Human Physiology, 14th edition (2016)

Cell Respiration and Metabolism 125

As mentioned earlier, lactic acid produced anaerobically
during exercise can be used for energy following the cessation
of exercise. The lactic acid, under aerobic conditions, is recon-
verted to pyruvic acid, which then enters the aerobic respiratory
pathway. The extra oxygen required to metabolize lactic acid
contributes to the oxygen debt following exercise (chapter 12,
section 12.4).
Acetyl CoA serves as a common intermediate in the forma-
tion of the molecules of the citric acid cycle, ketone bodies, and
fatty acids ( fig. 5.18 ). Acetyl CoA also serves as the donor of
two-carbon groups for the acetylation of proteins, including his-
tone proteins for epigenetic regulation (chapter 3, section 3.5).
In this way, metabolism and the availability of acetyl CoA could

influence gene activity.

Organ Glucose

Fatty

Acids

Ketone

Bodies

Lactic

Acid

Brain 111 2 1 2

Skeletal muscles

(resting)

1 111 1 2

Liver 1 111 11 1

Heart 11111

Table 5.4 | Relative Importance of Different
Molecules in the Blood with Respect to the
Energy Requirements of Different Organs

Clinical Investigation CLUES

Andrea went on a high-protein diet and was assured that she

could maintain an adequate blood glucose concentration.

• What are the steps by which amino acids are
  metabolized for energy?


• What are the steps by which amino acids can be
  used in gluconeogenesis to form glucose?

| CHECKPOINT

6a. Construct a flowchart to show the metabolic

pathway by which glucose can be converted to fat.

Indicate only the major intermediates involved (not all

of the steps of glycolysis).

6b. Define the terms lipolysis and b -oxidation and explain,

in general terms, how fat can be used for energy.

7a. Describe transamination and deamination and

explain their functional significance.

7b. List five blood-borne energy carriers and explain, in

general terms, how these are used as sources of energy.

Andrea was overweight, and her physician warned her

that she was in danger of developing metabolic syn-

drome, which would greatly increase her risk of develop-

ing coronary heart disease and diabetes, among other

diseases. A waist circumference greater than a normal

range, together with certain abnormal blood measure-

ments, characterizes metabolic syndrome. Andrea went

on a high-protein diet, which presumably is also low in

carbohydrates and total calories. Her blood glucose can

be maintained by glycogenolysis and gluconeogenesis

(using amino acids) by the liver. If she is losing triglyc-

eride (fat) in her adipose tissue, she will have a rise in

plasma ketone bodies (ketosis), produced by the liver

from fatty acids. Andrea checked her urine for ketone

bodies. Her exercising muscles became fatigued and

relied increasingly on the lactic acid pathway for energy.

Lactic acid can cause a burning muscle pain during exer-

cise, which is normal. However, Andrea’s chest pain may

indicate angina pectoris, which is a pain due to lactic

acid produced by ischemic heart muscle. This is a seri-

ous condition that could lead to myocardial infarction

(heart attack), and so requires medical attention.

See the additional chapter 5 Clinical Investigation on

Metabolic Disease in the Connect site for this text.

Clinical Investigation SUMMARY