Principles of Life, 2e



Chapter 3: The Chemistry and Energy of Life

Concept Checkpoint Questions

Concept 3.1 Nucleic Acids Are Informational Macromolecules

1. List the key differences between DNA and RNA and between purines and pyrimidines.

2. What are the differences between DNA replication and transcription?

3. If one strand of a DNA molecule has the sequence 5′-TTCCGGAT-3′, what is the sequence of the other strand of DNA? If RNA is transcribed from the 5′-TTCCGGAT-3′ strand, what would be its sequence? And if RNA is transcribed from the other DNA strand, what would be its sequence? (Note that it is conventional to write these sequences with the 5′ end on the left.)

4. How can DNA molecules be so diverse when they appear to be structurally similar?

Concept 3.2 Some Proteins Are Polymers with Important Structural and Metabolic Roles

1. Sketch the peptide bonding of the two amino acids glycine and leucine (in that order). Now add a third amino acid, alanine, in the position it would have if added within a biological system. What is the directionality of this process?

2. Examine the structure of sucrase (see Figure 3.9). Where in the protein might you expect to find the following amino acids: valine, proline, glutamic acid, and threonine? Explain your answers.

3. Detergents disrupt hydrophobic interactions by coating hydrophobic molecules with a molecule that has a hydrophilic surface. When hemoglobin is treated with a detergent, the four polypeptide chains separate and become random coils. Explain these observations.

4. Several small molecules interact with a protein. The chemical groups on the small molecules interact with specific amino acids as shown in the table below. Fill in the table to show the types of noncovalent interactions that occur between the small molecules and the amino acids.

Concept 3.3 Some Proteins Act as Enzymes to Speed up Biochemical Reactions

1. Explain how the structure of an enzyme makes that enzyme specific.

2. What is activation energy? How does an enzyme lower the activation energy needed to start a reaction?

3. Compare coenzymes with substrates. How do they work together in enzyme catalysis?

4. Compare the state of an enzyme active site at a low substrate concentration and at a high substrate concentration. How does this affect the rate of the reaction?

Concept 3.4 Regulation of Metabolism Occurs by Regulation of Enzymes

1. Explain and give examples of irreversible and reversible enzyme inhibitors.

2. The amino acid glutamic acid (see Table 3.2) is at the active site of an enzyme. Normally the enzyme is active at pH 7. At pH 4 (higher concentration of H+), the enzyme is inactive. Explain these observations.

3. An enzyme is subject to allosteric regulation. How would you design an inhibitor of the enzyme that was competitive? Noncompetitive? Irreversible?

4. Some organisms thrive at pH 2; other organisms thrive at a temperature of 65oC. Yet mammals cannot tolerate either environment in their tissues. Explain.

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