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LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures byErin Barley
Kathleen Fitzpatrick
Cellular Respiration and Fermentation
Chapter 9
Overview: Life Is Work
• Living cells require energy from outside sources
• Some animals, such as the chimpanzee, obtain energy by eating plants, and some animals feed on other organisms that eat plants
© 2011 Pearson Education, Inc.
Figure 9.1
• Energy flows into an ecosystem as sunlight and leaves as heat
• Photosynthesis generates O2 and organic molecules, which are used in cellular respiration
• Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work
© 2011 Pearson Education, Inc.
Figure 9.2
Lightenergy
ECOSYSTEM
Photosynthesisin chloroplasts
Cellular respirationin mitochondria
CO2 H2O O2
Organicmolecules
ATP powersmost cellular workATP
Heatenergy
Catabolic Pathways and Production of ATP
• The breakdown of organic molecules is exergonic
• Fermentation is a partial degradation of sugars that occurs without O2
• Aerobic respiration consumes organic molecules and O2 and yields ATP
• Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2
© 2011 Pearson Education, Inc.
• Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration
• Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose
C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat)
© 2011 Pearson Education, Inc.
Redox Reactions: Oxidation and Reduction
• The transfer of electrons during chemical reactions releases energy stored in organic molecules
• This released energy is ultimately used to synthesize ATP
© 2011 Pearson Education, Inc.
The Principle of Redox
• Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions
• In oxidation, a substance loses electrons, or is oxidized
• In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)
© 2011 Pearson Education, Inc.
Figure 9.UN02
becomes oxidized
becomes reduced
Oxidation of Organic Fuel Molecules During Cellular Respiration
• During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced
© 2011 Pearson Education, Inc.
Figure 9.UN03
becomes oxidized
becomes reduced
Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
• In cellular respiration, glucose and other organic molecules are broken down in a series of steps
• Electrons from organic compounds are usually first transferred to NAD+, a coenzyme
• As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration
• Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP
© 2011 Pearson Education, Inc.
Figure 9.4
Nicotinamide(oxidized form)
NAD
(from food)
Dehydrogenase
Reduction of NAD
Oxidation of NADH
Nicotinamide(reduced form)
NADH
• NADH passes the electrons to the electron transport chain
• Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction
• O2 pulls electrons down the chain in an energy-yielding tumble
• The energy yielded is used to regenerate ATP
© 2011 Pearson Education, Inc.
Figure 9.5
(a) Uncontrolled reaction (b) Cellular respiration
Explosiverelease of
heat and lightenergy
Controlledrelease ofenergy for
synthesis ofATP
Fre
e en
erg
y, G
Fre
e en
erg
y, G
H2 1/2 O22 H 1/2 O2
1/2 O2
H2O H2O
2 H+ 2 e
2 e
2 H+
ATP
ATP
ATP
Electro
n tran
spo
rt
chain
(from food via NADH)
The Stages of Cellular Respiration: A Preview
• Harvesting of energy from glucose has three stages
– Glycolysis (breaks down glucose into two molecules of pyruvate)
– The citric acid cycle (completes the breakdown of glucose)
– Oxidative phosphorylation (accounts for most of the ATP synthesis)
© 2011 Pearson Education, Inc.
Figure 9.6-3
Electronscarried
via NADH
Electrons carriedvia NADH and
FADH2
Citricacidcycle
Pyruvateoxidation
Acetyl CoA
Glycolysis
Glucose Pyruvate
Oxidativephosphorylation:electron transport
andchemiosmosis
CYTOSOL MITOCHONDRION
ATP ATP ATP
Substrate-levelphosphorylation
Substrate-levelphosphorylation
Oxidative phosphorylation
• Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
• For each molecule of glucose degraded to CO2
and water by respiration, the cell makes up to 32 molecules of ATP
© 2011 Pearson Education, Inc.
Figure 9.7
Substrate
Product
ADP
PATP
Enzyme Enzyme
Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
• Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate
• Glycolysis occurs in the cytoplasm and has two major phases
– Energy investment phase
– Energy payoff phase
• Glycolysis occurs whether or not O2 is present
© 2011 Pearson Education, Inc.
Figure 9.9-4
Glycolysis: Energy Investment Phase
ATP ATPGlucose Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate
Dihydroxyacetonephosphate
Glyceraldehyde3-phosphate
Tostep 6
ADP ADP
Hexokinase Phosphogluco-isomerase
Phospho-fructokinase
Aldolase
Isomerase
12 3 4
5
Figure 9.9-9
Glycolysis: Energy Payoff Phase
2 ATP 2 ATP2 NADH
2 NAD + 2 H
2 P i
2 ADP
1,3-Bisphospho-glycerate
3-Phospho-glycerate
2-Phospho-glycerate
Phosphoenol-pyruvate (PEP)
Pyruvate
2 ADP2 2 2
2 H2O
Phospho-glycerokinase
Phospho-glyceromutase
Enolase Pyruvatekinase
67 8
910
Triosephosphate
dehydrogenase
Figure 9.9a
Glycolysis: Energy Investment Phase
ATPGlucose Glucose 6-phosphate
ADP
Hexokinase
1
Fructose 6-phosphate
Phosphogluco-isomerase
2
Figure 9.9b
Glycolysis: Energy Investment Phase
ATPFructose 6-phosphate
ADP
3
Fructose 1,6-bisphosphate
Phospho-fructokinase
4
5
Aldolase
Dihydroxyacetonephosphate
Glyceraldehyde3-phosphate
Tostep 6Isomerase
Figure 9.9c
Glycolysis: Energy Payoff Phase
2 NADH2 ATP
2 ADP 2
2
2 NAD + 2 H
2 P i
3-Phospho-glycerate
1,3-Bisphospho-glycerate
Triosephosphate
dehydrogenase
Phospho-glycerokinase
67
Figure 9.9d
Glycolysis: Energy Payoff Phase
2 ATP
2 ADP2222
2 H2O
PyruvatePhosphoenol-pyruvate (PEP)
2-Phospho-glycerate
3-Phospho-glycerate
89
10
Phospho-glyceromutase
Enolase Pyruvatekinase
After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules
• In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed
© 2011 Pearson Education, Inc.
Oxidation of Pyruvate to Acetyl CoA
• Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle
• This step is carried out by a multienzyme complex that catalyses three reactions
© 2011 Pearson Education, Inc.
Figure 9.10
Pyruvate
Transport protein
CYTOSOL
MITOCHONDRION
CO2 Coenzyme A
NAD + HNADH Acetyl CoA
1
2
3
• The citric acid cycle, also called the Krebs cycle or TCA cycle, completes the break down of pyrvate to CO2
• The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn
© 2011 Pearson Education, Inc.
The Citric Acid Cycle
Figure 9.11Pyruvate
NAD
NADH
+ HAcetyl CoA
CO2
CoA
CoA
CoA
2 CO2
ADP + P i
FADH2
FAD
ATP
3 NADH
3 NAD
Citricacidcycle
+ 3 H
• The citric acid cycle has eight steps, each catalyzed by a specific enzyme
• The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
• The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle
• The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain
© 2011 Pearson Education, Inc.
Figure 9.12-8
NADH
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Fumarate
Malate
Citricacidcycle
NAD
NADH
NADH
FADH2
ATP
+ H
+ H
+ H
NAD
NAD
H2O
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
7
8
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
• Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food
• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
© 2011 Pearson Education, Inc.
The Pathway of Electron Transport
• The electron transport chain is in the inner membrane (cristae) of the mitochondrion
• Most of the chain’s components are proteins, which exist in multiprotein complexes
• The carriers alternate reduced and oxidized states as they accept and donate electrons
• Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O
© 2011 Pearson Education, Inc.
Figure 9.13
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
2 e
H2O
NAD
Multiproteincomplexes
(originally from NADH or FADH2)
III
III
IV
50
40
30
20
10
0
Fre
e e
ner
gy
(G)
rela
tiv
e to
O2 (
kcal
/mo
l)
FMN
FeS FeS
FAD
Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
FeS
• Electrons are transferred from NADH or FADH2 to the electron transport chain
• Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2
• The electron transport chain generates no ATP directly
• It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts
© 2011 Pearson Education, Inc.
Chemiosmosis: The Energy-Coupling Mechanism
• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
• H+ then moves back across the membrane, passing through the proton, ATP synthase
• ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
• This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work
© 2011 Pearson Education, Inc.
Figure 9.14INTERMEMBRANE SPACE
Rotor
StatorH
Internalrod
Catalyticknob
ADP+P i ATP
MITOCHONDRIAL MATRIX
A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient.
A stator anchored in the membrane holds the knob stationary.
A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob.
Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP.
Figure 9.15
Proteincomplexof electroncarriers
(carrying electronsfrom food)
Electron transport chain
Oxidative phosphorylation
Chemiosmosis
ATPsynth-ase
I
II
III
IVQ
Cyt c
FADFADH2
NADH ADP P i
NAD
H
2 H + 1/2O2
H
HH
21
H
H2O
ATP
• The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis
• The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work
© 2011 Pearson Education, Inc.
Proton Motive Force
• Is an electrochemical gradient• Two gradients drive the protons from the
intermembrane space into the matrix– It is more negative inside the matrix than in the
intermembrane space, so there is electric potential across the membrane
• H+ is attracted to the opposite negative charge inside the matrix
– There is a chemical or pH gradient as well• Greater concentration of H+ in the intermembrane space
causes the protons to move to an area of lower concentration inside the matrix
• About 85% of the proton motive force is derived from the electric or charge gradient while approximately 15% comes from the chemical gradient
An Accounting of ATP Production by Cellular Respiration
• During cellular respiration, most energy flows in this sequence:
glucose NADH electron transport chain proton-motive force ATP
• About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP
© 2011 Pearson Education, Inc.
ATP Synthase
• Results from numerous experiments show that, on average, 3 protons have to pass through ATP synthase for 1 ATP to be synthesized
• Therefore, 9 protons would be required to produce 3 ATP, and we’ve seen that the electrons from NADH lead to the translocation of 10 protons into the intermembrane space
• But, it’s not quite that simple
ATP Synthase
• 3 protons produce 1 ATP, but the ATP has to be exported out of the mitochondria since most of them are used in the cytoplasm
• Adenine nucleotide translocase exchanges mitochondrial ATP4- for cytosolic ADP3-, but it results in a -1 net charge loss in the matrix
• Formation of ATP also requires the import of phosphate through a phosphate transporter
• Phosphate (H2PO4-) enters with H+ through an
electroneutral symporter• The cost of moving phosphate and ADP in and ATP out is
approximately equal to the influx of 1 H+
• Thus, synthesis of 1 ATP requires 4 protons
P/O Ratio
• Ratio of molecules phosphorylated to atoms of oxygen reduced
• 2 electrons are required to reduce a single atom of oxygen
• For each pair of electrons that pass through the ETC, 10 protons are translocated across the membrane through ATP synthase
• Since 4 protons are needed for each molecule of cytoplasmic ATP, the P/O ratio = 10/4 = 2.5– This applies to NADH, but the electrons involved
in succinate oxidation skip complex I, so 6/4 = 1.5
Figure 9.16
Electron shuttlesspan membrane
MITOCHONDRION2 NADH
2 NADH 2 NADH 6 NADH
2 FADH2
2 FADH2
or
2 ATP 2 ATP about 26 or 28 ATP
Glycolysis
Glucose 2 Pyruvate
Pyruvate oxidation
2 Acetyl CoACitricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
CYTOSOL
Maximum per glucose:About
30 or 32 ATP
Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen
• Most cellular respiration requires O2 to produce ATP
• Without O2, the electron transport chain will cease to operate
• In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP
© 2011 Pearson Education, Inc.
Types of Fermentation
• Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis
• Two common types are alcohol fermentation and lactic acid fermentation
© 2011 Pearson Education, Inc.
• In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2
• Alcohol fermentation by yeast is used in brewing, winemaking, and baking
© 2011 Pearson Education, Inc.
Figure 9.17
2 ADP 2 ATP
Glucose Glycolysis
2 Pyruvate
2 CO22
2 NADH
2 Ethanol 2 Acetaldehyde
(a) Alcohol fermentation (b) Lactic acid fermentation
2 Lactate
2 Pyruvate
2 NADH
Glucose Glycolysis
2 ATP2 ADP 2 Pi
NAD
2 H
2 Pi
2 NAD
2 H
• In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2
• Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt
• Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce
© 2011 Pearson Education, Inc.
Comparing Fermentation with Anaerobic and Aerobic Respiration
• All use glycolysis (net ATP =2) to oxidize glucose and harvest chemical energy of food
• In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis
• Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
© 2011 Pearson Education, Inc.
• Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
• Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration
• In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
© 2011 Pearson Education, Inc.
Figure 9.18Glucose
CYTOSOLGlycolysis
Pyruvate
No O2 present:Fermentation
O2 present: Aerobic cellular respiration
Ethanol,lactate, or
other products
Acetyl CoA
MITOCHONDRION
Citricacidcycle
The Evolutionary Significance of Glycolysis
• Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere
• Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP
• Glycolysis is a very ancient process
© 2011 Pearson Education, Inc.
The Versatility of Catabolism
• Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration
• Glycolysis accepts a wide range of carbohydrates
• Proteins must be digested to amino acids; after removal of the amino group, the carbon skeleton can be converted into intermediates of glycolysis or the citric acid cycle
© 2011 Pearson Education, Inc.
• Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA)
• Fatty acids are broken down by beta oxidation and yield acetyl CoA
• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate
© 2011 Pearson Education, Inc.
Figure 9.19CarbohydratesProteins
Fattyacids
Aminoacids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3 Pyruvate
Acetyl CoA
Citricacidcycle
Oxidativephosphorylation
Biosynthesis (Anabolic Pathways)
• The body uses small molecules to build other substances
• These small molecules may come directly from food, from glycolysis, or from the citric acid cycle
© 2011 Pearson Education, Inc.
Regulation of Cellular Respiration via Feedback Mechanisms
• Feedback inhibition is the most common mechanism for control
• If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down
• Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway
© 2011 Pearson Education, Inc.
Figure 9.20
Phosphofructokinase
Glucose
GlycolysisAMP
Stimulates
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Pyruvate
Inhibits Inhibits
ATP Citrate
Citricacidcycle
Oxidativephosphorylation
Acetyl CoA