Human Physiology, 14th edition (2016)

Cell Respiration and Metabolism 119

5.4 METABOLISM OF LIPIDS

AND PROTEINS

Triglycerides can be hydrolyzed into glycerol and fatty acids.

The latter are of particular importance because they can be

converted into numerous molecules of acetyl CoA that can

enter citric acid cycles and generate a large amount of ATP.

Amino acids derived from proteins also may be used for

energy. This involves the removal of the amine group and

the conversion of the remaining molecule into either pyruvic

acid or one of the Krebs cycle molecules.

the glycolytic intermediates—phosphoglyceraldehyde and dihy-

droxyacetone phosphate—do not complete their conversion to

pyruvic acid, however, and acetyl CoA does not enter a citric

acid cycle. The acetic acid subunits of these acetyl CoA mol-

ecules can instead be used to produce a variety of lipids, includ-

ing cholesterol (used in the synthesis of bile acids and steroid

hormones), ketone bodies, and fatty acids ( fig. 5.13 ). Acetyl

CoA may thus be considered a branch point from which a num-

ber of different possible metabolic pathways may progress.

In the formation of fatty acids, a number of acetic acid

(two-carbon) subunits are joined together to form the fatty acid

chain. Six acetyl CoA molecules, for example, will produce

a fatty acid that is 12 carbons long. When three of these fatty

acids condense with 1 glycerol (derived from phosphoglyceral-

dehyde), a triglyceride (also called triacylglycerol ) molecule is

in the liver allows depleted skeletal muscle glycogen to be
restored within 48 hours.

| CHECKPOINT

4. Describe glycogenesis and glycogenolysis, and
   explain why the liver, but not skeletal muscles, can
   use stored glycogen to replenish blood glucose.


5. Explain the significance of gluconeogenesis and the
   Cori cycle in the homeostasis of blood glucose.

LEARNING OUTCOMES

After studying this section, you should be able to:

6. Describe how triglycerides can be used in aerobic
   cell respiration, and the nature of ketone bodies.


7. Describe how amino acids can be metabolized
   for energy, and explain how proteins, fats, and
   carbohydrates can be interconverted.

Energy can be derived by the cellular respiration of lipids and
proteins using the same aerobic pathway previously described
for the metabolism of pyruvic acid. Indeed, some organs
preferentially use molecules other than glucose as an energy
source. Pyruvic acid and the Krebs cycle acids also serve as
common intermediates in the interconversion of glucose, lip-
ids, and amino acids.
When food energy is taken into the body faster than it is
consumed, the concentration of ATP within body cells rises.
Cells, however, do not store extra energy in the form of extra
ATP. When cellular ATP concentrations rise because more
energy (from food) is available than can be immediately used,
ATP production is inhibited and glucose is instead converted
into glycogen and fat ( fig. 5.12 ).

Lipid Metabolism

When glucose is going to be converted into fat, glycolysis
occurs and pyruvic acid is converted into acetyl CoA. Some of

Figure 5.12 The conversion of glucose into glycogen

and fat. This occurs as a result of inhibition of respiratory

enzymes when the cell has adequate amounts of ATP. Favored

pathways are indicated by blue arrows.

Glucose 1-phosphate

Fructose 1,6-biphosphate

3-Phosphoglyceraldehyde

Glycogen

Acetyl CoA

C 6

Citric acid

C 5

α-Ketoglutaric acid

C 4

Oxaloacetic acid

Glucose 6-phosphate

Pyruvic acid

Fructose 6-phosphate

Glycerol

Fatty acids

Fat

Glucose

Citric acid

cycle