Carbohydrates. Next to proteids in importance to all living things come the carbohydrates. They are composed of carbon, hydrogen, and oxygen, with always twice as much hydrogen as oxygen, and varying amounts of carbon. Carbohydrates are found almost entirely in plants, whose tissues they largely compose. When animals eat them, they usually oxidize them as fuel to produce heat and energy. Some are converted into fats and stored as such.
Some common carbohydrates are:
Complicated forms found in wood, paper, cotton, linen. (Glycogen is an animal carbohydrate found in the liver of some animals and called " liver starch." It seems to be stored there for later use.)
It is a little strange to think of cotton and starch, or wood and sugar as being so nearly related, but they consist of the same three elements, and are produced by the plants from water and carbon dioxide. It would be a cheap diet, if we could take water from a reservoir and carbon dioxide from the air and make them into flour. Man has to depend on plants for this wonderful process, and can only begin where the plants leave off, using the plantmade carbohydrates for his food.
The Test for Starches.
No one test can be used for all the carbohydrates, but we can test for any starch by dissolving the substance supposed to contain it in hot water and then adding a drop of iodine. The solution will turn blue if starch be present. No substance other than starch will act this way under these conditions.
The Test for Grape Sugar.
There is no one test for all sugars, but grape sugar (glucose) is very common and can be easily distinguished from our household (beet or cane) sugar by what is known as the Fettling Test — so named from the man who devised it. Two solutions are used in the Fehling test, one colorless, and one blue. When these are added in equal amounts to a similar amount of the substance to be tested, and the mixture heated, a yellowbrown solid will form if grape sugar be present. Cane or beet sugar will not act this way.
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