CIE A Level Biology (9700) exams from 2022

Revision Notes

14.1.8 Test Strips & Biosensors

Test Strips & Biosensors

Measuring urine glucose concentration

  • People with diabetes cannot control their blood glucose concentration so that it remains within normal, safe limits
  • The presence of glucose in urine is an indicator that a person may have diabetes
    • If blood glucose concentration increases above a value known as the renal threshold, not all of the glucose from the filtrate in the proximal convoluted tubule is reabsorbed and some will be left in the urine
  • Test strips can be used to test urine for the presence and concentration of glucose
  • Two enzymes are immobilised on a small pad at one end of the test strip. These are:
    • glucose oxidase
    • peroxidase
  • The pad is immersed in the urine sample for a short time
  • If glucose is present:
    • Glucose oxidase catalyses a reaction in which glucose is oxidised to form gluconic acid and hydrogen peroxide
    • Peroxidase then catalyses a reaction between the hydrogen peroxide and a colourless chemical in the pad to form a brown compound and water
  • The colour of the pad is compared to a colour chart – different colours represent different concentrations of glucose (the higher the concentration of glucose present, the darker the colour)
  • Urine tests only show whether or not the blood glucose concentration was above the renal threshold whilst urine was collecting in the bladder – they do not indicate the current blood glucose concentration

Measuring blood glucose concentration

  • A biosensor can be used by people with diabetes to show their current blood glucose concentration
  • Similar to the test strips, a biosensor uses glucose oxidase (but no peroxidase) immobilised on a recognition layer
  • Covering the recognition layer is a partially permeable membrane that only allows small molecules from the blood to reach the immobilised enzymes
  • When a small sample of blood is tested, glucose oxidase catalyses a reaction in which any glucose in the blood sample is oxidised to form gluconic acid and hydrogen peroxide
  • The hydrogen peroxide produced is oxidised at an electrode that detects electron transfers
  • The electron flow is proportional to the glucose concentration of the blood sample
  • The biosensor amplifies the current, which is then read by a processor to produce a digital reading for blood glucose concentration
  • This process is complete within a matter of seconds

Exam Tip

The urine test strip will only produce a positive result for glucose. Other sugars such as fructose, sucrose and lactose will give a negative result. This is due to the specificity of the glucose oxidase enzyme.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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