarized as follows:
1
glucose + 6 O2
^ 6 CO2
+ 6 H2O + 36 ATP
418 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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