2.1: Introduction

Objectives

After completing this section, you should be able to

1. identify carbohydrates (sugars) as being polyhydroxylated aldehydes and ketones.

 

Introduction

All carbohydrates consist of carbon, hydrogen, and oxygen atoms and are polyhydroxy aldehydes or polyhydroxyketones or are compounds that can be broken down to form such compounds. Examples of carbohydrates include glucose, ribose, fructose,  table sugar, starch, and structural materials such as cellulose. The term carbohydrate had its origin in a misinterpretation of the molecular formulas of many of these substances.  The molecular formula for a carbohydrate can be considered as C_n(H₂O)n, where n is the number of carbon atoms. or example, because its formula is C₆H₁₂O₆, glucose was once thought to be a “carbon hydrate” with the structure C₆(H₂O)₆.

Fructose has a ketone group, so it is considered a polyhydroxy ketone. Image by Ayacop, Public domain, via Wikimedia Commons 

 

 

Example 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 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:

\[6CO_2 + 6H_2O + \text{2870 kJ} \rightarrow C_6H_{12}O_6 + 6O_2 \tag{\({1}\)}\]

(The 2870 kJ comes 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).

Contributors and Attributions

• Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University)

• The Basics of General, Organic, and Biological Chemistry by David W. Ball, John W. Hill, and Rhonda J. Scott.