Carbohydrates- Metabolism of Glycogen

Yay, last post on carbohydrates! I'm gonna binge on Skylanders after this because I'm really a 12-year-old at heart.

Be able to describe how carbohydrates are digested.

I've already mentioned a bit about digestion in previous posts about the digestive system, including Digestion and Absorption of Food- Part 1 and Digestion and Absorption of Food- Part 2. In the mouth, longer-chain sugars are broken down into shorter chains via the action of salivary amylase. Further down, pancreatic amylase from the pancreas breaks down these shorter chains further into disaccharides. Finally, disaccharides are broken down into monosaccharides via enzymes in the intestinal juice.

Once inside cells, sugar molecules can become oxidised in the mitochondria, reducing NADH, which heads over to the electron transport chain where it powers the production of ATP. ATP is an energy carrier within the cell.

Be able to describe the structure of glycogen and how it is stored in the body.

Be able to describe the role of glycogen in the liver.

Glycogen has a similar structure to amylopectin (described in an earlier post). It is a highly branched polysaccharide consisting wholly of glucose units. Some are joined in alpha(1->4) linkages while the branches are formed via alpha(1->6) links. There are more branches in glycogen than there are in amylopectin. The reducing end of glycogen is often bound to a protein molecule called glycogenin.

Glycogen is stored in liver and muscle. Part of the reason why glucose is stored as glycogen is because a large molecule has less osmotic effect than many small molecules, and thus glycogen can be stored without having to worry about too much water following by osmosis. Between meals, glycogen in the liver can be broken down, maintaining blood glucose levels between meals.

Be able to explain anabolism and catabolism.

Know that cleavage of glycosidic bonds does not require metabolic energy.

Know that formation of glycosidic bonds requires metabolic energy.

Anabolism is the synthesis of larger molecules from combining many small ones- for example forming glycosidic bonds between sugars to form longer sugars. This process often consumes energy. Catabolism is the opposite- it is the breakdown of larger molecules into smaller ones. One example is the cleavage of the glycosidic bonds between sugars to form shorter sugars. This process often does not require energy.

Know the pathway by which glycogen is synthesized from glucose.

The synthesis of glycogen is somewhat complex but I'll try and explain it as simply as possible.

Firstly, hexokinase adds a phosphate group to the C6 of glucose, using energy from ATP. This produces glucose 6-phosphate. Phosphoglucomutase then converts glucose 6-phosphate to glucose 1-phosphate. UDP-glucose pyrophosphorylase then catalyses the addition of UTP to glucose 1-phosphate, releasing two phosphate groups (i.e. a pyrophosphate) and producing UDP-glucose. UDP-glucose molecules can then join on to each other and to the growing glycogen chain via glycogen synthase.

Know the pathway by which glycogen is catabolised to glucose.

The catabolism of glycogen is somewhat simpler than its synthesis. Since no energy is required, fewer steps are required. Firstly, glycogen phosphorylase breaks the glycosidic bond to attach a phosphate group to the C1 carbon instead. This forms glucose 1-phosphate. Phosphoglucomutase then converts glucose 1-phosphate to glucose 6-phosphate. Finally, glucose 6-phosphatase removes the phosphate group, leaving glucose.

Yay! Carbohydrate revision is done for now!