Download Cell Respiration

Cellular Respiration:
Harvesting Chemical Energy
1
Cellular Respiration
•
Energy flows into an ecosystem
as sunlight and leaves as heat
• Photosynthesis generates
oxygen and organic molecules,
which are used in cellular
respiration
• All cells can harvest energy from
organic molecules to power
work
• To do this, they break down the
organic molecules and use the
energy that is released to make
ATP from ADP and phosphate
Light
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic + O
molecules 2
CO2 + H2O
Cellular respiration
in mitochondria
ATP
powers most cellular work
Heat
energy
2
Catabolic Pathways and Production of ATP
•
•
Heterotrophs live off the energy produced by
autotrophs - extracting energy from food via digestion
and catabolism
There are different catabolic pathways used in ATP
production:
• Fermentation - the partial degradation of sugars in
the absence of oxygen.
• Cellular respiration - A more efficient and
widespread catabolic process that consumes
oxygen as a reactant to complete the breakdown
of a variety of organic molecules.
3
Catabolic Pathways and Production of ATP
•
Although carbohydrates, fats, and proteins are all
consumed as fuel, it is helpful to trace cellular
respiration with the sugar glucose:
• The catabolism of glucose is exergonic with a
G of −686 kcal per mole of glucose.
• Some of this energy is used to produce ATP,
which can perform cellular work
C6H12O6 + 6O2
6CO2 + 6H2O + Energy (ATP + heat)
4
Redox Reactions
•
•
Catabolic pathways yield energy through the transfer
electrons from one reactant to another by oxidation and
reduction
Redox reactions
• In oxidation - A substance loses electrons, or is oxidized
• In reduction - A substance gains electrons, or is reduced
becomes oxidized
(loses electron)
Na
+
Cl
Na+
+
Cl–
becomes reduced
(gains electron)
5
Oxidation of Organic Fuel Molecules During
Cellular Respiration
•
Cellular respiration provides the energy for the cell
using the exergonic reaction:
becomes oxidized
C6H12O6 + 6O2
6CO2 + 6H2O + Energy ~686kcal/mole
becomes reduced
•
During cellular respiration glucose is oxidized and
oxygen is reduced
• Glucose oxidation is accomplished in a series of
steps
6
Glucose Oxidation
If electron transfer is not stepwise
• A large release of energy occurs
• As in the reaction of hydrogen and oxygen to form water
H2 + 1/2 O2
Free energy, G
•
Figure 9.5 A
Explosive
release of
heat and light
energy
(a) Uncontrolled reaction
H2O
7
Glucose Catabolism
•
Glucose catabolism is a series of redox reactions that release energy
by repositioning electrons closer to oxygen atoms.
The high energy electrons are stripped from glucose and picked up by
NAD+ and FAD.
2 e – + 2 H+
NAD+
H
•
Dehydrogenase
O
NH2
C
N+
O
CH2
O
O P O–
O
H
O P O– HO
O
CH2
H
OH
HO
Nicotinamide
(oxidized form)
H
HO
H
OH
Oxidation of NADH
H O
C
H
N
NH2
+ H
Nicotinamide
(reduced form)
N
H
N
Reduction of NAD+
NADH
NH2
N
O
+ 2[H]
(from food)
2 e– + H+
N
H
Figure 9.4
8
The Electron Transport Chain
•
2H
1/
+
2
O2
NADH
(from food via NADH)
2 H+ + 2 e–
50
Free energy (G) relative to O2 (kcal/mol)
•
Passes electrons in a series of steps instead of in one explosive reaction
Uses the energy from the electron transfer to form ATP
Eventually, the electrons, along with H+, are passed to a final acceptor.
Controlled
release of
energy for
synthesis of
ATP
ATP
Free energy, G
•
ATP
ATP
FADH2
40
FMN
I
Fe•S
Multiprotein
complexes
FAD
Fe•S II
Q
III
Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
20
Cyt a3
10
2 e–
1/
2
H+
2
O2
0
H2O
2 H+ + 1/2 O2
H2O9
Glucose Catabolism
•
•
•
If molecular oxygen (O2) is the final electron
acceptor, the process is called aerobic
respiration.
If some other inorganic molecule is the final
electron acceptor, the process is called
anaerobic respiration.
If an organic molecule is the final electron
acceptor, the process is called fermentation.
10
The Stages of Cellular Respiration
•
Respiration is a cumulative function of three
metabolic stages
•
Glycolysis - breaks down glucose into two
molecules of pyruvate
• The Citric Acid Cycle (Kreb’s) - completes the
breakdown of glucose
• Oxidative phosphorylation - driven by the
electron transport chain and Generates ATP
11
Cellular Respiration
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Glycolsis
Pyruvate
Glucose
Cytosol
ATP
Substrate-level
phosphorylation
Citric
acid
cycle
Oxidative
phosphorylation:
electron
transport and
chemiosmosis
Mitochondrion
ATP
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
12
Substrate Phosphorylation
•
Both glycolysis and the citric acid cycle can
generate ATP by substrate-level phosphorylation
Enzyme
Enzyme
ADP
Substrate
P
Product
+
ATP
PEP
P
P
Enzyme
P
ADP
Adenosine
13
Glycolysis
•
•
Glycolysis harvests energy by oxidizing
glucose to pyruvate
Glycolysis
• Means “splitting of sugar”
• Breaks down glucose into pyruvate
• Occurs in the cytoplasm of the cell
14
Glycolysis
•
Occurs in the cytoplasm of
the cell
• Results in the partial
breakdown of glucose
• Anaerobic – no oxygen is
used during glycolysis
• For each molecule of
glucose that passes
through glycolysis, the cell
nets two ATP molecules.
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidative
phosphorylation
ATP
15
Glycolysis
Glucose
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
ATP
•
Energy investment phase
Hexokinase
ADP
ATP/NADH Ledger
- 1 ATP
Glucose-6-phosphate
16
Glycolysis
Glucose
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
ATP
•
Total of 2 ATP invested
Hexokinase
ADP
ATP/NADH Ledger
- 2 ATP
Glucose-6-phosphate
Phosphoglucoisomerase
Fructose-6-phosphate
ATP
Phosphofructokinase
ADP
Fructose1, 6-bisphosphate
Aldolase
Isomerase
Dihydroxyacetone
phosphate
Glyceraldehyde3-phosphate
17
NAD+
Glycolysis
• Energy payoff
phase
NAD+
Triose phosphate
dehydrogenase
NADH
+ H+
Triose phosphate
dehydrogenase
NADH
+ H+
1, 3-Bisphosphoglycerate
1, 3-Bisphosphoglycerate
ADP
ADP
Phosphoglycerokinase
ATP
Phosphoglycerokinase
ATP
ATP/NADH Ledger
- 2 ATP
+ 2 ATP
+ 2 NADH
3-Phosphoglycerate
Phosphoglyceromutase
2-Phosphoglycerate
3-Phosphoglycerate
Phosphoglyceromutase
2-Phosphoglycerate
18
NAD+
Glycolysis
•
End-products of
glycolysis are 2
pyruvate molecules
NAD+
Triose phosphate
dehydrogenase
NADH
+ H+
Triose phosphate
dehydrogenase
NADH
+ H+
1, 3-Bisphosphoglycerate
1, 3-Bisphosphoglycerate
ADP
ADP
Phosphoglycerokinase
ATP
Phosphoglycerokinase
ATP
ATP/NADH Ledger
- 2 ATP
+ 4 ATP
+ 2 NADH
3-Phosphoglycerate
3-Phosphoglycerate
Phosphoglyceromutase
2-Phosphoglycerate
H2O
Enolase
Phosphoenolpyruvate
ADP
Phosphoglyceromutase
2-Phosphoglycerate
H2O
Phosphoenolpyruvate
ADP
Pyruvate kinase
ATP
Enolase
Pyruvate kinase
ATP
19
Pyruvate
Pyruvate
Glycolysis Summary
•
•
•
•
•
•
•
•
Occurs in the cytoplasm
Glucose converted to two
3-C chains
Anaerobic - no oxygen
2 ATP used, 4 ATP
produced
Inefficient - net yield only 2
ATPs
Not discarded by evolution
but used as starting point
for energy production
If no O2 - Fermentation
occurs
End products:
• 2 ATP
• Pyruvate (3 C)
• 2 x CO2
• 2 x NADH
Energy investment phase
Glucose
2 ADP + 2 P
2 ATP
used
4 ATP
formed
2 NADH
+ 2 H+
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
20
The Citric Acid (Krebs) Cycle
The Krebs cycle is named after Hans Krebs and is a metabolic event
that follows glycolysis. This process occurs in the fluid matrix of the
mitochondrion, uses the pyruvic acid from glycolysis and is aerobic. To
begin the Krebs cycle, pyruvic acid is converted to acetyl CoA.
21
Oxidation of Pyruvate
•
•
More energy can be extracted if oxygen is present
Within mitochondria, pyruvate is decarboxylated,
yielding acetyl-CoA, NADH, and CO2
MITOCHONDRION
CYTOSOL
NAD+
NADH
+ H+
Acetyl Co A
Pyruvate
Transport protein
CO2
Coenzyme A
22
The Citric Acid (Krebs) Cycle
•
•
•
•
Occurs in the
mitochondrial matrix
Aerobic – although O2 is
not used directly in this
pathway, it will not occur
unless enough is present
in the cell.
Main catabolic pathway
Acetyl-CoA is oxidized in
a series of nine reactions
23
Krebs Cycle
•
•
AcetylCoA reacts with
oxaloacetate using an
enzyme called citrate
synthase producing
citric acid.
Because of this, the
Krebs cycle is
sometimes called the
citric acid cycle.
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
Citric
acid
cycle
24
Krebs Cycle
•
•
•
The next 7 steps
decompose the
citrate back to
oxaloacetate,
Citric acid is
systematically
decarboxylated and
dehyrogenated in
order to use up the
acetyl groups that
were attached to the
oxaloacetate.
This allows
oxaloacetate and
CoA to be used in
the next cycle.
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
CO2
Citric
acid
cycle
NAD+
NADH
+ H+
Fumarate
a-Ketoglutarate
FADH2
NAD+
FAD
Succinate
GTP GDP
Pi
Succinyl
CoA
CO2
NADH
+ H+
ADP
ATP
25
Krebs Cycle
•
The NADH and
FADH2 produced
by the cycle relay
electrons extracted
from food to the
electron transport
chain
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
NADH
+ H+
H2O
NAD+
Oxaloacetate
Malate
Citrate
Isocitrate
CO2
Citric
acid
cycle
H2O
NAD+
NADH
+ H+
Fumarate
ATP/NADH Ledger
+ 2 ATP
+ 6 NADH
+ 2 FADH2
a-Ketoglutarate
FADH2
NAD+
FAD
Succinate
GTP GDP
Pi
Succinyl
CoA
CO2
NADH
+ H+
ADP
ATP
26
Krebs Cycle
27
ETC and Oxidative Phosphorylation
•
•
Occurs along the inner mitochondrial membrane (IMM) in the
cristae of the mitochondrion
NADH/FADH2 molecules carry electrons from glycolysis and the
citric acid cycle to the inner mitochondrial membrane, where they
transfer electrons to a series of membrane-associated proteins.
28
The Pathway of Electron Transport
•
•
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
water
NADH
50
FADH2
Free energy (G) relative to O2 (kcal/mol)
•
40
FMN
I
Multiprotein
complexes
FAD
Fe•S II
Fe•S
Q
III
Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
2 H+ + 1/2 O2
29
H2O
The Pathway of Electron Transport
•
The electron transport
chain generates no
ATP
The chain’s function is
to break the large freeenergy drop from food
to O2 into smaller steps
that release energy in
manageable amounts
NADH
50
FADH2
Free energy (G) relative to O2 (kcal/mol)
•
40
FMN
I
Multiprotein
complexes
FAD
Fe•S II
Fe•S
Q
III
Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
2 H+ + 1/2 O2
30
H2O
Electron Transport Phosphorylation
•
•
•
•
•
Electron transfer in the electron transport chain causes proteins to pump H+
from the mitochondrial matrix to the intermembrane space
The ETC uses energy from electrons to pump H+ across a membrane against
their concentration gradient - potential energy.
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
31
LE 9-15
Inner
mitochondrial
membrane
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidative
phosphorylation:
electron transport
and chemiosmosis
ATP
H+
H+
H+
H+
Intermembrane
space
Cyt c
Protein complex
of electron
carriers
Q
IV
III
I
ATP
synthase
II
Inner
mitochondrial
membrane
FADH2
NADH + H+
2H+ + 1/2 O2
H2O
FAD
NAD+
Mitochondrial
matrix
ATP
ADP + P i
(carrying electrons
from food)
H+
Electron transport chain
Electron transport and pumping of protons (H+),
Which create an H+ gradient across the membrane
Chemiosmosis
ATP synthesis powered by the flow
of H+ back across the membrane
Oxidative phosphorylation
32
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
Intermembrane
space
H+
H+
H+
H+
H+
H+
H+
H+
Rotor
Most of the ATP produced in cells is
made by the enzyme ATP synthase
Rod
•
•
The enzyme is embedded in the
membrane and provides a channel
through which protons can cross the
membrane down their concentration
gradient
The energy released causes the rotor
and the rod structures to rotate. This
mechanical energy is converted to
chemical energy with the formation of
ATP
Catalytic
head
ADP + Pi
ATP
H+
Mitochondrial matrix
33
LE 9-14
INTERMEMBRANE SPACE
H+
H+
H+
H+
H+
H+
A rotor within the
membrane spins
as shown when
H+ flows past
it down the H+
gradient.
H+
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.
H+
ADP
+
P
ATP
i
MITOCHONDRAL MATRIX
Three catalytic
sites in the
stationary knob
join inorganic
phosphate to
ADP to make
ATP.
34
Summary
of Glucose
Catabolism
35
Theoretical ATP Yield of Aerobic
Respiration
36
Catabolism of Proteins and Fats
•
Proteins are utilized by deaminating their amino acids, and
then metabolizing the product.
• Fats are utilized by beta-oxidation.
37
Regulating Aerobic Respiration
•
Control of glucose catabolism occurs at two key points in the catabolic
pathway.
• Glycolysis - phosphofructokinase
• Pyruvate Oxidation – pyruvate decarboxylase
38
Recycling NADH
•
•
•
As long as food molecules are available to
be converted into glucose, a cell can
produce ATP.
Continual production creates NADH
accumulation and NAD+ depletion.
NADH must be recycled into NAD+.
• Aerobic respiration - oxygen as electron
acceptor
• Fermentation - organic molecule
39
Anaerobic Respiration
•
•
•
•
•
•
Final electron acceptor is an inorganic
molecule other than oxygen
Some use NO3 -: E. coli
Some use SO42Important in nitrogen and sulfur cycles
ATP varies, less than 38
Only part of Krebs cycle & ETC used
40
Fermentation
•
In some cases, the high
energy electrons picked
up by NAD+ during
glycolysis are not donated
to an ETC.
•
Instead, NADH donates
its extra electrons and H+
directly to an organic
molecule, which serves as
the final electron acceptor.
41
Fermentation
•
•
•
•
•
•
Pyruvate converted to
organic product
NAD+ regenerated
Doesn’t require oxygen
Does not use Krebs cycle
or ETC
Organic molecule is final
electron acceptor
Produces 2 ATP max
42
Alcohol Fermentation
•
Occurs in single-celled fungi called yeast
• A terminal CO2 is removed from the pyruvic acid (3C)
produced during glycolysis, producing acetaldehyde (2C)
• Acetaldehyde accepts 2 e- and a H+ from NADH, producing
ethanol and NAD+
2Glucose
ADP + 2 P i
2 ATP
H
G
H C OH
L
CH3
Glucose
Y Glycolysis
2 Ethanol
2
NAD+
C
2 ATP
O
2 Pyruvate
L
22
NADH
Y +
2 NAD
NADH
2 CO2
H
+
O H
S
+2H
I
C O
O
C
CO2
S
CH3
O
C
2 Acetaldehyde
CH3
2 Pyruvic Acid
2 ADP
2 Ethanol
2 Acetaldehyde
43
Alcohol fermentation
Lactic Acid Fermentation
•
•
Used by most animal cells when O2 is not available
NADH donates 2 e- and a H+ directly to the pyruvate (3C)
produced during glycolysis, producing lactate (3C) and
NAD+
2 ADP + 2 P i
Glucose
Glucose
2 ADP
2 ATP
O
-
C
O
C
O
CH3
2 ATP
O–
G
C
O
Glycolysis
L
H C OH
Y
CH3
C
2 Lactate
2
NAD+
O
2 NAD+
2 NADH
2 CO2
L
+
+2H
Y
2 NADH 2 Pyruvate
S
I
S
2 Pyruvate
2 Lactate
Lactic acid fermentation
44
Fermentation
Alcoholic fermentation and lactic acid fermentation each
generate 2 ATP / glucose molecule compared to the
theoretical maximum of 36 ATP per glucose during aerobic
respiration.
45
46
47
Extra Slides
48
Glycolysis
ATP/NADH Ledger
- 2 ATP
The energy
investment phase
carbons
Energy
coupling
ATP  ADP + P: exergonic
Glu  Glu-6-P :endergonic
49
Glycolysis
ATP/NADH Ledger
-2ATP
+2 NADH
+2ATP
The energy
payoff phase
Redox reactions
Energy coupling
50
ATP/NADH Ledger
Glycolysis
-2ATP
+2ATP
+2ATP
+2 NADH
More energy
coupling
End-products of
glycolysis are 2 pyruvate
molecules
51
52
Flow of Energy in Living Things
•
•
•
Oxidation – Reduction
• Oxidation occurs when an atom or molecule loses an electron.
• Reduction occurs when an atom or molecule gains an electron.
Redox reactions occur because every electron that is lost by an atom
through oxidation is gained by some other atom through reduction.
During redox reactions, H+ are often transferred along with the
electrons.
Loss of electron (oxidation)
o
A
o
B
A
e–
B
–
+
A*
B*
Gain of electron (reduction)
Low energy
High energy
53
Electron carriers
•
•
•
•
•
•
Molecules that pick up electrons from substances being oxidized and
donate them to substances being reduced.
For example, during the breakdown of glucose :
• Enzymes remove 2 H atoms (2p and 2e) from glucose
• Both electrons and one proton are picked up by NAD+ to form
NADH
• The other proton is released as a hydrogen ion (H+)
Oxidized Form
NAD+
FAD
NADP+
2e- and 2H+
2e- and 2H+
2e- and 2H+
Reduced Form
NADH + H+
FADH2
NADPH + H+
54
Use of chemical cofactor (NAD+)
Energy-rich molecule
Enzyme
H
H
NAD+
NAD+
1. Enzymes that harvest
hydrogen atoms have a
binding site for NAD+
located near another
binding site. NAD+ and
an energy-rich
molecule bind to
the enzyme.
H
NAD+
2. In an oxidationreduction reaction,
a hydrogen atom
is transferred to
NAD+, forming
NADH.
Product
NAD H
NAD
H
3. NADH then
diffuses away and
is available to
other molecules.
55
Electron Transport
•
An electron transport chain (ETC) is a series of electron
carriers that are embedded in a membrane and that
pass electrons from one carrier to the next in a specific
sequence:
H+
H+
H+
C
Q
e–
e–
FADH2
NADH
+ H+
NAD+
FAD
2H+ +
½O2
H 2O
56
Electron Flow
57
Electron Transport
•
As electrons are passed from one carrier to the next in the
ETC, some of their energy is released. This energy can be
used to make ATP:
Electrons from food
e–
High energy
Energy for
synthesis of
ATP
Electron
transport
chain
Low energy
e–
Formation of water
58
ATP
•
Adenosine Triphosphate (ATP) is the energy
currency of the cell.
• Drives movement
• Used in endergonic reactions
59
ATP - Energy Coupling
•
ATP hydrolysis can be coupled to other reactions
Endergonic reaction: ∆G is positive, reaction
is not spontaneous
NH2
Glu
+
Glutamic
acid
NH3
Glu
Ammonia
Glutamine
∆G = +3.4 kcal/mol
Exergonic reaction: ∆ G is negative, reaction
is spontaneous
ATP
+
H2O
ADP +
Coupled reactions: Overall ∆G is negative;
together, reactions are spontaneous
P
∆G = - 7.3 kcal/mol
∆G = –3.9 kcal/mol
60