Chapter 34 HEIN
Chapter 34 HEIN
Hein * Best * Pattison * Arena
© John Wiley and Sons, Inc.
34.1 Metabolic Pathways
34.2 Exercise and Energy
34.3 The Bloodstream: A
34. 4 Anaerobic Sequence
34.5 Citric Acid Cycle
34.7 Overview of Complex
34.9 Blood Glucose and
A metabolic pathway is a
series of biochemical
reactions that serve a
Everything we do requires metabolic
Metabolism is a complex interplay:
chemical reactions within cells lead
to chemical transport between cells.
Carbohydrate catabolism is designed
to release energy relatively quickly,
so this form of catabolism is
activated during strenuous muscular
• When a muscle contracts, energy is
• Muscle contraction uses ATP; ATP is in
• Muscle tissue can contract for no more
than several seconds before the supply of
high-energy phosphate bonds is depleted.
• After the initial contraction, the muscle cells
look for other energy sources.
• Muscle glycogen is the next available source.
• This polymer breaks down to glucose, which is
oxidized to replenish the ATP supply.
• Because glucose oxidation is a complex process,
muscle contraction must proceed at a slower rate.
• This energy supply is only useful for about 2
minutes of work; muscles rapidly deplete their
glycogen stores and build up lactic acid.
The Bloodstream: A
• The bloodstream transports
chemicals from one cell to another.
• Nutrients (e.g. glucose, amino
acids, and fatty acids) and oxygen
are delivered; metabolic products
(e.g. lactate) and carbon dioxide
Figure 34.1 Overview of carbohydrate metabolism
• When blood glucose is in excess, it is
converted to glycogen in the liver and in
• Glycogen is a storage polysaccharide; it
quickly hydrolyzes to replace depleted
glucose supplies in the blood.
• The synthesis of glycogen from glucose is
• The hydrolysis, or breakdown, of glycogen
to glucose is known as glycogenolysis.
• In the absence of oxygen, glucose in
living cells is converted to a variety of
end products, including lactic acid (in
muscle) and alcohol (in yeast).
• At least a dozen reactions, many
different enzymes, ATP, and inorganic
phosphate (Pi) are required.
• Such a sequence of reactions from a
particular reactant to end product is
called a metabolic pathway.
• The anaerobic conversion of glucose to
pyruvate is known as the EmbdenMyerhof pathway.
• The sequence is a catabolic one in which
glucose is oxidatively degraded.
• When lactate is the final product of
anaerobic glucose catabolism, the pathway
is termed glycolysis.
• What glycolysis does for the cell can be
summarized with the following net
C6H12O6 + 2 ADP + 2 Pi
2CH3CH(OH)COO- + 2 ATP + 150 kJ
Citric Acid Cycle
Since only a small fraction of the energy
that is potentially available from glucose
is liberated during anaerobic conversion
to lactate (glycolysis), lactate remains
valuable to the cells.
The lactate formed may be:
1) Circulated back to the liver and converted to
glycogen at the expense of some ATP
2) Converted back to pyruvate in order to enter
the citric acid cycle.
The citric acid
one cycle, (1)
blue enter the
cycle, and (2)
marked in red
are lost as
citric acid cycle,
+ 30 ATP
The formation of glucose from
noncarbohydrate sources is called
Most of the glucose formed during
gluconeogenesis comes from
lactate, certain amino acids, and the
glycerol of fats.
Figure 34.4 An overview of gluconeogenesis.
All transformations except lactate to pyruvate
require a series of reactions.
Overview of Complex
Figure 34.5 A single-step oxidation process compared with a
multiple process: In the pathway, A, B, and C represent 26
hypothetical pathway intermediates.
• Hormones are chemical substances that act
as control agents in the body, often
regulating metabolic pathways.
• Hormones help to adjust physiological
processes such as:
• Hormones are often called the chemical
messengers of the body.
• They do not fit into any single structural
• Polypeptides or proteins
• Phenol or amino acid derivatives
Blood Glucose and
• An adequate blood-glucose level must be
maintained to ensure good health.
• To achieve this goal, hormones regulate
and coordinate metabolism in specific
• The hormones control selected enzymes,
which, in turn, regulates the rates of
reaction in the appropriate metabolic
related to the
glucose in the
Figure 34.8 Typical responses to a glucose-tolerance test.