Lecture 5-Cellular Respiration
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Transcript of Lecture 5-Cellular Respiration
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Biology 11- Lecture 5- Cellular Respiration
1
Cellular Respiration
This handout is for lecture use only and not for commercial reproduction and
distribution.
Lightenergy
ECOSYSTEM
Photosynthesisin chloroplasts
CO2 + H2O
Cellular respirationin mitochondria
Organicmolecules
+ O2
ATP powers most cellular work
Heatenergy
ATP
energy flows into an ecosystem as sunlightand leaves as heat
photosynthesisgenerates O2 and organic molecules, which are used in cellular respiration
cells use chemical energy stored in organic molecules to regenerate ATP, which powers work
Catabolic Pathways and Production of ATP
fermentation - partial degradation of sugars that occurs without O2
aerobic respiration consumes organic molecules and O2 and yields ATP
anaerobic respiration consumes compounds other than O2
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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)
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Cellular respirationNAD+= Nicotinamide adenine dinucleotide; a coenzyme occurring in most living cells and used as an oxidizing or reducing agent in various metabolic processes.
NADH= The reduced form of NAD.
Photophosphorylation =addition of a phosphate group
Reduction =addition of hydrogen or electrons or removal of oxygen
Oxidation =removal of hydrogen or electrons or addition of oxygen.
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Biology 11- Lecture 5- Cellular Respiration
2
Oxidation of Organic Fuel Molecules
during cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced:
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
becomes oxidized
becomes reduced
Reduction =addition of hydrogen or electrons or removal of oxygen Oxidation =removal of hydrogen or electrons or addition of oxygen
Glucose Oxygen
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
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
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
DehydrogenaseReduction of
NAD+Oxidation of NADH
2 e + 2 H+ 2 e
+ H+
NAD+ + 2[H]
NADH
+
H+
H+
Nicotinamide(oxidized form)
Nicotinamide(reduced form)
The full name for NAD+, nicotinamide adenine dinucleotide, describes its structure: the molecule consists of two nucleotidesjoined together at their phosphate groups (shown in yellow). Nicotinamide is a nitrogenous base.
In oxidation reactions, each electron (e ) travels with a proton (H+ )-thus, as a hydrogen atom.
The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme NAD+ .As an electron acceptor, NAD+ functions as an oxidizing agent during respiration.
Enzyme called dehydrogenase removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (glucose, in this example), thereby oxidizing it. The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+. The other proton is released as a hydrogen ion (H+) into the surrounding solution:
The enzymatic transfer of 2 electrons and 1 proton (H+) from an organic molecule in food to NAD+ reduces the NAD+ to NADH; the second proton (H+) is released
NAD+ as an electron shuttle The Stages of Cellular Respiration
three stages:
Glycolysis (breaks down glucose into two molecules of pyruvate)
Citric acid cycle (completes the breakdown of glucose)
Oxidative phosphorylation (accounts for most of the ATP synthesis)
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Mitochondrion
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
Substrate-levelphosphorylation
ATP
Electrons carriedvia NADH and
FADH2
Oxidativephosphorylation
ATP
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosisGlycolysis, which occurs in the cytosol, begins
the degradation process by breaking glucose
into two molecules of a compound called pyruvate
Pyruvate enters the mitochondrion. where
the citric acid cycle oxidizes it to carbon dioxide.
NADH and electron carrier coenzyme called FADH transfer electrons derived from glucose to electron transport chains which are built into the inner mitochondrial membrane.
During oxidative phosphorylation, electron transportchains convert the chemical energy to a formused for ATP synthesis in the process calledchemiosmosis.
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Biology 11- Lecture 5- Cellular Respiration
3
Mode of ATP synthesis
Oxidative Phosphorylation
Substrate-level Phosphorylation
process that generates most of the ATP (powered by redox reactions)
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accounts for almost 90% of the ATP generated by cellular respiration
Oxidative Phosphorylation
Enzyme
ADP
P
Substrate
Enzyme
ATP+
Product
- forms a smaller amount of ATP in glycolysis and the citric acid cycle
Substrate-level Phosphorylation
- occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation
- "Substrate molecule" here refers to an organic molecule generated as an intermediate during the catabolism of glucose.
breaks down glucose into two molecules of pyruvate (splitting of sugar)
occurs in the cytoplasm and has two major phases:
energy investment phase
energy payoff phase
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1. Glycolysis
Energy investment phase
Glucose
2 ADP + 2 P 2 ATP used
formed4 ATP
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+
2 Pyruvate + 2 H2O
2 Pyruvate + 2 H2OGlucoseNet
4 ATP formed 2 ATP used 2 ATP
2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+
Glucose, a six-carbon sugar, is split into two three-carbon sugars. Smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.)
Cell actually spends ATP
Investment is repaid with
interest during the energy payoff phase, when ATP is produced by substrate-level phosphorylation
and NAD+ is reduced to NADH by electrons
released from the oxidation of glucose.
Net energy yield from glycolysis,
per glucose molecule
A Closer Look at Glycolysis.
Glucose enters the cell and is phosphorylated by the enzyme hexokinase, which transfers a phosphate group from ATP to the sugar.
Glucose-6-phosphate is converted to its isomer, fructose-6-phosphate.
Phosphofructokinase enzyme transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis. So far, 2 ATP have been used. With phosphate groups on its opposite ends, the sugar is now ready to be split in half. This is a key step for regulation of glycolysis.
Aldolase cleaves the sugar molecule into two different three carbon sugars: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. These two sugars are isomers of each other.
Isomerase catalyzes the reversible conversion between the two three-carbon sugars.
ENERGY INVESTMENT PHASE
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Biology 11- Lecture 5- Cellular Respiration
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ENERGY PAYOFF PHASE
This enzyme catalyzes two sequential reactions while it holds glyceraldehyde-3-phosphate in its active site. Sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH (a redox reaction).Coefficient 2 precedes all molecules in the energy payoff phase; these steps occur after glucose has been split into two three-carbon sugars
This step produces 2 ATP, since every product after the sugarsplitting step is doubled. This ATP debt has now been repaid. Glucose has been converted to two molecules of 3-phosphoglycerate, which is not a sugar.
This enzyme relocates the remaining phosphate group, preparing the substrate for the next reaction.
This enzyme causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP).
The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP, a second instance of substrate level phosphorylation. Since this step occurs twice for each glucose molecule, 2 ATP are produced.
A Closer Look at Glycolysis.
ENERGY INVESTMENT PHASE
ENERGY PAYOFF PHASE
Energy investment phase
Glucose
2 ADP + 2 P 2 ATP used
formed4 ATP
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+
2 Pyruvate + 2 H2O
2 Pyruvate + 2 H2OGlucoseNet
4 ATP formed 2 ATP used 2 ATP
2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+
Glucose, a six-carbon sugar, is split into two three-carbon sugars. Smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.)
Cell actually spends ATP
Investment is repaid with
interest during the energy payoff phase, when ATP is
produced by substrate-level phosphorylation
and NAD+ is reduced to NADH by electrons
released from the oxidation of glucose.
Net energy yield from glycolysis,
per glucose molecule
in the presence of O2, pyruvate enters the mitochondrion
pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis
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2. Citric acid cycle (Krebs cycle)
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Conversion of pyruvate to acetyl CoA
-Pyruvate is a charged molecule, so in eukaryotic cells it must enter the mitochondnon via active transport, with the help of a transport protein. A complex of several enzymes catalyzes the three numbered steps 1. Pyruvate's carboxyl group (-COO-) is removed and given off as a molecule of CO22. The remaining two-carbon fragment is oxidized, forming a compound named acetate. An enzyme transfers the
extracted electrons to NAD+, storing energy in the form of NADH.3. coenzyme A (CoA), a sulfur containing compound is attached to the acetate by an unstable bond that makes the
acetyl group very reactive.
This molecule is now ready to feed its acetyl group into the citric acid cycle for further oxidation
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Biology 11- Lecture 5- Cellular Respiration
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takes place within the mitochondrial matrix
oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
2. Citric acid cycle (Krebs cycle)Acetyl CoA
CoASH
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
CO2
-Keto-glutarate
CoASH
CO2NAD+
NADH
+ H+SuccinylCoA
CoASH
P i
GTP GDP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
CitricacidcycleH2O
Malate
Oxaloacetate
NADH
+H+
NAD+
1
2
3
4
5
6
7
8
has eight steps, each catalyzed by a specific enzyme
acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
the next seven steps decomposethe citrate back to oxaloacetate, making the process a cycle
Acetyl CoA
CoASH
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
CO2
-Keto-glutarate
CoASH
CO2NAD+
NADH
+ H+SuccinylCoA
CoASH
P i
GTP GDP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
CitricacidcycleH2O
Malate
Oxaloacetate
NADH
+H+
NAD+
1
2
3
4
5
6
7
8
Acetyl CoA adds its two-carbon acetyl group to oxaloacetate, producing
citrate
Citrate is converted to its isomer, isocitrate, by removal
of one water molecule and addition of another
Isocitrate is oxidized, reducing NAD+ to NADH.
Then the resulting compound loses a CO2molecule.
Another CO2 is lost, and the resulting compound is oxidized,
reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an
unstable bond.
CoA is displaced by a phosphate group, which is transferred to GDP forming GTP, a molecule with function similar to ATP that, in some cases, is used to generate ATP
Two hydrogens are transferred to FAD (flavin adenine dinucleotide),
forming FADH2 and oxidizing succinate.
Addition of a water molecule rearranges
bonds in the substrate
The substrate is oxidized, reducing
NAD+ to NADH and regenerating oxaloacetate
Pyruvate
NAD+
NADH
+ H+ Acetyl CoA
CO2
CoA
CoA
CoA
Citricacidcycle
FADH2
FAD
CO22
3
3 NAD+
+ 3 H+
ADP + P i
ATP
NADH
NADH and FADH2produced by the cycle relay electrons extracted from food to the electron transport chain
ATP is also produced
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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
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
3. Oxidative Phosphorylation
intermembrane space
inner mitochondrial membrane
mitochondrial matrix Electron transport and pumping of protons (H
+),which create an H+ gradient across the membrane
ATP synthesis powered by the flow of H+ back across the membrane
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Biology 11- Lecture 5- Cellular Respiration
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Electron Transport Chain
found in the cristae of the mitochondrion
NADH
NAD+2
FADH2
2 FAD
MultiproteincomplexesFAD
FeS
FMN
FeS
Q
FeS
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
IV
50
40
30
20
10 2
(from NADHor FADH2)
0 2 H+ + 1/2 O2
H2O
e
e
e
the carriers alternate reduced and oxidized states as they accept and donate electrons
electrons are passed through a number of proteins including cytochromes
electrons are finally passed to O2, forming H2O
Electron transport chain
NADH
NAD+2
FADH2
2 FAD
MultiproteincomplexesFAD
FeS
FMN
FeS
Q
FeS
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
IV
50
40
30
20
10 2
(from NADHor FADH2)
0 2 H+ + 1/2 O2
H2O
e
e
e
Electron transport chain
Electrons removed from glucose by NAD+, during glycolysis and the citric acid cycle, are
transferred from NADH to the first molecule of the electron transport chain in complex I. This molecule is a flavoprotein, called flavin
mononucleotide(FMN).
Flavoprotein returns to its oxidized form as it
passes electrons to an iron-sulfur protein (Fe-S in complex I)
Iron-sulfur protein then passes the
electrons to a compound called ubiquinone (Q)
Most of the remaining electron carriers between
ubiquinone and oxygen are proteins called cytochromes, each a
different protein with a slightly different electron-carrying heme group
Each oxygen atom also picks up a pair of hydrogen
ions from the aqueous solution, forming water.
Another source of electrons for the transport chain is FADH2, which adds
its electrons to the electron transport chain at complex II a lower energy level than NADH does
The last cytochrome of the chain, cyt a3, passes its
electrons to oxygen, which is very electronegative
electron transfer in the electron transport chain causes proteins to pump H+ ions from the intermembrane space to the mitochondrial matrix
H+ then moves back across the membrane, passing through channels in ATP synthase
Chemiosmosis: The Energy-Coupling Mechanism
Chemiosmosis - a process in which a proton gradient across a mitochondrial membrane and ATP synthesis pump metabolites across a membrane
- generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration.
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
Chemiosmosis: The Energy-Coupling Mechanism
-ATP synthase, the enzyme that actually makes ATP from ADP and inorganic phosphate.
- ATP synthase uses the energy of an existing ion gradient to power ATP synthesis.
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
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Chemiosmosis: The Energy-Coupling Mechanism
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Biology 11- Lecture 5- Cellular Respiration
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intermembrane space
inner mitochondrial membrane
mitochondrial matrix
NADH and FADH2 shuttle high-energy electrons extracted from food during glycolysis and the citric acid cycle to an electron transport chain built into the inner mitochondrial membrane. The gold arrows trace the transport of electrons, which finally pass to oxygen at the "downhill" end of the chain, forming water.
intermembrane space
inner mitochondrial membrane
mitochondrial matrix
Most of the electron carriers of the chain are grouped into four complexes. Two mobile carriers, ubiquinone (Q) and cytochrome c (Cyt c),move rapidly, ferrying electrons between the large complexes. As complexes I, III, and IV accept and then donate electrons, they pump protons from the mitochondrial matrix into the intermembrane space.
intermembrane space
inner mitochondrial membrane
mitochondrial matrix
During chemiosmosis, the protons flow back down their gradient via ATP synthase, which is built into the membrane nearby. The ATP synthase harnesses the protonmotive force to phosphorylate ADP forming ATP.
intermembrane space
inner mitochondrial membrane
mitochondrial matrix Electron transport and pumping of protons (H
+),which create an H+ gradient across the membrane
ATP synthesis powered by the flow of H+ back across the membrane
Mitochondrion
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
Substrate-levelphosphorylation
ATP
Electrons carriedvia NADH and
FADH2
Oxidativephosphorylation
ATP
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
An Accounting of ATP Production
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Biology 11- Lecture 5- Cellular Respiration
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Per molecule of GlucoseSubstrate-level phosphorylation: 2 ATP from glycolysis + 2 ATP (directly GTP) from Krebs cycle
Oxidative phosphorylation2 NADH+H+ from glycolysis: 2 1.5 ATP (if glycerol phosphate shuttle transfers hydrogen atoms) or 2 2.5 ATP (malate-aspartate shuttle)2 NADH+H+ from the oxidative decarboxylation of pyruvate and 6 from Krebs cycle: 8 2.5 ATP2 FADH2 from the Krebs cycle: 2 1.5 ATP
Altogether this gives 4 + 3 (or 5) + 20 + 3 = 30 (or 32) ATP per molecule of glucose
Fermentation and anaerobic respiration
glycolysis can produce ATP with or without O2(in aerobic or anaerobic conditions)
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Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate
Fermentation uses phosphorylation instead of an electron transport chain to generate ATP
Fermentation
consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis
two common types:
alcohol fermentation
lactic acid fermentation
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pyruvate is converted to ethanol in two steps, with the first releasing CO2
used in brewing, winemaking, and baking (yeast)
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Alcohol Fermentation
pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2
used to make cheese and yogurt (fungi and bacteria)
used by human muscle to generate ATP when O2 is scarce
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Lactic Acid Fermentation Fermentation vs Aerobic Respiration
both use glycolysis to oxidize glucose and other organic fuels to pyruvate
have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration
cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
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http://en.wikipedia.org/wiki/Substrate-level_phosphorylationhttp://en.wikipedia.org/wiki/Substrate-level_phosphorylationhttp://en.wikipedia.org/wiki/Substrate-level_phosphorylationhttp://en.wikipedia.org/wiki/Glycolysishttp://en.wikipedia.org/wiki/Krebs_cyclehttp://en.wikipedia.org/wiki/Oxidative_phosphorylationhttp://en.wikipedia.org/wiki/Oxidative_phosphorylationhttp://en.wikipedia.org/wiki/Pyruvate_decarboxylationhttp://en.wikipedia.org/wiki/Pyruvate_decarboxylation
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Biology 11- Lecture 5- Cellular Respiration
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Glucose
Glycolysis
Pyruvate
CYTOSOL
No O2 present:Fermentation
O2 present:
Aerobic cellularrespiration
MITOCHONDRION
Acetyl CoAEthanolor
lactateCitricacidcycle
Proteins Carbohydrates
Aminoacids
Sugars
Fats
Glycerol Fattyacids
Glycolysis
Glucose
Glyceraldehyde-3-
Pyruvate
P
NH3
Acetyl CoA
Citricacidcycle
Oxidativephosphorylation
The catabolism of various molecules from food.
- Glycolysis and the citric acid cycle connect to many other metabolic pathways.
- Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these molecules enter glycolysis orthe citric acid cycle at various points.
- Glycolysis and the citric acid cycle are catabolic funnels through which electrons from all kinds of organicmolecules flow on their exergonic fall to oxygen.