Control of blood glucose

• carbohydrate is transported in the bloodstream in form of glucose in solution in blood plasma
• glucose is converted into glycogen (a polysaccharide, large insoluble molecule made up of many glucose units linked by 1-4 glycosidic bonds with 1-6 branching points)
• glycogen is a short-term energy store found in liver and muscle cells and is easily converted to glucose
• in a healthy human, each 100cm3 of blood normally contains between 80 and 120mg of glucose
• if concentration decreases below this, cells may not have enough glucose for respiration and maybe unable to carry out their normal activities
• this is especially important for cells that can respire only glucose, such as brain cells
• very high concentrations of glucose again upset normal behaviour of cells
• the homostatic control of blood glucose concentration is carried out by two hormones secreted by endocrine tissue in the pancreas 
• this tissue consists of groups of cells, known as the islets of Langerhans, which are scattered throughout the pancreas
  • islet means a small island
• the islets contain two types of cells:
  • alpha cells secrete glucagon
  • beta cells secrete insulin
    

The alpha and beta cells act as the receptors and the central control for this homeostatic mechanism; the hormones glucagon and insulin coordinate the actions of the effectors

After consuming a meal containing carbohydrate, glucose from digested food is absorbed from the small intestine and passes into the blood

• as this blood flows through the pancreas, the alpha and beta cells detect the increase in glucose concentration
• alpha cells respond by stopping secretion of glucagon
• beta cells respond by secreting insulin into the blood plasma
  • the insulin is carried to all parts of the body in the blood
• insulin is a signalling molecule
  • since it is a protein, it cannot pass through cell membranes to stimulate the mechanisms directly
  • instead insulin binds to a receptor in the cell surface membrane and affects the cell indirectly through the mediation of intracellular messengers
• there are insulin receptors on many cells, such as those in the liver, muscle and adipose (fat storage) tissue
• insulin stimulates cells with these receptors to increase rate of glucose absorption from the blood, convert it into glycogen and use it in respiration
• this results in a decrease in the concentration of glucose in the blood

Glucose can only enter cells through transporter proteins known as GLUT

• there are several types of GLUT proteins; muscles have the type called GLUT4
• normally GLUT proteins are kept in cytoplasm in the same way as the aquaporins in collecting duct cells
• when insulin molecules bind to receptors on muscle cells, vesicles with GLUT4 proteins are moved to the cell surface membrane and fuse with it
• GLUT4 proteins facilitate the movement of glucose into the cell
• brain cells have GLUT1 proteins and liver cells have GLUT2 proteins, which are always in the cell surface membrane, and their distribution is not altered by insulin

Insulin also stimulates the activation of the enzyme glucokinase, which phosphorylates glucose.