Controlling blood glucose concentration 5.4.3 Flashcards
What are three ways in which glucose can enter the bloodstream?
- Absorption in the gut following carbohydrate digestion
- Hydrolysis of glycogen stores
- Non carbohydrates such as lipids, lactate and amino acids that have been converted to glucose
How does the amount of glucose absorbed in the blood vary?
Through digestion - as some meals may be much more carbohydrate rich than others
What are the hormones that control blood glucose concentration?
Insulin and glucagon
What happens when blood glucose concentration decreases?
Detected by a and b cells in the pancreas (a=glucagon rise, b=insulin stops)
- Decrease in insulin reduces use of glucose by muscles and livers
- Glucagon binds to receptors in liver membrane, causing a conformational change
- This activates a G protein which activates the enzyme adenylyl cyclase
- Adenylyl cyclase catalyses the conversion of ATP to cyclic AMP
- cAMP binds to protein kinase A enzymes
- Activate protein kinase A activates phosphorylase kinase enzymes by adding phosphate groups to them
- Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
- Active glycogen phoshorylase enzymes catalyse the breakdown of glycogen to glucose
How do B cells respond when blood glucose concentration is too low?
By stopping the secretion of insulin
How do a cells respond when blood glucose concentration is too low?
By secreting glucagon
What is glycogenolysis?
Breakdown of glycogen to glucose
How does adrenaline increase the concentration of blood glucose?
It binds to different receptors on the surface of liver cells that activate the same enzyme cascade and lead to the same end result - which is the breakdown of glycogen by glycogen phosphorylase (glycogenolysis)
It also stimulates the breakdown of glycogen stores in muscle during exercise
What happens when blood glucose concentration increases?
- B cells detect this rise
- When the concentration of glucose is high, glucose molecules enter the B cells by facilitated diffusion
- The cells respire this glucose and produce ATP
- High concentrations of ATP causes the potassium channels in the B cells to close, producing a change in the membrane potential
- This causes voltage gates calcium channels to open
- In response to the influx of calcium ions, the B cells secrete insulin and insulin containing vesicles move towards the cell surface membrane where theyre released by exocytosis
- Once in the bloodstream, insulin circulates around the body
- It stimulates the uptake of glucose by muscle, fat, and liver cells
What cells possess glucose transporter proteins
Muscle cells, fat storage cells, adipose tissue, and liver cells
Which are the target cells of insulin
What do glucose transporter proteins allow for?
Facilitated diffusion of the uptake of glucose molecules
What is the rate of glucose uptake limited by?
The number of glucose transporter proteins
What happens when insulin binds to specific receptors on the membranes of target cells?
- Stimulates them to activate/add more glucose transporter proteins to their cell surface membrane which increases the permeability of their cells
- As a result the rate of facilitated diffusion increases
What is blood glucose concentration regulated by?
Negative feedback control mechanisms
What happens in the negative feedback systems
- Receptors detect whether a specific level is too low or too high
- This information is communicated through the hormonal or nervous system to effectors
- Effectors react to counteract the change by bringing the level back to normal
Describe the negative feedback mechanism of blood glucose concentration
- a and b cells in the pancreas act as the receptors
- They release the hormones glucagon (secreted by a cells) and insulin (secreted by B cells)
- Liver cells act as the effectors in response to glucagon and fat cells act as the effectors in response to insulin
What is the role of the liver in hormonal communication
Plays a vital role in the conversion between glycogen and glucose, which helps to regulate glucose concentration
What is glycogenesis?
The synthesis of glycogen from glucose molecules
What triggers glycogenesis?
Insulin after it detects an increased blood glucose concentration
What does the synthesis of glycogen do?
Removes glucose molecules from the bloodstream and decreases blood glucose concentration to within a normal range
What does the breakdown of glycogen do?
Releases more glucose molecules to the bloodstream and increases the blood glucose concentration to within the normal range
What is gluconeogenesis?
The synthesis of glucose molecules from non carbohydrate molecules
What triggers gluconeogenesis?
Glucagon
Describe gluconeogenesis
- Glucagon triggers this by activating enzymes within the liver
- These enzymes convert other molecules, such as fatty acids and amino acids, into glucose molecules
- Glucose molecules are released into the bloodstream
