14.3 regulation of blood glucose level concentration

normal blood glucose concentration

90mg cm^-3

reasons blood glucose concentration can increase

diet - when you eat carbohydrate rich food such as pasta and rice and sweet foods, the carbohydrates which they contain are broken down into the bloodstream to release glucose. The glucose released is absorbed into the bloodstream and the blood glucose concentration rises

glycogenesis - 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 and amino acids. This glucose is released into the bloodstream and causes an increase in blood glucose concentration

decreasing blood glucose concentration

respiration - some of the glucose in the blood is used by cells to release energy. This is required to perform normal bodily functions. However, during exercise, more glucose is needed as the body needs to generate more energy in order to 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

beta cells role in blood glucose concentration

when there is a rise in blood glucose concentration, beta cells detect this and respond by secreting insulin directly into the bloodstream

insulin receptors on blood cells

• all body cells (except red blood cells) have insulin receptors on their cell surface membrane

• 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 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 alpha cells of the islets of langerhans

why does insulin need to constantly be secreted

it is broken down by enzymes in the cells of the liver

negative feedback of blood glucose concentration

when it falls below a set level, the beta cells reduce their secretion of insulin. This insures that changes are reversed and returned back to the set level

alpha cells role in maintaining blood glucose concentration

if the blood glucose concentration is too low, the alpha cells detect this falls and respond by secreting glucagon directly into the bloodstream

the only cells in the body which has glucagon receptors are the liver cells and fat cells - therefore these are the only cells that can response to glucagon

glucagon raises blood glucose concentration by:

glycogenesis - 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

mechanism by which insulin secretion is controlled

1. at normal blood glucose concentration levels, potassium channels in the plasma membrane of the beta cells are open and potassium ions diffuse out of the cell. This inside of the cell is at a potential of -70mV with respect to 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 -30mV and depolarisation occurs

6. depolarisation causes the voltage gated calcium channels to open

7. calcium ions enter the cells and cause secretory vesicles to release the insulin they contain by exocytosis