Flashcards in Control of Blood Glucose (Chapter 14) Deck (45)
What happens to carbohydrate in the body?
1) it is transported through the blood stream in the form of glucose in solution in the blood plasma
2) glucose is converted into the polysaccharide glycogen, a short-term energy store found in liver and muscle cells
What is the concentration of glucose in a healthy human?
80-120mg of glucose per 100cm3 of blood
What happens when there is a low concentration of glucose in the blood?
There is not enough glucose for respiration, ∴ cells can't carry out normal functions - especially important for brain cells, which can only respire glucose
What happens when there is a very high concentration of glucose in the blood?
It can cause major problems, upsetting normal behaviour of cells
What is the homeostatic control of blood glucose concentration carried out by?
2 hormones, glucagon and insulin, secreted by endocrine tissue in the pancreas
What does the endocrine tissue in the pancreas consist of?
Groups of cells (islets of Langerhans), scattered throughout the pancreas
What 2 types of cells does the islets of Langerhans contain and what does each 1 secrete?
1) alpha cells secrete glucagon
2) beta cells secrete insulin
What do alpha and beta cells act as?
The receptors and central control for the control of blood glucose concentration
What do glucagon and insulin coordinate?
The actions of effectors
What happens after a meal when glucose is absorbed into the small intestine and passes into the blood?
1) as this blood flows through the pancreas, alpha and beta cells detect this increase in glucose concentration
2) alpha cells respond by stopping secretion of glucagon, which liver cells respond to and ∴ there is no breakdown of glycogen
3) beta cells respond by secreting insulin into the blood plasma
4) insulin is carried to all parts of the body, in the blood
What is insulin?
A signalling molecule
Why can't insulin stimulate mechanisms within the cell directly?
Because it is a protein and can ∴ not pass through CSMs
How does the release of insulin result in the decrease in the concentration of blood glucose?
1) insulin binds to a receptor on the CSM and affects the cell indirectly through the mediation of intracellular messengers
2) insulin stimulates cells with insulin receptors e.g. muscle cells, to move vesicles containing GLUT4 proteins from the cytoplasm to the CSM and fuse with it
3) GLUT4 proteins facilitate the diffusion of glucose into the cell, down its concentration gradient ∴ increasing the rate at which they absorb glucose from the blood, convert it to glycogen and use it in respiration
4) this results in the decrease of blood glucose concentration
What is the only way that glucose can enter cells?
Through a transporter protein called GLUT
Where are GLUT4 proteins normally kept?
In the cytoplasm
Which GLUT proteins are always in the CSM and not affected by insulin?
GLUT1 in brain cells and GLUT2 in liver cells
How does insulin also trap glucose inside cells?
1) insulin stimulates the activation of the enzyme glucokinase, which phosphorylates glucose
2) phosphorylated glucose cannot pass through the GLUT transporters in the CSM bc it is too big and charged ∴ the glucose is trapped inside the cells
What other 2 enzymes does insulin activate and what do they do?
Glucofructokinase and glycogen synthase, which add glucose molecules to glycogen, increasing the size of glycogen granules in the cell
What happen when a decrease in blood glucose concentration is detected by alpha and beta cells?
1) alpha cells respond by secreting glucagon
2) beta cells respond by stopping the secretion of insulin
What is the effect of decreasing the concentration of insulin in the blood?
The rate of uptake and use of glucose in the liver and muscle cells decreases - uptake still continues, but at a lower rate
How does glucagon increase the concentration of blood glucose?
1) glucagon binds to receptor molecules on the CSMs of liver cells, causing the start of cell signalling
2) this activates a G protein that in turn activates and enzyme within the membrane that catalyses the conversion of ATP to cyclic AMP (a second messenger)
3) cyclic AMP binds to kinase enzymes within the cytoplasm that activate other enzymes by adding phosphate groups to them
4) this enzyme cascade amplifies the original signal from glucagon
5) at the end of the enzyme cascade, glycogen phosphorylase is activated and catalyses the breakdown of glycogen to glucose by removing glucose units from the numerous 'ends' of glycogen
6) this increases the glucose concentration inside the cell, so that it diffuses out through GLUT2 transporter proteins into the blood ∴ increasing blood glucose concentration
How else can glucose be made?
From amino acids and lipids in a process called gluconeogenesis
What else happens as a result of glucagon secretion?
The liver releases extra glucose to increase blood glucose concentration
Why do muscle cells not respond to glucagon?
Bc they do not have receptors for glucagon
What kind of homeostatic mechanism is the control of blood glucose concentration?
Glucagon and insulin work together as part of the negative feedback system in which any deviation of blood glucose concentration from the set point stimulates actions by effectors (liver and muscle cells) to bring it back to normal
Why does blood glucose concentration never remain constant?
- Bc of the time delay between a change in blood glucose concentration and the onset of actions to correct it
- These time delays in control systems result in oscillation, where blood glucose concentration sometimes rises slightly above or drop lightly below the required level
What other hormone increases the blood glucose concentration?
How does adrenaline increase the blood glucose concentration?
1) adrenaline binds to receptors on the surface of liver cells that activate the same enzyme cascade as glucagon and leads to the same end result - the breakdown of glycogen by glycogen phosphorylase
2) adrenaline also stimulates the breakdown of glycogen stores in muscle cells during exercise - the glucose produced remains in muscle cells where it is needed for respiration
What is 1 of the most common metabolic diseases in humans?
Sugar diabetes (diabetes mellitus)