Question

1.

A) Why is it essential that glucose is stored as large single particles of glycogen polymerized from up to 55000 glucose molecules, as opposed to large numbers of individual glucose molecules?

B) Glucose 1-phosphate is released from the nonreducing ends of glycogen in the liver. What takes place in order for glucose to enter the blood supply?

C) Why cannot glucose be released to the blood supply from other tissues?

D) Glycogen is a highly branched structure with many nonreducing ends. Why is this important for the function of glycogen?

Answer 1

A) Unlike glucose, glycogen molecule is water insoluble and therefore does not get dissolve in water. It needs to be broken down to release glucose units during its requirement as an energy source. Thus, the insoluble property of glycogen allows it to be stored as a major source of energy for extreme cases and requirements. If individual molecules of glucose would be stored as energy source, then it would not be possible to use it afterwards because of its solubility in water, thus getting used up at once.

B) Glycogen breakdown occurs by release of glucose 1-phosphate and further conversation into glucose 6-phosphate. This glucose 6-phosphate can enter into glycolysis, or could get metabolize by Pentose phosphate pathway and also can be released directly into the bloodstream in the form of free glucose units. This occurs mostly in liver and to some extent in the intestine and kidney.

C) Glycogen is not as reduced as fatty acids are and consequently not as energy rich. However, glycogen is an important fuel reserve for several reasons. The controlled breakdown of glycogen and release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels. Glycogen's role in maintaining blood-glucose levels is especially important because glucose is virtually the only fuel used by the brain, except during prolonged starvation. Moreover, the glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity.

D) Branching in glycogen is important because it increases its solubility. Also, branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase. Thus, branching increases the rate of glycogen synthesis and degradation.