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Lecture Presentation
Chapter 12
Food as Fuel—A
Metabolic Overview
Julie Klare
Fortis College
Smyrna, GA
© 2014 Pearson Education, Inc.
Outline
• 12.1 How Metabolism Works
• 12.2 Metabolically Relevant Nucleotides
• 12.3 Digestion—From Food Molecules to Hydrolysis
Products
• 12.4 Glycolysis—From Hydrolysis Products to Common
Metabolites
• 12.5 The Citric Acid Cycle—Central Processing
• 12.6 Electron Transport and Oxidative Phosphorylation
• 12.7 ATP Production
• 12.8 Other Fuel Choices
© 2014 Pearson Education, Inc.
12.1 How Metabolism Works
• Animals get energy from the covalent bonds
contained in carbohydrates, fats, and proteins.
• In the first stage of metabolism, biomolecules
in food are digested into smaller units through
hydrolysis reactions.
• Polysaccharides are hydrolyzed into
monosaccharide units.
• Triglycerides are broken down to glycerol and
fatty acids.
• Proteins are hydrolyzed into their amino acid
units.
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12.1 How Metabolism Works
• The molecules produced by the breakdown are
absorbed through the intestinal wall into the
bloodstream and transported to different tissues
for use by the cells.
• In the cells, the hydrolysis products are broken
down into a few common metabolites containing
two or three carbons.
• Metabolites are chemical intermediates formed
by enzyme-catalyzed reactions in the body.
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12.1 How Metabolism Works
• As long as cells have oxygen
and are producing energy,
two-carbon acetyl groups can
be broken down further to
carbon dioxide through the
citric acid cycle.
• This cycle works to produce
the molecules ATP,
nicotinamide adenine
dinucleotide (NADH), and
flavin adenine dinucleotide
(FADH2).
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12.1 How Metabolism Works
• Chemical reactions that occur in living systems
are biochemical reactions.
• Chemical reactions occur in a series called a
metabolic pathway.
• The sugar molecule glucose (containing six
carbons) is broken down to two molecules of
pyruvate (three carbons each) through a series
of chemical reactions referred to as glycolysis.
• Metabolism can be considered in two parts,
catabolism and anabolism.
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12.1 How Metabolism Works
• Catabolism refers to
chemical reactions in which
larger molecules are broken
down into a few common
metabolites. These reactions
tend to be exergonic (-G).
• Anabolism refers to chemical
reactions in which metabolites
combine to form larger
molecules. These reactions
tend to be endergonic (+G).
• The energy released during
catabolic reactions is
captured in ATP and used to
drive anabolic reactions.
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12.1 How Metabolism Works
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12.1 How Metabolism Works
• In animals, a cell membrane separates the
materials inside the cell from the exterior
aqueous environment.
• The nucleus contains DNA that controls cell
replication and protein synthesis for the cell.
• The cytoplasm consists of all the material
between the nucleus and the cell membrane.
• The cytosol is the fluid part of the cytoplasm.
It is the aqueous solution of electrolytes and
enzymes that catalyzes many of the cell’s
chemical reactions.
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12.1 How Metabolism Works
• Within the cytoplasm are organelles.
• Ribosomes are the sites of protein synthesis.
• Mitochondria are the energy-producing
factories of the cells.
• A mitochondrion consists of an outer
membrane, an inner membrane, and an
intermembrane matrix.
• Enzymes in the matrix and inner membrane
catalyze the oxidation of carbohydrates, fats,
and amino acids.
© 2014 Pearson Education, Inc.
12.1 How Metabolism Works
© 2014 Pearson Education, Inc.
12.1 How Metabolism Works
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12.2 Metabolically Relevant Nucleotides
• Nucleotides act as energy exchangers and
can also be coenzymes.
• All of these nucleotides have two forms:
a high-energy form and a low-energy form.
• They consist of some basic components:
the nucleoside adenosine, a phosphate, and
a five-carbon sugar. Many of these
molecules also have a vitamin within their
structure.
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12.2 Metabolically Relevant Nucleotides
© 2014 Pearson Education, Inc.
12.2 Metabolically Relevant Nucleotides
© 2014 Pearson Education, Inc.
12.2 Metabolically Relevant Nucleotides
© 2014 Pearson Education, Inc.
12.2 Metabolically Relevant Nucleotides
• ATP is often referred to as the energy currency of the cell.
• ATP can undergo hydrolysis: during hydrolysis, energy is released as
a product, so in this case, ATP is the high-energy form and ADP is
the low-energy form.
• The energy given off during the hydrolysis of ATP can be coupled to
drive a chemical reaction that requires energy.
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12.2 Metabolically Relevant Nucleotides
NADH/NAD+ and FADH2/FAD
• Nicotinamide adenine dinucleotide (NAD+) and flavin
adenine dinucleotide (FAD) are energy-transferring
compounds with a high-energy form that is reduced
(hydrogen added) and a low-energy form that is
oxidized (hydrogen removed).
• The abbreviations for these forms are NADH (reduced
form) and NAD+ (oxidized form) and FADH2 (reduced
form) and FAD (oxidized form).
• The active end of each molecule contains a vitamin
component.
• Nicotinamide is derived from the vitamin niacin (B3),
and riboflavin (B2) is found in FAD.
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12.2 Metabolically Relevant Nucleotides
Acetyl Coenzyme A and Coenzyme A
• Another important energy exchanger is coenzyme A (CoA).
• The two forms of this compound are acetyl coenzyme A
(high energy) and coenzyme A (low energy).
• Energy is released from acetyl coenzyme A when the
C—S bond in the thioester functional group is hydrolyzed,
producing an acetyl group and coenzyme A.
• CoA contains adenosine, three phosphates, and
a pantothenic acid (vitamin B5)-derived portion.
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12.3 Digestion—From Food Molecules to Hydrolysis Products
Carbohydrates
• Starch (amylose and amylopectin) begins to be digested in
your mouth by alpha-amylase in saliva.
• This salivary amylase hydrolyzes some of the α-glycosidic
bonds in the starch molecules, producing glucose, the
disaccharide maltose, and oligosaccharides.
• Only monosaccharides are small enough to be transported
into the bloodstream.
• To complete the digestion of starch, enzymes in the small
intestine hydrolyze starch and disaccharides into
monosaccharides.
• Cellulose cannot be digested because we lack the enzyme
cellulase that hydrolyzes its β-glycosidic bonds.
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12.3 Digestion—From Food Molecules to Hydrolysis Products
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12.3 Digestion—From Food Molecules to Hydrolysis Products
Fats
• Dietary fats are nonpolar molecules, so to assist in digestion, bile
is excreted from the gall bladder into the stomach during digestion.
• Bile contains bile salts, which are amphipathic: they place their
nonpolar face toward the dietary fats and their polar face toward the
water, forming micelles.
• Breaking up larger nonpolar globules into smaller droplets
(micelles) is called emulsification.
• The micelles move the dietary fats closer to the intestinal cell wall
so cholesterol can be absorbed and triglycerides hydrolyzed.
• Once across the intestinal wall, free fatty acids and monoglycerides
are reassembled as triglycerides while the cholesterol is linked to
another free fatty acid forming a cholesterol ester.
• These are repackaged as a lipoprotein called a chylomicron.
• Chylomicrons transport triglycerides to the tissues, where they are
used for energy production or stored.
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12.3 Digestion—From Food Molecules to Hydrolysis Products
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12.3 Digestion—From Food Molecules to Hydrolysis Products
Proteins
•
•
•
Protein digestion begins in the stomach, where proteins are denatured (unfolded)
by the acidic digestive juices.
Digestive enzymes like pepsin, trypsin, and chymotrypsin hydrolyze peptide bonds.
Amino acids are absorbed into the bloodstream for delivery to the tissues.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
The Chemical Reactions in Glycolysis
• In the body, energy must be transferred in small
amounts to minimize the heat released in the process.
• Reactions that produce energy are coupled with
reactions that require energy, thereby helping to
maintain a constant body temperature.
• In glycolysis, energy is transferred through phosphate
groups undergoing condensation and hydrolysis
reactions. There are 10 chemical reactions in glycolysis
that result in the formation of two molecules of pyruvate
from one molecule of glucose.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
The Chemical Reactions in Glycolysis
• The first five reactions require an energy investment of two
molecules of ATP, which are used to add two phosphate
groups to the sugar molecule.
• This molecule is split into two sugar phosphates.
• Reactions 6 through 10 generate two high-energy NADH
molecules during the addition of two more phosphates and
four ATP molecules when the four phosphates are removed
from the sugar phosphates.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
© 2014 Pearson Education, Inc.
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
Regulation of Glycolysis
• The main step of regulation in glycolysis is step 3.
• The enzyme phosphofructokinase, which
catalyzes the phosphorylation of fructose-6phosphate to fructose-1,6-bisphosphate, is heavily
regulated by the cells.
• ATP acts as an inhibitor of phosphofructokinase.
• If cells have plenty of ATP, glycolysis slows down.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
The Fates of Pyruvate
• Aerobic conditions: pyruvate produces more energy
for the cell when the carboxylate functional group of
pyruvate is liberated as CO2 during oxidative
decarboxylation.
• The acetyl group binds to coenzyme A during the
oxidation through a sulfur atom, creating a thioester
functional group and acetyl CoA.
• This reaction occurs in the mitochondria.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
The Fates of Pyruvate
• Anaerobic conditions: the middle carbonyl in pyruvate
is reduced (hydrogen added) to an alcohol group,
and lactate is formed.
• The hydrogen (and energy) required for this reaction
is supplied by NADH and H+, producing NAD+.
• The NAD+ produced funnels back into glycolysis to
oxidize more glyceraldehyde-3-phosphate (step 6),
providing a small amount of ATP. This reaction
occurs in the cytosol.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
• Yeast converts pyruvate to ethanol under
anaerobic conditions.
• This process is called fermentation.
• In the preparation of alcoholic beverages, yeast
produces pyruvate from glucose in grape juices
and under low-oxygen conditions transforms
pyruvate into ethanol.
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12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
Fructose and Glycolysis
• Fructose is readily taken up in the muscle and liver.
• In the muscles, it is converted to fructose-6-phosphate,
entering glycolysis at step 3. In the liver, it is converted
to the trioses used in step 5.
• Fructose that enters a cell flows from reaction 5 to 10.
• Fructose uptake by the cells is not regulated by insulin:
all fructose in the bloodstream is forced into catabolism.
• Glycolysis is regulated at step 3. The triose products
created in the liver provide an excess of reactants that
create excess pyruvate and acetyl CoA that, if not
required for energy by the cells, is converted to fat.
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12.5 The Citric Acid Cycle—Central Processing
• During aerobic catabolism, glucose, amino acids, and
fatty acids funnel into and out of the citric acid cycle.
• The citric acid cycle degrades two-carbon acetyl groups
from acetyl CoA into CO2 and generates the high-energy
molecules NADH and FADH2.
• The initial reaction is a condensation reaction between
acetyl CoA and the four-carbon molecule oxaloacetate.
• The six-carbon citrate loses first one and then a second
carbon as CO2, forming the four-carbon succinyl CoA.
• These carbon–carbon bond-breaking reactions transfer
energy and produce NADH from the coenzyme NAD+.
• Succinyl CoA then runs through a set of reactions
regenerating oxaloacetate, and the cycle begins again.
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12.5 The Citric Acid Cycle—Central Processing
Reactions of the Citric Acid Cycle
• Reaction 1, Formation of Citrate: The acetyl group from
acetyl CoA (two carbons) combines with oxaloacetate
(four carbons), forming citrate (six carbons) and CoA.
• Reaction 2, Isomerization to Isocitrate: The –OH and
one of the –H atoms are swapped in citrate to form
isocitrate. This rearrangement is necessary because
isocitrate is oxidized in the next reaction.
• Reaction 3, First Oxidative Decarboxylation (Release
of CO2): An alcohol undergoes oxidation (two hydrogens
removed) to a ketone called -ketoglutarate, and NAD+
is reduced to NADH, accepting the proton and electrons
removed during the oxidation. The six-carbon isocitrate
is decarboxylated to the five-carbon -ketoglutarate.
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12.5 The Citric Acid Cycle—Central Processing
Reactions of the Citric Acid Cycle
• Reaction 4, Second Oxidative Decarboxylation: The
thiol group of CoA is oxidized (loses a hydrogen), and
another NAD+ is reduced to NADH. Alpha-ketoglutarate
(five carbons) is decarboxylated into a succinyl group
(four carbons). The CoA is bonded to the succinyl group,
thus producing succinyl CoA.
• Reaction 5, Hydrolysis of Succinyl CoA: Succinyl CoA
undergoes hydrolysis to succinate and coenzyme A. The
energy produced produces the high-energy nucleotide
guanosine triphosphate or GTP from GDP and Pi. GTP
is converted to ATP in the cell.
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12.5 The Citric Acid Cycle—Central Processing
Reactions of the Citric Acid Cycle
• Reaction 6, Dehydrogenation of Succinate: One
hydrogen is eliminated from each of the two central
carbons of succinate, forming a trans C=C bond, thus
producing fumarate. These two hydrogens reduce the
coenzyme FAD to FADH2.
• Reaction 7, Hydration of Fumarate: Water adds to the
trans double bond of fumarate as –H and –OH forming
malate.
• Reaction 8, Oxidation of Malate: As in reaction 3, the
secondary alcohol of malate is oxidized to a ketone
forming oxaloacetate, providing protons and electrons to
reduce the coenzyme NAD+ to NADH.
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12.5 The Citric Acid Cycle—Central Processing
• One turn of the citric acid cycle produces a net energy
output of three NADH, one FADH2, and one GTP
(which forms ATP).
• Two CO2 and one CoA also are produced.
• The net reaction for one turn of this eight-step cycle is
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12.5 The Citric Acid Cycle—Central Processing
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12.6 Electron Transport and Oxidative Phosphorylation
• Two ATP are produced in glycolysis and two ATP in the
citric acid cycle. Where is all the energy?
• NADH and FADH2 are produced in glycolysis (two NADH
per glucose), from pyruvate oxidation to acetyl CoA
(two NADH per glucose), and in the citric acid cycle
(six NADH and two FADH2 per glucose).
• High-energy reduced forms of the nucleotides transfer
electrons and hydrogens through the inner mitochondrial
membrane and to form H2O.
• The energy generated as a result of this process is used
to drive the reaction of ADP to form ATP.
• This is called oxidative phosphorylation.
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12.6 Electron Transport and Oxidative Phosphorylation
• Mitochondria are the ATP factories of the cell.
• Reduced nucleotides from the citric acid cycle are produced
here, and their energy upon oxidation is used to generate
ATP.
• The reactions of the citric acid cycle occur in the matrix of the
mitochondria. The reduced nucleotides, NADH and FADH2,
begin their journey through the inner membrane here.
• Enzyme complexes I through V are embedded in the inner
membrane of the mitochondria and electron carriers that
transport the electrons and protons of NADH and FADH2
through the inner mitochondrial membrane.
• Two of the electron carriers, coenzyme Q and cytochrome c,
are not firmly attached to any one complex and shuttle
electrons between the complexes.
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12.6 Electron Transport and Oxidative Phosphorylation
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12.6 Electron Transport and Oxidative Phosphorylation
• Complex I, NADH Dehydrogenase: At complex I,
NADH enters electron transport. During its oxidation,
two electrons and two protons are transferred to the
electron transporter coenzyme Q, reducing its two
ketone groups to alcohols (see figure at left). NAD+ is
regenerated and returns to a catabolic pathway as in the
citric acid cycle. The overall reaction at complex I is
NADH + H+ + Q → NAD + + QH2
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12.6 Electron Transport and Oxidative Phosphorylation
• Complex II, Succinate Dehydrogenase: FADH2 enters
electron transport after the reduced nucleotide is
produced in the conversion of succinate to fumarate in
the citric acid cycle. Two electrons and two protons from
FADH2 are also transferred to coenzyme Q to yield QH2.
FADH2 + Q → FAD + QH2
• Complex III, Coenzyme Q—Cytochrome c Reductase:
At complex III, the reduced coenzyme Q (QH2) molecules
are reoxidized to ubiquinone (Q), and the electrons pass
through a series of electron acceptors until they arrive at
cytochrome c, which moves the electron from complex III
to complex IV.
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12.6 Electron Transport and Oxidative Phosphorylation
• Complex IV, Cytochrome c Oxidase:
At complex IV, single electrons are transferred
from cytochrome c through another set of
electron acceptors to combine with hydrogen
ions and oxygen (O2) to form water. This is the
final stop for the electrons:
4H+ + 4e− + O2 → 2H2O
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12.6 Electron Transport and Oxidative Phosphorylation
• Oxidative phosphorylation: The chemiosmotic model
links electron transport to the generation of a proton (H+)
gradient across the inner membrane.
• In this model, three of the complexes (I, III, and IV) span
the inner membrane and pump (relocate) protons out of
the matrix and into the intermembrane space as electrons
are shuttled through the complexes.
• The formation of the proton gradient across the inner
mitochondrial membrane provides the energy for ATP
synthesis.
• Protons move back into the matrix through a protein
complex, called complex V, or ATP synthase. As protons
flow back into the matrix through complex V, the resulting
release of energy drives the synthesis of ATP.
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12.6 Electron Transport and Oxidative Phosphorylation
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12.6 Electron Transport and Oxidative Phosphorylation
Thermogenesis
• If ATP cannot be produced, the energy that would have been
harnessed as ATP is released as heat. This is called thermogenesis.
• Some animals adapted to cold climates produce small organic
molecules called uncouplers, which uncouple electron transport and
oxidative phosphorylation and thereby assist in regulating their body
temperature through thermogenesis.
• These animals have a higher amount of a tissue called brown fat,
which appears brown due to the high concentrations of mitochondria
present. The cytochrome molecules present in mitochondria contain
an iron ion that is responsible for the brown color.
• Newborn babies have higher levels of brown fat than do adults
because newborns do not have much stored fat. Brown fat deposits
are located near major blood vessels that carry the warmed blood
through the body, allowing a newborn to generate more heat to warm
its body surface.
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12.7 ATP Production
Glycolysis: The oxidation of glucose produces two
NADH molecules and two ATP molecules.
• Glycolysis occurs in the cytosol, and electron
transport draws NADH from the matrix inside the
mitochondria.
• The two NADH from glycolysis must be shuttled into
the matrix to enter electron transport. This results in
the production of five ATP.
Oxidation of pyruvate: After glycolysis, the two
pyruvates enter the mitochondria, where they are
oxidized to produce two acetyl CoA, two CO2, and two
NADH. The oxidation of two pyruvates leads to the
production of five ATP.
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12.7 ATP Production
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12.8 Other Fuel Choices
• If glycogen and glucose are not available, cells can
oxidize fatty acids to acetyl CoA through beta
oxidation (β oxidation).
• β oxidation includes four reactions that convert the
–CH2– of the  carbon to a  ketone.
• Once this ketone is formed, the two-carbon acetyl
group splits from the fatty acyl carbon chain. One
cycle of β oxidation yields one FADH2 and one
NADH.
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12.8 Other Fuel Choices
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12.8 Other Fuel Choices
Reaction 1, Oxidation (Dehydrogenation). The first reaction
removes one hydrogen from the alpha and beta carbons, and a
double bond is formed. These hydrogens are transferred to FAD
to form FADH2.
Reaction 2, Hydration. In reaction 2, water is added to the
 and β carbon double bond as –H and –OH, respectively.
Reaction 3, Oxidation (Dehydrogenation). The alcohol formed
on the β carbon is oxidized to a ketone. As we have seen before
in the citric acid cycle, the hydrogen from the alcohol reduces
NAD+ to NADH.
Reaction 4, Removal of Acetyl CoA. In the fourth reaction of
the cycle, the bond between the  and β carbon is broken and
a second CoA is added, forming an acetyl CoA and a fatty acyl
CoA shortened by two carbons. The fatty acyl CoA can be run
through the cycle again.
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12.8 Other Fuel Choices
Ketosis
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•
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In the absence of carbohydrates, the body breaks down body fat
through  oxidation to continue ATP production.
The liver will produce glucose from pyruvate through gluconeogenesis
for tissues like the brain.
The oxidation of large amounts of fatty acids can cause acetyl CoA
to accumulate in the liver. When accumulation occurs, the two carbon
acetyl units condense in the liver, forming the four-carbon ketone
molecules β-hydroxybutyrate and acetoacetate and the molecule
acetone. These are collectively referred to as ketone bodies.
Ketosis occurs when an excessive amount of ketone bodies is present
in the body.
Because two of the ketones are carboxylic acids, the excessive
formation of ketone bodies can cause metabolic acidosis.
Acetone vaporizes easily, giving someone suffering from ketosis an
odd, sweet-smelling breath upon exhalation similar to that of someone
who has been drinking alcohol.
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12.8 Other Fuel Choices
Energy from Amino Acids
• Amino acids produce nitrogen when metabolized in
the body. When excess protein is ingested, amino
acids must be degraded. The α-amino group of an
amino acid is removed, yielding an α-keto acid
through transamination.
• The α-keto acid can be converted into intermediates
for other metabolic pathways.
• The ammonium ions produced in this process must
be excreted from the body.
• The urea cycle converts ammonium ions (NH4+) into
urea, which can be excreted in the urine.
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12.8 Other Fuel Choices
Energy from Amino Acids
• Amino acids offer a way to replenish the intermediates
in the citric acid cycle.
• Amino acids like alanine, containing three carbons, can
enter the pathways as pyruvate.
• Amino acids with four carbons are converted to
oxaloacetate.
• Five-carbon amino acids are converted to  -ketoglutarate.
• Some amino acids can enter at more than one point
depending on cellular requirements.
• Amino acids provide only about 10% of the required
energy under normal conditions.
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12.8 Other Fuel Choices
Putting It Together: Linking the Pathways
• Degradation of food biomolecules begins with digestion.
• When the cell requires energy and oxygen is plentiful,
larger molecules are metabolized into smaller
metabolites that ultimately funnel into the citric acid
cycle, electron transport, and oxidative phosphorylation.
• Through anabolic pathways, larger molecules can be
synthesized from the smaller metabolites when
necessary.
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12.8 Other Fuel Choices
Putting It Together: Linking the Pathways
• The biological hydrolysis products can be shifted into
anabolic or catabolic pathways depending on the
requirements of the cell.
• Glucose can be degraded to acetyl CoA entering the
citric acid cycle to produce energy or be converted to
glycogen for storage in the cells.
• Amino acids provide nitrogen for anabolism of nitrogen
compounds, but their carbons can enter the citric acid
cycle as α-keto acids if necessary.
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12.8 Other Fuel Choices
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Chapter Twelve Summary
12.1 How Metabolism Works
• Metabolism refers to biochemical reactions occurring in the body.
• Catabolism refers to reactions that break down larger molecules
into smaller ones. Catabolic reactions are exergonic overall, and
the processes are oxidative.
• Anabolism refers to reactions that synthesize larger biological
molecules from smaller ones. Anabolic reactions are endergonic
overall and are reductive.
• In the body, biochemical reactions are usually grouped into
pathways. Biochemical reactions that produce energy tend to be
coupled to reactions requiring energy.
• Metabolic pathways tend to be compartmentalized in different
parts of the cell.
• The mitochondria are the energy-producing factories of the cells.
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Chapter Twelve Summary
12.2 Metabolically Relevant Nucleotides
•
Nucleotides are used in metabolism to transfer energy throughout the cell.
ATP is considered the main energy currency of the cell and produces
energy when hydrolyzed to ATP + Pi. NADH and FADH2 contain a
nucleotide portion and a vitamin portion. They are important coenzymes
that transport hydrogens and electrons in the cell. Coenzyme A (CoA)
also contains a nucleotide and vitamin portion. Each of these nucleotides
has a high-energy and a low-energy form.
12.3 Digestion—From Fuel Molecules to Hydrolysis Products
•
Food molecules are broken down into their component parts through
hydrolysis prior to absorption into the bloodstream.
•
Monosaccharides and amino acids travel directly through the bloodstream
to the cells for absorption.
•
Triglycerides are packaged into lipoproteins called chylomicrons for
delivery.
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Chapter Twelve Summary
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
•
Glycolysis is a series of 10 reactions that catabolizes a six-carbon glucose
molecule to two three-carbon pyruvate molecules. These 10 reactions yield
two ATP and two NADH molecules per glucose.
•
Under aerobic conditions, pyruvate is further oxidized in the mitochondria
to acetyl CoA.
•
In the absence of oxygen, pyruvate is reduced to lactate and regenerates
NAD+ so that glycolysis can continue.
•
Glucose can be stored as glycogen in the liver and muscle for later use.
•
Fructose can undergo glycolysis. In the liver, its catabolism is unregulated
and can produce excess products that become stored in the body as fat.
© 2014 Pearson Education, Inc.
Chapter Twelve Summary
12.5 The Citric Acid Cycle—Central Processing
• The citric acid cycle occurs in the mitochondrial matrix
and combines acetyl CoA (two carbons) with
oxaloacetate (four carbons), producing citric acid.
• Citric acid undergoes several oxidations, decarboxylation,
dehydrogenation and a hydration yielding two CO2, GTP,
three NADH, and one FADH2. The cycle regenerates
oxaloacetate to begin again. GTP readily converts to ATP
in the cell.
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Chapter Twelve Summary
12.6 Electron Transport and Oxidative Phosphorylation
• The reduced nucleotides NADH and FADH2 become oxidized,
transporting H+ and electrons through a series of enzyme complexes
in the inner mitochondrial membrane.
• The final electron acceptor in this process is O2, which combines with
H+ to form H2O. The complexes act as proton pumps, moving
protons from the matrix to the inner membrane space during electron
transport.
• This produces an electrochemical gradient. The protons can return to
the matrix through complex V, ATP synthase, which generates ATP.
• This process is known as oxidative phosphorylation.
• When ATP and proton transport are uncoupled, the energy in the
proton gradient is released as heat, which assists in maintaining
body temperature in a process called thermogenesis.
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Chapter Twelve Summary
12.7 ATP Production
• For every four H+ pumped into the inner membrane space and
returned to the matrix, one ATP can be synthesized.
• The oxidation of one NADH provides enough energy to synthesize
2.5 ATP. One FADH2 provides energy to synthesize 1.5 ATP.
• Under aerobic conditions, the complete oxidation of one molecule of
glucose produces a total of 32 ATP.
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Chapter Twelve Summary
12.8 Other Fuel Choices
• Fatty acids produce ATP when glucose supplies are low. Fatty acids
link to coenzyme A, forming activated fatty acyl CoA, which is
transported to the mitochondria for catabolism in a reaction cycle
called β oxidation.
• Fatty acyl CoA is oxidized, producing a new fatty acyl CoA that is
two carbons shorter and one molecule of acetyl CoA. Each turn
of β oxidation produces one NADH and one FADH2.
• High levels of acetyl CoA in the cell activate the ketogenesis
pathway, forming ketone bodies that can lead to ketosis and acidosis.
• Amino acids can produce ATP when other fuel supplies are low and
the cell does not require other nitrogen-containing compounds.
• When amines are removed from amino acids as ammonium ions,
they are converted to urea for excretion.
• The carbons from amino acids can feed into oxidative catabolism as
different intermediates depending on the amino acid.
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Chapter Twelve Study Guide
12.1 Overview of Metabolism
– Distinguish catabolism from anabolism.
– Identify reactions as catabolic or anabolic.
– Name the parts of a cell associated with metabolism.
12.2 Metabolically Relevant Nucleotides
– Identify the metabolically relevant nucleotides.
– Distinguish the low-energy and high-energy forms of the relevant
nucleotides.
12.3 Digestion—From Food Molecules to Hydrolysis
Products
– Compare digestion of carbohydrates, lipids, and proteins.
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Chapter Twelve Study Guide
12.4 Glycolysis—From Hydrolysis Products to Common Metabolites
– Follow a molecule of glucose through the ten reactions of glycolysis.
– Discuss anaerobic and aerobic fates of pyruvate.
– Contrast glycolysis for glucose and fructose.
12.5 The Citric Acid Cycle—Central Processing
– Identify the reactions in the citric acid cycle.
– List the energy output in the citric acid cycle.
12.6 Electron Transport and Oxidative Phosphorylation
– Describe the function of each enzyme complex (I–IV) during
electron transport.
– Discuss the function of coenzyme Q and cytochrome c.
– Describe the production of ATP at complex V using the
chemiosmotic model.
© 2014 Pearson Education, Inc.
Chapter Twelve Study Guide
12.7 ATP Production
– Convert the number of reduced nucleotides produced (NADH,
FADH2) to a corresponding number of ATP.
– Calculate the number of ATP produced during the oxidative
catabolism of a molecule of glucose.
12.8 Other Fuel Choices
– Calculate the number of ATP produced from a saturated fatty
acid undergoing  oxidation.
– Describe the metabolic pathways of  oxidation, transamination,
and the urea cycle.
– Identify catabolic and anabolic pathways in the cell.
© 2014 Pearson Education, Inc.
Reaction Summary
© 2014 Pearson Education, Inc.
Reaction Summary
© 2014 Pearson Education, Inc.