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Plants make compounds called carbohydrates which have a wide variety of uses including foods and fuels. All carbohydrates contain the elements carbon, hydrogen and oxygen. Two of the most common carbohydrates are glucose and starch.

Glucose (C6H12O6) is a simple sugar unit. From the formula, you can see that it contains twice as many hydrogen atoms as carbon atoms. Starch is a much larger, more complicated molecule. Plants produce glucose during the process of photosynthesis and convert it into starch to store energy. Starch is made by joining together many glucose units.

Distinguishing glucose from starch

A test to distinguish starch from glucose is to shine a beam of light through 'solutions' of each.

This is also called the Tyndall Beam Effect. Dispersal only happens in starch as the large starch molecules are big enough to affect the light. This is a physical test.

A chemical test for starch is to add iodine solution (yellow/brown) and look for a colour change. In the presence of starch, iodine turns a blue/black colour. It is possible to distinguish starch from glucose (and other carbohydrates) using this iodine solution test. For example, if iodine is added to a peeled potato then it will turn black.

A potato is peeled. Iodine is dropped onto the exposed area, which turns black.

Benedict’s reagent can be used to test for glucose. The test involves heating a solution of the sugar to be tested with Benedict’s reagent and observing the colour change of blue to orange.

Benedict’s reagent will give a positive test result for glucose but not for starch.

CarbohydrateBenedict’s ReagentIodine solutionTyndall Beam Experiment
GlucoseBlue to orange--
Starch-Yellow/brown to blue/blackLight beam visibly dispersed

Digestion of carbohydrates

During digestion starch is broken down into glucose. Glucose is small enough to pass through the gut wall but starch cannot. This is done in the body using enzymes (biological catalysts) which work best at body temperature. The glucose molecules can then be transported around the body in the blood stream so that they can be used for the process of respiration.