AQA A-level Biology: Homeostasis and Control of Blood Glucose (7402)

This guide covers sections 3.6.4.1 (principles of homeostasis and negative feedback) and 3.6.4.2 (control of blood glucose concentration) of the AQA AS and A-level Biology specification for first teaching 2015, for A-level Biology (7402). Both sections are A-level only. It ends with Required practical 11. Condensed recall is in the revision notes and exam-style questions are in the practice set. Course pages: the AQA A-level Biology hub and the printable checklist. Free diagnostics show where to start.

What this unit covers

Section	What you must be able to do
3.6.4.1	Explain why core temperature, blood pH and blood glucose concentration are kept within limits; describe negative feedback; explain why separate mechanisms for opposite changes give greater control; interpret examples of negative and positive feedback
3.6.4.2	Explain the factors that influence blood glucose; the roles of the liver in glycogenesis, glycogenolysis and gluconeogenesis; the actions of insulin, glucagon and adrenaline; the second messenger model; the causes and control of types I and II diabetes; evaluate the positions of health advisers and the food industry
Required practical 11	Make a dilution series of a glucose solution, use colorimetry to build a calibration curve and read off the concentration in an unknown “urine” sample

3.6.4.1 Homeostasis and negative feedback

Homeostasis in mammals is the maintenance of a stable internal environment, within restricted limits, by physiological control systems. “Stable” does not mean fixed. Values swing a little either side of a set point.

Why stable values matter

• Core temperature and blood pH, because of enzymes. Below the optimum temperature, molecules have less kinetic energy, so there are fewer collisions and fewer enzyme-substrate complexes per second. Above it, the tertiary structure unravels as hydrogen bonds break, the active site loses its shape and the enzyme denatures. A change in pH alters the charges on amino acid R groups, which disrupts ionic and hydrogen bonds and also changes the shape of the active site.
• Blood glucose concentration, for two reasons. Glucose is the respiratory substrate, so a low concentration leaves cells, especially neurones, short of the substrate for ATP production. A high concentration makes the blood water potential more negative, so cells lose water by osmosis and shrink.

Negative feedback restores a system to its original level. A receptor detects a departure from the set point, a coordinator (the nervous or endocrine system) processes the information, and an effector acts to reverse the change. When the level returns to normal, the stimulus for the response shrinks, so the response switches off.

Separate mechanisms in opposite directions. Each variable has one mechanism that corrects a rise and a different one that corrects a fall. Glucose has insulin to lower it and glucagon and adrenaline to raise it. Skin temperature control has vasodilation and sweating against vasoconstriction and shivering. Two controls let the level be pushed back from either side, which gives more precise control than one mechanism.

Positive feedback amplifies a change, so the system moves further from its starting point. The opening of voltage-gated sodium ion channels is one example: entry of sodium ions depolarises the membrane, which opens more channels. Oxytocin release during labour is another. Because positive feedback drives the system away from a steady state, it is not a homeostatic mechanism.

Worked example: interpreting a feedback data set

A volunteer sat in rooms at three air temperatures. Skin blood flow was measured in arbitrary units and core temperature in degrees Celsius.

Air temperature / °C	Skin blood flow / arbitrary units	Core temperature / °C
20	6	37.0
30	14	37.0
35	27	37.1

Calculate the percentage increase in skin blood flow from 20 °C to 35 °C and decide whether the data show negative feedback.

1. Increase = 27 − 6 = 21 units.
2. Percentage increase = 21 ÷ 6 × 100 = 350%.
3. Air temperature rose by 15 °C but core temperature changed by only 0.1 °C. The effector response (more blood flow to the skin, so more heat loss) grew as the stimulus grew. Core temperature was held almost constant by a response that opposes the change, which is negative feedback.

3.6.4.2 Control of blood glucose concentration

Factors that influence blood glucose

Blood glucose rises with the absorption of glucose after digesting carbohydrate and with glycogenolysis, the hydrolysis of glycogen to glucose. It also rises with gluconeogenesis, the production of glucose from non-carbohydrate sources. It falls when cells use glucose in respiration, and when glucose is stored by glycogenesis.

The liver

Liver cells (hepatocytes) carry out three processes:

• Glycogenesis: the conversion of glucose to glycogen, which lowers blood glucose.
• Glycogenolysis: the hydrolysis of glycogen to glucose, which raises it.
• Gluconeogenesis: the formation of glucose from non-carbohydrates, namely glycerol and amino acids.

The three hormones

Blood glucose is detected by the beta cells of the islets of Langerhans in the pancreas. The alpha cells of the islets also respond to blood glucose and secrete glucagon. Adrenaline comes from the adrenal glands.

Hormone	Released when	How it acts on target cells	Overall effect
Insulin	Blood glucose is too high	Binds to receptors on the cell surface membrane; increases the number of glucose channel proteins in the membrane; activates enzymes that convert glucose to glycogen	Lowers blood glucose
Glucagon	Blood glucose is too low	Binds to receptors on the cell surface membrane; activates enzymes that convert glycogen to glucose; activates enzymes that convert glycerol and amino acids to glucose	Raises blood glucose
Adrenaline	Stress or danger, giving a need for more glucose	Binds to receptors on the cell surface membrane; activates enzymes that convert glycogen to glucose	Raises blood glucose

Insulin does not itself enter the cell. It binds to a receptor, and the cell responds by adding more glucose channel proteins to its surface membrane. More facilitated diffusion of glucose into the cell follows, and the concentration gradient between blood and cell is kept up because the glucose is turned into glycogen.

The second messenger model

Adrenaline and glucagon do not enter their target cells. The hormone is the first messenger. The sequence is:

1. Adrenaline or glucagon attaches to its receptor in the cell’s outer membrane, which changes the receptor’s shape on the inside.
2. This activates the enzyme adenylate cyclase.
3. Adenylate cyclase makes cyclic AMP (cAMP) from ATP. This is the second messenger.
4. cAMP then switches on protein kinase (protein kinase A).
5. Protein kinase activates other enzymes by phosphorylation, in a cascade, which includes those that convert glycogen to glucose.
6. Glucose leaves the cell and the blood glucose concentration rises.

A small amount of hormone gives a large response because each active enzyme makes many product molecules, so the signal is amplified at each step.

Diabetes

• Type I diabetes is caused by the body’s immune system destroying the beta cells, so the pancreas makes little or no insulin. Blood glucose stays high after meals. It is controlled by monitoring blood glucose and injecting insulin, together with managing the timing and amount of carbohydrate in the diet.
• Type II diabetes is caused by receptors on target cells losing responsiveness to insulin, sometimes together with inadequate insulin secretion. Risk factors include obesity, a diet high in refined carbohydrate and fat, inactivity, age and family history. It is controlled by manipulating the diet (regulating carbohydrate intake, reducing fat and total energy intake) and by exercise, and in some cases by insulin or drugs.

Worked example: a glucose tolerance data set

Two adults drank a glucose solution after fasting. Blood glucose was measured every 30 minutes.

Time / min	0	30	60	90	120
Person A / mmol dm⁻³	4.9	7.8	6.6	5.4	4.9
Person B / mmol dm⁻³	7.4	13.2	14.0	12.1	10.4

1. Person A: percentage rise in the first 30 minutes = (7.8 − 4.9) ÷ 4.9 × 100 = 59%.
2. Person A: mean rate of fall from the peak = (7.8 − 4.9) ÷ 90 = 0.032 mmol dm⁻³ min⁻¹.
3. Person B: mean rate of fall from the peak = (14.0 − 10.4) ÷ 60 = 0.060 mmol dm⁻³ min⁻¹.
4. Interpretation: person A returns to the starting value and peaks early, which fits an intact negative feedback loop. Person B starts high, peaks later and is still 3.0 mmol dm⁻³ above the starting value at 120 minutes. Person B’s rate of fall is numerically larger, but the starting value was already raised and was not restored, which is consistent with diabetes.

Evaluating health advice and the food industry

The specification asks you to evaluate the positions of health advisers and the food industry on the increased incidence of type II diabetes. A balanced answer sets out:

• Health advisers: link a diet high in refined sugar, fat and total energy, and low activity, to obesity and type II diabetes. They recommend lower sugar and energy intake, clear labelling, regulation of marketing and more exercise. Their evidence comes from population studies, which show correlation.
• Food industry: emphasises personal choice, says its products are safe within a balanced diet, and points to reformulated products, smaller portions and labelling. It also has a commercial interest in sales.
• Evaluation: judge the quality of the evidence, the sample size, whether a correlation proves cause, who funded the work and what each party gains.

Required practical 11: dilution series and calibration curve

Aim: find the glucose concentration in an unknown “urine” sample using colorimetry.

1. Make a dilution series from a stock glucose solution using a measured volume of stock and distilled water in each tube.
2. In a typical method (follow your school’s version), add the same volume of Benedict’s reagent to each tube, heat in a water bath at the same temperature for the same time, and cool.
3. Remove the precipitate (by filtering or centrifuging) so that only the remaining blue colour is measured.
4. Use a colorimeter with a red filter, zeroed on distilled water. Less glucose leaves more blue copper(II) ions, so the red light absorbance is higher at low glucose concentration.
5. Plot a calibration curve of absorbance (y) against glucose concentration (x) and read off the unknown.

Control volumes, heating time, temperature and the filter. A tube with no glucose gives the baseline. Wear eye protection and take care with hot water.

Worked example: making the dilutions

You need 10 cm³ of each concentration from a 200 mmol dm⁻³ stock. Volume of stock = (required concentration × 10) ÷ 200.

Concentration / mmol dm⁻³	Stock / cm³	Distilled water / cm³
200	10.0	0.0
160	8.0	2.0
120	6.0	4.0
80	4.0	6.0
40	2.0	8.0
0	0.0	10.0

A serial dilution that halves the concentration each time gives 200, 100, 50, 25 and 12.5 mmol dm⁻³.

Worked example: reading the calibration curve

Absorbance readings for the series above (200 down to 0 mmol dm⁻³, in that order) were 0.13, 0.23, 0.32, 0.43, 0.52 and 0.62.

1. The points lie close to a line. A line of best fit has slope −0.00245 per mmol dm⁻³ and intercept 0.62.
2. An unknown sample gives an absorbance of 0.37.
3. Concentration = (0.37 − 0.62) ÷ (−0.00245) = about 102 mmol dm⁻³, which is 100 mmol dm⁻³ to two significant figures.
4. If the unknown had been diluted 1 in 2 before testing, multiply the result by 2.

Read from the curve only within the range of the standards. If the sample is beyond the highest standard, dilute it and repeat.

Common errors

• Saying the pancreas “releases glucose” or “converts glucose to glycogen”. The liver does the conversion.
• Describing insulin as entering the cell. It binds to a surface receptor.
• Mixing up glycogenesis (making glycogen) and glycogenolysis (breaking it down).
• Leaving out adenylate cyclase or the conversion of ATP to cAMP in a second messenger answer.

Official syllabus

AQA AS and A-level Biology specification for first teaching 2015, published by AQA (sections 3.6.4.1 and 3.6.4.2, with Required practical 11).