Human Physiology, 14th edition (2016)

120 Chapter 5

White Adipose Tissue

White adipose tissue, or white fat, is where most of the triglyc-

erides in the body are stored. When fat stored in adipose tissue

is going to be used as an energy source, lipase enzymes hydro-

lyze triglycerides into glycerol and free fatty acids in a process

called lipolysis. These molecules (primarily the free fatty acids)

serve as blood-borne energy carriers that can be used by the

liver, skeletal muscles, and other organs for aerobic respiration.

When adipocytes (adipose cells) hydrolyze triglycerides,

the glycerol leaves through certain protein channels in the

plasma membrane and enters the blood. The glycerol released

into the blood is mostly taken up by the liver, which converts it

into glucose through gluconeogenesis. By this means, glycerol

released from adipocytes during exercise or fasting can be an

important source of liver glucose.

However, the most significant energy carriers provided by

lipolysis are the free fatty acids. Most fatty acids consist of a

long hydrocarbon chain with a carboxyl group (COOH) at one

end. In a process known as b-oxidation ( b is the Greek letter

beta ), enzymes remove two-carbon acetic acid molecules from

the acid end of a fatty acid chain. This results in the formation

of acetyl CoA, as the third carbon from the end becomes oxi-

dized to produce a new carboxyl group. The fatty acid chain is

thus decreased in length by 2 carbons. The process of oxida-

tion continues until the entire fatty acid molecule is converted

to acetyl CoA ( fig. 5.14 ).

A sixteen-carbon-long fatty acid, for example, yields 8 ace-

tyl CoA molecules. Each of these can enter a citric acid cycle and

produce 10 ATP per turn of the cycle, producing 8 3 10 5 80

ATP. In addition, each time an acetyl CoA molecule is formed

and the end carbon of the fatty acid chain is oxidized, 1 NADH

and 1 FADH 2 are produced. Oxidative phosphorylation produces

2.5 ATP per NADH and 1.5 ATP per FADH 2. For a sixteen-

carbon-long fatty acid, these 4 ATP molecules would be formed

seven times (producing 4 3 7 5 28 ATP). Not counting the sin-

gle ATP used to start b -oxidation ( fig. 5.14 ), this fatty acid could

yield a grand total of 28 1 80, or 108 ATP molecules!

produced. The formation of fat, or lipogenesis, occurs primar-
ily in adipose tissue and in the liver when the concentration of
blood glucose is elevated following a meal.
Fat stored in adipose cells ( adipocytes ) of white adipose tis-
sue (or white fat ) serves as the major form of energy storage
in the body. One gram of fat provides 9 kilocalories of energy,
compared to 4 kilocalories for a gram of carbohydrates or pro-
tein. In a non-obese 70-kilogram (155-pound) man, 80% to 85%
of the body’s energy is stored as fat, which amounts to about
140,000 kilocalories. Stored glycogen, by contrast, accounts
for less than 2,000 kilocalories, most of which (about 350 g) is
stored in skeletal muscles and is available for use only by the
muscles. The liver contains between 80 and 90 grams of gly-
cogen, which can be converted to glucose and used by other
organs. Protein accounts for 15% to 20% of the stored calories in
the body, but protein usually is not used extensively as an energy
source because that would involve the loss of muscle mass.

Figure 5.13 Divergent metabolic pathways for acetyl
coenzyme A. Acetyl CoA is a common substrate that can be
used to produce a number of chemically related products.

Ketone bodies

Cholesterol

Bile acids Steroids

Triacylglycerol

(triglyceride)

Phospholipids

Fatty acids

Citric acid

Acetyl CoA (Krebs cycle) CO 2

FITNESS APPLICATION

Metabolic syndrome is a combination of abnormal mea-

surements—including central obesity (excess abdominal

fat), hypertension (high blood pressure), insulin resistance

(prediabetes), type 2 diabetes mellitus, high plasma triglyc-

erides, and high LDL cholesterol—that greatly increase the

risk of coronary heart disease, stroke, diabetes mellitus, and

other conditions. The incidence of metabolic syndrome has

increased alarmingly in recent years because of the increase

in obesity. Eating excessive calories, particularly in the form

of sugars (including high fructose corn syrup), stimulates

insulin secretion. Insulin then promotes the uptake of blood

glucose into adipose cells, where (through lipogenesis) it is

converted into stored triglycerides (see figs. 5.12 and 5.13 ).

Conversely, the lowering of insulin secretion, by diets that

prevent the plasma glucose from rising sharply, promotes

lipolysis (the breakdown of fat) and weight loss.

Clinical Investigation CLUES

Andrea was in danger of developing metabolic syndrome.

She went on a diet and checked her urine for ketone bod-

ies to see if she was breaking down fat.

• What are the symptoms and dangers of metabolic
  syndrome?


• What are ketone bodies, and how are they
  produced?

Brown Adipose Tissue

Brown adipose tissue, or brown fat, has a different pri-

mary function than white adipose tissue: it is the major site

for thermogenesis (heat production) in the newborn, who have