Living cells require energy from outside Some animals, such as … · 2018. 9. 6. · Concept 9.5:...
Transcript of Living cells require energy from outside Some animals, such as … · 2018. 9. 6. · Concept 9.5:...
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Chapter 9 – Cellular Respiration Overview: Life Is Work
• Living cells require energy from outside
sources
• Some animals, such as the giant panda, obtain
energy by eating plants, and some animals
feed on other organisms that eat plants
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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
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Remember
Metabolism is the TOTAL Chemical activity
within an organism!
Starts at food that is broken into
macromolecules.
The breakdown of organic molecules is
exergonic
Overview: Metabolism
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Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels
• Several processes are central to cellular
respiration and related pathways
• 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 does not consume O2
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• 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)
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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 also called the Kreb
Cycle (completes the breakdown of glucose)
– Oxidative phosphorylation (accounts for
most of the ATP synthesis)
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Fig. 9-6-3
Mitochondrion
Substrate-level
phosphorylation
2 ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electrons
carried
via NADH
Substrate-level
phosphorylation
2 ATP
Electrons carried
via NADH and
FADH2
Oxidative
phosphorylation
30 ATP
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
- 2 ATP
For active transport
About 32 ATP maximum produced per glucose
2 Acetyl CoA
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Concept 9.2: 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
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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+
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Fig. 9-10
CYTOSOL MITOCHONDRION
NAD+ NADH + H+
2
1 3
Pyruvate
Transport protein
CO2 Coenzyme A Acetyl CoA
In the presence of O2, pyruvate enters the mitochondria
Pyruvate converts to acetyl CoA to start the citric acid
cycle (essentially, one C is cut off to make Pyruvate
turn into Acetyl CoA)
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• The citric acid cycle, (also
called the Krebs cycle),
takes place within the
mitochondrial matrix
• This cycle constitutes the
major source of energy in all
living organisms.
Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules
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Fig. 9-11
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 oxidizes
organic fuel
derived from
pyruvate
This ultimately
generates
• 1 ATP,
• 3 NADH,
• 1 FADH2
for each turn of
the cycle
Pyruvate is converted
to Acetyl CoA
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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, making the process a
cycle
• The NADH and FADH2 produced by the cycle
relay electrons extracted from food to the
electron transport chain
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Fig. 9-12-8
Acetyl CoA
CoA—SH
Citrate
H2O
Isocitrate NAD+
NADH
+ H+
CO2
-Keto- glutarate
CoA—SH
CO2 NAD+
NADH
+ H+ Succinyl CoA
CoA—SH
P i
GTP GDP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
Citric acid cycle H2O
Malate
Oxaloacetate
NADH
+H+
NAD+
1
2
3
4
5
6
7
8
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Concept 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 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
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Fig. 9-13
NADH
NAD+ 2
FADH2
2 FAD
Multiprotein
complexes FAD
Fe•S
FMN
Fe•S
Q
Fe•S
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
IV
50
40
30
20
10 2
(from NADH
or FADH2)
0 2 H+ + 1/2 O2
H2O
e–
e–
e–
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• 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
• The chain’s function is to break the large free-
energy drop from food to O2 into smaller steps
that release 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
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• 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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Fig. 9-16
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
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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 40% of the energy in a glucose molecule
is transferred to ATP during cellular respiration,
making about 36 to 38 ATP
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Fig. 9-17
Maximum per glucose: About 36 or 38 ATP
+ 2 ATP + 2 ATP + about 32 or 34 ATP
Oxidative phosphorylation: electron transport
and chemiosmosis
Citric acid cycle
2 Acetyl CoA
Glycolysis
Glucose 2
Pyruvate
2 NADH 2 NADH 6 NADH 2 FADH2
2 FADH2
2 NADH CYTOSOL Electron shuttles
span membrane
or
MITOCHONDRION
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Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the 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 consists of glycolysis plus
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
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Fig. 9-18a
2 ADP + 2 P i 2 ATP
Glucose Glycolysis
2 Pyruvate
2 NADH 2 NAD+
+ 2 H+ CO2
2 Acetaldehyde 2 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. 9-19
Glucose
Glycolysis
Pyruvate
CYTOSOL
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Acetyl CoA Ethanol or
lactate
Citric acid cycle
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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