Chapter 9 Gen Bio.pdf
Transcript of Chapter 9 Gen Bio.pdf
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Cellular Respiration Chapter 9
You should be able to:
1. Explain in general terms how redox reactions
are involved in energy exchanges
2. Name the three stages of cellular respiration;
for each, state the region of the eukaryotic
cell where it occurs, the products that result,
& the major intermediate steps
3. In general terms, explain the role of the
electron transport chain in cellular respiration
4. Explain where and how the respiratory
electron transport chain creates a proton
gradient
5. Distinguish between fermentation and
anaerobic respiration
6. Distinguish between obligate and facultative
anaerobes
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o Energy flows into an ecosystem as sunlight
and leaves as heat
o Photosynthesis (Chapter 10) generates O2
and organic molecules, which are used in
Cellular Respiration
o Cells use chemical energy stored in organic
molecules to regenerate ATP, which powers
work
Fig. 9-2
Light energy
ECOSYSTEM
Photosynthesis in chloroplasts
CO2 + H2O
Cellular respiration in mitochondria
Organic molecules
+ O2
ATP powers most cellular work
Heat energy
ATP
Catabolic Pathways & Production of ATP
o The breakdown of organic molecules is exergonic
o Aerobic respiration consumes organic molecules & O2 and yields ATP
o Anaerobic respiration is similar to aerobic respiration, but consumes compounds other than O2
o Fermentation is a partial degradation of sugars that occurs without O2
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o Cellular respiration includes both aerobic
and anaerobic respiration, but is most often
used to refer to aerobic respiration
o 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)
The Principle of Redox o Chemical reactions that transfer electrons
between reactants are called oxidation-reduction
reactions, or redox reactions
o In oxidation, a substance loses electrons, or is oxidized
o In reduction, a substance gains electrons, or is
reduced (the amount of positive (+) charge is
reduced)
LEO goes GER OIL RIG
Losing Electrons is Oxidation Oxidation Is Loss
Gaining Electrons is Reduction Reduction Is Gain
Fig. 9-UN1
becomes oxidized (loses electron)
becomes reduced (gains electron)
becomes oxidized
becomes reduced
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o The electron donor (Na, Xe-) is called the reducing agent (causes reduction, undergoes oxidation, loses electrons)
o The electron receptor (Cl, Y) is called the oxidizing agent (causes oxidation, undergoes reduction, gains electrons)
Redox: Electron Position
o Some redox reactions do NOT transfer electrons, but change the electron sharing in covalent bonds
oChange position in relation to the nuclei
o An example is the reaction between methane and O2
Fig. 9-3
Reactants
becomes oxidized
becomes reduced
Products
Methane (reducing
agent)
Oxygen (oxidizing
agent)
Carbon dioxide Water
When electrons are closer to electronegative nuclei, they have
less free energy
So when electrons are transferred to a more electronegative atom
(say, from C to O) energy is released-Exergonic Reaction
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Oxidation of Organic Fuel Molecules
During Cellular Respiration
During cellular respiration, the fuel (ex. the C in
glucose) is oxidized, and O2 is reduced:
becomes oxidized
becomes reduced
o In cellular respiration, glucose and other organic
molecules are broken down in a series of steps
o Electrons (with protons) from organic compounds are
usually first transferred to NAD+, a coenzyme
o As an electron acceptor, NAD+ is an oxidizing agent
during cellular respiration
o Each NADH (the reduced form of NAD+) represents stored
energy that is tapped to synthesize ATP
o NAD+ w/o extra e-, NADH w/ extra e-
e-
e-
NAD+
Stepwise Energy Harvest via NAD+ & the
Electron Transport Chain
NADH
e-
e-
NADH
NAD+
NAD+
Hi! I am NAD+
(I have 1 more
proton than
electron)
Hi! I am glucose
and I am going to
donate 2 e-s and 1 H+ to NAD+
NADH
Hi! I am NADH! I
am the reduced
form of NAD+ and
will carry my e-s to the ETC!
Dehydrogenase
The Electron Taxi Cab:
NAD+ to NADH
1 H+ drifts
away
Glucose total loss in the
presence of dehydrogenase
= 2 hydrogen atoms
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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)
Dehydrogenase
2 Electrons & Protons from Glucose
2 es & 1 H+ to NAD+
1 H+ drifts away
Dehydrogenase removes a pair of H
atoms (2 es and 2 protons) from glucose, oxidizing it. The enzyme
delivers the 2 es along w/ 1 proton to NAD+ and the other H+ is released as a
H+ ion
o NADH passes the electrons to the electron
transport chain
o Unlike an uncontrolled reaction, the electron
transport chain passes electrons in a series of
steps instead of one explosive reaction
o O2 pulls electrons down the chain in an energy-
yielding tumble
o The energy is used to regenerate ATP
Fig. 9-5
(a) Uncontrolled reaction
H2 + 1/2 O2
Explosive release of
heat and light energy
(b) Cellular respiration
Controlled release of energy for
synthesis of ATP
2 H+ + 2 e
2 H 1/2 O2
(from food via NADH)
1/2 O2 How we power
the space shuttle Same reaction but
occurs in stages
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Overall e- movement in cellular respiration
GlucoseNADHelectron transport chainO2
The Stages of Cellular Respiration: A Preview
o Cellular respiration has three stages:
o 1. Glycolysis (breaks down 1 glucose into 2
molecules of pyruvate)
o 2. The Citric Acid Cycle (completes the
breakdown of glucose)
o 3. Oxidative phosphorylation (accounts for
most of the ATP synthesis)
Mitochondrion
Substrate-level
phosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electrons
carried
via NADH
Substrate-level
phosphorylation
ATP
Electrons carried
via NADH and
FADH2
Oxidative
phosphorylation
ATP
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
PLEASE NOTE WHERE EACH OCCURS
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o The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions
o Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
o A smaller amount of ATP is formed in glycolysis & the citric acid cycle by substrate-level phosphorylation
oBioFlix: Cellular Respiration
Enzyme
ADP
P
Substrate
Enzyme
ATP +
Product
Substrate-Level Phosphorylation
oSome ATP is made by direct transfer of a phosphate group from
an organic substrate with more energy than ATP has to ADP, by
an enzyme
oThe ATP is more stable than the original molecule, so this is
spontaneous
oVersus oxidative phosphorylation which adds an inorganic
phosphate to ADP
Glycolysis harvests chemical energy
by oxidizing Glucose Pyruvate
o Glycolysis (splitting of sugar) breaks down Glucose into 2 molecules of Pyruvate
o Glycolysis occurs in the cytoplasm (cytosol) and
has two major phases:
1. Energy investment phase
ATP is used to phosphorylate Glucose (2x)
2. Energy payoff phase
Substrate-Level Phosphorylation (4x)
o Glycolysis occurs whether or not O2 is present
Glucose (6 C) 2 Pyruvate (3 C) + 2 ATP (net)+ 2 NADH
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Fig. 9-8
Energy investment phase
Glucose
2 ADP + 2 P 2 ATP used
formed 4 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 H2O Glucose Net
4 ATP formed 2 ATP used 2 ATP
2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+
Figure 9.9a
Glycolysis: Energy Investment Phase
ATP Glucose Glucose 6-phosphate
ADP
Hexokinase
1
Fructose 6-phosphate
Phosphogluco-
isomerase
2
Figure 9.9b
Glycolysis: Energy Investment Phase
ATP Fructose 6-phosphate
ADP
3
Fructose 1,6-bisphosphate
Phospho-
fructokinase
4
5
Aldolase
Dihydroxyacetone phosphate
Glyceraldehyde 3-phosphate
To step 6
Isomerase
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Figure 9.9c
Glycolysis: Energy Payoff Phase
2 NADH 2 ATP
2 ADP 2
2
2 NAD + 2 H
2 P i
3-Phospho-
glycerate
1,3-Bisphospho-
glycerate
Triose
phosphate
dehydrogenase
Phospho-
glycerokinase
6 7
Figure 9.9d
Glycolysis: Energy Payoff Phase
2 ATP
2 ADP 2 2 2 2
2 H2O
Pyruvate Phosphoenol-
pyruvate (PEP)
2-Phospho-
glycerate
3-Phospho-
glycerate
8 9
10
Phospho-
glyceromutase
Enolase Pyruvate
kinase
Preparation of Pyruvate for the
Citric Acid Cycle
CYTOSOL MITOCHONDRION
NAD+ NADH + H+
2
1 3
Pyruvate
Transport protein
CO2 Coenzyme A
Acetyl CoA
o In the presence of O2, pyruvate enters the
mitochondrion
o Before the citric acid cycle can begin, pyruvate
must be converted to acetyl CoA, which links the
cycle to glycolysis (note loss of CO2)
1. Pyruvates COO- which
is fully oxidized is
removed and given off
as CO2
2. 2C fragment is oxidized
forming acetate and e-
are transferred to
NAD+
3. CoA attaced to acetate
forming acetyl CoA
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o The citric acid cycle, (Krebs cycle), takes place within the mitochondrial matrix
o Oxidizes organic fuel derived from pyruvate
o Generates: 1 ATP, 3 NADH, and 1 FADH2 per turn (per Acetyl CoA)
Pyruvate
NAD+
NADH + H+
Acetyl CoA
CO2
CoA
CoA
CoA
Citric acid cycle
FADH2
FAD
CO2 2
3
3 NAD+
+ 3 H+
ADP +
P i
ATP
NADH
The Citric Acid Cycle completes the energy-
yielding oxidation of organic molecules
o The citric acid cycle has 8 steps, each catalyzed
by a specific enzyme
o The acetyl group of acetyl CoA joins the cycle by
combining with oxaloacetate, forming citrate
o The next 7 steps decompose the citrate back to
oxaloacetate, making the process a cycle
o The NADH and FADH2 (from FAD) produced by
the cycle relay electrons extracted from food to the
electron transport chain
Figure 9.12-1
1
Acetyl CoA
Citrate
Citric
acid
cycle
CoA-SH
Oxaloacetate
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Figure 9.12-2
1
Acetyl CoA
Citrate Isocitrate
Citric
acid
cycle
H2O
2
CoA-SH
Oxaloacetate
Figure 9.12-3
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Citric
acid
cycle
NADH
+ H
NAD
H2O
3
2
CoA-SH
CO2
Oxaloacetate
Figure 9.12-4
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Succinyl
CoA
Citric
acid
cycle
NADH
NADH
+ H
+ H
NAD
NAD
H2O
3
2
4
CoA-SH
CO2
CoA-SH
CO2
Oxaloacetate
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Figure 9.12-5
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Succinyl
CoA
Succinate
Citric
acid
cycle
NADH
NADH
ATP
+ H
+ H
NAD
NAD
H2O
ADP
GTP GDP
P i
3
2
4
5
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Figure 9.12-6
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Succinyl
CoA
Succinate
Fumarate
Citric
acid
cycle
NADH
NADH
FADH2
ATP
+ H
+ H
NAD
NAD
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Figure 9.12-7
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Succinyl
CoA
Succinate
Fumarate
Malate
Citric
acid
cycle
NADH
NADH
FADH2
ATP
+ H
+ H
NAD
NAD
H2O
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
7
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
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Figure 9.12-8
NADH
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Succinyl
CoA
Succinate
Fumarate
Malate
Citric
acid
cycle
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
We are through with the Glucose molecule
But.
Remember all those high-energy electrons taken by NAD+ & FAD?
During Oxidative Phosphorylation,
Chemiosmosis couples electron
transport to ATP synthesis
o NADH and FADH2 account for most of the
energy extracted from food
o 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
o The electron transport chain is in the cristae
of the mitochondrion
o Most of the chains components are proteins, which exist in multiprotein complexes
o The carriers alternate reduced and oxidized
states as they accept & donate electrons
o Electrons drop in free energy as they go
down the chain and are finally passed to O2,
forming H2O
o Electrons are transferred from
NADH or FADH2 (from
glycolysis and citric acid cycle)
to the electron transport chain
o Electrons are passed through a
number of proteins including
cytochromes (each with an iron
atom) to O2
o The electron transport chain
generates no ATP
oThe chains function is to break the large free-energy drop from food to O2 into smaller steps that release
energy in manageable amounts
Chemiosmosis:
The Energy-Coupling Mechanism
o Electron transfer in the electron
transport chain causes proteins to pump
H+ (protons) from the matrix intermembrane space (uphill)
o H+ then moves back across the
membrane (downhill), passing through channels in ATP synthase
o ATP synthase uses the exergonic flow
of H+ to drive phosphorylation of ATP
o This is an example of chemiosmosis,
the use of energy in a H+ gradient to
drive cellular work
INTERMEMBRANE SPACE
Rotor
H+ Stator
Internal rod
Cata- lytic knob
ADP +
P ATP i
MITOCHONDRIAL MATRIX
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o The energy stored in a H+ gradient across a
membrane couples the redox reactions of
the electron transport chain to ATP
synthesis
o The H+ gradient is referred to as a
proton-motive force, emphasizing its
capacity to do work
Protein complex of electron carriers
H+
H+ H+
Cyt c
Q
V
FADH2 FAD
NAD+ NADH
(carrying electrons from food)
Electron transport chain
2 H+ + 1/2O2 H2O
ADP + P i
Chemiosmosis
Oxidative phosphorylation
H+
H+
ATP
synthase
ATP
2 1
Notice flow of e- from NADH and FADH2 to H2O
1.Electron transport and pumping of the protons (H+) create H+ gradient across the
membrane
2.ATP synthesis powered by flow of H+ back across the membrane
Accounting of ATP Production by
Cellular Respiration
o During cellular respiration, most energy
flows in this sequence:
glucose NADH electron transport
chain proton-motive force ATP
o About 34% of the energy in a glucose
molecule is transferred to ATP during
cellular respiration, making about 32 ATP
o There are several reasons why the number
of ATP is not known exactly
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Figure 9.16
Electron shuttles span membrane
MITOCHONDRION 2 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 CoA
Citric acid cycle
Oxidative phosphorylation: electron transport
and chemiosmosis
CYTOSOL
Maximum per glucose: About
30 or 32 ATP
Fermentation & Anaerobic Respiration
enable ATP without oxygen
o Most cellular respiration requires O2 to
produce ATP
o Glycolysis can produce ATP with or without
O2 (in aerobic or anaerobic conditions)
o In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP
Anaerobic respiration uses an electron
transport chain with a different electron
acceptor (other than O2).
Example: Sulfate (SO42-)
Fermentation uses phosphorylation instead
of an electron transport chain to generate
ATP
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Types of Fermentation
There must be a place to drop off electrons from NADH, or all the NAD+ will be rapidly used up
o Fermentation = Glycolysis + reactions that regenerate NAD+, which can be reused by glycolysis
o 2 common types are alcohol fermentation and lactic acid fermentation
Animation: Fermentation Overview
o Alcohol fermentation--Pyruvate is converted to
ethanol in two steps, with the first releasing CO2
o Alcohol fermentation by yeast is used in brewing,
winemaking, and baking
o Lactic acid fermentation--Pyruvate is reduced
directly, forming lactate, with no release of CO2
oLactic acid fermentation by some fungi and bacteria is used to
make cheese and yogurt.
oHuman muscle cells use lactic acid fermentation to generate ATP
when O2 is scarce.
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Fermentation and Aerobic
Respiration Compared
o All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food
o In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis
o The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration
o Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
o Obligate anaerobes carry out fermentation
or anaerobic respiration and cannot survive
in the presence of O2
o Yeast and many bacteria are facultative
anaerobes--they can survive using either
fermentation or cellular respiration
Glucose
Glycolysis
Pyruvate
CYTOSOL
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Acetyl CoA Ethanol or
lactate
Citric acid cycle
In a
facultative
anaerobe,
pyruvate is
a fork in the
metabolic
road that
leads to two
alternative
catabolic
routes
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The Evolutionary Significance of
Glycolysis
o Glycolysis occurs in nearly all organisms
o Glycolysis probably evolved in ancient
prokaryotes before there was oxygen in the
atmosphere
o 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
o Glycolysis is a very ancient process
Glycolysis & the Citric Acid Cycle
connect to many other metabolic
pathways
o Gycolysis & the Citric Acid Cycle are major intersections to various catabolic and anabolic pathways
oMany other Macromolecules can feed into these pathways
oYou dont eat only Glucose
oMany things the cell and/or body needs are originally part of these pathways (or can be made from a chemical which is part of these pathways)
oBut are shuttled off before the next bioenergetic step
The Versatility of Catabolism
o Catabolic pathways funnel electrons from
many kinds of organic molecules into cellular
respiration
o Carbs: Glycolysis accepts a wide range of
carbohydrates
o Proteins must be digested to amino acids;
amino groups can feed glycolysis or the citric
acid cycle
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o Fats are digested to glycerol (used in glycolysis)
and fatty acids (used in generating acetyl CoA)
o Fatty acids are broken down by beta oxidation
and yield acetyl CoA
o An oxidized gram of fat produces more than twice
as much ATP as an oxidized gram of carbohydrate
Carbohydrate = 4 calories/gram
Fat = 9 calories/gram
Why it is so much harder to lose weight eating a high fat so much stored energy
Fig. 9-20
Proteins Carbohydrates
Amino acids
Sugars
Fats
Glycerol Fatty acids
Glycolysis
Glucose
Glyceraldehyde-3-
Pyruvate
P
NH3
Acetyl CoA
Citric acid cycle
Oxidative phosphorylation
Biosynthesis (Anabolic
Pathways)
o The body uses small molecules to build
other substances
o 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
o Feedback inhibition is the most common
mechanism for control
o If ATP concentration begins to drop,
respiration speeds up; when there is plenty
of ATP, respiration slows down
o Control of catabolism is based mainly on
regulating the activity of enzymes at
strategic points in the catabolic pathway
Fig. 9-21 Glucose
Glycolysis
Fructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphate Inhibits
AMP
Stimulates
Inhibits
Pyruvate
Citrate Acetyl CoA
Citric
acid cycle
Oxidative
phosphorylation
ATP
+
Figure 9.UN06
Inputs Outputs
Glucose
Glycolysis
2 Pyruvate 2 ATP 2 NADH
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Figure 9.UN07
Inputs Outputs
2 Pyruvate 2 Acetyl CoA
2 Oxaloacetate Citric
acid
cycle
2
2 6
8 ATP NADH
FADH2 CO2