synthesize necessary materials for cells and tissues. A
simple summary of these reactions is shown in Fig.
17–4. The three food types and their end products of
digestion are at the bottom of the picture, and the
arrows going upward indicate synthesis and lead to the
products formed. You may wish to refer to Fig. 17–4
as you read the next sections.
Glucose
Glucose is the raw material for the synthesis of
another important monosaccharide, the pentose sug-
arsthat are part of nucleic acids. Deoxyribose is the
five-carbon sugar found in DNA, and ribose is found
in RNA. This function of glucose is very important,
for without the pentose sugars our cells could neither
produce new chromosomes for cell division nor carry
out the process of protein synthesis.
Any glucose in excess of immediate energy needs or
the need for pentose sugars is converted to glycogen
in the liver and skeletal muscles. Glycogen is then an
energy source during states of hypoglycemia or during
exercise. If still more glucose is present, it will be
changed to fat and stored in adipose tissue.Amino Acids
As mentioned previously, the primary uses for amino
acids are the synthesis of the non-essential amino
acidsby the liver and the synthesis of new proteinsin
all tissues. By way of review, we can mention some
proteins with which you are already familiar: keratin
and melanin in the epidermis; collagen in the dermis,
tendons, and ligaments; myosin, actin, and myoglobin
in muscle cells; hemoglobin in RBCs; antibodies pro-
duced by WBCs; prothrombin and fibrinogen for406 Body Temperature and Metabolism
Table 17–4 HORMONES THAT
REGULATE METABOLISMHormone (Gland) Effects
Thyroxine (thyroid
gland)Growth hormone
(anterior pituitary)Insulin (pancreas)Glucagon (pancreas)Cortisol (adrenal
cortex)Epinephrine
(adrenal medulla)- Increases use of all three
food types for energy (glu-
cose, fats, amino acids) - Increases protein synthesis
- Increases amino acid trans-
port into cells - Increases protein synthesis
- Increases use of fats for
energy - Increases glucose transport
into cells and use for energy - Increases conversion of glu-
cose to glycogen in liver
and muscles - Increases transport of amino
acids and fatty acids into
cells to be used for synthe-
sis (notenergy production) - Increases conversion of
glycogen to glucose - Increases use of amino acids
and fats for energy - Increases conversion of glu-
cose to glycogen in liver - Increases use of amino acids
and fats for energy - Decreases protein synthesis
except in liver and GI tract - Increases conversion of
glycogen to glucose - Increases use of fats for
energy
BOX17–3 KETOSIS
When fats and amino acids are to be used for
energy, they are often converted by the liver to
ketones. Ketones are organic molecules such as
acetone that may be changed to acetyl CoA and
enter the Krebs cycle. Other cells are able to use
ketones as an energy source, but they do so
slowly. When ketones are produced in small
amounts, as they usually are between meals, the
blood level does not rise sharply.
A state of ketosisexists when fats and pro-
teins become the primary energy sources, and
ketones accumulate in the blood faster than cells
can utilize them. Because ketones are organic
acids, they lower the pH of the blood. As the
blood ketone level rises, the kidneys excrete
ketones, but they must also excrete more water
as a solvent, which leads to dehydration.
Ketosis is clinically important in diabetes mel-
litus, starvation, and eating disorders such as
anorexia nervosa. Diabetics whose disease is
poorly controlled may progress to ketoacido-
sis, a form of metabolic acidosis that may lead to
confusion, coma, and death. Reversal of this
state requires a carbohydrate energy source and
the insulin necessary to utilize it.