5.1: Carbohydrates

 Learning Objectives

• To recognize carbohydrates as aldehydes or ketones containing many hydroxyl groups.

Three Important Organic Structures

Before we can begin our study of carbohydrates, you will need to become familiar with three organic structures: the hydroxyl group -OH, the ketone, and the aldehyde. These three structures become branches off the main carbon backbone of a carbohydrate molecule.

A hydroxyl group -OH in an organic compound creates an alcohol functional group that has the following general structure:

R1, R2, and R3 could be hydrogen atoms, other carbon atoms, rings of carbon atoms, or any combination thereof.  For example:

Two other important structures appear in carbohydrates. Both of these include a carbonyl group, identified by a carbon atom double-bonded to an oxygen atom: C=O. The term carbonyl is pronounced, "carbon-eel."

When the carbonyl group appears, it will qualify a compound to join a new class of molecules.  When the carbonyl group is bonded to two other carbon atoms, the compound is called a ketone, and the carbonyl earns the name "the ketone group."

When the carbonyl group is bonded to a hydrogen atom, it creates a structure called an aldehyde. The other bond to the carbonyl group may be either a second hydrogen atom or a carbon atom. For example:

The CHO group in an aldehyde is called the aldehyde group.

Carbohydrates are aldehydes or ketones with extra hydroxyl groups

All carbohydrates consist of carbon, hydrogen, and oxygen atoms and are polyhydroxy aldehydes or ketones or are compounds that can be broken down to form such compounds. Examples of carbohydrates include starch, fiber, the sweet-tasting compounds called sugars, and structural materials such as cellulose. The term carbohydrate had its origin in a misinterpretation of the molecular formulas of many of these substances. For example, because its formula is C₆H₁₂O₆, glucose was once thought to be a “carbon hydrate” with the structure C₆·6H₂O.

Example \(\PageIndex{1}\)

Which compounds would be classified as carbohydrates?

1. 
2. 
3. 
4. 

Solution

1. This is a carbohydrate because the molecule contains an aldehyde functional group with OH groups on the other two carbon atoms.
2. This is not a carbohydrate because the molecule does not contain an aldehyde or a ketone functional group.
3. This is a carbohydrate because the molecule contains a ketone functional group with OH groups on the other two carbon atoms.
4. This is not a carbohydrate; although it has a ketone functional group, one of the other carbons atoms does not have an OH group attached.

Exercise \(\PageIndex{1}\)

Which compounds would be classified as carbohydrates?

1. 
2. 
3. 
4. 

Green plants are capable of synthesizing glucose (C₆H₁₂O₆) from carbon dioxide (CO₂) and water (H₂O) 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, linen, rayon), shelter (wood), fuel (wood), and paper (wood).

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. All these so-called higher saccharides can be hydrolyzed back to their constituent monosaccharides.

Compounds that cannot be hydrolyzed will not react with water to form two or more smaller compounds.

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.