Topic 6—C: Homeostasis- 2. Control of blood glucose concentration Flashcards
What’s monitored by cells in the pancreas?
Glucose
When does blood glucose concentration rise?
- After eating food containing carbohydrate
When does blood glucose concentration fall?
- After exercise as more glucose is used in respiration to release energy
What 2 hormones does the hormonal system use to control blood glucose concentration?
- Insulin
- Glucagon
What are insulin and glucagon?
- hormones
- Chemical messengers that travel in the blood to their target cells (effectors)
Where are glucagon and insulin secreted by?
- They’re both secreted by clusters of cells in the pancreas called the islets of langerhans
What is the name of the 2 cells that the islets of langerhans contain?
- Beta cells
- Alpha cells
What does b cells secrete?
They secrete insulin into the blood
What does a cells secrete?
- They secrete glucagon into the blood
What do insulin and glucagon do?
- They act on effectors, which respond to restore the blood glucose concentration to the normal level
Insulin
- It lowers blood glucose concentration when its too high
- It binds to specific receptors on the cell membranes of muscle cells and liver cells
- It increases the permeability of muscle-cell membranes to glucose so the cell takes up more glucose
- This involves increasing the number of channel proteins in the cell membranes
- It also activates enzymes in muscle and liver cells that convert glucose into glycogen
- The cells are bale to store glycogen in their cytoplasm as an energy source
- Insulin increases the rate of respiration of glucose especially in muscle cells
What is the process of forming glycogen from glucose?
- Glycogenesis
Glucagon
- It raises blood glucose concentration when it’s too low
- It binds to specific receptors on the cell membranes of liver cells and activates enzymes to break down glycogen into glucose
- It also activates enzymes that are involved in the formation of glucose from glycerol (a component of lipids) and amino acids
- Glucagon decreases the rate of respiration of glucose in cells
What is the name of the process of breaking down glycogen into glucose?
- Glycogenolysis
What is the name of the process of forming glucose from glycerol/amino acids?
- Gluconeogenesis
Why are the responses produced by hormones slower than those produced by nervous impulses?
- Because they travel in the blood to their target cells and nerve impulses are quick
- Also means that responses to hormones can occur all over the body if their target cells are widespread, unlike nerve impulses that are localised to one area
- Hormones are not broken down as quickly as neurotransmitters though so their effect tends to last for longer
Negative feedback mechanisms (glucose concentration) - Rise in blood glucose concentration
- When the pancreas detects blood glucose concentration is too high, the b cells secrete insulin and the a cells stop secreting glucagon
- Insulin then binds to receptors on liver and muscle cells (effectors)
- The liver and muscle cell respond to decrease the blood glucose concentration e.g. glyogenesiss is activated
- Blood glucose concentration returns to normal
Negative feedback mechanism model (rise in blood glucose concentration)
Normal blood glucose concentration-> rise in blood glucose concentration-> pancreas detects change-> Pancreas secretes insulin, stops glucagon secretion-> liver and muscle cells respond: - Cells take up more glucose, glycogenesis is activated, cells respire more glucose-> blood glucose concentration falls back to normal blood glucose concentration
Negative feedback mechanisms (glucose concentration) - fall in blood glucose concentration
- When the pancreas detects blood glucose is too low, the a cells secrete glucagon and the b cells stop secreting insulin
- Glucagon then binds to receptors on liver cells (effectors)
- The liver cells respond to increase the blood glucose concentration e.g. glycogenolysis is activated
- Blood glucose concentration then returns to normal
Negative feedback mechanism model (fall in blood glucose concentration)
Normal blood glucose concentration-> fall in blood glucose concentration-> Pancreas detects change-> Pancreas secretes glucagon, stops insulin secretion-> Liver cells respond: Glycogenolysis is activated, gluconeogenesis is activated, cells respire less glucose-> blood glucose concentration rises back to normal blood glucose concentration
Glucose transporters
- They are channel proteins
- They allow glucose to be transported across a cell membrane
- Skeletal and cardiac muscle cells contain a glucose transporter called GLUT4
- When insulin levels are low, GLUT4 is stored in vesicles in the cytoplasm of cells but when insulin binds to receptors on the cell surface membrane it triggers the movement of GLUT4 to the membrane
- Glucose can be transported into the cell through the GLUT4 protein by facilities diffusion
Adrenaline
- It’s a hormone that’s secreted from your adrenal glands
- It’s secreted when there is a low concentration of glucose in your blood, when your stressed and when your exercising
- Adrenaline binds to receptors in the cell membrane of liver cells and does these things to increase blood glucose concentration
- Activates Glycogenolysis (breakdown of glycogen to glucose)
- Inhibits glycogenesis (synthesis of glycogen from glucose)
- It also activates glucagon secretion and inhibits insulin secretion which increases glucose concentration
- Adrenaline gets the body ready for action by making more glucose available for muscles to respire
Second messengers
- Adrenaline and glucagon can activate Glycogenolysis inside a cell even though they bind to receptors on the outside of the cell
- They do this by the second messenger model
- The binding of the hormone to cell receptors activates an enzyme on the inside of the cell membrane which then produces a chemical known as the second messenger
- The second messenger activates other enzymes in the cells to bring about a response
- Receptors for adrenaline and glucagon have specific tertiary structures that make them complementary in shape to their respective hormones
- To activate Glycogenolysis, adrenaline and glucagon bind to their receptors and activate an enzyme called adenylate cyclase
- Activated adenylate cyclase converts ATP into a chemical called cyclic AMP (cAMP) which is a second messenger
- cAMP activates an enzyme called protein kinase A
- Protein kinase A activates a cascade (chain of reactions) that break down glycogen into glucose (Glycogenolysis)
What does glycogenesis convert?
- Glucose to glycogen
What is glycogenesis activated by?
- Insulin
What does Glycogenolysis convert?
- Glycogen to glucose
What is Glycogenolysis activated by?
- Glucagon and adrenaline
What does gluconeogenesis convert?
- Glycerol/amino acids to glucose
What is gluconeogenesis activated by?
Glucagon
