Control of blood glucose concentration

In a nutshell

Blood glucose concentration needs to be controlled. Glucagon and insulin are the effectors utilised to alter blood glucose concentration. Diabetes is a chronic health condition which prevents reduction of blood glucose concentration after eating.

Blood glucose concentration

There are factors which effect the blood glucose concentration which you need to be aware of. Glucose provides energy through respiration and is required for many cellular functions so the concentration of blood glucose must be strictly regulated.

The blood glucose concentration is kept around $90 \space mg$ per $100\space cm^3$ of blood. The level is overseen by cells in the pancreas. Eating foods with carbohydrates increases the blood glucose concentration. Exercise lowers blood glucose concentration, as glucose is used more rapidly to provide energy.

Blood glucose control

The pancreas monitors the blood glucose concentration and releases hormones to respond to changes in the blood glucose concentration. Insulin is released when blood glucose concentration is too high and glucagon is released when blood glucose concentration is too low.

The pancreas contains groups of cells in the islets of Langerhans called $\beta$ and $\alpha$ cells. $\beta$ cells release insulin and $\alpha$ cells release glucagon.

Insulin

When blood glucose concentration is too high insulin is secreted into the blood by $\beta$ cells. Insulin then interacts with receptors presented on the membrane of liver and muscle cells. The interaction causes these cells to present more glucose channel proteins which makes them more permeable to glucose so glucose is taken up by cells.

Enzymes are activated by insulin that convert glucose to glycogen. This process is called glycogenesis. Glycogen can be stored in the cell cytoplasm and means there is not excess glucose in the bloodstream.

Glucagon

When blood glucose concentration is too low glucagon is secreted into the blood by $\alpha$ cells. Glucagon interacts with receptors presented on the membrane of liver cells. The interaction activates intracellular enzymes which convert glycogen into glucose. This process is called glycogenolysis.

Glucagon also stimulates the activation of other enzymes which catalyse the production of glucose from glycerol and amino acids. This process is called gluconeogenesis.

Mechanism of action

Insulin

Insulin interacts with receptors that leads to the increased permeability of cells to glucose. GLUT $4$ is a glucose transporter channel. If insulin blood levels are low then GLUT $4$ is stored in vesicles in the cytoplasm of skeletal and cardiac muscle cells.

If insulin blood levels are high then the interaction between cell receptors and insulin causes the GLUT $4$ channel to be presented on the surface of cells. This allows facilitated diffusion of glucose from the blood into the cell.

Glucagon and adrenaline

Adrenaline and glucagon are secreted into the blood when blood glucose concentration is low. Adrenaline is secreted from the adrenal glands. Adrenaline activates the same processes as glucagon including glycogenolysis and gluconeogenesis. Furthermore, adrenaline stops insulin release and promotes the secretion of glucagon.

Glucagon and adrenaline both induce intracellular changes by interacting with cell membrane receptors and using a secondary messenger called cyclic AMP (cAMP).

Biology; Homeostasis; KS5 Year 12; Control of blood glucose concentration — 1. Hormone, 2. Receptor, 3. Adenylate cyclase, 4. Second messenger (cyclic AMP), 5. Protein kinase A, 6. Cascade, 7. Glycogen, 8. Glucose, 9. Cell membrane, A. Outside the cell, B. Inside the cell.

A.	Following the interaction of glucagon/adrenaline with receptors, an enzyme called adenylate cyclase is activated.
B.	Adenylate cyclase converts ATP into a chemical signal molecule called cAMP.
C.	cAMP then activates an enzyme called protein kinase A which initiates a cascade which eventually leads to the conversion of glycogen into glucose.

Diabetes

Type $1$

Type $1$ diabetes is caused by faulty immune cells which attack $\beta$ cells in the islets of Langerhans. This means the $\beta$ cells are unable to produce insulin to respond to rising blood glucose concentration.

When a meal is consumed the blood glucose level rises extremely high and cannot be controlled. When the blood glucose level rises high it is called hyperglycaemia.

Type $1$ diabetes is treated with insulin therapy. Patients are given insulin injections at regular periods throughout the day. People with type $1$ diabetes must control their carbohydrate intake. Type $1$ diabetes is incurable.

Type $2$

Type $2$ diabetes is usually caused by an unhealthy lifestyle. Many people with type $2$ diabetes also have issues with obesity. Type $2$ diabetes is caused either by $\beta$ cells that don't produce enough insulin or cells which are insensitive to insulin and therefore do not take up blood glucose through facilitated diffusion. Both of these issues lead to an increased blood glucose level.

Type $2$ diabetes can be cured through a healthy lifestyle. If no action is taken, patients with type $2$ diabetes could eventually have to take insulin injections.

Colorimetry

Colorimetry is a method used to find out the glucose concentration of a solution.

Normally urine glucose concentration is between $0$ and $0.8\space mM$ . However, illnesses like diabetes may cause a higher value than this.

Biology; Homeostasis; KS5 Year 12; Control of blood glucose concentration — 1. Transfer $5\ cm^3$ and then mix, A. $10\ cm^3$ of $4\ mM$ sucrose solution, B. $5\ cm^3$ of distilled water.

1.	Firstly, five serial dilutions of $4\space mM$ glucose solutions need to be made. These five glucose solutions should have concentrations of $2\space mM, 1\space mM, 0.5\space mM, 0.25\space mM$ .
2.	Quantitative Benedict's reagent interacts with solutions, it is different to normal Benedict's reagent as it loses its blue colour when heated with higher glucose concentrations and no brick-red precipitate is formed. Perform this test on the five serial dilutions. Using a colorimeter, determine the absorbance of each serial dilution.
3.	Using the colorimeter results, plot a calibration curve for absorbance against glucose concentration.
4.	Testing an unknown glucose concentration for absorbance following a quantitative Benedict's test will reveal the glucose concentration as you can use the calibration curve.

Example

A solution with an unknown glucose concentration was tested. The absorbance value was $0.74$ . Using the calibration curve, you can work out that the concentration of glucose present in the sample is $\approx0.75\ mM$ .

Biology; Homeostasis; KS5 Year 12; Control of blood glucose concentration