Essentials of Anatomy and Physiology

new proteins. Excess amino acids, however, those not
needed immediately for protein synthesis, may be
used for energy production. In the liver, excess amino
acids are deaminated, that is, the amino group (NH 2 )
is removed. The remaining portion is converted to a
molecule that will fit into the Krebs cycle. For exam-
ple, a deaminated amino acid may be changed to a
three-carbon pyruvic acid or to a two-carbon acetyl
group. When these molecules enter the Krebs cycle,
the results are just the same as if they had come from
glucose. This is diagrammed in Fig. 17–3.
Fats are made of glycerol and fatty acids, which are
the end products of fat digestion. These molecules may
also be changed to ones that will take part in the Krebs
cycle, and the reactions that change them usually take
place in the liver. Glycerol is a three-carbon molecule
that can be converted to the three-carbon pyruvic acid,
which enters the Krebs cycle. In the process of beta-
oxidation, the long carbon chains of fatty acids are split
into two-carbon acetyl groups, which enter a later step
in the Krebs cycle (see Fig. 17–3).
Both amino acids and fatty acids may be converted
by the liver to ketones, which are two- or four-carbon
molecules such as acetone and acetoacetic acid.
Although body cells can use ketones in cell respira-
tion, they do so slowly. In situations in which fats or
amino acids have become the primary energy sources,
a state called ketosismay develop; this is described in
Box 17–3: Ketosis. Excess amino acids may also be

converted to glucose; this is important to supply the

brain when dietary intake of carbohydrates is low. The

effects of hormones on the metabolism of food are

summarized in Table 17–4.

Energy Available from

the Three Nutrient Types

The potential energy in food is measured in units

called Caloriesor kilocalories. A calorie (lowercase

“c”) is the amount of energy needed to raise the tem-

perature of 1 gram of water 1°C. A kilocalorie or

Calorie (capital “C”) is 1000 times that amount of

energy.

One gram of carbohydrate yields about 4 kilocalo-

ries. A gram of protein also yields about 4 kilocalories.

A gram of fat, however, yields 9 kilocalories, and

a gram of alcohol yields 7 kilocalories. This is why a

diet high in fat is more likely to result in weight gain

if the calories are not expended in energy-requiring

activities.

You may have noticed that calorie content is part of

the nutritional information on food labels. On such

labels the term calorieactually means Calorie or kilo-

calories but is used for the sake of simplicity.

SYNTHESIS USES OF FOODS

Besides being available for energy production, each of

the three food types is used in anabolic reactions to

Body Temperature and Metabolism 405

Table 17–3 SUMMARY OF CELL RESPIRATION

Molecules That Vitamins or

Stage Enter the Process Results Minerals Needed

Glycolysis

(cytoplasm)

Krebs citric acid cycle

(mitochondria)

Cytochrome

transport system

(mitochondria)

• 2 ATP (net)


• 2 NADH 2 (to cytochrome
  transport system)


• 2 pyruvic acid (aerobic:
  to Krebs cycle; anaerobic:
  lactic acid formation)
  •CO 2 (exhaled)


• ATP (2 per glucose)


• 3 NADH 2 and 1 FADH 2 (to
  cytochrome transport system)


• A 4-carbon molecule is regen-
  erated for the next cycle


• 34 ATP


• Metabolic water
  
  
  • Niacin (part of NAD)
  
  
  • Thiamine (for removal
    of CO 2 )
  
  
  • Niacin (part of NAD)
  
  
  • Riboflavin (part of FAD)
  
  
  • Pantothenic acid (part
    of coenzyme A)
  
  
  • Iron and copper (part
    of some cytochromes)
  
  
  
  

Glucose—ATP needed as

energy of activation

Pyruvic acid—from glucose

or glycerol or excess

amino acids

or

Acetyl CoA—from fatty acids

or excess amino acids

NADH 2 and FADH 2 —from

glycolysis or the Krebs

cycle