Control of blood glucose concentrationAQA A-Level Biology: Revision notes
Section 1
Factors affecting blood glucose and the role of the liver
Blood glucose concentration rises after digestion and absorption of carbohydrate, and when glycogen is hydrolysed or glucose is made from other substances. It falls as cells use glucose in respiration, especially during exercise.
The liver adjusts the level using three processes:
- Glycogenesis: glucose to glycogen (lowers blood glucose)
- Glycogenolysis: glycogen to glucose (raises blood glucose)
- Gluconeogenesis: glycerol and amino acids to glucose (raises blood glucose)
Section 2
Insulin lowers blood glucose
When blood glucose is high, insulin is secreted. Insulin binds to receptors on the surface of target cells (liver and muscle) and:
- Controls glucose uptake by regulating the inclusion of glucose channel proteins in the surface membrane, so more glucose enters by facilitated diffusion
- Activates enzymes that convert glucose to glycogen, which keeps the concentration gradient steep
Blood glucose therefore falls back to normal, and insulin secretion falls (negative feedback).
Section 3
Glucagon and adrenaline raise blood glucose
When blood glucose is low, glucagon binds to receptors on liver cells and activates enzymes that convert glycogen to glucose and enzymes that convert glycerol and amino acids to glucose.
Adrenaline (released in stress or exercise) binds to receptors on liver cells and activates enzymes that convert glycogen to glucose.
Glucose then leaves the liver cells and enters the blood.
Section 4
The second messenger model
Glucagon and adrenaline cannot cross the surface membrane, so they act through a second messenger:
- The hormone binds to a specific receptor on the surface membrane.
- This activates adenylate cyclase.
- Adenylate cyclase converts ATP to cyclic AMP (cAMP), the second messenger.
- cAMP activates protein kinase.
- Protein kinase activates the enzymes that convert glycogen to glucose.
The cascade amplifies the signal, so a small amount of hormone causes a large response.
Section 5
Diabetes and diet
Type I: the (beta) cells of the pancreas do not produce enough insulin, often because of an autoimmune attack. It is controlled by insulin injections, with attention to carbohydrate intake.
Type II: insulin is produced, but target cells are less responsive (receptors lose responsiveness). Risk factors include obesity, a diet high in refined carbohydrate and sugar, lack of exercise, age and genes. It is controlled by manipulating diet (less carbohydrate and energy), exercise and weight loss, sometimes with drugs or insulin.
Evaluating positions: health advisers link high sugar and energy intake with rising type II diabetes and urge taxes and labelling; the food industry points to multiple causes and to reformulation. Judge the evidence (correlation versus cause), any commercial interest, and whether a combined approach is best.
Type II is not caused by a lack of insulin; the problem is that cells do not respond to it.
Section 6
Required practical 11: calibration curve for glucose
To estimate the glucose concentration of an unknown 'urine' sample:
- Make a dilution series of glucose solutions of known concentration from a stock solution, using measured volumes of stock and distilled water.
- Add the same volume of Benedict's reagent to each, heat for the same time at the same temperature, and measure the absorbance with a colorimeter (zeroed with water).
- Plot a calibration curve of absorbance against glucose concentration.
- Treat the unknown in exactly the same way and read its concentration from the curve, multiplying by any dilution factor.
Worked example: absorbance falls 0.90 per mmol dm⁻³. A sample diluted 1 in 10 reads 0.45, so its concentration is (0.90 − 0.45) ÷ 0.90 = 0.50 mmol dm⁻³. The urine is 0.50 × 10 = 5.0 mmol dm⁻³.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Control of blood glucose concentration
- A healthy adult eats no food for 14 hours. During this time the concentration of glucose in her blood stays at about 4.5 mmol dm⁻³, even though no glucose is being absorbed from her gut. Her liver cells are responsible for much of this regulation.Glucagon is released when blood glucose concentration falls. State two ways in which glucagon increases the concentration of glucose in the blood.2 marks
- A researcher adds insulin to cultured muscle cells. Within minutes, the uptake of glucose by the cells increases, and the number of glucose channel proteins in the cell surface membrane is found to be greater than before. The concentration of glucose in the surrounding solution is the same throughout.Insulin also activates enzymes inside the muscle cells. Explain how this helps to increase the uptake of glucose.2 marks
- A clinic is testing the urine of two patients with diabetes: patient 1 has type I diabetes and patient 2 has type II diabetes. The clinic estimates glucose concentration using Benedict's reagent and a colorimeter with a red filter. Standard glucose solutions of 0.0, 0.2, 0.4, 0.6 and 0.8 mmol dm⁻³ gave absorbances of 0.90, 0.72, 0.54, 0.36 and 0.18, because less blue copper(II) remains as glucose concentration increases. A urine sample from patient 2 was diluted by mixing 1 cm³ of urine with 9 cm³ of distilled water, and the diluted sample gave an absorbance of 0.45.Explain why patient 1 needs insulin injections, but patient 2 may be able to control their blood glucose concentration by changing their diet.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).