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Transcript of Living cells require energy from outside sources Energy flows into an ecosystem as sunlight and...
![Page 1: Living cells require energy from outside sources Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O 2 and organic.](https://reader036.fdocuments.us/reader036/viewer/2022062423/56649eb15503460f94bb7a29/html5/thumbnails/1.jpg)
• Living cells require energy from outside sources
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Ch 9 – Cellular Respiration: Harvesting Chemical Energy
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Fig. 9-2
Lightenergy
ECOSYSTEM
Photosynthesis in chloroplasts
CO2 + H2O
Cellular respirationin mitochondria
Organicmolecules+ O2
ATP powers most cellular work
Heatenergy
ATP
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• The breakdown of organic molecules is exergonic (releases energy)
• Fermentation is a partial degradation of sugars that occurs without O2 (anaerobic)
• Aerobic respiration consumes organic molecules and O2 and yields ATP
• Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
9.1: Catabolic pathways yield energy by oxidizing organic fuels
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• Cellular respiration includes both aerobic and anaerobic processes 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)
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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
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The Principle of Redox
• Reactions that transfer electrons are called oxidation-reduction or redox reactions
• In oxidation, a substance loses electrons, or is oxidized (LEO: Loss of Electrons = Oxidation)
– The electron donor = the “reducing agent”
• In reduction, a substance gains electrons, or is reduced (GER: Gain of Electrons = Reduction)
– The electron acceptor = the “oxidizing agent”
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becomes oxidized(loses electron)
becomes reduced(gains electron)
Na is the reducing agent; it becomes oxidized as it loses / donates its electron to Cl
Cl is the oxidizing agent; it becomes reduced as it receives / accepts the electron from Na
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In this redox reaction, electrons are not transferred but there has been a change in electron sharing in covalent bonds.
Reactants
becomes oxidized
becomes reduced
Products
Methane(reducing
agent)
Oxygen(oxidizing
agent)
Carbon dioxide Water
Loses electrons Accepts electrons
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becomes oxidized
becomes reduced
During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced:
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Dehydrogenase
• 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
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Fig. 9-4
Dehydrogenase
Reduction of NAD+
Oxidation of NADH
2 e– + 2 H+
2 e– + H+
NAD+ + 2[H]
NADH
+
H+
H+
Nicotinamide(oxidized form)
Nicotinamide(reduced form)
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• NADH passes the electrons to the electron transport chain (ETC)
• The ETC passes electrons in a series of steps, instead of one explosive reaction
• O2 pulls electrons down the chain in an energy-yielding tumble
– In aerobic respiration, O2 is the final electron acceptor in the ETC
• The energy yielded is used to regenerate ATPCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 9-5
Fre
e en
erg
y, G
Fre
e en
erg
y, G
(a) Uncontrolled reaction
H2O
H2 + 1/2 O2
Explosiverelease of
heat and lightenergy
(b) Cellular respiration
Controlledrelease ofenergy for
synthesis ofATP
2 H+ + 2 e–
2 H + 1/2 O2
(from food via NADH)
ATP
ATP
ATP
1/2 O22 H+
2 e–E
lectron
transp
ort
chain
H2O
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The Stages of Cellular Respiration: A Preview
• Cellular respiration 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)
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Fig. 9-6-1
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
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Fig. 9-6-2
Mitochondrion
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
Substrate-levelphosphorylation
ATP
Electrons carriedvia NADH and
FADH2
Citricacidcycle
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Fig. 9-6-3
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
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• The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions
• 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
BioFlix: Cellular RespirationBioFlix: Cellular Respiration
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Enzyme
ADP
PSubstrate
Enzyme
ATP+
Product
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9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
• Glycolysis (“splitting of sugar”) breaks down glucose (6C) into two molecules of pyruvate (3C each) -- animation
• Glycolysis occurs in the cytoplasm and has two major phases:
– Energy investment phase (put in 2 ATP)
– Energy payoff phase (make 4 ATP)
• Net gain of 2 ATP
• Also produces 2 NADH (electron carrier)
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Fig. 9-8
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+
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Site of Respiration
• Glycolysis occurs in the cytoplasm.
• The rest of aerobic respiration occurs in the mitochondrion
– The Krebs / Citric Acid cycle occurs in the mitochondrial matrix
– The ETC occurs in the mitochondrial membrane (cristae)
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Mitochondria: Chemical Energy Conversion
• Mitochondria are in nearly all eukaryotic cells
• They have a smooth outer membrane and an inner membrane folded into cristae
• The inner membrane creates two compartments: intermembrane space and mitochondrial matrix
• Cristae present a large surface area for enzymes that synthesize ATP
– Structure / function! Surface area!
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Fig. 6-17
Free ribosomesin the mitochondrial matrix
Intermembrane space
Outer membrane
Inner membraneCristae
Matrix
0.1 µm
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9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules
• In the presence of O2, pyruvate enters the mitochondrion
• Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA
– Prior to this, one C from pyruvate is removed as CO2
– CoA = coenzyme A (organic substance that helps enzymes function)
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Fig. 9-10
CYTOSOL MITOCHONDRION
NAD+ NADH + H+
2
1 3
Pyruvate
Transport protein
CO2Coenzyme A
Acetyl CoA
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• The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix
• The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn
– Remember, the Krebs cycle turns 2x for each glucose molecule broken down by glycolysis
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Fig. 9-11
Pyruvate
NAD+
NADH
+ H+Acetyl CoA
CO2
CoA
CoA
CoA
Citricacidcycle
FADH2
FAD
CO22
3
3 NAD+
+ 3 H+
ADP + P i
ATP
NADH
1C leaves as CO2
The remaining 2C compound joins with CoA to form acetyl CoA
2 CO2 are produced / released
3 NADH are produced
1 ATP is produced
1 FADH2 is produced
Oxaloacetate is regenerated for the next round
CoA is removed and the acetyl group enters the Krebs cycle
The acetyl group combines with oxaloacetate to form citrate
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• 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
• The NADH and FADH2 produced by the cycle relay electrons extracted from food to the ETC
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
How the Krebs Cycle Works (animation)
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9.4: 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
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The Pathway of Electron Transport
• The electron transport chain is in the cristae of the mitochondrion
• Most of the chain’s components are proteins, which exist in multiprotein complexes
• The carriers alternate between 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 – that energy is harnessed to make ATP
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Fig. 9-13
NADH
NAD+2FADH2
2 FADMultiproteincomplexesFAD
Fe•S
FMN
Fe•S
Q
Fe•S
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
IV
Fre
e en
erg
y (G
) r e
lat i
ve t
o O
2 (
kcal
/mo
l)
50
40
30
20
10 2
(from NADHor FADH2)
0 2 H+ + 1/2 O2
H2O
e–
e–
e–
Notice that electrons from NADH and FADH2 enter the ETC at different points; this is because the electrons in NADH have more energy than the electrons in FADH2
Oxygen is the final electron acceptor; when the electrons combine with oxygen, water is produced / released.
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• Electrons from NADH and FADH2 are passed through a number of proteins, including cytochromes, to O2
• The electron transport chain generates no ATP in and of itself
• The chain’s function is to break the large free-energy drop from food to O2 into smaller steps, releasing energy in manageable amounts
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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 channels in 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
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Fig. 9-14
INTERMEMBRANE SPACE
Rotor
H+
Stator
Internalrod
Cata-lyticknob
ADP+P ATP
i
MITOCHONDRIAL MATRIX
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
Protons diffuse back into the mitochondrial matrix through ATP synthase
This causes a conformational change in the ATP synthase molecule, catalyzing the formation of ATP
Animation
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Fig. 9-16
Protein complexof electroncarriers
H+
H+H+
Cyt c
Q
V
FADH2 FAD
NAD+NADH
(carrying electronsfrom food)
Electron transport chain
2 H+ + 1/2O2H2O
ADP + P i
Chemiosmosis
Oxidative phosphorylation
H+
H+
ATP synthase
ATP
21
Intermembrane space
Mitochondrial matrix
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Fig. 9-17
Maximum per glucose: About36 or 38 ATP
+ 2 ATP+ 2 ATP + about 32 or 34 ATP
Oxidativephosphorylation:electron transport
andchemiosmosis
Citricacidcycle
2AcetylCoA
Glycolysis
Glucose2
Pyruvate
2 NADH 2 NADH 6 NADH 2 FADH2
2 FADH2
2 NADHCYTOSOL Electron shuttles
span membrane
or
MITOCHONDRION
Cellular Respiration Summary
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9.5: Fermentation and anaerobic respiration enable cells to produce ATP withoutthe use of oxygen
• Most cellular respiration requires O2 to produce ATP
• Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP
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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
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Types of Fermentation
• Fermentation starts with glycolysis and continues with reactions that regenerate NAD+, which can be reused by glycolysis
• Two common types are alcohol fermentation and lactic acid fermentation
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• 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
Animation: Fermentation OverviewAnimation: Fermentation Overview
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Fig. 9-18a
2 ADP + 2 P i 2 ATP
Glucose Glycolysis
2 Pyruvate
2 NADH2 NAD+
+ 2 H+CO2
2 Acetaldehyde2 Ethanol
(a) Alcohol fermentation
2
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• 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
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Fig. 9-18b
Glucose
2 ADP + 2 P i 2 ATP
Glycolysis
2 NAD+ 2 NADH+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
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Fermentation and Aerobic Respiration Compared
• Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate
• The processes 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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• 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
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Fig. 9-19Glucose
Glycolysis
Pyruvate
CYTOSOL
No O2 present:Fermentation
O2 present:
Aerobic cellular respiration
MITOCHONDRION
Acetyl CoAEthanolor
lactateCitricacidcycle
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The Evolutionary Significance of Glycolysis
• Glycolysis occurs in nearly all organisms
• Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere
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• Catabolic pathways are versatile; they funnel electrons from many kinds of organic molecules (not just glucose!) into cellular respiration
• Glycolysis accepts a wide range of carbohydrates
• Proteins must first be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle
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9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways
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• 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
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Fig. 9-20Proteins
Carbohydrates
Aminoacids
Sugars
Fats
Glycerol Fattyacids
Glycolysis
Glucose
Glyceraldehyde-3-
Pyruvate
P
NH3
Acetyl CoA
Citricacidcycle
Oxidativephosphorylation
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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
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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
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Fig. 9-21Glucose
GlycolysisFructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphateInhibits
AMP
Stimulates
Inhibits
Pyruvate
CitrateAcetyl CoA
Citricacidcycle
Oxidativephosphorylation
ATP
+
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