26.1. Carbohydrates http://www.ck12.org
used for simple carbohydrates with 3-6 carbon atoms in their primary chain are shown below:
triose –three carbonstetrose –four carbons
pentose –five carbons
hexose –six carbonsSome of the most important simple carbohydrates studied in biochemistry include ribose and deoxyribose (pentoses
involved in the structure of nucleic acids) and glucose, fructose, and galactose (hexoses that make major contribu-
tions to our diet).
The simple carbohydrates discussed above are used as the building blocks for complex carbohydrates. Monosac-
charideslike glucose or fructose can be linked together to form larger structures. When two monosaccharides are
bound together, the result is adisaccharide. Some common disaccharides are shown in theTable26.1.
TABLE26.1:
Disaccharide Made up of
sucrose glucose + fructose
lactose glucose + galactose
maltose glucose + glucoseLonger chains of monosaccharides are referred to aspolysaccharides. Starch, glycogen, and cellulose are three
extremely common polysaccharides made entirely out of glucose molecules. The differences lie in the types of
bonds between the glucose units and the extent of branching in the carbohydrate chain. Starch is found in plants
and is consumed as part of our diet.Glycogenis made by the body and is a storage form of glucose for when the
cells need extra energy. Cellulose is another long-chain polysaccharide found in plants. Unlike starch and glycogen,
the human body cannot break cellulose down into molecules of glucose. Although cellulose does not have any
nutritional value in terms of calories, it is a major component of dietary fiber, which has other digestive benefits.
Carbohydrate Intake and Utilization
Dietary Consumption and Digestion of Carbohydrates
There are several dietary sources of carbohydrates. Starch is found in many vegetables (especially potatoes) and
baked goods. Sucrose (table sugar) is included as part of many prepared foods. Lactose is found in milk products,
and maltose is present in some vegetables.
A small amount of starch digestion will occur in the mouth, but the bulk of this process occurs in the small
intestine. Amylase, an enzyme produced in the pancreas and released into the interior of the small intestine, breaks
complex carbohydrates like starch into disaccharides and trisaccharides. These molecules are then cleaved into their
corresponding monosaccharides by specific enzymes located in the interior wall of the small intestine.
TABLE26.2:
Disaccharide Enzyme Products
sucrose sucrase glucose + fructose
lactose lactase glucose + galactose
maltose maltase glucose + glucose