Control and storage of glucose

As we discovered earlier, glucose, like fat, is stored by the body as an energy reserve. An average 80kg person stores 500 grams of glucose as glycogen. Approximately 400 grams is stored in muscles while 100 grams is stored in the liver. Muscle glycogen serves as the major source of energy for active muscles during exercise. Depletion of glycogen, through exercise or starvation, stimulates a process called gluconeogenesis to take place. Gluconeogenesis is the formation of glucose from non-glucose molecules such as amino acids. During gluconeogenesis, proteins are broken down into amino acids which are then modified by removing nitrogen and using the remainder of the molecule to synthesise glucose.

 
Hormones regulate the level of blood sugar circulating in the body at any given moment. Elevated blood sugar levels, as occurs after ingesting a meal high in carbohydrates, trigger the secretion of insulin by special cells, called beta cells, found in the pancreas. Insulin facilitates the uptake of glucose by muscle cells and therefore helps to keep the blood sugar concentration at an appropriate level. The image on the right shows insulin at work. It binds with special molecules found on the surface of specific cells. This causes special gates to open and allow glucose to enter the cell. a
However, if the blood sugar level drops below normal a condition known as hypoglycemia occurs. In response to hypoglycemia different cells in the pancreas, known as alpha cells, secrete a hormone called glucagon. Glucagon promotes glycogenolysis in the liver which releases glucose into the blood. Glycogenolysis is the process by which glycogen is broken down to form glucose. Glucagon also promotes gluconeogenesis in an effort to increase blood glucose levels.

The body's reserves of glycogen are limited and active people can easily deplete their glycogen stores if daily carbohydrate intake is not maintained. However, too much of a good thing is not always better. Excess glucose that can not be converted into glycogen is converted into fat and stored in adipose tissue.

 

Not only is glucose used as a fuel directly but it also influences the use of fat and protein as sources of energy.
- Appropriate carbohydrate consumption directly impacts on the preservation of tissue proteins. Protein is essential for repair and growth of tissues. When glycogen levels are low gluconeogenesis, synthesises glucose from amino acids derived from the breakdown of protein. This helps maintain blood sugar levels in times of prolonged exercise or starvation.

- Insufficient glucose metabolism acts to stimulate the use of fat as an energy source. Using fat provides energy at a slower rate than glucose metabolism and exercise intensity falls sharply.
List two important functions of carbohydrates in the body.
Under what conditions is the hormone glucagon secreted by the pancreas?
What does glucagon do when released into the blood stream by the pancreas?
Explain the differences between glucagon and insulin.
Under what conditions is the hormone insulin secreted by the pancreas?
What does insulin do when released into the blood stream?
What is gluconeogenesis?
What conditions are likely to result in gluconeogenesis?
During long and frequent periods of exercise an athlete is likely to reduce in muscle mass. Explain why
What is one possible outcome of low levels of glycogen in muscle cells?
Continue with Are all carbohydrates the same?