Biology (Holt)

Glucose

(monosaccharide)

A potato contains many granules

that are filled with starch.

Starch contains hundreds of

linked glucose molecules.

Starch

(polysaccharide)

Section 3 Chemistry of Cells

34 CHAPTER 2Chemistry of Life

Carbon Compounds

Most matter in your body that is not water is made of organic

compounds. Organic compounds contain carbon atoms that are

covalently bonded to other elements—typically hydrogen, oxygen,

and other carbon atoms. Four principal classes of organic com-

pounds are found in living things: carbohydrates, lipids, proteins, and

nucleic acids. Without these compounds, cells could not function.

Carbohydrates

are organic compounds made of carbon, hydrogen,

and oxygen atoms in the proportion of 1:2:1. Carbohydrates are a

key source of energy, and they are found in most foods—especially

fruits, vegetables, and grains. The building blocks of carbohydrates

are single sugars, called (mahn oh SAK uh

reyedz), such as glucose, C 6 H 12 O 6 , and fructose. Simple sugars such

as glucose are a major source of energy in cells. Disaccharides are

double sugars formed when two monosaccharides are joined. For

example, sucrose, or common table sugar, consists of both glucose

and fructose. Polysaccharides such as starch, shown in Figure 8, are

chains of three or more monosaccharides. A polysaccharide is an

example of a macromolecule, a large molecule made of many

smaller molecules.

In organisms, some polysaccharides function as storehouses of the

energy contained in sugars. Two polysaccharides that store energy in

this way are starch, which is made by plants, and glycogen, which is

made by animals. Both starch and glycogen are made of hundreds of

linked glucose molecules. Cellulose is a polysaccharide that provides

structural support for plants. Humans cannot digest cellulose. Thus,

you cannot digest wood, which is mostly cellulose.

monosaccharides

Carbohydrates

Objectives

Summarizethe characteris-
tics of organic compounds.

Comparethe structures and
function of different types of
biomolecules.

Describethe components of
DNA and RNA.

Statethe main role of ATP
in cells.

Key Terms

carbohydrate

monosaccharide

lipid

protein

amino acid

nucleic acid

nucleotide

DNA

RNA

ATP

Figure 8 Structure of

polysaccharides.Starch is a

long chain of many linked

glucose molecules.

CO

CC

C C

H

OH H

H OH

CH 2 OH

H

OH

OH

H

9A

6A

9A