8.1: Carbohydrates

 Learning Objectives

• To recognize carbohydrates and classify them as mono-, di-, or polysaccharides.

Structure of Carbohydrates 

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 (6 carbon atoms bonded to 6 water molecules). Instead, there is an oxygen atom bonded to each carbon atom as shown below. 

Carbohydrates can be classified as monosaccharides, disaccharides, oligosaccharides, or polysaccharides. Monosaccharides have equal numbers of carbon and oxygen atoms and twice as many hydrogen atoms, C_nH_2nO_n. Monosaccharides that are acyclic like the one shown above contain one aldehyde or ketone functional group and all other carbon atoms are bonded to a hydroxy group (OH). Monosaccharides are also frequently cyclic. In that case the molecule contains -O- instead of an aldehyde or ketone functional group.

Two monosaccharides bonded together create a disaccharide. When more than two monosaccharides are bonded together the result is an oligosaccharide or polysaccharide. During the reaction that forms these larger carbohydrates water molecules are lost so the molecular formula no longer has the pattern C_nH_2nO_n. These larger carbohydrates will be discussed in more detail later in the chapter. 

Example \(\PageIndex{1}\)

Which compounds would be classified as monosaccharides?

1. 
2. 
3. 
4. 

Solution

1. This is a monosaccharide because the molecule contains an aldehyde functional group with OH groups on the other two carbon atoms.
2. This is not a monosaccharide because the molecule does not contain an aldehyde or a ketone functional group.
3. This is a monosaccharide because the molecule contains a ketone functional group with OH groups on the other two carbon atoms.
4. This is not a monosaccharide. 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 monosaccharides?

1. 
2. 
3. 
4. 

Answer

1. Not a monosaccharide. One of the carbons does not have an oxygen atom bonded to it.
2. Monosaccharide. One carbon has an aldehyde functional group; the others are each bonded to a hydroxy group. 
3. Monosaccharide. One carbon has a ketone functional group; the others are each bonded to a hydroxy group.
4. Not a monosaccharide. One of the carbons does not have an oxygen atom bonded to it. None of the carbons has an aldehyde or ketone functional group.

Exercise \(\PageIndex{2}\)

Which compounds would be classified as monosaccharides?

1. 
2. 
3. 

Answer

1. Monosaccharide. In the cyclic form a monosaccharide is a ring of carbons with -O-. There is still a hydroxy group on each of the carbon atoms.
2. Monosaccharide. The abbreviation CHO stands for an aldehyde functional group. The rest of the carbon atoms have hydroxy groups.
3. Not a monosaccharide. Two of the carbon atoms do not have an oxygen atom bonded to them. 

Production and Use of Carbohydrates 

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). 

We get 4 Calories (Cal) of energy from each gram of carbohydrate. For comparison, protein also provides 4 Calories per gram and fat provides 9 Calories per gram. Nutritional Calories are usually written with a capital C. They are equal to 1 kilocalorie (kcal) or 1000 calories. 

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.