Human Physiology, 14th edition (2016)

38 Chapter 2

are sufficient amounts of ketone bodies in the blood to lower

the blood pH, the condition is called ketoacidosis. Severe

ketoacidosis, which may occur in diabetes mellitus, can lead

to coma and death.

Phospholipids

The group of lipids known as phospholipids includes a num-

ber of different categories of lipids, all of which contain a

phosphate group. The most common type of phospholipid mol-

ecule is one in which the three-carbon alcohol molecule glyc-

erol is attached to two fatty acid molecules; the third carbon

atom of the glycerol is attached to a phosphate group, and the

phosphate group, in turn, is bound to other molecules. If the

phosphate group is attached to a nitrogen-containing choline

Ketone Bodies

Hydrolysis of triglycerides within adipose tissue releases free

fatty acids into the blood. Free fatty acids can be used as an

immediate source of energy by many organs; they can also be

converted by the liver into derivatives called ketone bodies

( fig. 2.21 ). These include four-carbon-long acidic molecules

(acetoacetic acid and b -hydroxybutyric acid) and acetone (the

solvent in nailpolish remover). A rapid breakdown of fat, as

may occur during strict low-carbohydrate diets and in uncon-

trolled diabetes mellitus, results in elevated levels of ketone

bodies in the blood. This is a condition called ketosis. If there

Figure 2.20 The formation of a triglyceride (triacylglycerol) molecule from glycerol and three fatty acids by

dehydration synthesis reactions. A molecule of water is produced as an ester bond forms between each fatty acid and the

glycerol. Sawtooth lines represent hydrocarbon chains, generally 16 to 22 carbons long, which are symbolized by an R.

Glycerol

Carboxylic

acid

Fatty acids

R

Hydrocarbon chain

Triglyceride

Glycerol

Ester

bond

Hydrocarbon

chain

3H 2 O

H

HC

HC

OH

OH

H

H

C

H

HC C

O

O R

C

O

O R

C

O

O R

HC

H

H

OH C

OHHHHHH

HHHHHH

HOCCCCCCC

OHHHHHH

HHHHHH

HOCCCCCCC

OHHHHHH

HHHHHH

HOCCCCCCC

+

Figure 2.21 Ketone bodies. Acetoacetic acid, an acidic
ketone body, can spontaneously decarboxylate (lose carbon
dioxide) to form acetone. Acetone is a volatile ketone body that
escapes in the exhaled breath, thereby lending a “fruity” smell to
the breath of people with ketosis (elevated blood ketone bodies).

O

COH

H CH

CO

H CH

H

Acetoacetic

acid

H

H CH

CO

H CH

H

Acetone

+ CO 2

Clinical Investigation CLUES

When Brian’s urine is tested in the laboratory, they dis-

cover that he has ketonuria (an elevated level of ketone

bodies in his urine).

• What are ketone bodies, and how do they originate?


• What is the relationship between Brian’s ketonuria
  and his weight loss?