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Organic Molecules

The chemistry of carbon accounts for the diversity of organic molecules found in living things. Carbon can bond with as many as four other atoms. It can also bond with itself to form both chains and rings. Differences in the carbon skeleton and attached functional groups cause organic molecules to have different chemical properties. The chemical properties of a molecule determine how it interacts with other molecules and the role the molecule plays in the cell. Some functional groups are hydrophobic and others are hydrophilic.

There are four classes of biomolecules in cells: carbohydrates, lipids, proteins, and nucleic acids (Table 3.4). Polysaccharides, the largest of the carbohydrates, are polymers of simple sugars called monosaccharides. The polypeptides of proteins are polymers of amino acids, and nucleic acids are polymers of nucleotides. Polymers are formed by the joining together of monomers. For each bond formed during a dehydration reaction, a molecule of water is removed, and for each bond broken during a hydrolysis reaction, a molecule of water is added.

Organic Compounds in Cells
  Categories Elements Examples Functions
Carbohydrates Monosaccharides C, H, O    

6-carbon sugar

5-carbon sugar



Deoxyribose, ribose

Immediate energy source

Found in DNA, RNA


12-carbon sugar

C, H, O Sucrose Transport sugar in plants


Polymer of glucose

C, H, O Starch, glycogen, Cellulose

Energy storage in plants, animals

Plant cell wall structure



1 glycerol + 3 fatty acids

C, H, O Fats, oils Long-term energy storage


Like triglyceride except the head group contains phosphate

C, H, O, P Lecithin Plasma membrane phospholipid bilayer


Backbone of 4 fused rings

C, H, O Cholesterol Testosterone, estrogen

Plasma membrane component

Sex hormones


Fatty acid + alcohol

C, H, O



Protective covering in plants

Protective wax in ears



Polymer of amino acids

C, H, O, N, S


Myosinand actin




Speed cellular reactions

Movement of muscle cells

Hormonal control of blood sugar

Transport of oxygen in blood

Fibrous support of body parts

Nucleic Acids

Nucleic acids

Polymer of nucleotides

C, H, O, N, P



Genetic material

Protein synthesis




Energy carrier

Assist enzymes


Monosaccharides, disaccharides, and polysaccharides are all carbohydrates. Therefore, the term carbohydrate includes both the monomers (e.g., glucose) and the polymers (e.g., starch, glycogen, and cellulose). Glucose is the immediate energy source of cells. Polysaccharides such as starch, glycogen, and cellulose are polymers of glucose. Starch in plants and glycogen in animals are energy storage compounds, but cellulose in plants and chitin in crabs and related animals, as well as fungi, have structural roles. Chitin's monomer is glucose with an attached amino group.


Lipids include a wide variety of compounds that are insoluble in water. Fats and oils, which allow long-term energy storage, contain one glycerol and three fatty acids. Both glycerol and fatty acids have polar groups, but fats and oils are nonpolar, and this accounts for their insolubility in water. Fats tend to contain saturated fatty acids, and oils tend to contain unsaturated fatty acids. Saturated fatty acids do not have carbon–carbon double bonds, but unsaturated fatty acids do have double bonds in their hydrocarbon chain. The double bond causes a kink in the molecule that accounts for the liquid nature of oils.

In a phospholipid, one of the fatty acids is replaced by a phosphate group. In the presence of water, phospholipids form a bilayer because the head of each molecule is hydrophilic and the tails are hydrophobic. Steroids have the same four-ring structure as cholesterol, but each differs by the groups attached to these rings. Waxes are composed of a fatty acid with a long hydrocarbon chain bonded to an alcohol, also with a long hydrocarbon chain.

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Proteins carry out many diverse functions in cells and organisms, including support, metabolism, transport, defense, regulation, and motion. Proteins contain polymers of amino acids.

A polypeptide is a long chain of amino acids joined by peptide bonds. There are 20 different amino acids in cells, and they differ only by their R groups. Presence or absence of polarity is an important aspect of the R groups. A polypeptide has up to four levels of structure: The primary level is the sequence of the amino acids, which varies between polypeptides; the secondary level contains α helices and β (pleated) sheets held in place by hydrogen bonding between amino acids along the polypeptide chain; and the tertiary level is the final folding of the polypeptide, which is held in place by bonding and hydrophobic interactions between R groups. Proteins that contain more than one polypeptide have a quaternary level of structure as well.

The shape of an enzyme is important to its function. Both high temperatures and a change in pH can cause proteins to denature and lose their shape.

Nucleic Acids

The nucleic acids DNA and RNA are polymers of nucleotides. Variety is possible because the nucleotides can be in any order. Each nucleotide has three components: a phosphate (phosphoric acid), a 5-carbon sugar, and a nitrogen-containing base.

DNA, which contains the sugar deoxyribose, is the genetic material that stores information for its own replication and for the order in which amino acids are to be sequenced in proteins. DNA, with the help of mRNA, specifies protein synthesis. DNA, which contains phosphate, the sugar deoxyribose, and nitrogen-containing bases, is a double-stranded helix in which A pairs with T and C pairs with G through hydrogen bonding. RNA, containing phosphate, the sugar ribose, and the bases A, U, C, and G, is single stranded.

ATP, with its unstable phosphate bonds, is the energy currency of cells. Hydrolysis of ATP to ADP + releases energy, which is used by the cell to make a product or do any other type of metabolic work.

understanding the terms

Match the terms to these definitions:

  1. Class of organic compounds that includes monosaccharides, disaccharides, and polysaccharides.

  2. Class of organic compounds that tend to be soluble in nonpolar solvents such as alcohol but insoluble in water.

  3. Biomolecule consisting of covalently bonded monomers.

  4. Molecules that have the same molecular formula but a different structure and, therefore, shape.

  5. Two or more amino acids joined together by covalent bonding.

reviewing this chapter
  1. How do the chemical characteristics of carbon affect the structure of organic molecules? 38–39

  2. Give examples of functional groups, and discuss the importance of these groups being hydrophobic or hydrophilic. 39

  3. What biomolecules are monomers of the polymers studied in this chapter? How do monomers join to produce polymers, and how are polymers broken down to monomers? 40

  4. Name several monosaccharides, disaccharides, and polysaccharides, and give a function of each. How are these molecules structurally distinguishable? 41–42

  5. What is the difference between a saturated and an unsaturated fatty acid? Explain the structure of a fat molecule by stating its components and how they join together. 44–45

  6. How does the structure of a phospholipid differ from that of a fat? How do phospholipids form a bilayer in the presence of water? 46

  7. Describe the structure of a generalized steroid. How does one steroid differ from another? 46–47

  8. Draw the structure of an amino acid and a peptide, pointing out the peptide bond. 48

  9. Discuss the four possible levels of protein structure, and relate each level to particular bonding patterns. 50–51

  10. How do nucleotides bond to form nucleic acids? State and explain several differences between the structure of DNA and that of RNA. 52–53

  11. Discuss the structure and function of ATP. 53–54

testing yourself

Choose the best answer for each question.

  1. Which of these is not a characteristic of carbon?

    1. forms four covalent bonds

    2. bonds with other carbon atoms

    3. is sometimes ionic

    4. can form long chains

    5. sometimes shares two pairs of electrons with another atom

  2. The functional group is

    1. acidic.

    2. basic.

    3. never ionized.

    4. found only in nucleotides.

    5. All of these are correct.

  3. Page 57
  4. A hydrophilic group is

    1. attracted to water.

    2. a polar and/or ionized group.

    3. found at the end of fatty acids.

    4. the opposite of a hydrophobic group.

    5. All of these are correct.

  5. Which of these is an example of a hydrolysis reaction?

    1. denaturation of a polypeptide

    2. Both a and b are correct.

    3. Both b and c are correct.

  6. Which of these makes cellulose nondigestible in humans?

    1. a polymer of glucose subunits

    2. a fibrous protein

    3. the linkage between the glucose molecules

    4. the peptide linkage between the amino acid molecules

    5. The carboxyl groups ionize.

  7. A fatty acid is unsaturated if it

    1. contains hydrogen.

    2. contains carbon–carbon double bonds.

    3. contains a carboxyl (acidic) group.

    4. bonds to glycogen.

    5. bonds to a nucleotide.

  8. Which of these is not a lipid?

    1. steroid

    2. fat

    3. polysaccharide

    4. wax

    5. phospholipid

  9. The difference between one amino acid and another is found in the

    1. amino group.

    2. carboxyl group.

    3. R group.

    4. peptide bond.

    5. carbon atoms.

  10. The shape of a polypeptide is

    1. maintained by bonding between parts of the polypeptide.

    2. ultimately dependent on the primary structure.

    3. necessary to its function.

    4. All of these are correct.

  11. Which of these illustrates a peptide bond?

  12. Nucleotides

    1. contain a sugar, a nitrogen-containing base, and a phosphate group.

    2. are the monomers of fats and polysaccharides.

    3. join together by covalent bonding between the bases.

    4. are present in both DNA and RNA.

    5. Both a and d are correct.

  13. ATP

    1. is an amino acid.

    2. has a helical structure.

    3. is a high-energy molecule that can break down to ADP and phosphate.

    4. provides enzymes for metabolism.

    5. is most energetic when in the ADP state.

  14. Label the following diagram using the terms H2O, monomer, hydrolysis reaction, dehydration reaction, and polymer. Terms can be used more than once and a term need not be used.

  15. The monomer of a carbohydrate is

    1. an amino acid.

    2. a nucleic acid.

    3. a monosaccharide.

    4. a fatty acid.

  16. The joining of two adjacent amino acids is called

    1. a peptide bond.

    2. a dehydration reaction.

    3. a covalent bond.

    4. All of these are correct.

  17. The characteristic globular shape of a polypeptide is the

    1. primary structure.

    2. secondary structure.

    3. tertiary structure.

    4. quaternary structure.

  18. The shape of a polypeptide

    1. is maintained by bonding between parts of the polypeptide.

    2. is ultimately dependent on the primary structure.

    3. involves hydrogen bonding.

    4. All of these are correct.

  19. Which of the following pertains to an RNA nucleotide and not to a DNA nucleotide?

    1. contains the sugar ribose

    2. contains a nitrogen-containing base

    3. contains a phosphate molecule

    4. becomes bonded to other nucleotides following a dehydration reaction

  20. Which is a carbohydrate?

    1. disaccharide

    2. amino acid

    3. dipeptide

    4. Both a and c are correct.

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For questions 20–27, match the items to those in the key. Some answers are used more than once.


  1. carbohydrate

  2. fats and oils

  3. protein

  4. nucleic acid

  1. contains the bases adenine, guanine, cytosine, and thymine

  2. the 6-carbon sugar, glucose

  3. polymer of amino acids

  4. glycerol and fatty acids

  5. enzymes

  6. long-term energy storage

  7. genes

  8. plant cell walls

  9. muscle cells

  10. butter

  11. potato

  12. Which of these does not apply to DNA?

    1. sequence of nucleotides

    2. sugar-phosphate backbone

    3. A-T and C-G

    4. sequence of amino acids

    5. Both a and c do not apply.

  13. Which is a correct statement about carbohydrates?

    1. All polysaccharides serve as energy storage molecules.

    2. Glucose is broken down for immediate energy.

    3. Glucose is not a carbohydrate, only polysaccharides are.

    4. Starch, glycogen, and cellulose have different monomers.

    5. Both a and c are correct.

  14. In phospholipids,

    1. heads are polar.

    2. tails are nonpolar.

    3. heads contain phosphate.

    4. All of these are correct.

For questions 34–38, match the items to those in the key.


  1. Most enzymes are globular.

  2. DNA is a double helix.

  3. Steroids differ by their attached groups.

  4. The tails of a phospholipid can contain nonsaturated fatty acids.

  5. Hydrogen bonding occurs between microfibrils of cellulose.

  1. Strands held together by hydrogen bonding between strands.

  2. Four fused rings plus functional groups.

  3. Tertiary level of organization of a protein.

  4. Provides added strength for plant cell wall.

  5. Makes plasma membrane a fluid bilayer.

thinking scientifically
  1. The seeds of temperate plants tend to contain unsaturated fatty acids, while the seeds of tropical plants tend to have saturated fatty acids. a. How would you test your hypothesis. b. Assuming your hypothesis is supported, give an explanation.

  2. Chemical analysis reveals that an abnormal form of an enzyme contains a polar amino acid at the location where the normal form has a nonpolar amino acid. Formulate a testable hypothesis concerning the abnormal enzyme.

bioethical issue
Organic Pollutants

Organic compounds include the carbohydrates, proteins, lipids, and nucleic acids that make up our bodies. Modern industry also uses all sorts of organic compounds that are synthetically produced. Indeed, our modern way of life wouldn't be possible without synthetic organic compounds.

Pesticides, herbicides, disinfectants, plastics, and textiles contain organic substances that are termed pollutants when they enter the natural environment and cause harm to living things. Global use of pesticides has increased dramatically since the 1950s, and modern pesticides are ten times more toxic than those of the 1950s. The Centers for Disease Control and Prevention in Atlanta reports that 40% of children working in agricultural fields now show signs of pesticide poisoning. The U.S. Geological Survey estimates that 32 million people in urban areas and 10 million people in rural areas are using groundwater that contains organic pollutants. J. Charles Fox, an official of the Environmental Protection Agency, says that “over the life of a person, ingestion of these chemicals has been shown to have adverse health effects such as cancer, reproductive problems, and developmental effects.”

At one time, people failed to realize that everything in the environment is connected to everything else. In other words, they didn't know that an organic chemical can wander far from the site of its entry into the environment and that eventually these chemicals can enter our own bodies and cause harm. Now that we are aware of this outcome, we have to decide as a society how to proceed. We might decide to do nothing if the percentage of people dying from exposure to organic pollutants is small. Or we might decide to regulate the use of industrial compounds more strictly than has been done in the past. We could also decide that we need better ways of purifying public and private water supplies so that they do not contain organic pollutants.

Biology website

The companion website for Biology provides a wealth of information organized and integrated by chapter. You will find practice tests, animations, videos, and much more that will complement your learning and understanding of general biology.