Download glycogen disappears

Glycolysis & the Oxidation of Pyruvate
Glycolysis, the major pathway for glucose
metabolism, occurs in the cytosol of all cells.
•
• It is unique in that it can function either
aerobically or anaerobically.
• Erythrocytes, which lack mitochondria, are
completely reliant on glucose as their
metabolic fuel and metabolize it by anaerobic
glycolysis.
The pathway
of glycolysis.
• The end product of glycolysis is
pyruvate under aerobic conditions.
• However, to oxidize glucose beyond
pyruvate, glycolysis requires both
oxygen and mitochondrial enzyme
systems such as the pyruvate
dehydrogenase complex, the citric acid
cycle, and the respiratory chain.
GLYCOLYSIS CAN FUNCTION
ANAEROBIC CONDITIONS
UNDER
• When a muscle contracts in an anaerobic
medium, i.e, one from which oxygen is
excluded, glycogen disappears and lactate
appears as the principal end product.
• When oxygen is admitted, aerobic recovery
takes place and lactate disappears.
The overall equation for glycolysis from
glucose to lactate is as follows:
Summary of glycolysis. − , blocked by anaerobic
conditions or by absence of mitochondria containing
key respiratory enzymes, eg, as in erythrocytes.
Metabolism of Glycogen
• Glycogen
is
the
major
storage
carbohydrate in animals, corresponding to
starch in plants; it is a branched polymer of
α-D-glucose.
• It occurs mainly in liver (up to 6%) and
muscle, where it rarely exceeds 1%.
• Muscle glycogen is a readily available
source of glucose for glycolysis within the
muscle itself.
• Liver glycogen functions to store and
export glucose to maintain blood glucose
between meals.
• After 12–18 hours of fasting, the liver
glycogen is almost totally depleted.
• Glycogenesis: it is the process of glycogen
synthesis and is controlled by the enzyme
glycogen synthase.
• Glycogenolysis is the process of glycogen
degradation into glucose in blood and
lactate in muscle, it is controlled by the
enzyme glycogen phosphorylase.
• Glycogenolysis is not the reverse of
glycogenesis but a separate pathway.
Pathway of glycogenesis and of
glycogenolysis in the liver.
SUMMARY
• Glycogen represents the principal storage form of
carbohydrate in the mammalian body, mainly in the liver and
muscle.
• In the liver, its major function is to provide glucose for
extrahepatic tissues. In muscle, it serves mainly as a ready
source of metabolic fuel for use in muscle.
• Glycogen is synthesized from glucose by the pathway of
glycogenesis. It is broken down by a separate pathway known
as glycogenolysis.
• Glycogenolysis leads to glucose formation in liver and lactate
formation in muscle owing to the respective presence or
absence of glucose-6-phosphatase.