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Chapter 4
Cellular Metabolism
4.1 Introduction
1. Define metabolism. (p. 115)
Metabolism is the sum total of chemical reactions in the cell.
2. Explain how metabolic pathways are linked and intersect. (p. 115)
In many cases, products of one reaction are starting materials for the next. These reactions form cycles and
pathways that may intersect where they share intermediate compounds. Each step may be catalyzed by an
enzyme.
4.2 Metabolic Processes
3. Distinguish between catabolism and anabolism. (p. 115
Anabolism uses energy to build large molecules from smaller ones. Catabolism releases energy by
breaking large molecules into smaller ones.
4. Distinguish between dehydration synthesis and hydrolysis. (p. 115)
Dehydration synthesis is a special form of anabolism where larger molecules are formed by removing an
–OH (hydroxyl group) from the end of one molecule and an –H (hydrogen atom) from the end of another.
The –OH and –H combine to form H2O (water), and the ends of the two molecules join by sharing the
remaining oxygen atom.
Hydrolysis is the opposite of dehydration synthesis. In hydrolysis, a large molecule is split apart at a
certain point and a hydrogen atom is attached to one of the new molecules, while a hydroxyl group is
attached to the other.
Both of these processes can occur over and over until the original molecule is altered to the cell’s needs. In
short, dehydration synthesis dehydrates a molecule and hydrolysis rehydrates it.
5. Give examples of a dehydration synthesis reaction and a hydrolysis reaction. (p. 115)
Dehydration synthesis joins many simple sugar molecules to form larger molecules of glycogen. Glycerol
and fatty acid molecules join to form larger fat molecules. Hydrolysis of a disaccharide yields two
monosaccharide molecules. Fats are broken down into glycerol and fatty acids.
4.3 Control of Metabolic Reactions
6. Describe how an enzyme interacts with its substrate? (p. 117)
The surface of an enzyme contains areas called active sites that will bind to a specific substrate only.
When the correct substrates are attached to the active sites (called an enzyme-substrate complex), the
enzyme alters the shapes of the substrates in a way that promotes the reaction. All enzymes demonstrate
this specificity to its substrates. To illustrate, an enzyme-substrate complex is like a “lock-and-key” model
with the enzyme as the lock and the substrate as the key. Although many keys may fit the lock, only one
type of key will make it work.
7. Define active site. (p. 117)
During an enzyme-catalyzed reaction, regions of the enzyme molecule, called active sites, temporarily
combine with portions of the substrate, forming an enzyme-substrate complex.
8. State two factors that control the rate of an enzyme-catalyzed reaction. (p. 117)
The factors are:
a. an increase in the enzyme concentration.
b. an increase in the substrate concentration.
9. A cell has _________ of types of enzymes and metabolic reactions. (p. 117)
b. hundreds of
10. Explain the importance of a rate-limiting enzyme. (p. 118)
The rate at which a metabolic pathway functions is often determined by a regulatory enzyme that catalyzes
one of its steps
11. Describe how negative feedback involving a rate-limiting enzyme controls a metabolic pathway.
(p. 118)
The rate of a metabolic pathway is often determined by a regulatory enzyme responsible for one of
its steps. This regulatory enzyme is present in limited quantity. Consequently, it can become
saturated with substrate molecules whenever the substrate concentration increases above a certain
level. Once the enzyme is saturated, increasing the number of substrate molecules will no longer
affect the reaction rate.
12. Define cofactor. (p. 119)
A cofactor is a separate non-protein molecule that binds to an enzyme to aid in the reaction. Usually, a
cofactor is a non-organic molecule. A coenzyme is a cofactor that is an organic molecule.
13. Discuss the relationship between a coenzyme and a vitamin. (p. 119)
Many coenzymes are composed of vitamin molecules or incorporate altered forms of vitamin molecules
into their structures.
.
4.4 Energy for Metabolic Reactions
14. Define energy. (p. 119)
Energy, by definition, is the ability to do work.
15. Explain the importance of ATP, and its relationship to ADP. (p. 119)
ATP is the primary energy-carrying molecule in the cell. It acts as a rechargeable battery for cellular
processes by carrying energy in the terminal bond of the phosphate molecule and returning to recapture
energy when it is used. Without ATP, the cell would die. ATP releases its energy by breaking off the third,
or terminal, phosphate molecule. When this occurs, it becomes ADP (with only two phosphate molecules).
The ADP returns to “recharge” by picking up a third phosphate molecule with energy, and the cycle
repeats.
16. Explain how the oxidation of molecules inside cells differs from the burning of substances outside
cells. (p. 120)
The burning of a substance outside the cell usually requires large amounts of energy to start the reaction.
This burning indiscriminately breaks all chemical bonds in the substance and releases the energy as light
and heat.
Oxidation inside the cell utilizes enzymes that require less activation energy, controls the by-products
released, and uses certain energy capturing molecules to trap about one-half of the released energy for use
elsewhere.
The rest is lost as heat.
4.5 Cellular Respiration
17. Define cellular respiration. (p. 120)
The controlled, sequential process of oxidation and energy recapture is referred to as cellular respiration.
18. Distinguish between anaerobic and aerobic phases of cellular respiration. (p. 120)
During cellular respiration, the oxidative processes that occur in the absence of oxygen are called
anaerobic respiration. The oxidative processes that require the presence of oxygen for their reactions are
called aerobic respiration.
19. Match the part of cellular respiration to the associated activities. (p. 120)
1. Electron transport chain—C
2. Glycolysis—A
3. Citric acid cycle—B
20. Identify the final acceptor of the electrons released in the reactions of cellular respiration. (p.
123)
Oxygen is the final electron carrier.
4.6 Nucleic Acids and Protein Synthesis
21. Excess glucose in cells may link and be stored as: (p. 124)
Glycogen.
4.6 Nucleic Acids and Protein Synthesis
22. The genetic code is ________. (p. 124)
b. the correspondence between DNA triplet and amino acid
23. DNA information provides instructions for the cell to __________. (p. 125
construct a great variety of protein molecules, each with a specific function.
24. Distinguish between a gene and a genome. (p. 125)
The portion of a DNA molecule that contains the genetic information for making a particular protein is
called a gene. The complete set of genetic instructions in a cell constitutes the genome.
25. Define gene expression. (p. 125)
Gene expression refers to which proteins a cell produces and how many copies form under particular
conditions.
26. If a DNA strand has the sequence ATGCGATCCGC then the sequence on the complementary
strand is: ________. (p. 127)
TACGCTAGGCG.
27. Explain why DNA replication is essential. (p. 127)
In order for the newly formed daughter cells to function correctly, a complete copy of the DNA molecule is
required during replication to ensure that metabolism and all cell functions can be carried out.
28. Describe the events of DNA replication. (p. 127)
During a phase before cell reproduction an enzyme called DNA polymerase breaks the hydrogen bonds
between the complimentary base pairs of the DNA molecule. As the DNA molecule splits and unwinds,
new nucleotides bond with the exposed nucleotides of the parental strand and a new sugar-phosphate
backbone is built. The result is two complete DNA molecules, each containing one parental strand and one
daughter strand. These DNA molecules are then separated so that each new cell receives one complete
DNA molecule.
29. Identify the part of a DNA molecule that encodes information. (p. 130)
Gene.
30. List 3 ways that RNA differs from DNA. (p. 130)
1. RNA molecules are single-stranded.
2. RNA nucleotides have ribose instead of deoxyribose sugar.
3. In place of thymine nucleotides, RNA molecules have uracil nucleotides.
31. If one strand of a DNA molecule has the sequence ATTCTCGACTAT, the complementary
mRNA has the sequence. (p. 131)
UAAGAGCUGAUA
32. Distinguish between transcription and translation. (p. 131)
The process by which an mRNA molecule is formed from DNA is called transcription. The synthesis of
protein molecules from the mRNA is called translation.
33. Distinguish between the functions of MRNA, RRNA, and tRNA. (p. 132)
A messenger RNA (mRNA) molecule is a special type of RNA that is made of the complementary base
sequences, necessary for the production of a protein, from the DNA molecule. Transfer RNA (tRNA) is a
group of RNA molecules that bind to activated amino acids in the cytoplasm and bring them to the mRNA
molecule for assembly into a protein. A tRNA molecule can bind with only one kind of amino acid.
Therefore, since there are twenty different amino acids, there must be at least twenty different kinds of
tRNA molecules. In protein synthesis, a ribosome, composed of ribosomal RNA (rRNA) and protein
molecules, moves along with the mRNA and knits together a chain of amino acids by attaching itself to a
portion of the mRNA and bonding with the complementary amino acid on a tRNA molecule. As it moves
along the mRNA, it attaches each amino acid in sequence and releases the empty tRNA back into the
cytoplasm.
34. List the steps of protein synthesis. (p. 132)
See Table 4.3 for a summary of the events comprising transcription and translation during protein
synthesis.
35. Describe the function of a ribosome in protein synthesis. (p. 132)
In protein synthesis, a ribosome, composed of rRNA and protein molecules, moves along with the mRNA
and knits together a chain of amino acids by attaching itself to a portion of the mRNA and bonding with the
complementary amino acid on a tRNA molecule. As it moves along the mRNA, it attaches each amino acid
in sequence and releases the empty tRNA back into the cytoplasm.
36. Calculate the number of amino acids that a DNA sequence of 27 nucleotides encodes. (p. 132)
Each of the twenty different types of amino acids is represented in a DNA molecule by a triplet code,
consisting of sequences of three nucleotides. Therefore, a DNA sequence of 27 nucleotides encodes for 9
amino acids.
4.7 Changes in Genetic Information
37. Distinguish among mutations, SNPs, and copy number variants. (p. 135)
The rare distinctions in DNA sequence that affect how we look or feel are called mutations. More common
genetic variants with no detectable effects are called single nucleotide polymorphisms (SNPs).
Polymorphism is a term used to describe copy number variants.
38. Discuss two major ways that mutation occurs. (p. 136)
A mutation is defined as a mistake in, or damage to, the DNA strand that is not corrected and is passed on
to the new cells. Mutations can occur during replication when an incorrect nucleotide or extra nucleotides
bind to the parental DNA strand. Mutations can also occur if sections of the DNA strands are deleted,
misplaced, or attached to the wrong chromosome.
39. Define DNA damage response. (p. 136)
If the mutation to a DNA molecule occurs only on one strand, the cell uses special enzymes called repair
enzymes to clip out the defective portion and rebuild it correctly.
40. Discuss three ways that the genetic code protects against the persistence of a mutation. (p. 137)
Because of the importance of the DNA molecule, there are many safeguards against mutation. For instance,
there are sixty-one codons that specify the twenty amino acids. If the mutation occurs in the third
nucleotide of a codon, it is likely that this mutation will still yield the correct amino acid. If the mutation is
in the second codon, the new sequence will generally yield a structure similar enough to the original amino
acid that the effect would not be significant. There are two copies of each chromosome in an adult cell. If
one chromosome is mutated, the genes of the second chromosome will usually provide enough normal
“blueprints” to maintain the health of the cell. If a mutation occurs in a cell of an adult, it will probably go
unnoticed because of the many normal cells around it. If the mutation occurs in the cell of an embryo or
child, the results can be catastrophic because that cell may be the first, or be the parent, to many other cells
as the infant grows.