Objectives After completing this section, you should be able to identify carbohydrates (sugars) as being polyhydroxylated aldehydes and ketones. describe, briefly, the process of photosynthesis, and identify the role played by carbohydrates as an energy source for living organisms. Key Terms Make certain that you can define, and use in context, the key term below. carbohydrate Introduction 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 C 6 H 12 O 6 , glucose was once thought to be a “carbon hydrate” with the structure C 6 ·6H 2 O. Example 1 Which compounds would be classified as carbohydrates? Solution This is a carbohydrate because the molecule contains an aldehyde functional group with OH groups on the other two carbon atoms. This is not a carbohydrate because the molecule does not contain an aldehyde or a ketone functional group. This is a carbohydrate because the molecule contains a ketone functional group with OH groups on the other two carbon atoms. 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? 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 : \[\ce{6CO2 + 6H2O + 2870 kJ -> C6H12O6 + 6O2} \nonumber \] (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).