Biology » The Chemistry of Life » Organic Compounds

Carbohydrates Continued

In the previous lesson, we saw that carbohydrates are made up of monomers known as monosaccharides, and that the monosaccharide that makes up most carbohydrates is glucose. In this lesson, we will look at some experiments which can be used to test for the presence of carbohydrates.

Investigation: How to Test for the Presence of Starch

Aim

To test for the presence of starch.

Apparatus

  • piece of potato or bread
  • lettuce leaf
  • petri dish
  • iodine solution
  • dropper

Method

  1. Place a piece of potato or bread and the lettuce leaf in the petri dish.
  2. Using the dropper add a few drops of iodine solution onto the potato or bread.

Observations

Record your observations.

Questions

Can this method be used to determine how much starch is present? Explain your answer.

Certain monosaccharides, such as glucose, are known as reducing sugars. These are defined as sugars that can easily undergo oxidation reactions (i.e. lose an electron or gain an oxygen atom) and act as a reducing agent. In order to test for carbohydrates we typically test for the presence of reducing sugars using either the Benedict’s or Fehling’s test. Both solutions (Benedict’s and Fehling’s) contain copper sulphate which reacts with reducing sugars to produce a colour change.

Video Demonstration

You might be interested in watching a video demonstration of the test for glucose below by Michelle Sanders-Limonthas.

Investigation: Testing for the Presence of Reducing Sugars

Aim

To test for presence of sugars using Benedict’s or Fehling’s test.

Apparatus

  • 4 heat resistant test tubes
  • 1 beaker
  • bunsen burner or water bath with hot water (+50°C)
  • test tube rack (if using a water bath)
  • glucose solution
  • albumen solution or egg white
  • starch solution
  • water
  • Benedict’s solution
  • Fehling’s solution
  • marking pen to mark the test tubes
  • thermometer
  • 10 ml syringe or measuring cylinder

Safety precautions

  • Follow the safety procedures when lighting your Bunsen burner. Do not light it in a shelf or near any lights and remove all notebooks, papers and excess chemicals from the area. Tie back any long hair, dangling jewelry and loose clothing and never leave an open flame unattended while it is burning.
  • When heating your test tubes in the boiling water in the beakers ensure that the mouth of the test tubes point away from you and fellow learners.
  • When handling the test tubes, especially when they are hot, use a test tube holder and wear goggles.

Method

Prepare a water bath by filling a beaker to the halfway mark with water. Place the beaker on a tripod stand over a bunsen flame as shown in the figure below. This will serve as your water bath.

Whilst waiting for the water to boil or the water to reach temperature in the water bath, carry out the following instructions:

  1. Label the test-tubes 1–4.
  2. Using the syringe or measuring cylinder, add the following to the test tubes.
    1. Test tube 1: 5 ml of 1% starch solution
    2. Test tube 2: 5 ml of 10% glucose solution
    3. Test tube 3: 5 ml 1% albumen solution
    4. Test tube 4: 5 ml water.
  3. Add 5 ml Benedict’s solution to each tube.
  4. Place the test-tubes in the beaker of hot water on the tripod.
  5. Use a thermometer to monitor the water temperature and adjust the flame to maintain the water temperature at approximately 50°C.
  6. If using the water bath, place the test tubes into the test tube rack and place into the water bath with temperature set to 50°C.
  7. After about 5 minutes, when a colour change has occurred in some of the test tubes, extinguish the flame, or remove the test tubes from the water bath.
  8. Place the four tubes in a test-tube rack and compare the colours.

Image credit: Siyavula

Results

Construct a table to record the results of this experiment. It is important to observe and record any changes that have taken place.

Tube numberObservations in each tube
  
  

Questions

  1. What colour changes (if any) did you observe after heating the samples with Benedict’s solution?
  2. The three solutions tested are examples of the chemical substances found in cells: glucose, starch, protein (albumen). Which of the samples tested positive when the Benedict’s solution was added and the test tube was heated?
  3. Other than the colour, what change took place in the consistency of the Benedict’s solution?
  4. What can you conclude from the investigation?
  5. Why was water included in test tube 4?

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