Glucose or grape-sugar is the most important sugar known from the standpoint of the physiological chemist. This sugar is normally found in considerable quantities in human blood, and is absolutely essential to the life-process, a fact which forms an amusing contrast with the popular conception of the term glucose as something injurious or poisonous. [Sidenote: Sources of glucose] Glucose is found in honey, and in nearly all fruits, grains, and sweets. (For "Sweets" see Lesson VIII, Vol. II, p. 324). It may be taken into the human body directly from such fruits, or it may originate by the digestion of other carbohydrates. Pure glucose crystallizes and resembles cane-sugar, but is not so sweet. The glucose of commerce, sold as sirup, is a product manufactured from corn, or other starches, and will be considered more in detail under the heading _starch_. (See "Polysaccharids," p. 114). 2 PENTOSES (C₅H₁₀O₅) [Sidenote: Sources of pentoses] Pentoses form a group of sugars, the chemical formula of which contains five atoms of carbon. Each different pentose could be studied in detail by the chemist, but the pentoses are of no particular interest to the food scientist. They exist, however, in the coarse parts of plants, such as stalks and leaves, and are of considerable importance in animal feeding. From the standpoint of human food we will remember that the carbohydrates of green plants contain a percentage of these pentoses, but as they are never removed from the plant separately, as are other sugars, we must consider their physiological effect in the particular plant rather than separately. 3 LEVULOSE (C₆H₁₂O₆) This is the companion sugar to glucose and exists in many fruits. Levulose is often called "fruit-sugar." The composition of levulose is exactly the same as glucose, but the atoms are combined in different ways. Levulose, for all practical purposes, may be considered the equivalent of glucose in the human body. It is sweeter than glucose and more closely resembles cane-sugar. 4 GALACTOSE (C₆H₁₂O₆) Galactose, which is of the same composition as levulose, is another companion sugar to glucose, and is formed by the digestion of lactose or milk-sugar. b DISACCHARIDS 1 CANE-SUGAR (C₁₂H₂₂O₁₁) Just as there are three monosaccharid sugars with six carbon atoms each, so there are three disaccharid sugars which have twelve carbon atoms each. The first of these is cane-sugar. It is commercially made from either sugar-cane or sugar-beets, and is identical in chemical composition from either source. Cane sugar, when digested in the human body, or by artificial means, combines with water, and forms glucose and levulose, as shown by the following equation: C₁₂H₂₂O₁₁ + H₂O = C₆H₁₂O₆ + C₆H₁₂O₆ Cane-sugar + Water = Glucose + Levulose 2 MALTOSE (C₁₂H₂₂O₁₁) [Sidenote: Maltose--how formed]