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carbon, twelve hydrogen, and six oxygen) found in glucose. But fructose has a different structure. Its atoms are arranged in a different order. The chemical definition of this is that fructose and glucose are isomers. Isomers are compounds that have the same chemical formula (same number of each atom) but different arrangements of these atoms.
    Fischer projection formulas of the isomers glucose and fructose, showing the different order of hydrogen and oxygen atoms at C#1 and C#2. Fructose has no H atoms at C#2.
    Fructose exists mainly in the cyclic form, but it looks a bit different from glucose since fructose forms a five-membered ring, shown below as a Haworth formula, rather than the six-membered ring of glucose. As with glucose, there are α and β forms of fructose, but as it is carbon number 2 that joins the ring oxygen in fructose, it is around this carbon atom that we designate OH below the ring as α and OH above the ring as β.

    Sucrose contains equal amounts of glucose and fructose but not as a mixture of two different molecules. In the sucrose molecule one glucose and one fructose are joined together through the removal of a molecule of water (H 2 O) between the OH at carbon number 1 of α-glucose and the OH on carbon number 2 of β-fructose.

    Removal of a molecule of H 2 O between glucose and fructose forms sucrose. The fructose molecule has been turned 180° and inverted in these diagrams.

    Structure of the sucrose molecule
    Fructose is largely found in fruit but also in honey, which is about 38 percent fructose and 31 percent glucose, with another 10 percent of other sugars including sucrose. The remainder is mainly water. Fructose is sweeter than sucrose or glucose, so because of its fructose component, honey is sweeter than sugar. Maple syrup is approximately 62 percent sucrose with only 1 percent of each of fructose and glucose.
    Lactose, also called milk sugar, is a disaccharide formed from one unit of glucose and one unit of another monosaccharide, galactose. Galactose is an isomer of glucose; the only difference is that in galactose the OH group at carbon number 4 is above the ring and not below the ring as it is in glucose.

    Î² -galactose with arrow showing C#4 OH above the ring compared to β -glucose where the C#4 OH is below the ring. These two molecules combine to form lactose.

    Galactose on the left is joined through C#1 to C#4 of glucose on the right.
    Again, having an OH above or below the ring may seem like a very minor difference, but for those people who suffer from lactose intolerance, it is not. To digest lactose and other disaccharides or larger sugars, we need specific enzymes that initially break down these complex molecules into simpler monosaccharides. In the case of lactose, the enzyme is called lactase and is present in only small amounts in some adults. (Children generally produce greater amounts of lactase than adults.) Insufficient lactase makes the digestion of milk and milk products difficult and causes the symptoms associated with lactose intolerance: abdominal bloating, cramps, and diarrhea. Lactose intolerance is an inherited trait, easily treated with over-the-counter preparations of the lactase enzyme. Adults and children (but not babies) from certain ethnic groups, such as some African tribes, are missing the lactase enzyme completely. For these people, powdered milk and other milk products, often found in food aid programs, are indigestible and even harmful.
    The brain of a normal healthy mammal uses only glucose for fuel. Brain cells are dependent on a minute-to-minute supply from the bloodstream, as there are essentially no fuel reserves or storage in the brain. If blood glucose level falls to 50 percent of the normal level, some symptoms of brain dysfunction appear. At 25 percent of the normal level, possibly from an overdose of insulin—the hormone that maintains the level of glucose in the blood—a coma may result.

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    What makes