7.1: Introduction to Carbohydrates
- To recognize carbohydrates and classify them as mono-, di-, or polysaccharides.
- To recognize monosaccharides based on functional groups and the number of carbon atoms.
All carbohydrates consist of carbon, hydrogen, and oxygen atoms and are polyhydroxy aldehydes or ketones. Examples of carbohydrates include starch, cellulose (fiber), sucrose (the sweet-tasting compound called sugar). The term carbohydrate had its origin in a misinterpretation of the molecular formulas of many of these substances. For example, because its formula is C 6 H 12 O 6 , glucose was once thought to be a “carbon hydrate” with the structure C 6 ·6H 2 O.
Figure \(\PageIndex{1}\): Glucose
Green plants are capable of synthesizing glucose (C 6 H 12 O 6 ) from carbon dioxide (CO 2 ) and water (H 2 O) by using solar energy in the process known as photosynthesis:
\[\ce{6CO_2 + 6H_2O} + \text{686 kcal} \rightarrow \ce{C_6H_{12}O_6 + 6O_2} \label{\(\PageIndex{1}\)} \]
(The 686 kcal come from solar energy.) Plants can use the glucose for energy or convert it to larger carbohydrates, such as starch or cellulose. Starch provides energy for later use, perhaps as nourishment for a plant’s seeds, while cellulose is the structural material of plants. We can gather and eat the parts of a plant that store energy—seeds, roots, tubers, and fruits—and use some of that energy ourselves. Carbohydrates are also needed for the synthesis of nucleic acids and many proteins and lipids.
Animals, including humans, cannot synthesize carbohydrates from carbon dioxide and water and are therefore dependent on the plant kingdom to provide these vital compounds. We use carbohydrates not only for food (about 60%–65% by mass of the average diet) but also for clothing (cotton), shelter (wood), fuel (wood), and paper (wood).
Classification of Carbohydrates
The simplest carbohydrates—those that cannot be hydrolyzed to produce even smaller carbohydrates—are called monosaccharides. Two or more monosaccharides can link together to form chains that contain from two to several hundred or thousand monosaccharide units. Prefixes are used to indicate the number of such units in the chains. Disaccharide molecules have two monosaccharide units, trisaccharide molecules have three units, and so on. Chains with many monosaccharide units joined together are called polysaccharides. Polysaccharides, disaccharides, and trisaccharides can be hydrolyzed back to their constituent monosaccharides.
Classification of Monosaccharides
The classification of monosaccharides are based on the functional group present and the number of carbon atoms. Monosaccharides such as glucose are called aldoses since they contain an aldehyde functional group. Glucose also contain six carbon atoms and hence it is a hexose. Therefore glucose is an aldohexose since it is an aldehyde monosaccharide with six carbon atoms.
Fructose is a ketose since it contains a ketone functional group. Fructose also contain six carbon atoms and hence it is a hexose. Therefore fructose is an ketohexose since it is an ketone monosaccharide with six carbon atoms.
Ribose and 2-Deoxyribose are monosaccharides found in nucleic acids RNA and DNA. Both monosaccharides are aldopentoses.
Figure \(\PageIndex{2}\) Monosaccharides are compounds that cannot be hydrolyzed. They will not react with water to form two or more smaller compounds.
Classify the following compounds based on the functional group and the # of carbon atoms.
Solution
- Aldotriose.
- Ketotriose.
- Aldotetrose.
- Ketotetrose.
Summary
Carbohydrates are an important group of biological molecules that includes sugars and starches. Photosynthesis is the process by which plants use energy from sunlight to synthesize carbohydrates. A monosaccharide is the simplest carbohydrate and cannot be hydrolyzed to produce a smaller carbohydrate molecule. Disaccharides contain two monosaccharide units, and polysaccharides contain many monosaccharide units.