Control of blood glucose concentration: Students should be able to extract information and interpret data from graphs that show the effect of insulin in blood glucose levels in both people with diabetes and people without diabetes.

The idea

Blood glucose is controlled by insulin, which lowers glucose when it is too high. On a graph, read the vertical axis for glucose concentration, often in mmol/dm³, and the horizontal axis for time after eating or after an insulin dose. In a person without diabetes, glucose rises after a meal, then insulin is released and glucose falls back to a stable level. In a person with diabetes, the rise may be larger or the fall slower, or injected insulin may cause a sharper fall. To extract information, quote values from the graph. To interpret, link the shape to insulin action: a steeper downward slope shows a faster fall in glucose. Compare curves by giving specific values and times, not by saying one is higher.

What earns the marks

• Read the vertical axis as blood glucose concentration, commonly in mmol/dm³, and the horizontal axis as time, so that each point gives a glucose value at a stated time.
• Describe the pattern in a person without diabetes: glucose rises after a meal, then falls back towards a stable level as insulin causes cells to take up glucose and store it as glycogen.
• Describe the pattern in a person with diabetes: the glucose rise may be greater or the return to the stable level slower, or an injected insulin dose may produce a more rapid fall.
• Compare curves using data, for example the peak glucose value in mmol/dm³ and the time taken to return to the starting value, rather than using vague words such as higher or faster alone.
• Interpret a downward slope as insulin lowering blood glucose, and a smaller or absent fall as reduced insulin action or insufficient insulin.
• Use the graph to support a conclusion about how well blood glucose is controlled, quoting at least one value from each curve.

Common mistakes and how to put them right

• Reading the axes the wrong way round, for example treating time as glucose concentration.

  Put it right: State the quantity and unit on each axis before quoting any value.

• Confusing the roles of insulin and glucagon.

  Put it right: Insulin lowers blood glucose; glucagon raises it, and only insulin is the focus of these graphs.

• Describing a curve as going up or down without giving values.

  Put it right: Quote a glucose concentration and its time, such as 7.0 mmol/dm³ at 60 minutes, to support each comparison.

• Assuming all people with diabetes show identical graphs.

  Put it right: Describe the general pattern shown by the data, and note that individual responses and treatments vary.

Exam tips

• Before answering, label each curve, for example 'without diabetes' and 'with diabetes', and note the units on both axes.
• When asked to compare, use the phrase 'at X minutes, the person without diabetes had Y mmol/dm³, whereas the person with diabetes had Z mmol/dm³'.
• If asked to suggest why a curve differs, link your answer to insulin: less insulin released, insulin not working effectively, or injected insulin acting at a different time.
• Check that every value you quote has a unit and a time, and that your conclusion follows from the data you have quoted.