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Chapter 7
How Cells Release
Chemical Energy
2
Two Main Metabolic Pathways
 Aerobic metabolic pathways (using oxygen) are used by most eukaryotic
cells
 Anaerobic metabolic pathways (which occur in the absence of oxygen)
are used by prokaryotes and protists in anaerobic habitats
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Aerobic Respiration
 In modern eukaryotic cells, most of the aerobic respiration pathway takes
place inside mitochondria
 Like chloroplasts, mitochondria have an internal folded membrane system
that allows them to make ATP efficiently
 Electron transfer chains in this membrane set up hydrogen ion gradients
that power ATP synthesis
 At the end of these chains, electrons are transferred to oxygen molecules
Overview of Carbohydrate Breakdown Pathways
4
energy
 Photoautotrophs make ATP during
photosynthesis and use it to
synthesize glucose and other
carbohydrates
 Most organisms, including
photoautotrophs, make ATP by
breaking down glucose and other
organic compounds
Photosynthesis
glucose
CO2
O2
H2O
Aerobic Respiration
energy
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Overview of Aerobic Respiration
 Three stages
 Glycolysis
 Acetyl-CoA formation and Krebs cycle
 Electron transfer phosphorylation (ATP formation)
C6H12O6 (glucose) + O2 (oxygen)
→
CO2 (carbon dioxide) + H2O (water)
 Coenzymes NADH and FADH2 carry electrons and hydrogen
Oxidation & reduction
Reduction
Oxidation
adding O
removing H
loss of electrons
releases energy
exergonic
removing O
adding H
gain of
electrons
stores energy
endergonic
oxidation
C6H12O6 +
6O2
 6CO2 + 6H2O + ATP
reduction
Aerobic Respiration
glucose
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In the Cytoplasm
2 ATP
Glycolysis
4 ATP (2
net)
2 NADH 2 pyruvate
Krebs
Cycle
6 CO2
2 ATP
8 NADH, 2 FADH2
oxygen
Electron Transfer
Phosphorylation
In the Mitochondrion
H2O
32 ATP
Figure 7-3 p119
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Aerobic Respiration vs.
Anaerobic Fermentation
 Aerobic respiration and fermentation both begin with glycolysis, which
converts one molecule of glucose into two molecules of pyruvate
 After glycolysis, the two pathways diverge
 Fermentation is completed in the cytoplasm, yielding 2 ATP per glucose
molecule
 Aerobic respiration is completed in mitochondria, yielding 36 ATP per glucose
molecule
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Glycolysis
Carbohydrate
breakdown pathways
start in the cytoplasm,
with glycolysis.
Fermentation
concludes in
cytoplasm.
In eukaryotes,
aerobic respiration
concludes inside
mitochondria.
Figure 7-4 p119
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Take-Home Message: How do cells access the
chemical energy in carbohydrates?
 Most cells convert the chemical energy of carbohydrates to chemical energy
of ATP by aerobic respiration or fermentation
 Aerobic respiration and fermentation pathways start in cytoplasm, with
glycolysis
 Fermentation is anaerobic and ends in the cytoplasm
 Aerobic respiration requires oxygen. In eukaryotes, it ends in mitochondria
Glycolysis – Glucose Breakdown Starts
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 The reactions of glycolysis convert one molecule of glucose to two molecules of
pyruvate for a net yield of two ATP
 An energy investment of ATP is required to start glycolysis
 Two ATP are used to split glucose and form 2 PGAL, each with one phosphate
group
 Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH
 Four ATP are formed by substrate-level phosphorylation (net 2 ATP)
 Glycolysis ends with the formation of two three-carbon pyruvate molecules
ATP-Requiring Steps
1 An enzyme (hexokinase) transfers a phosphate group
from ATP to glucose, forming glucose-6-phosphate.
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2 A phosphate group from a second ATP is transferred to
the glucose-6phosphate. The resulting molecule is unstable,
and it splits into two three carbon molecules. The molecules
are interconvertible, so we will call them both PGAL
(phosphoglyceraldehyde). Two ATP have now been invested
in the reactions.
ATP-Generating Steps
3 Enzymes attach a phosphate to the two PGAL, and
transfer two electrons and a hydrogen ion from each PGAL
to NAD+. Two PGA (phosphoglycerate) and two NADH are the
result.
4 Enzymes transfer a phosphate group from each PGA to
ADP. Thus, two ATP have formed by substrate-level
phosphorylation. The original energy investment of two ATP
has now been recovered.
5 Enzymes transfer a phosphate group from each of two
intermediates to ADP. Two more ATP have formed by
substrate-level phosphorylation. Two molecules of pyruvate
form at this last reaction step.
6 Summing up, glycolysis yields two NADH, two ATP (net),
Stepped Art
and two pyruvate for each glucose molecule. Depending on
the type of cell and environmental conditions, the pyruvate
may enter the second stage of aerobic respiration or it may
be used in other ways, such as in fermentation.
Figure 7-5 p121
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Take-Home Message:
What is glycolysis?
 Glycolysis is the first stage of carbohydrate breakdown in both aerobic
respiration and fermentation
 The reactions of glycolysis occur in the cytoplasm
 Glycolysis converts one molecule of glucose to two molecules of pyruvate,
with a net energy yield of two ATP; two NADH also form
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Second Stage of Aerobic Respiration
 The second stage of aerobic respiration completes the breakdown of
glucose that began in glycolysis
 Occurs in mitochondria
 Includes two sets of reactions: acetyl CoA formation and the Krebs cycle
(each occurs twice in the breakdown of one glucose molecule)
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Acetyl CoA Formation
 In the inner compartment of the mitochondrion, enzymes split pyruvate, forming acetyl
CoA and CO2 (which diffuses out of the cell)
 NADH is formed
The Krebs
Cycle
• Krebs cycle
• A sequence of enzyme-mediated reactions that break down 1
acetyl CoA into 2 CO2
• Oxaloacetate is used and regenerated
• 3 NADH and 1 FADH2 are formed
• 1 ATP is formed
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Second Stage of Aerobic Respiration
cytoplasm
outer
membrane
inner
membrane
matrix
The breakdown of 2
pyruvate to 6 CO2 yields 2
ATP and 10 reduced
coenzymes (8 NADH, 2
FADH2). The coenzymes will
carry their cargo of
electrons and hydrogen
ions to the third stage of
aerobic respiration.
Acetyl–CoA Formation and the Krebs Cycle
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1 An enzyme splits a pyruvate
coenzyme A NAD+ molecule into a
two-carbon acetyl group and CO2.
Coenzyme A binds the acetyl group
(forming acetyl–CoA). NAD+
combines with released hydrogen
ions and electrons,
forming NADH.
2 The Krebs cycle starts as one
carbon atom is transferred from
acetyl–CoA tooxaloacetate.
Citrate forms, and coenzyme
A is regenerated.
3 A carbon atom is
removed from an
intermediate and leaves the
cell as CO2. NAD+ combines
with released hydrogen ions
and electrons, forming
NADH.
4 A carbon atom is
removed from another
intermediate and leaves the
cell as CO2, and another
NADH forms.
Pyruvate’s three carbon
atoms have now exited the
cell, in CO2.
8 The final steps
of the Krebs cycle
regenerate
oxaloacetate.
Krebs
Cycle
7 NAD+ combines
with hydrogen ions
and electrons,
forming NADH.
6 The coenzyme
FAD combines with
hydrogen
ions and electrons,
forming FADH2.
5 One ATP forms
by substrate-level
phosphorylation.
Stepped Art
Figure 7-7 p123
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Take-Home Message: What happens during
the second stage of aerobic respiration?
 The second stage of aerobic respiration, acetyl–CoA formation and the
Krebs cycle, occurs in the inner compartment (matrix) of mitochondria
 The pyruvate that formed in glycolysis is converted to acetyl–CoA and CO2;
the acetyl–CoA enters the Krebs cycle, which breaks it down to CO2
 For two pyruvate molecules broken down in the second-stage reactions,
two ATP form, and ten coenzymes (eight NAD+; two FAD) are reduced
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Aerobic Respiration’s Big Energy Payoff
 Many ATP are formed during the third and final stage of aerobic respiration
 Electron transfer phosphorylation
 Occurs in mitochondria
 Results in attachment of phosphate to ADP to form ATP
 Coenzymes NADH and FADH2 donate electrons and H+ to electron transfer chains
 Active transport forms a H+ concentration gradient in the outer mitochondrial
compartment
 H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP
 Finally, oxygen accepts electrons and combines with H+, forming water
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Electron Transfer Phosphorylation
Summary: The
Energy Harvest
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• Typically, the
breakdown of
one glucose
molecule
yields 36 ATP
• Glycolysis: 2
ATP
• Acetyl CoA
formation
and Krebs
cycle: 2 ATP
• Electron
transfer
phosphorylat
ion: 32 ATP
Figure 7-9 p125
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Take-Home Message: What happens during
the third stage of aerobic respiration?
•
In electron transfer phosphorylation, energy released by electrons flowing
through electron transfer chains is captured in the attachment of phosphate to
ADP; a typical net yield of aerobic respiration is thirty-six ATP per glucose
•
The reactions begin when coenzymes that were reduced in the first and second
stages of reactions deliver electrons and hydrogen ions to electron transfer
chains in the inner mitochondrial membrane
•
Energy released by electrons as they pass through electron transfer chains is
used to pump H+ from the mitochondrial matrix to the intermembrane space
•
The H+ gradient that forms across the inner mitochondrial membrane drives the
flow of hydrogen ions through ATP synthases, which results in ATP formation
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Fermentation
 Fermentation pathways break down carbohydrates without using oxygen
 The final steps in these pathways regenerate NAD+ but do not produce ATP
 Glycolysis is the first stage of fermentation
 Forms 2 pyruvate, 2 NADH, and 2 ATP
 Pyruvate is converted to other molecules, but is not fully broken down to CO2
and water
 Regenerates NAD+ but doesn’t produce ATP
 Provides enough energy for some single-celled anaerobic species
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Two Fermentation Pathways
 Alcoholic fermentation
 Pyruvate is split into acetaldehyde and CO2
 Acetaldehyde receives electrons and hydrogen from NADH, forming NAD+ and
ethanol
 Lactate fermentation
 Pyruvate receives electrons and hydrogen from NADH, forming NAD+ and
lactate
Glycolysis
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glucose
2 NAD+
2
2
4
pyruvate
Alcoholic
Fermentation
2 CO2
acetaldehyde
2
2 NAD+
ethanol
Figure 7-10a p127
Glycolysis
26
glucose
2 NAD+
2
2
4
pyruvate
Lactate
Fermentation
2 CO2
2
2 NAD+
lactate
Figure 7-11a p127
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Red and White Muscle Fibers
•
•
Red muscle fibers make ATP by aerobic respiration
•
Have many mitochondria
•
Myoglobin stores oxygen
•
Sustain prolonged activity
White muscle fibers make ATP by lactate fermentation
•
Have few mitochondria and no myoglobin
•
Sustain short bursts of activity
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Figure 7-11b p127
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Figure 7-11c p127
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Take-Home Message:
What is fermentation?
 ATP can form by carbohydrate breakdown in fermentation pathways, which are
anaerobic
 The end product of lactate fermentation is lactate. The end product of alcoholic
fermentation is ethanol
 Both pathways have a net yield of two ATP per glucose molecule; the ATP forms during
glycolysis
 Fermentation reactions regenerate the coenzyme NAD+, without which glycolysis (and ATP
production) would stop
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Alternative Energy Sources in Food
 Aerobic respiration can produce ATP from the breakdown of complex
carbohydrates, fats, and proteins
 As in glucose metabolism, many coenzymes are reduced, and the energy
of the electrons they carry ultimately drives the synthesis of ATP in electron
transfer phosphorylation
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Energy From Complex Carbohydrates
 Enzymes break starch and other complex carbohydrates down to monosaccharide
subunits
 Monosaccharides are taken up by cells and converted to glucose-6-phosphate, which
continues in glycolysis
 A high concentration of ATP causes glucose-6-phosphate to be diverted away from
glycolysis and into a pathway that forms glycogen
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Energy From Fats
 Enzymes cleave fats into glycerol and fatty acids
 Glycerol products enter glycolysis
 Fatty acids are converted to acetyl Co-A and enter
the Krebs cycle
 Compared to carbohydrates, fatty acid
breakdown yields more ATP per carbon atom
 When blood glucose level is high, acetyl CoA is
diverted from the Krebs cycle and into a
pathway that makes fatty acids
Energy from Proteins
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 Enzymes split dietary proteins into amino acid subunits, which are used to build proteins or
other molecules
 The amino group is removed and converted into ammonia (NH3), a waste product
eliminated in urine
 Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle forms, depending on the
amino acid
alanine (an amino acid)
pyruvate
Food
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Fats
fatty acids
2
acetyl–CoA
glycerol
3
Complex Carbohydrates
Proteins
glucose, other simple
sugars
1
amino acids
4
acetyl–CoA
PGAL
Glycolysis
NADH pyruvate
intermediate of
Krebs cycle
Krebs
Cycle
NADH, FADH2
Electron Transfer
Phosphorylation
Figure 7-12b p128
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Take-Home Message: Can organic molecules
other than glucose be used for energy?
 Complex carbohydrates, fats, and proteins can be oxidized in aerobic
respiration to yield ATP
 First the digestive system and then individual cells convert molecules in
food into intermediates of glycolysis or the Krebs cycle