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Carbohydrates
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.
Benedict鈥檚 reagent can be used to test for glucose. The test involves heating a solution of the sugar to be tested with Benedict鈥檚 reagent and observing the colour change of blue to orange.
Benedict鈥檚 reagent will give a positive test result for glucose but not for starch.
| Carbohydrate | Benedict鈥檚 Reagent | Iodine solution | Tyndall Beam Experiment |
| Glucose | Blue to orange | - | - |
| Starch | - | Yellow/brown to blue/black | Light beam visibly dispersed |
| Carbohydrate | Glucose |
|---|---|
| Benedict鈥檚 Reagent | Blue to orange |
| Iodine solution | - |
| Tyndall Beam Experiment | - |
| Carbohydrate | Starch |
|---|---|
| Benedict鈥檚 Reagent | - |
| Iodine solution | Yellow/brown to blue/black |
| Tyndall Beam Experiment | Light 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.