5.4.3 Controlling Blood Glucose Concentrations Flashcards
What are the factors that affect blood glucose concentration?
There 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
The amount of glucose that gets absorbed into the blood from the products of digestion can vary substantially
Some meals may be much more carbohydrate-rich than others
Control systems within the body help to manage the concentration of glucose in the blood via the hormones insulin and glucagon
When there is excess glucose in the blood from a carbohydrate-dense meal it is removed
This occurs through increased glucose uptake into muscle, fat and liver cells and glycogenesis
When there is insufficient glucose in the blood for metabolic needs it is rapidly released from storage molecules
This occurs through glycogenolysis and gluconeogenesis
The levels of insulin and glucagon present in the blood are constantly regulated and adjusted in order to maintain the blood glucose concentration at a mostly constant level
How is blood glucose level controlled?
If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and may not be able to function normally
If the concentration of glucose in the blood increases above a certain level, this can also disrupt the normal function of cells, potentially causing major problems
The control of blood glucose concentration is a key part of homeostasis
Blood glucose concentration is controlled by two hormones secreted by endocrine tissue in the pancreas
This tissue is made up of groups of cells known as the islets of Langerhans
The islets of Langerhans contain two cell types:
α cells that secrete the hormone glucagon
β cells that secrete the hormone insulin
These α and β cells act as the receptors and initiate the response for controlling blood glucose concentration
The control of blood glucose concentration by glucagon can be used to demonstrate the principles of cell signalling
What happens when there is a decrease in blood glucose concentration?
If a decrease in blood glucose concentration occurs, it is detected by the α and β cells in the pancreas:
The α cells respond by secreting glucagon
The β cells respond by stopping the secretion of insulin
The decrease in blood insulin concentration reduces the use of glucose by liver and muscle cells
Glucagon binds to receptors in the cell surface membranes of liver cells
This binding causes a conformational change in the receptor protein that activates a G protein
This activated G protein activates the enzyme adenylyl cyclase
Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
cAMP binds to protein kinase A enzymes, activating them
Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose
This process is known as glycogenolysis
The enzyme cascade described above amplifies the original signal from glucagon and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level
The hormone adrenaline also increases the concentration of blood glucose
It does this by binding to different receptors on the surface of liver cells that activate the same enzyme cascade and lead to the same end result – the breakdown of glycogen by glycogen phosphorylase
Adrenaline also stimulates the breakdown of glycogen stores in muscle during exercise
The glucose produced remains in the muscle cells where it is needed for respiration
What happens if there is an increase in blood glucose levels?
When the blood glucose concentration increases to above the normal range it is detected by the β cells in the pancreas
When the concentration of glucose is high glucose molecules enter the β cells by facilitated diffusion
The cells respire this glucose and produce ATP
High concentrations of ATP causes the potassium channels in the β cells to close, producing a change in the membrane potential
This change in the membrane potential causes the voltage-gated calcium channels to open
In response to the influx of calcium ions, the β cells secrete the hormone insulin
Insulin-containing vesicles move towards the cell-surface membrane where they release insulin into the capillaries
Once in the bloodstream, insulin circulates around the body
It stimulates the uptake of glucose by muscles cells, fat cells and the liver
What is the action of insulin?
Muscle cells, fat storage cells, adipose tissue and liver cells possess glucose transporter proteins in their surface membranes
They are the target cells of insulin
These membrane proteins allow for the uptake of glucose molecules via facilitated diffusion
The rate of glucose uptake for these cells is limited by the number of glucose transporter proteins present
The glucose transporter proteins on target cells are insulin-sensitive
Insulin binds to specific receptors on the membranes of target cells
This stimulates them to activate/add more glucose transporter proteins to their cell surface membrane which increases the permeability of the cells to glucose
As a result, the rate of facilitated diffusion increases
Insulin also helps to increase the uptake of glucose in the liver by stimulating glycogenesis
Once glucose has entered a liver cell an enzyme rapidly converts it to glucose phosphate
Different enzymes then convert glucose phosphate into glycogen
This helps to lower glucose concentration within the liver cell
A steep diffusion gradient is maintained between the blood in the capillaries and the liver cells
How is blood glucose regulated by negative feedback?
Blood glucose concentration is regulated by negative feedback control mechanisms
In 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
In the control of blood glucose concentration:
α and β cells in the pancreas act as the receptors
They release the hormones glucagon (secreted by α cells) and insulin (secreted by β cells)
Liver cells act as the effectors in response to glucagon and liver, muscle and fat cells act as the effectors in response to insulin
What is the role of the liver?
The liver plays a vital role in the conversion between glycogen and glucose
The conversion between these molecules helps to regulate blood glucose concentration
Both insulin and glucagon have specific receptors on the membranes of liver cells
When these hormones bind to their receptors they trigger several processes within the liver
What is gylcogenesis?
Glycgogenesis is the synthesis of glycogen from glucose molecules
Insulin triggers this process after it detects an increased blood glucose concentration
The synthesis of glycogen removes glucose molecules from the bloodstream and decreases the blood glucose concentration to within a normal range
Glycogen acts as a compact and efficient carbohydrate storage molecule
What is glycogenolysis?
Glycogenolysis is the breakdown of glycogen to produce glucose molecules
Glucagon triggers this process after it detects a decreased blood glucose concentration
It activates enzymes within the liver that breakdown glycogen molecules into glucose
The breakdown of glycogen releases more glucose molecules to the bloodstream and increases the blood glucose concentration to within the normal range
What is gluconeogenesis?
Gluconeogenesis is the synthesis of glucose molecules from non-carbohydrate molecules
Glucagon also 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 which increases the blood glucose concentration to within the normal range
