Page 1008 - Fundamentals of anatomy physiology

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25-3 Lipid metabolism involves lipolysis, beta-oxidation, and
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25-1 Metabolism refers to all the chemical reactions
in the body, and energetics refers to the flow and
transformation of energy p. 962
1. Energetics is the study of the flow of energy and its change(s)
from one form to another. Its focus includes understanding a
range of energy requirements from cells to the whole body.
2. In general, during cellular metabolism, cells break down excess car-
bohydrates first and then lipids, while conserving amino acids.
Only about 40 percent of the energy released through catabo-
lism is captured in ATP; the rest is released as heat. (Figure 25–1)
3. Cells synthesize new compounds (anabolism) to (1) perform
structural maintenance or repairs, (2) support growth, (3) pro-
duce secretions, and (4) build and store nutrient reserves.
4. Cells “feed” small organic molecules to their mitochondria; ­
in return, the cells get the ATP they need to perform cellular
functions. (Figure 25–2)
25-2 Carbohydrate metabolism involves glycolysis, ATP
production, and gluconeogenesis p. 965
5. Most cells generate ATP and other high-energy compounds
through the breakdown of carbohydrates.
6. Glycolysis and aerobic metabolism, or cellular respiration,
provide most of the ATP used by typical cells. Glycogen can be 25-4 Protein catabolism involves transamination and
broken down to glucose molecules. In glycolysis, each mol-
ecule of glucose yields 2 molecules of pyruvic acid (as pyru-
vate ions), a net 2 molecules of ATP, and 2 NADH molecules.
(Figure 25–3)
7. In the presence of oxygen, pyruvate molecules enter mito-
chondria, where they are broken down completely in the
citric acid cycle. Carbon and oxygen atoms are lost as carbon
dioxide (decarboxylation); hydrogen atoms are passed to
coenzymes, which initiate the oxygen-consuming and ATP-
generating reaction oxidative phosphorylation. (Figure 25–4)
8. Cytochromes of the electron transport system (ETS) pass
electrons to oxygen, resulting in the formation of water. As this
transfer occurs, the ETS generates ATP. (Figure 25–5)
9. For each glucose molecule processed through glycolysis, the
citric acid cycle, and the ETS, most cells gain 36 molecules of
ATP. (Figure 25–6)
10. Cells can break down other nutrients to provide substrates for
the citric acid cycle if supplies of glucose are limited.
11. Gluconeogenesis, the synthesis of glucose from noncarbohy-
drate precursors such as lactate, glycerol, or amino acids, en-
ables a liver cell to synthesize glucose molecules when carbo-
hydrate reserves are depleted. Glycogenesis is the process of
glycogen formation. Glycogen is an important energy reserve
Chapter 25 Metabolism and Energetics 995
when cells cannot obtain enough glucose from interstitial
fluid. (Figure 25–7)
the transport and distribution of lipids as free fatty acids
and lipoproteins p. 973
™ 12. During lipolysis (lipid catabolism), lipids are broken down
into pieces that can be converted into pyruvate or channeled
™ into the citric acid cycle.
13. Triglycerides, the most abundant lipids in the body, are split
into glycerol and fatty acids. The glycerol enters the glycolytic
pathways (glycolysis and gluconeogenesis), and the fatty acids
enter the mitochondria.
14. Beta-oxidation is the breakdown of a fatty acid molecule into
2-carbon fragments that can be used in the citric acid cycle.
The steps of beta-oxidation cannot be reversed, and the body
cannot manufacture all the fatty acids needed for normal
metabolic operations. (Figure 25–8)
15. Lipids cannot provide large amounts of ATP quickly. However,
cells can shift to lipid-based energy production when glucose
reserves are limited.
16. In lipogenesis (the synthesis of lipids), almost any organic
substrate can be used to form glycerol. Essential fatty acids
are those that cannot be synthesized and must be included in
the diet.
17. Most lipids circulate as lipoproteins (lipid–protein com-
plexes that contain large glycerides and cholesterol). The larg-
est lipoproteins, chylomicrons, carry absorbed lipids from the
intestinal tract to the bloodstream. All other lipoproteins are
derived from the liver and carry lipids to and from various tis-
sues of the body. (Figure 25–9)
18. Capillary walls of adipose tissue, skeletal muscle, cardiac
muscle, and the liver contain lipoprotein lipase, an enzyme
that breaks down lipids, releasing a mixture of fatty acids and
monoglycerides into the interstitial fluid. (Figure 25–9)
deamination, whereas protein synthesis involves
amination and transamination p. 978
19. If other energy sources are inadequate, mitochondria can
25break down amino acids in the citric acid cycle to generate
ATP. In the mitochondria, the amino group can be removed by
either transamination (an exchange reaction) or deamina-
tion. (Figure 25–10)
20. Protein catabolism is impractical as a source of quick energy.
21. Roughly half the amino acids needed to build proteins can be
synthesized. There are 10 essential amino acids, which must
be acquired through the diet. Amination, the attachment of
an amino group to a carbon framework, is an important step
in the synthesis of nonessential amino acids. (Figure 25–10,
Spotlight Figure 25–11)
25-5 The body experiences two patterns of metabolic activity:
the absorptive and postabsorptive states p. 982
22. No one cell of a human can perform all the anabolic and cata-
bolic operations necessary to support life. Homeostasis can be
preserved only when metabolic activities of different tissues
are coordinated.
23. The body has five metabolic components: the liver, adipose
tissue, skeletal muscle, neural tissue, and other peripheral