Chapter 7
Lecture
Slides
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7.1 Where Is the Energy in Food?
• The energy for living is obtained by
breaking down the organic molecules
originally produced in plants
 the energy invested in building the organic
molecules is retrieved by stripping away
electrons and using them to make ATP
 this process is called cellular respiration
7.1 Where Is the Energy in Food?
• Eukaryotes produce the majority of their
ATP by harvesting electrons from the food
molecule glucose:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy
• The reactants are glucose and oxygen,
and the products are carbon dioxide,
water, and energy (heat or ATP)
7.1 Where Is the Energy in Food?
• Electrons pass from atoms or molecules in
many chemical reactions, such as those
involved in cellular respiration
 oxidation is when an atom or molecule loses
an electron
 reduction is when an atom or molecule gains
an elections
 these reactions always occur together
• called oxidation-reduction (redox) reactions
7.1 Where Is the Energy in Food?
• In many redox reactions, oxygen is a common electron
acceptor
 in cellular respiration, oxygen is the final electron acceptor
• Energy follows the electrons, such that the reduced form
of an organic molecule has a higher level of energy than
the oxidized form
Figure 7.2
Redox
reactions
7.1 Where Is the Energy in Food?
•
Cellular respiration takes place in two
stages
1. Glycolysis



occurs in the cytoplasm
does not require oxygen to generate ATP
a very ancient pathway
2. Krebs cycle



occurs within the mitochondrion
harvests energy-rich electrons through a cycle of
oxidation reactions
the electrons are passed to an electron transport chain
in order to power the production of ATP
Figure 7.3 An overview of cellular respiration
7.2 Respiration Without Oxygen:
Glycolysis
• Glycolysis is a sequence of reactions that form a
biochemical pathway
 in ten enzyme-catalyzed reactions, the six-carbon
sugar glucose is broken into two three-carbon
pyruvate molecules
 the breaking of the bonds yields energy that is used
to phosphorylate ADP to ATP
• this process is called substrate-level phosphorylation
• in addition, electrons and hydrogens are donated to NAD+ to
form NADH
 this is the only way organisms can derive energy from
food in the absence of oxygen
Essential Biological Process 7A: Glycolysis
Animation: How Glycolysis Works
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7.3 Respiration With Oxygen: the
Krebs Cycle
• When oxygen is available, the cell
harvests pyruvate’s energy in two steps:
 the first step is to oxidize pyruvate to form
acetyl-CoA
 the second step is to oxidize acetyl-CoA in the
Krebs cycle
7.3 Respiration With Oxygen: the
Krebs Cycle
• When pyruvate is oxidized, a single of its three
carbons is cleaved off by the enzyme pyruvate
dehydrogenase
 this carbon leaves as part of a CO2 molecule
 in addition, a hydrogen and electrons are removed
from pyruvate and donated to NAD+ to form NADH
 the remaining two-carbon fragment of pyruvate is
joined to a cofactor called coenzyme A (CoA)
 the final compound is called acetyl-CoA
Figure 7.4 Producing acetyl-CoA
Essential Biological Process 7B:
Transferring Hydrogen Atoms
Animation: How NAD+ Works
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7.3 Respiration With Oxygen: the
Krebs Cycle
• The fate of acetyl-CoA depends on the
availability of ATP in the cell
 if there is insufficient ATP, then the acetyl-CoA
heads to the Krebs cycle
 if there is plentiful ATP, then the acetyl-CoA is
diverted to fat synthesis for energy storage
7.3 Respiration With Oxygen: the
Krebs Cycle
•
The Krebs cycle occurs in mitochondria and is
a series of nine reactions that can be broken
down into three stages of oxidative respiration
1. Acetyl-CoA enters the cycle and binds to a fourcarbon molecule, forming a six-carbon molecule
2. Two carbons are removed as CO2 and their
electrons donated to NAD+. In addition, an ATP is
produced.
3. The four-carbon molecule is recycled and more
electrons are extracted, forming NADH and FADH2.
Essential Biological Process 7C: The Krebs Cycle
Animation: How the Krebs Cycle
Works
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7.3 Respiration With Oxygen: the
Krebs Cycle
• In the process of cellular respiration, the
glucose is entirely consumed
 the energy from its chemical bonds has been
transformed into
• four ATP molecules
• 10 NADH electron carriers
• 2 FADH2 electron carriers
7.4 Using the Electrons to Make
ATP
• NADH and FADH2 transfer their electrons to a
series of molecules embedded in the
mitochondrial membrane
 This series of membrane-associated molecules is
called the electron transport chain
 many of the proteins in the electron transport chain
operate as proton pumps, pumping protons into the
intermembrane space of the mitochondrion
 the last transport protein donates the electrons to
hydrogen and oxygen in order to form water
7.4 Using the Electrons to Make
ATP
• Mitochondria use chemiosmosis to make
ATP
 as the concentration of protons builds up in
the intermembrane space, the protons reenter the matrix via ATP synthase channels
 their passage powers the production of ATP
from ADP
Figure 7.5 The electron transport chain
Animation: Electron Transport
System and ATP Synthesis
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Animation: Electron Transport
System and Formation of ATP
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7.5 Cells Can Metabolize Food
Without Oxygen
• In the absence of oxygen, fermentation, rather
than oxidation, is the pathway that occurs after
glycolysis
 in fermentation, an organic molecule accepts
hydrogen atoms from the NADH generated by
glycolysis, thus recycling NAD+ and allowing
glycolysis to continue.
• bacteria carryout more than a dozen kinds of fermentation
• eukaryotic cells are capable of only a few types of
fermentation
7.5 Cells Can Metabolize Food
Without Oxygen
• In ethanol fermentation, which occurs in
yeasts, NAD+ and ethanol are produced;
this is the source of ethanol for wine and
beer
• In lactic acid fermentation, carried out in
the muscle cells of animals, NAD+ and
lactate (lactic acid) are produced
Figure 7.7 Fermentation
7.6 Glucose Is Not the Only Food
Molecule
• Cells also get energy from foods other than
sugars
 proteins are first broken down into their individual
amino acids
• deamination reactions convert the amino acids into
molecules that can take part in the Krebs cycle
 fats are first broken down into fatty acids
• a process called -oxidation then converts the fatty acid
tails into acetyl groups that can be combined with
coenzyme A to form acetyl-CoA, which feeds into the
Krebs cycle
Figure 7.8 How cells obtain energy from foods
Inquiry & Analysis
• About how much of the
total lactic acid is
released after exercise
stops?
• Is this result consistent
with the hypothesis that
fish maintain blood pH
levels by delaying the
release of lactic acid from
muscles?
How Do Swimming Fish Avoid
Low Blood pH?