Basics of Carbohydrate Utilisation during exercise

Question 1

Why are carbohydrates a source of energy?

Answer 1

the breakdown of glucose is associated with a negative change in gibbs free energy

Question 2

How are carbohydrates digested?

Answer 2

• carbohydrate digestion begins in the mouth and then small intestine where the enzyme amylase hydrolyses glycosidic bonds in starch to short chain carbohydrates
• oligosaccharides are then broken down into disaccharides in the villi of the small intestine
• specific enzymes such as lactate, maltase and sucrose then breakdown disaccharides to monosaccharides
• monosaccharides are then absorbed into the cytosol of enterocytes and transported into capillaries that empty into venous blood and the portal vein that supplied the liver

Question 3

Where is glycogen stored?

Answer 3

liver (3-7%) and muscle (1-1.5%)

Question 4

What is glycogen synthesis?

Answer 4

• reaction: anabolic, condensation
• process: uridine triphosphate reacts with glucose to provide energy to build the glycogen chain
• enzymes: glycogenin, glycogen synthase and branching enzyme

Question 5

What is glycogen breakdown?

Answer 5

• reaction: catabolic, hydrolysis
• process: a phosphate group is added to glucose to release it from the glycogen chain
• enzymes: glycogen phosphorylase and debranching enzyme

Question 6

How does exercise regulate glycogen breakdown?

Answer 6

• the rate of glycogen breakdown in skeletal muscle increases rapidly within seconds of muscle contraction due to the high demand for ATP
• glycogen breakdown is also increased in the liver to supply the muscle with more glucose
• glycogen phosphorylase activity in muscle is increased by:
  
  • increased Pi
  • Increased AMP/ATP ration
  • increased adrenaline
  • increased Ca2+
• glycogenolysis in the liver is increased by adrenaline

Question 7

How does exercise regulate glycogen synthesis?

Answer 7

• glycogen synthesis directly relates to glycogen content in muscle
• over a short term period of hard exercise, glycogen synthase activity is inhibited. Prolonged periods of exercise may activate glycogen synthesis to maintain energy balance. glycogen synthesis can be rapidly activated after exercise
• phosphorylation is a quick and energetically favourable control mechanism for regulating enzyme function, without the need to deconstruct a protein