What happens to the glucose when it is utilized by the body?

I am studying Bachelors of Science in Biotechnology and my Molecular Biology lecturer asked me the question "Why is Glucose not stored in the body, but Glycogen is stored?"

I couldn't answer her question in class...so i came home and "googled" and ofcourse i found the answer!

Glucose is a ready source of energy, since its carbon atoms are easily oxidised to form carbon dioxide, releasing energy in the process ( so we now know why is glucose instant energy provider). Due to the numerous OH groups in glucose it readily hydrogen-bonds with water molecues, making it highly soluble in water. This allows easy transportation within the biological systems.

If large amount of glucose was to remain in blood, the osmotic balance between the blood and the cell fluids would be disrupted and the cells would be damaged. However this does not occur, since the glucose does not remain in the bloodstream, but its converted into Glycogen in the liver. Glycogen is suitable for storage because it is insoluble and cannot pass through cell membranes.

When glucose sugar is phosphorylated on carbon 6 we get glucose 6-Phosphate. its two major metaboilc pathways are:

1. Glycolysis: If the cells need energy or carbon skeletons for sysnthesis then glucose 6-phosphate is targeted for glycolysis.
2. Pentose phosphate pathway: when the ratio of NADP+:NADPH increses, the body realizes it needs to produce more NADPH. This will cause the G6P to be dehydrogenated by glucose 6-phosphate dehydrogenase. This reversible reaction is the initial step of the pentose phosphate pathway, which generates the useful co factor NADPH as well as Ribulose 5-phosphate, a carbon source for the synthesis of other molecules.

If the blood sugar levels are high, the body needs a way to store the excess glucose. After being converted to G6P, an isomerase ( phosphoglucose mutase) can turn the molecule into glucose 1 phosphate. G1P can be combined with Uridine TriPhosphate (UTP) to form UDP-glucose, This reaction is driven by the hydrolysis of pyrophosphate that is released in the reaction. Now, the activated UDP-glucose can add to a growing glycogen molecue with the help of glycogen synthase. This is a very efficient storage mechanism for glucose as it requires only 1 ATP to store the 1 glucose molecule and virtually no energy to remove it from storage.

Note that G6P is an allosteric activator of glycogen synthase, which makes sense because when the level of glucose is high the body should store the excess glucose as glycogen. On the other hand, glycogen synthase is inhibited when it is phosphorylated by protein kinase during times of high stress or low blood glucose levels.

And when energy is needed by the body..glycogen is broken down into glucose and used up.

I hope this explanation was enough! you can also visit these sites for more information.

http://www.brianmac.co.UK/nutrit.htm

http://www.carbs-information.com/glucose-energy.htm

I hope this will help you!!

Thankyou!

Amukta!!