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Chapter 4
Cellular
Metabolism
 4.1 Introduction
A. The total of chemical reactions in a
cell -> Metabolism
B. Special type of protein called
enzymes control the rate of these
reactions.
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4.2 Metabolic Processes
A. Metabolic reactions are of two types:
1. Anabolic reactions -> larger
molecules are constructed from smaller
ones (requires energy)
2. Catabolic reactions -> larger
molecules are broken down
(releasing energy)
*The reactions of metabolism are often
reversible.*
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B. Anabolism
1. Anabolism provides the
substances needed for growth
and repair.
2. These reactions occur by
dehydration synthesis, removing
a molecule of water to join two
smaller molecules.
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3. Polysaccharides, lipids, and proteins
are constructed by dehydration synthesis.
a. To form fats, glycerol and
fatty acids bond.
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b. The bond between two
amino acids is a peptide bond;
two bound amino acids form a
dipeptide, while many joined
form a polypeptide.
c. Monosaccharides are bonded
to produces a disaccharide.
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C. Catabolism
1. Catabolism breaks apart larger
molecules into their building
blocks.
2. These reactions occur by
hydrolysis, where a molecule of
water is inserted into a polymer
which is split into two smaller
molecules.
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3. Hydrolysis is the reverse of
dehydration synthesis.
4. Like dehydration synthesis,
hydrolysis requires specific
enzymes, discussed in the next
section.
4.3 Control of Metabolic Reactions:
A. Enzymes control the rates of all
the metabolic reactions of the cell.
B. Enzyme Action
1. Enzymes are complex proteins
that function to lower the
activation energy of a reaction so
it may begin and proceed more
rapidly.
*Enzymes are called catalysts.*
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2. Each enzyme is specific, acting
on only one kind of substrate.
~ Enzyme names are often
derived from the name of their
substrate, with the suffix –ase
added.
ex. Sucrase - sucrose, maltase –
maltose & lactase - lactose.
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3. Active sites on the enzyme
combine with the substrate and a
reaction occurs.
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4. Enzymes work in small quantities
and are recycled by the cell.
5. The speed of enzymatic reactions
depends on the number of enzyme
and substate molecules available.
C. Factors That Alter Enzymes
1. Enzymes (proteins) can be denatured
by heat, pH extremes, chemicals,
electricity, radiation, and by other
causes.
2. “extremophiles,” live in conditions of
high or low heat, salinity, or pH.
* Their enzymes have evolved under
these conditions and are useful in
industrial processes that are too harsh
to use other enzymes.*
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D. Cofactors & Coenzymes
1. An enzyme may be inactive until it
combines with a non-protein
component that either helps the
active sit change shape or helps bind
the enzyme to its substrate.
~ cofactor – ion of an element, such
as copper, iron, or zinc
~ coenzyme – small organic
molecule
2. Vitamins are essential organic
molecules that humans cannot
synthesize (or may not make in
sufficient quantities) and must
come from the diet.
~ Vitamins provide coenzymes
& are required in very small
quantities.
~ Why such small amounts?
4.4Energy for Metabolic Reactions:
A. Energy is the capacity to do work.
1. Forms include heat, light, sound,
and electrical, mechanical, and
chemical energy.
2. Energy can be changed from one
form to another.
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ex. Engine changes chemical energy
in fuel to heat & mechanical
energy
? Light bulb
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B. Release of Chemical Energy
1. Release of chemical energy in
the cell often occurs through the
oxidation of glucose.
2. “Burning” glucose requires
energy to begin the process.
(enzymes reduce activation
energy)
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3. The end-products of these
reactions are heat (maintain body
temperature) as well as stored
energy.
4. About ½ of the energy is
captured in special energycarrying molecules such as ATP.
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C. Cellular Respiration
~ Series of three reactions
glycolysis
citric acid cycle
electron transport chain
~ Products
Carbon Dioxide
Water
Energy
~ ½ Energy is used to create ATP
1. ATP Molecules
a. Up to 38 molecules of ATP are
produced for each molecule of
glucose oxidized.
b. Adenosine triphosphate (ATP)
is a molecule that carries energy
in a form that the cell can use.
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c. Each ATP molecule consists of
three main parts: adenine, ribose
and 3 phosphates in a chain.
d. Release & storage of energy is in the form of
high energy bonds between the phosphate
groups.
2. Anaerobic Respiration
a. The first part of cellular
respiration is the splitting of 6-C
glucose that occurs through a
series of enzyme-catalyzed steps
called glycolysis.
b. The result is two 3-C molecules
of pyruvate.
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c. Glycolysis occurs in the cytosol
and does not require oxygen
(anaerobic).
d. Energy from ATP is used to
start the process but there is a net
gain of energy as a result.
2 ATP – start
4 ATP – result
2 ATP - net
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3. Aerobic Respiration
a. Oxygen is needed for aerobic
respiration in the mitochondria.
1. Pyruvic acid is converted
into an intermediate
molecule called acetyl-CoA.
2. The citric acid cycle (Hans
Kreb German-British
biochemist) releases
carbon dioxide & H+ resulting
in the formation of ATP.
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3. Carrier molecules move efrom the citric acid cycle to the
e- transport chain, where water
& more ATP are formed.
Summary
Glycolosis
2 ATP
Cytric Acid Cycle
2 ATP
Electron Transport
34 ATP
Yield
38 ATP
b. There is a much greater gain of
ATP molecules from aerobic
respiration.
c. The actual number of ATP varies
for different types of cells.
ex. Brain cells – 32 ATP
4.5 Metabolic Pathways:
A. The enzymes controlling either
an anabolic or catabolic sequence of
reactions must act in a specific order.
B. A sequence of enzyme-controlled
reactions is called a metabolic
pathway.
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C. Regulation of Metabolic
Pathways
1. The rate of a metabolic pathway
is determined by a regulatory
enzyme responsible for one of its
steps.
2. A rate-limiting enzyme is the
first step in a series.
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4.6 Nucleic Acids
~ Deoxyribonucleic acid (DNA)
contains the genetic code needed for
the synthesis of each protein (including
enzymes).
A. Genetic Information
1. A gene is a portion of a DNA
that contains the genetic
information for making a single
protein.
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2. Because enzymes control
synthesis reactions, all four
groups of organic molecules
depend on proteins.
3. The genome is all the DNA
(genetic instructions) in a cell .
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B. DNA Molecules
1. Nucleotides are the building
blocks of nucleic acids.
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2. A polynucleotide chain consists
of nucleotides connected by a
sugar-phosphate backbone.
(Model)
3. A DNA molecule consists of two
polynucleotide chains. (Model)
~ Notice that the sugars point in
opposite directions
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4. Nitrogen bases project from the
backbone of one strand and bind by
hydrogen bonds to the base of the
2nd strand. (Model)
a. Four types of Bases
A, T, G, C
b. Pairing
A – T, C – G
~ Called complementary base pairs
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5. The DNA molecule
twists to form a
double helix and
may be millions of
base pairs long.
Hint: DNA profiling
p. 82
4.7 DNA Replication
1. Each new cell must be provided
with an exact replica of the parent
cell's DNA.
2. DNA replication occurs during
interphase.
a. The DNA molecule splits.
b. Nucleotides form
complementary pairs with
the original strands.
(DNA Polymerase)
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3. Each new DNA molecule
consists of one parental strand
and one newly synthesized strand
of DNA.
Hint: Topic of interest p. 83 Mutations
Topic of interest p. 87 Proteomics
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4.8 Protein Synthesis
~ Genetic Code
1. The sequence of nucleotides in
a DNA molecule gives the order
of aa for a protein.
2. This method of storing
information for protein synthesis
is the genetic code.
3. RNA molecules copy & transfer
this information to the cytoplasm
where proteins are manufactured.
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A. Transcription
1. RNA Molecules
a. RNA molecules are singlestranded and contain ribose
rather than deoxyribose, and
uracil rather than thymine.
b. Messenger RNA (mRNA)
molecules are synthesized in
the nucleus in a sequence
complementary to the DNA
template.
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c. Each amino acid corresponds to a
triplet of DNA nucleotides; a triplet of
nucleotides in mRNA is called a codon.
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d. Messenger RNA can move out
of the nucleus and attach to a
ribosomes in the cytoplasm
where the protein will be made in
a process called translation.
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B. Translation
1. In the cytoplasm transfer RNA has
a triplet of nucleotides called the
anticodon, which is complementary
to nucleotides of the mRNA codon.
2. The ribosome holds the mRNA in
position while the tRNA carries in
the correct amino acid in sequence,
with anticodons matching up to
codons.
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3. The ribosome contains
enzymes needed to join the
amino acids together.
4. As the amino acids are joined,
the new protein molecule
changes into its unique shape.
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