: a disaccharide of
glucose (left) and
fructose (right) Scientifically,
sugar loosely refers to a number of compounds typically with the formula (CH2O)n. Some large classes of sugars, ranked in increasing order of molecular weight are
monosaccharides,
disaccharides, or oligosaccharides.
Monosaccharides Monosaccharides are also called "simple sugars", the most important being glucose. Most monosaccharides have a formula that conforms to with n between 3 and 7 (
deoxyribose being an exception).
Glucose has the
molecular formula . The names of typical sugars end with -
ose, as in "glucose" and "
fructose". Such labels may also refer to any types of these compounds. Fructose, galactose and glucose are all simple sugars, monosaccharides, with the general formula . They have five hydroxyl groups (−OH) and a carbonyl group (C=O) and are cyclic when dissolved in water. They each exist as several
isomers with dextro- and laevo-rotatory forms that cause polarised light to diverge to the right or the left. •
Fructose, or fruit sugar, occurs naturally in fruits, some root vegetables, cane sugar and honey and is the sweetest of the sugars. It is one of the components of sucrose or table sugar. It is used as a
high-fructose syrup, which is manufactured from hydrolysed corn starch that has been processed to yield
corn syrup, with enzymes then added to convert part of the glucose into fructose. •
Galactose generally does not occur in the free state but is a constituent with glucose of the disaccharide lactose or milk sugar. It is less sweet than glucose. It is a component of the antigens found on the surface of
red blood cells that determine
blood groups. •
Glucose occurs naturally in fruits and plant juices and is the primary product of
photosynthesis.
Starch is converted into glucose during digestion, and glucose is the form of sugar that is transported around the bodies of animals in the bloodstream. Although in principle there are two
enantiomers of glucose (mirror images one of the other), naturally occurring glucose is D-glucose. This is also called
dextrose, or
grape sugar because drying grape juice produces crystals of dextrose that can be
sieved from the other components. The
acyclic monosaccharides (and disaccharides) contain either
aldehyde groups or
ketone groups. These
carbon-oxygen double bonds (C=O) are the reactive centres. All
saccharides with more than one ring in their structure result from two or more monosaccharides joined by
glycosidic bonds with the resultant loss of a molecule of water () per bond.
Disaccharides Lactose, maltose and sucrose are disaccharides, also called "compound sugars". They share the formula . They are formed by the condensation of two monosaccharide molecules with the expulsion of a molecule of water. •
Maltose is formed during the germination of certain grains, the most notable being
barley, which is converted into
malt, the source of the sugar's name. A molecule of maltose is formed by the combination of two molecules of glucose. It is less sweet than glucose, fructose or sucrose. •
Sucrose is found in the stems of sugarcane and roots of sugar beet. It also occurs naturally alongside fructose and glucose in other plants, in particular fruits and some roots such as carrots. The different proportions of sugars found in these foods determines the range of sweetness experienced when eating them.
Polysaccharides Longer than disaccharides are oligosaccharides and polysaccharides.
Cellulose and
chitin are polymers, often crystalline, found in diverse plants and insects, respectively. Cellulose cannot be digested directly by animals. Starch is an amorphous polymer of glucose that is found in many plants and is widely used in the
sugar industry. == Sources ==