regulation of blood glucose Flashcards
increasing blood glucose concentration
Glucose is a small, soluble molecule that is carried in the blood plasma. Blood glucose is normally maintained at a concentration of around 90 mg cm-3 of blood. Blood glucose concentration can increase as a result of:
• Diet - when you eat carbohydrate-rich foods such as pasta and rice (which are rich in starch) and sweet foods such as cakes and fruit (which contain high levels of sucrose), the carbohydrates they contain are broken down in the digestive system to release glucose. The glucose released is absorbed into the bloodstream, and the blood glucose concentration rises.
• Glycogenolysis - glycogen stored in the liver and muscle cells is broken down into glucose which is released into the bloodstream increasing blood glucose concentration.
-Gluconeogenesis - the production of glucose from non-carbohydrate sources. For example, the liver is able to make glucose from glycerol (trom lipids) and amino acids. This glucose is released into the bloodstream and causes an increase in blood glucose concentration.
decreasing blood glucose concentration
Blood glucose concentration can be decreased by:
-Respiration - some of the glucose in the blood is used by cells to release energy. This is required to perform normal body functions.
However, during exercise, more glucose is needed as the body needs to generate more energy in order for muscle cells to contract.
The higher the level of physical activity, the higher the demand for glucose and the greater the decrease of blood glucose concentration.
• Glycogenesis - the production of glycogen. When blood glucose concentration is too high, excess glucose taken in through the diet is converted into glycogen which is stored in the liver.
role of insulin
Insulin is produced by the ß cells of the islets of Langerhans in the pancreas. If the blood glucose concentration is too high, the B cells detect this rise in blood glucose concentration and respond by secreting insulin directly into the bloodstream.
Virtually all body cells have insulin receptors on their cell surface membrane (an exception being red blood cells). When insulin binds to its glycoprotein receptor, it causes a change in the tertiary structure of the glucose transport protein channels. This causes the channels to open allowing more glucose to enter the cell. Insulin also activates enzymes within some cells to convert glucose to glycogen and fat.
Insulin therefore lowers blood glucose concentration by:
• increasing the rate of absorption of glucose by cells, in particular skeletal muscle cells
• increasing the respiratory rate of cells - this increases their need for glucose and causes a higher uptake of glucose from the blood
• increasing the rate of glycogenesis - insulin stimulates the liver to remove glucose from the blood by turning the glucose into glycogen and storing it in the liver and muscle cells
• increasing the rate of glucose to fat conversion
• inhibiting the release of glucagon from the a cells of the islets of Langerhans.
Insulin is broken down by enzymes in the cells of the liver. Therefore, to maintain its effect it has to be constantly secreted. Depending on the food eaten, insulin secretion can begin within minutes of the food entering the body and may continue for several hours after eating.
As blood glucose concentration returns to normal, this is detected by the B cells of the pancreas. When it falls below a set level, the ß cells reduce their secretion of insulin. This is an example of negative feedback.
Negative feedback ensures that, in any control system, changes are reversed and returned back to the set level.
role of glucagon
Glucagon is produced by the a cells of the islets of Langerhans in the pancreas. If the blood glucose concentration is too low, the a cells detect this fall in blood glucose concentration and respond by secreting glucagon directly into the bloodstream.
Unlike insulin, the only cells in the body which have glucagon receptors are the liver cells and fat cells - therefore these are the only cells that can respond to glucagon.
Glucagon raises blood glucose concentration by:
• glycogenolysis - the liver breaks down its glycogen store into glucose and releases it back into the bloodstream
• reducing the amount of glucose absorbed by the liver cells
• increasing gluconeogenesis - increasing the conversion of amino acids and glycerol into glucose in the liver.
As blood glucose concentration returns to normal, this is detected by the a cells of the pancreas. When it rises above a set level, the a cells reduce their secretion of glucagon. This is another example of negative feedback. The feedback causes the corrective measures to be switched off, returning the system to its original (normal) level.
interaction of insulin and glucagon
Figure 4 shows how insulin and glucagon work together to maintain a constant blood glucose concentration. Insulin and glucagon are antagonistic hormones, that is, they work against each other. The system of maintaining blood glucose concentration is said to be self-regulating, as it is the level of glucose in the blood that determines the quantity of insulin and glucagon that is released. Blood glucose concentration is not constant, but fluctuates around a set point as the result of negative feedback. In times of stress adrenaline is released by the body. One of the effects of this hormone is to raise the blood glucose concentration to allow more respiration to occur. You will find out more about the fight and flight response in Topic 14.5, Coordinated responses.
control of insulin secretion
When blood glucose concentration rises above the set level, this is detected by the ß cells in the islets of Langerhans and insulin is released. The mechanism by which this occurs is as follows:
1) At normal blood glucose concentration levels, potassium channels in the plasma membrane of B cells are open and potassium ions diffuse out of the cell. The inside of the cell is at a potential of -70 mV with respect to the outside of the cell.
2)When blood glucose concentration rises, glucose enters the cell by a glucose transporter.
3)The glucose is metabolised inside the mitochondria, resulting in the production of ATP.
4)The ATP binds to potassium channels and causes them to close.
They are known as ATP-sensitive potassium channels.
5)As potassium ions can no longer diffuse out of the cell, the potential difference reduces to around -30 mV and depolarisation occurs.
6)Depolarisation causes the voltage-gated calcium channels to open.
7)Calcium ions enter the cell and cause secretory vesicles to release the insulin they contain by exocytosis.
