14.1 Homeostasis in Mammals (Blood Glucose) Flashcards
In a healthy human, what would their blood glucose concentration be
100cm cubed of blood normally contains around 100mg of glucose.
If blood glucose concentration falls well below 100mg in 100cm cubed of blood, the person is said to be
Hypoglycaemic
- cells do not have enough glucose to carry out respiration and so metabolic reactions may not be able to take place –> like in the brain –> unconsciousness
If blood glucose concentration increases well above 100mg in 100cm cubed of blood, the person is said to be
Hyperglycaemic
- the high glucose concentration decreases the water potential of blood and tissue fluid, so water moves out of the cells –> unconsciousness
The two hormones released for blood glucose control is secreted by which endocrine tissue
The islets of Langerhans in the Pancreas
- islet - small islands
What are the cells in the islets of Langerhans?
- α cells secrete glucagon
- β cells secrete insulin.
The α and β cells act as the receptors and the central
control for this homeostatic mechanism; the hormones
glucagon and insulin coordinate the actions of the
effectors.
β cells function
Sense when blood glucose concentration is high by secreting insulin which has several effects including:
- causing muscle and adipose tissue cells (fat cells) to absorb more glucose from the blood
- causing liver cells to convert glucose to glycogen for storage
These effects cause blood glucose concentration to fall
α cells function
Sense when blood glucose concentration is low by secreting glucagon which has several effects including:
- causing liver cells to break down glycogen to glucose, and releasing it into the blood
- causing liver cells to produce glucose from other substances such as amino acids or lipids
These effects cause blood glucose concentration to rise
Negative feedback control of high blood glucose concentration
- α and β cells in the islets of Langerhans act as receptors that detect a rise in blood glucose concentration
- Decrease in glucagon secreted, increase in insulin secreted by pancreas
- Liver cells respond to less glucagon - no glycogen breakdown
- Liver, muscle and fat cells respond to more insulin - increased uptake and use of glucose
Negative feedback control of low blood glucose concentration
- α and β cells in the islets of Langerhans act as receptors that detect a fall in blood glucose concentration
- Decrease in insulin secreted, increase in glucagon secreted by pancreas
- Liver cells respond to more glucagon by breaking down glycogen to glucose
- Liver, muscle and fat cells respond to less insulin - reduced uptake of glucose
Glucose Transport Proteins
GLUT proteins
- Muscle cells have GLUT4
- Brain cells have GLUT1 (not affected by insulin)
- Liver cells have GLUT2
Insulin action
- Occurs at muscle cells, liver cells, adipose cells
- Insulin binds to a receptor in the cell surface membrane.
- The receptor signals to the cell and makes vesicles carrying glucose transporter proteins (GLUT4 in muscles, GLUT2 in liver) merge with the cell surface membrane.
- Glucose can now diffuse into the cell down its concentration gradient.
- It stimulates the activation of the enzyme
glucokinase, which phosphorylates glucose. This traps
glucose inside cells, because phosphorylated glucose cannot pass through the transporters in the cell surface
membrane. - stimulates the activation of two other enzymes, (glycogen synthase), which together add glucose molecules to glycogen. This increases the size of the glycogen granules inside the cell
Glucagon action
- Occurs at liver cells (not muscles)
1) Glucagon binds to membrane receptor of liver cells
2) This activates a G protein that in turn activates an enzyme within the membrane that catalyses the conversion of ATP to cyclic AMP, which is a second messenger (Cascade of enzyme reactions occur)
3) Cyclic AMP binds to kinase enzymes within the cytoplasm that activate other enzymes.
4) Kinase enzymes activate enzymes by adding phosphate groups to them in a process known as phosphorylation. This enzyme cascade amplifies the
original signal from glucagon.
5) Glycogen phosphorylase is at the end of the enzyme
cascade: when activated, it catalyses the breakdown of
glycogen to glucose.
6) This increases the concentration of glucose inside the cell so that it diffuses out through GLUT2 transporter proteins into the blood.
Adrenaline action
- also increases the concentration of blood glucose
- 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 - 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.
Dip Sticks
- Dip sticks for detecting glucose in urine contain the enzymes glucose oxidase and peroxidase and a pigment called Cromagen.
- These two enzymes are immobilised on a small pad
at one end of the stick. - Glucose (reacts with glucose oxidase) → Gluconic acid + Hydrogen Peroxide (H2O2)
- Hydrogen peroxide reacts with Cromagen (pigment): Green → Brown
- The resulting colour of the pad is matched against a colour chart.
Biosensors
- Biosensors turn the colour readings into a number - digital readout
- Glucose oxidase catalyses the reaction to produce gluconolactone and at the same time a tiny electric current is generated.
- Each biosensor will only detect one thing
- Exact reading
- put in blood
