Pyruvic acid. This end product of glycolysis can be
metabolized further to create ATP or can be converted
to make amino acids or glucose, depending on the ATP
level, blood glucose level, and amino acid requirements.
If oxygen is unavailable, pyruvic acid is converted into
lactic acid and then must be transformed back into a
more functional material once oxygen become available.
Acetyl CoA. This product of pyruvic breakdown, fatty
acid oxidation, and deamination of certain amino
acids can be metabolized in the Krebs cycle for ATP
production, depending on the cell’s ATP levels and
oxygen availability. As already noted, under certain
conditions, acetyl CoA can also be converted to ketones
for transport to other cells.
The fate of these molecules can also be influenced by vari-
ous hormones, including insulin, glucagon, cortisol, epi-
nephrine, and norepinephrine.
1.
Which
metabolic pathway occurs exclusively in
the cytosol?
2.
During
which part of cellular respiration is most
ATP created?
3.
Where
does aerobic respiration take place in
the cell?
4.
What
is gluconeogenesis, and what nutrients
feed into its pathways?
5.
Where
do the products of lipolysis feed into the
pathways of cellular respiration?
6.
What
is the first step in catabolizing amino
acids?
Diabetes and Obesity Are Metabolic Disorders
LEARNING OBJECTIVES
1.
Distinguish
between type 1 and type 2 diabetes.
2.
Explain
the physiological consequences of dia-
betes.
3.
Define
obesity
and explain its consequences.
4.
Describe
the possible treatments for obesity.
iabetes mellitus is one of many disorders
that can affect the endocrine system (see
Chapter 9). This disorder involves the hor-
mone insulin. Although its causes are endo-
crine in nature, diabetes may be best understood when you
consider its effects on metabolism. Diabetes is also closely
linked with obesity, which has reached epidemic propor-
tions in the United States. Obesity is a complex metabolic
disease involving the balance between food intake and en-
ergy expenditure. Let’s begin our discussion of metabolic
disorders by examining diabetes.
Diabetes Is Like Prolonged Starvation
In most people, blood sugar levels do not change much
after eating a meal. However, eating can be a dangerous
undertaking for the millions of Americans affected by dia-
betes mellitus. As mentioned in Chapter 9, there are two
major types of diabetes mellitus. Type 1 diabetes involves
a lack of insulin because the patient’s immune system has
destroyed the pancreatic P cells. Type 2 diabetes involves
decreased sensitivity of target cells to insulin and is often
associated with obesity. Patients with type 1
diabetes have
no insulin in their bloodstream, while those with type 2
diabetes may have normal, or even elevated, levels of in-
sulin in their blood. The major symptoms of both types of
diabetes include elevated blood glucose, increased urine
flow, increased appetite, and poor circulation (especially
in the hands and feet). How does diabetes produce these
symptoms?
Diabetes is a disease that essentially involves the inef-
fectiveness of insulin, either due to lack of the hormone
or insensitivity to it. As a diabetic patient eats, glucose,
triglycerides, fatty acids, and amino acids are absorbed
into the blood but are not taken up by cells. (Remember
that cellular uptake of these substances requires insulin.)
This increases the concentrations of these substances; el-
evated concentrations of glucose, triglycerides, fatty acids,
and amino acids have important consequences for kidney
function, peripheral circulation, respiration, nervous sys-
tem activity, and vision (Figure 14.15).
422 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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