Understanding Nutrition, 8e

1
Understanding Nutrition, 8e

• Chapter 7 - Metabolism Transformations and
  Interactions

2
Anabolic and Catabolic Reactions Compared
Note You need not memorize a color code to
understand the figures in this chapter but you
may find it helpful to know that blue is used for
carbohydrates, yellow for fats, and red for
proteins.
3
ATP (Adenosine Triphosphate)
ATP is one of the bodys quick-energy molecules.
Notice that the bonds connecting the three
phosphate groups have been drawn as wavy lines,
indicating a high-energy bond. When these bonds
are broken, a large amount of energy is released.
4
Transfer of Energy by ATP 1 of 3
Before the transfer of energy.
5
Transfer of Energy by ATP 2 of 3
During the transfer of energy.
6
Transfer of Energy by ATP 3 of 3
After the transfer of energy.
7
A Typical Cell (Simplified Diagram)
8
Common Metabolic Pathway
This simple overview introduces the metabolism
that is presented in the upcoming text and
detailed in the summary figure on p. 214.
9
Glycolysis Glucose-to-Pyruvate Pathway
Glucose splits to two 3-carbon compounds that
become pyruvate. The pathway is called glycolysis
(glucose splitting) and occurs in anaerobic
conditions (does not require oxygen).
10
Pyruvate-to-Acetyl CoA (Aerobic)
Each pyruvate loses a carbon as carbon dioxide
and picks up a molecule of CoA, becoming acetyl
CoA. The arrow goes only one way (down), because
the step is not reversible. Result (from 1
glucose) 2 carbon dioxide and 2 acetyl CoA.
11
The Breakdown of Acetyl CoA
The complete oxidation of acetyl CoA is
accomplished through the reactions of the TCA
(tricarboxylic acid) cycle, or Krebs cycle (named
for the biochemist who elucidated them), and the
electron transport chain. In the TCA cycle, the
acetyl CoA carbons are converted to carbon
dioxide. Each CoA returns to pick up another
acetate (coming from glucose, glycerol, fatty
acids, and amino acids). The net result is that
acetyl CoA splits, the carbons combine with
oxygen, and the energy originally in the acetyl
CoA is stored in ATP and similar compounds, thus
becoming available for the bodys use. Chapter 10
describes how the B vitamin coenzymes participate
in these metabolic pathways. For more details,
see the text and Appendix C.
12
Fats-to-Energy Pathway 1 of 2
Glycerol enters the metabolic path about midway
between glucose and pyruvate and can be converted
to either fatty acids are broken down into
2-carbon fragments that combine with CoA to form
acetyl CoA. Net from an 18-carbon fatty acid 9
acetyl CoA molecules, which are converted to 18
carbon dioxide molecules. Notice that you have
seen parts of this figure beforein Figure 7-9.
13
Fats-to-Energy Pathway 2 of 2
Glycerol enters the metabolic path about midway
between glucose and pyruvate and can be converted
to either fatty acids are broken down into
2-carbon fragments that combine with CoA to form
acetyl CoA. Net from an 18-carbon fatty acid 9
acetyl CoA molecules, which are converted to 18
carbon dioxide molecules. Notice that you have
seen parts of this figure beforein Figure 7-9.
14
Fatty Acid Oxidation
During oxidation, fatty acids are taken apart to
2-carbon fragments that combine with CoA to make
acetyl CoA. Fatty acid oxidation is a series of
aerobic reactions.
15
The Carbons of a Typical Triglyceride
A typical triglyceride contains only one small
molecule of glycerol (3 C), but has three fatty
acids (each about 18 C on the average, or about
54 C). Only the glycerol portion of a
triglyceride can yield glucose.
16
Keto Acids
(a) The deamination of an amino acid produces
ammonia (NH3) and a keto acid. (b) Given a source
of NH3,,the body can make nonessential amino
acids from keto acids.
17
Transamination
The body can transfer amino groups (NH2) from an
amino acid to a keto acid, forming a new
nonessential amino acid and a new keto acid.
Transamination reactions require the vitamin B6
coenzyme.
18
Urea Synthesis
When amino nitrogen is stripped from amino acids,
ammonia is produced. The liver detoxifies ammonia
before releasing it into the bloodstream by
combining it with another waste product, carbon
dioxide, to produce urea. See Appendix C for
details.
19
Urea Excretion Amino Acids
The liver and kidneys both play a role in
disposing of excess nitrogen. Can you see why the
person with liver disease has high blood ammonia,
while the person with kidney disease has high
blood urea? (Figure 3-8 provides details of how
the kidneys work.)
20
Urea Excretion Urea
The liver and kidneys both play a role in
disposing of excess nitrogen. Can you see why the
person with liver disease has high blood ammonia,
while the person with kidney disease has high
blood urea? (Figure 3-8 provides details of how
the kidneys work.)
21
Ketone Body Formation
(1) The first step in the formation of ketone
bodies is the condensation of two molecules of
acetyl CoA and the removal of the CoA to form a
compound that is converted to the first ketone
body. (2) this ketone body may lose a molecule
of carbon dioxide to become another ketone. (3)
Or, the acetoacetate may add two hydrogens,
becoming another ketone body (beta-hydroxbutyrate)
.
22
Alcohol Metabolism
The conversion of alcohol to acetyl CoA requires
the B vitamin niacin in its role as NAD. When the
enzymes oxidize alcohol, they remove H atoms and
attach them to NAD. Thus NAD is used up, and NADH
accumulates. (Note More accurately, NAD1 is
converted to NADH 1 H1.) For simplicitys sake,
the process has been described here as if one
hydrogen were added to NAD, but, in reality, two
are added.)
23
Alternate Route for Acetyl CoA To Fat
Acetyl CoA molecules are blocked from getting
into the TCA cycle by the high level of NADH.
Instead of being used for energy, the acetyl CoA
molecules become building blocks for fatty
acids. For simplicitys sake, the process has
been described here as if one hydrogen were added
to HAD, but, in reality, two are added.)
24
Alcohol Doses and Blood Levels
2) Speech and vision centers are affected next.
If the drinker drinks faster than the rate at
which the liver can oxidize the alcohol, blood
alcohol concentrations rise the speech and
vision centers of the brain become sedated. (3)
Voluntary muscular control is then affected. At
still higher concentrations, the cells in the
cerebellum responsible for coordination of
voluntary muscles are affected including those
used in speech, eye, and limb movements. At this
point people under the influence stagger or weave
when they try to walk, or they may slur their
speech. (4) Respiration and heart action are the
last to be affected. Finally, the conscious brain
is completely subdued, and the person passes out.
Now the person can drink no more this is
fortunate because higher doses have an anesthetic
effect that could reach the deepest brain
centers, which control breathing and heartbeat,
and the person could die.
25
Alcohols Effects on the Brain
(1) Judgment and reasoning centers are most
sensitive to alcohol. When alcohol flows to the
brain, it first sedates the frontal lobe, the
reasoning part. As the alcohol molecules diffuse
into the cells of these lobes, they interfere
with reasoning and judgment. (2) Speech and
vision centers are affected next. If the drinker
drinks faster than the rate at which the liver
can oxidize the alcohol, blood alcohol
concentrations rise the speech and vision
centers of the brain become sedated. (3)
Voluntary muscular control is then affected. At
still higher concentrations, the cells in the
cerebellum responsible for coordination of
voluntary muscles are affected including those
used in speech, eye, and limb movements. At this
point people under the influence stagger or weave
when they try to walk, or they may slur their
speech. (4) Respiration and heart action are the
last to be affected. Finally, the conscious brain
is completely subdued, and the person passes out.
Now the person can drink no more this is
fortunate because higher doses have an anesthetic
effect that could reach the deepest brain
centers, which control breathing and heartbeat,
and the person could die.
Exit