Diabetes: Types, Control, and Glucose Concentration Measurement Revision Notes | A Level AQA Biology

1. Why Diabetes Matters: The Homeostasis Connection

Your body keeps blood glucose concentration within a narrow range — roughly 4 to 6 mmol per litre — because every cell depends on a reliable glucose supply. When that regulation breaks down, the consequences are serious and long-term. Diabetes is the name for that breakdown, and understanding it sits right at the heart of the AQA homeostasis topic.

There are two distinct types of diabetes, with different causes, different mechanisms, and different management strategies. Examiners expect you to distinguish them precisely — vague answers about 'not enough insulin' will not score full marks for type II. Let's build that precision now.

2. Type I Diabetes: Cause and Control

Type I diabetes is an autoimmune condition. The body's own immune system mistakenly identifies the beta cells in the islets of Langerhans (in the pancreas) as foreign and destroys them. Because beta cells are the only source of insulin, the result is a complete absence of insulin production.

Without insulin, GLUT4 transporters are not translocated to the cell membrane in muscle and adipose cells, so glucose uptake into these cells is greatly reduced. In the liver, insulin is required to stimulate glycogen synthesis; without insulin, glucose is not stored as glycogen efficiently, contributing to elevated blood glucose concentration after meals. Left untreated, this leads to diabetic ketoacidosis — a life-threatening condition. The onset is typically in childhood or early adulthood, and genetic factors play a significant role, though the precise trigger for the autoimmune attack is still being researched.

Control of type I diabetes relies on insulin replacement therapy. Because insulin is a protein, it would be digested if taken orally, so it must be injected or delivered via an insulin pump. Patients monitor their blood glucose concentration and adjust their insulin dose accordingly — a process that effectively mimics the feedback loop the pancreas would normally carry out automatically.

Diet management also plays a supporting role: controlling carbohydrate intake helps prevent sharp spikes in blood glucose, reducing the insulin dose required at any one time. However, diet alone cannot control type I diabetes — insulin administration is non-negotiable.

3. Type II Diabetes: Cause and Control

Type II diabetes has a fundamentally different mechanism. The beta cells still produce insulin — often in normal or even elevated amounts — but the target cells (particularly in the liver and muscles) become less sensitive or completely unresponsive to it. This is called insulin resistance.

Picture the insulin molecule as a key and the receptor on the target cell as a lock. In type II diabetes, the lock becomes damaged or less responsive, so even when the key is present, the door does not open properly. Glucose remains in the blood rather than entering cells, and blood glucose concentration stays elevated.

Risk factors for type II diabetes include obesity (particularly excess abdominal fat), a diet high in refined carbohydrates and sugars, physical inactivity, and genetic predisposition. The condition develops gradually, and early stages can often be managed — or even reversed — through dietary changes and increased physical activity.

Management strategies for type II diabetes include reducing dietary sugar and refined carbohydrate intake, increasing physical activity to improve insulin sensitivity, and in more advanced cases, using medication such as metformin. In later stages, insulin injections may also be required if beta cell function declines.

The incidence of type II diabetes has risen sharply in recent decades, and this increase sits at the centre of a significant public health debate — one that AQA explicitly asks you to evaluate.

4. Evaluating the Debate: Health Advisers vs the Food Industry

The rising incidence of type II diabetes has prompted a public disagreement between health advisers (including bodies such as the NHS, Public Health England, and the World Health Organisation) and the food industry. AQA requires you to evaluate both positions — not describe them. That means weighing the evidence and reaching a justified conclusion.

Health advisers argue that the food industry's marketing and product formulation practices have contributed directly to the obesity epidemic and, by extension, to the rise in type II diabetes. Their position rests on strong epidemiological evidence: populations with higher consumption of ultra-processed foods and added sugars show higher rates of type II diabetes. They advocate for clearer food labelling, restrictions on advertising high-sugar products to children, and reformulation of products to reduce sugar content.

The food industry's position is more complex. Industry representatives often argue that individual consumer choice and lifestyle factors — rather than specific products — are responsible for rising diabetes rates. They point out that many companies have voluntarily reduced sugar content in products and that a single food or ingredient cannot be held solely responsible for a multifactorial disease. They also argue that excessive regulation could harm economic activity and consumer freedom.

Evaluating these positions requires you to consider the quality and source of evidence on each side. Health advisers draw on large-scale longitudinal studies and meta-analyses, which carry significant scientific weight. Industry-funded research, by contrast, has been shown in peer-reviewed literature to be more likely to reach conclusions favourable to the industry — a form of publication bias and conflict of interest that weakens the credibility of those claims.

A balanced evaluation acknowledges that type II diabetes is genuinely multifactorial: genetics, physical inactivity, socioeconomic factors, and diet all contribute. However, the weight of independent scientific evidence supports the conclusion that high dietary sugar intake and the marketing of high-calorie, low-nutrient foods are significant modifiable risk factors — and that the food industry has a responsibility to address these, not merely to defend current practices.

5. Required Practical 11: Dilution Series and Colorimetric Calibration

Now that you understand what diabetes is and how blood glucose regulation fails, the next question is: how do we actually measure glucose concentration in a biological sample? Required Practical 11 answers this directly — and it is a favourite source of exam questions, particularly on method design, variables, and data interpretation.

The practical has two linked stages. First, you produce a dilution series of a known glucose solution to create a set of standards with precisely known concentrations. Second, you use colorimetry — measuring how much light a coloured solution absorbs — to link those known concentrations to measurable absorbance values, producing a calibration curve. That curve then allows you to read off the glucose concentration of an unknown sample.

5a. Producing a Dilution Series

A dilution series is a set of solutions of decreasing concentration, each made by diluting the previous one (or a stock solution) with distilled water in a known ratio. For this practical, you typically start with a stock glucose solution of known concentration — for example, 1.0 mol per litre — and produce a series of lower concentrations such as 0.8, 0.6, 0.4, 0.2, and 0.0 mol per litre (the last being a distilled water blank).

Worked example: Producing a Dilution Series from a 1.0 mol/l Stock Glucose Solution

You have a 1.0 mol/l glucose stock solution and distilled water. Describe how to produce five standard solutions with concentrations of 0.8, 0.6, 0.4, 0.2, and 0.0 mol/l, each with a total volume of 10 cm³.

1. $C_1 V_1 = C_2 V_2$ — This relationship tells us: the concentration of the stock multiplied by the volume of stock needed equals the target concentration multiplied by the total final volume. Rearranging: $V_1 = \frac{C_2 \times V_2}{C_1}$
2. $V_1 = \frac{0.8 \times 10}{1.0} = 8.0 \text{ cm}^3$ stock; add $2.0 \text{ cm}^3$ distilled water — 8.0 cm³ of 1.0 mol/l stock diluted to 10 cm³ total gives 0.8 mol/l.
3. 0.6 mol/l: 6.0 cm³ stock + 4.0 cm³ water 0.4 mol/l: 4.0 cm³ stock + 6.0 cm³ water 0.2 mol/l: 2.0 cm³ stock + 8.0 cm³ water 0.0 mol/l: 0.0 cm³ stock + 10.0 cm³ water (blank) — Each calculation uses the same formula. The blank (0.0 mol/l) contains only distilled water and serves as the colorimeter zero reference.
4. Use a calibrated pipette or burette to measure each volume. Transfer to clearly labelled test tubes. — Accuracy here is critical — any error in volume measurement produces an error in concentration that propagates through the entire calibration curve.
5. Invert or gently swirl each tube after adding both components. — Incomplete mixing produces a non-uniform concentration, which will give an unreliable colorimetric reading.

Answer: A dilution series of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol/l glucose solutions, each with a total volume of 10 cm³, produced by combining the calculated volumes of stock solution and distilled water.

5b. Colorimetric Technique and Producing the Calibration Curve

Benedict's reagent is added to each standard solution and to the unknown sample, then each tube is heated in a water bath at approximately 80°C for a fixed time. Benedict's reagent reacts with reducing sugars (glucose is a reducing sugar) to produce a coloured precipitate — the more glucose present, the more the colour shifts from blue through green, yellow, and orange to brick red.

After heating, each tube is centrifuged or allowed to settle, and the supernatant (the liquid above the precipitate) is transferred to a cuvette. A colorimeter set to an appropriate wavelength — typically around 540 nm (green light, because the solution absorbs green when it appears red/orange) — measures the absorbance of each solution. Higher glucose concentration produces a more intensely coloured solution, which absorbs more light and gives a higher absorbance reading.

Plotting absorbance (on the y-axis) against known glucose concentration (on the x-axis) for all your standards produces the calibration curve. You then read the absorbance of the unknown sample on the y-axis and draw a horizontal line across to the curve, then a vertical line down to the x-axis to read off the corresponding glucose concentration.

Notice that the relationship between absorbance and glucose concentration is approximately linear across the range shown. This linearity is what makes the calibration curve reliable — if your unknown sample gives an absorbance reading that falls within the range of your standards, you can read off its concentration with confidence. If the reading falls outside the range, you would need to dilute the sample and repeat.

6. Interpreting the Calibration Curve: Linking Back to Diabetes

Recall from earlier that in uncontrolled diabetes, blood glucose concentration rises well above the normal range. When blood glucose exceeds the renal threshold (approximately 10 mmol/l in most people), the kidneys can no longer reabsorb all the glucose from the filtrate, and glucose appears in the urine — a condition called glycosuria. This is why a urine sample is used in the practical as a proxy for blood glucose status.

A urine sample from a healthy individual would give an absorbance reading close to the blank (0.0 mol/l standard), because negligible glucose is present. A sample from a person with poorly controlled diabetes would give a significantly higher absorbance, corresponding to a measurable glucose concentration on the calibration curve. This connects the laboratory technique directly to the clinical reality of diabetes diagnosis and monitoring.

7. Exam Technique: Securing Full Marks on Diabetes and Colorimetry Questions

Diabetes questions appear across a range of command words — describe, explain, compare, and evaluate — and the colorimetry practical generates data-handling and method-design questions. Here is how to approach each type with precision.

For 'compare' questions on type I and type II diabetes, you must address both similarities and differences. A common error is to describe each type separately without explicitly comparing them. Use comparative language: 'In type I... whereas in type II...' or 'Both types result in elevated blood glucose, but the mechanism differs: in type I... while in type II...'

For method-design questions on the practical, examiners look for specific control variables with reasons. Stating 'keep temperature the same' earns one mark; explaining 'keep temperature the same because higher temperatures increase the rate of the Benedict's reaction, which would alter the absorbance reading independently of glucose concentration' earns the explanation mark as well.

When reading a concentration from a calibration curve in an exam, always show your method: state the absorbance value you are reading from, draw (or describe drawing) a horizontal line to the curve and a vertical line to the x-axis, and state the concentration with units. Omitting units is a consistent mark-losing error.

Bringing this lesson together: diabetes represents a failure of the homeostatic control of blood glucose concentration, whether through loss of insulin production (type I) or loss of cellular responsiveness to insulin (type II). The rising incidence of type II diabetes is a public health challenge that sits at the intersection of biology, diet, and policy — and AQA expects you to evaluate that debate with scientific rigour. Required Practical 11 gives you the laboratory tools to quantify glucose concentration directly, connecting the clinical reality of diabetes to measurable biochemical data.