Class 11 Biology Notes Chapter 11 (Chapter 11) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 11 from your Lab Manual, which deals with a very important diagnostic test: Testing for the presence of sugar in urine. This practical skill is not just for your lab record, but the underlying principles are crucial for understanding certain physiological conditions and are often tested in competitive government exams.

Chapter 11: Test for Presence of Sugar in Urine

1. Introduction:
Normally, urine contains water, urea, salts, uric acid, and small amounts of other metabolic wastes. Glucose, a simple sugar, is usually completely reabsorbed by the kidney tubules back into the bloodstream. However, if the blood glucose level exceeds a certain limit (the renal threshold, typically around 180 mg/dL), the kidneys cannot reabsorb all the glucose, and it starts appearing in the urine. This condition is known as Glycosuria. The most common cause of glycosuria is Diabetes Mellitus. Therefore, testing for sugar in urine is a fundamental screening test for this condition.

2. Aim:
To detect the presence of reducing sugar (specifically glucose) in the given sample of urine.

3. Principle:
The test commonly used is Benedict's Test.

• Benedict's Reagent: This is an alkaline solution containing copper sulphate (CuSO₄), sodium carbonate (Na₂CO₃), and sodium citrate. The sodium carbonate provides the alkaline medium, and sodium citrate prevents the precipitation of copper carbonate by keeping cupric ions (Cu²⁺) in solution.
• Reaction: Glucose is a reducing sugar. In an alkaline medium and upon heating, reducing sugars donate electrons and reduce the blue cupric ions (Cu²⁺) from copper sulphate in Benedict's reagent to red/orange/yellow cuprous oxide (Cu₂O), which is insoluble and forms a precipitate.
  • Cu²⁺ (Blue, soluble) + Reducing Sugar --- (Alkaline medium + Heat) ---> Cu⁺ (as Cu₂O precipitate - Yellow/Green/Orange/Red, insoluble)
• Colour Change: The colour of the final mixture depends on the concentration of reducing sugar present. The change progresses from blue (no sugar) to green, yellow, orange, and finally brick-red (high concentration of sugar).

4. Materials Required:

• Test tube
• Test tube holder
• Spirit lamp or Bunsen burner
• Droppers
• Urine sample
• Benedict's reagent

5. Procedure:

• Take a clean, dry test tube.
• Using a dropper, add about 2 ml of the urine sample to the test tube.
• Add about 2 ml (or sometimes specified as 5 ml, follow reagent instructions) of Benedict's reagent to the same test tube.
• Mix the contents gently by shaking the test tube.
• Using a test tube holder, heat the mixture carefully over the flame of a spirit lamp or Bunsen burner.
• Bring the mixture to a boil. Caution: Heat gently and keep the mouth of the test tube pointed away from yourself and others to avoid spurting.
• Allow the test tube to cool down or place it in a test tube stand.
• Observe any change in colour and the formation of precipitate.

6. Observation:
Record the colour change observed after heating and cooling. The results can be interpreted semi-quantitatively:

7. Inference/Result:
Based on the observed colour change, report whether reducing sugar is present or absent in the given urine sample. If present, indicate the approximate concentration based on the colour (e.g., "Reducing sugar is present in high concentration as indicated by the brick-red precipitate").

8. Precautions:

• Use clean and dry test tubes and droppers to avoid contamination.
• Measure the quantities of urine and Benedict's reagent accurately.
• Heat the mixture gently and carefully to avoid spurting. Do not point the test tube towards anyone while heating.
• Do not overheat the mixture. Bring it just to a boil.
• Allow the solution to cool before making the final observation, as the precipitate settles down.
• Handle Benedict's reagent carefully; it is alkaline.

9. Clinical Significance:

• The presence of glucose in urine (Glycosuria) is a primary indicator of Diabetes Mellitus.
• It can also occur temporarily after a very heavy carbohydrate meal or during pregnancy in some individuals.
• Other conditions like renal tubular diseases can also cause glycosuria, but diabetes is the most common cause.
• Benedict's test detects reducing sugars. While glucose is the most common one implicated in diabetes, other reducing sugars like lactose (sometimes found in urine of pregnant/lactating women) or fructose could also give a positive test. However, in the context of routine screening, a positive test strongly suggests the need for further investigation for diabetes (like blood glucose tests).

Multiple Choice Questions (MCQs):

1. Benedict's test is used to detect the presence of which substance in urine?
   a) Albumin
   b) Urea
   c) Reducing Sugar
   d) Bile Salts

2. The principle of Benedict's test involves the:
   a) Oxidation of cupric ions to cuprous ions
   b) Reduction of cupric ions to cuprous ions
   c) Precipitation of copper sulphate
   d) Hydrolysis of sodium carbonate

3. What is the initial colour of Benedict's reagent?
   a) Red
   b) Yellow
   c) Blue
   d) Colourless

4. A brick-red precipitate in Benedict's test indicates:
   a) Absence of sugar
   b) Traces of sugar
   c) Moderate concentration of sugar
   d) High concentration of sugar

5. The condition characterized by the presence of glucose in urine is termed:
   a) Ketonuria
   b) Proteinuria
   c) Glycosuria
   d) Hematuria

6. Which of the following provides the alkaline medium required for Benedict's test?
   a) Copper sulphate
   b) Sodium citrate
   c) Sodium carbonate
   d) Glucose

7. What action is essential during the procedure of Benedict's test after adding the reagent to the sample?
   a) Cooling in ice
   b) Vigorous shaking for 5 minutes
   c) Heating the mixture
   d) Adding acid

8. Which of the following is NOT a necessary precaution while performing Benedict's test?
   a) Using clean apparatus
   b) Heating the test tube with its mouth pointing towards you
   c) Adding appropriate volumes of sample and reagent
   d) Allowing the mixture to cool before final observation

9. The renal threshold for glucose is approximately:
   a) 50 mg/dL
   b) 100 mg/dL
   c) 180 mg/dL
   d) 300 mg/dL

10. Benedict's test gives a positive result with:
    a) Sucrose
    b) Starch
    c) Glucose
    d) All carbohydrates

Answer Key for MCQs:

1. c) Reducing Sugar
2. b) Reduction of cupric ions to cuprous ions
3. c) Blue
4. d) High concentration of sugar
5. c) Glycosuria
6. c) Sodium carbonate
7. c) Heating the mixture
8. b) Heating the test tube with its mouth pointing towards you (This is dangerous and should NOT be done)
9. c) 180 mg/dL
10. c) Glucose (It detects reducing sugars; sucrose and starch are non-reducing in this context).

Remember to correlate this practical knowledge with the concepts you learn in the theory chapters on digestion, absorption, and excretion, especially kidney function and hormonal regulation of blood sugar. Good luck with your preparation!