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Cellular Respiration:
Harvesting Chemical Energy
Chapter 9
Cellular Respiration
• Metabolic pathways that release stored
energy by breaking down complex
molecules are called catabolic pathways.
• There are two types of catabolic
processes- fermentation and cellular
respiration.
Catabolic Pathways
•Catabolic pathways yield energy by oxidizing organic fuels
•The breakdown of organic molecules is exergonic
Catabolic Pathways
Fermentation
Is the partial degradation of sugars that
occurs without the help of oxygen
Cellular Respiration
This is the most efficient and prevalent catabolic
pathway, in which oxygen is consumed as a
reactant along with organic fuel.
Energy flow and Recycling
• Energy
– Flows into an ecosystem as sunlight and leaves as heat
Light energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic
+ O2
CO2 + H2O
Cellular
molecules
respiration
in mitochondria
ATP
powers most cellular work
Heat
Energy
Respiration harvests energy stored in
organic molecules to generate ATP, Which
powers most cellular work. The waste
products are used by chloroplasts For
photosynthesis. Thus chemicals essential to
life are recycled.
Redox Reaction
• Example of Redox Reaction
becomes oxidized
(loses electron)
Na
+
Cl
Na+
+
Cl–
becomes reduced
(gains electron)
Some redox reactions
Do not completely exchange electrons
Change the degree of electron sharing in covalent bonds
Redox Reactions
•
Redox reactions
– Transfer electrons from one reactant to another by oxidation and
reduction
•
In oxidation
– A substance loses electrons, or is oxidized
In reduction
– A substance gains electrons, or is reduced
•
Cellular Respiration
• During cellular respiration
– Glucose is oxidized and oxygen is reduced
becomes oxidized
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
becomes reduced
Stepwise Harvest
•
•
•
Cellular respiration
–
Oxidizes glucose in a series of steps
Electrons from organic compounds
–
Are usually first transferred to NAD+, a coenzyme
NADH, the reduced form of NAD+
–
Passes the electrons to the electron transport chain
•
If electron transfer is not stepwise
–
A large release of energy occurs
–
As in the reaction of hydrogen
–
and oxygen to form water
Free energy, G
H2 + 1/2 O2
Explosive
release of
heat and light
energy
H2O
(a) Uncontrolled reaction
Electron Transport Chain
• The electron transport chain
– Passes electrons in a series of steps instead of in one explosive
reaction
– Uses the energy from the electron transfer to form ATP
2H
1/
+
2
O2
(from food via NADH)
2 H+ + 2 e–
Controlled
release of
energy for
synthesis of
ATP
Free energy, G
ATP
ATP
ATP
2 e–
1/
2
H+
H2O
(b) Cellular respiration
2
O2
Processes of Cellular Respiration
•
Respiration is a cumulative function of three metabolic stages
– Glycolysis
– The citric acid cycle
– Oxidative phosphorylation
• Glycolysis
– Breaks down glucose into two molecules of pyruvate
• The citric acid cycle
– Completes the breakdown of glucose
•
Oxidative phosphorylation
– Is driven by the electron transport chain
– Generates ATP
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolsis
Pyruvate
Glucose
Cytosol
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
ATP
Substrate-level
phosphorylation
ATP
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
•
Both glycolysis and the citric acid cycle
– Can generate ATP by substrate-level phosphorylation
Enzyme
Enzyme
ADP
Substrate
P
+
Product
ATP
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
Glycolysis consists of two major phases
– Energy investment phase
– Energy payoff phase
– Glycolysis
– Can produce ATP with or without oxygen, in aerobic or anaerobic
conditions
– Couples with fermentation to produce ATP
Glycolysis
ATP
•The energy input and output
•of glycolysis
Citric
acid
cycle
Oxidative
phosphorylation
ATP
ATP
Energy investment phase
Glucose
2 ATP + 2
P
2 ATP
used
Energy payoff phase
4 ADP + 4
P
4 ATP
2 NADH
2 NAD+ + 4 e- + 4 H +
formed
+ 2 H+
2 Pyruvate + 2 H2O
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H +
2 Pyruvate + 2 H2O
2 ATP
2 NADH
+ 2 H+
CH2OH
H
H
H
HO
HO
OH
H OH
H
Glycolysis
Glucose
ATP
1
Hexokinase
ADP
CH2OH
P
O H
H
H
OH H
HO
H OH
Glucose-6-phosphate
2
Phosphoglucoisomerase
CH2O
P
O
CH2OH
H HO
HO
H
HO H
Fructose-6-phosphate
3
ATP
Phosphofructokinase
ADP
P O CH2 O CH2 O
HO
H
OH
HO H
Fructose1, 6-bisphosphate
P
4
Aldolase
5
P O CH2
C
O
CH2OH
Dihydroxyacetone
phosphate
H
Isomerase
C O
CHOH
CH2 O
P
Glyceraldehyde3-phosphate
Citric
acid
cycle
Oxidative
phosphorylation
•Glucose enters the cell and is
phosphorylated by the enzyme hexokinase
which transfers a phosphate group fro ATP
to sugar
•The charge of the phosphate group traps
the sugar in the cell because the plasma
membrane is impermeable to membranes.
•Phosphorylation makes glucose more
reactive chemically.
6
2 NAD+
2
Triose phosphate
dehydrogenase
Pi
2
NADH
+ 2 H+
2
P
O
C
O
CHOH
P
CH2
O
1, 3-Bisphosphoglycerate
2 ADP
7
Phosphoglycerokinase
2 ATP
O–
2
C
CHOH
O
CH2
3-Phosphoglycerate
P
8
Phosphoglyceromutase
O–
2
C
H
C
O
P
O
CH2OH
2-Phosphoglycerate
9
Enolase
2 H2 O
2
O–
C
O
C
O
P
CH2
Phosphoenolpyruvate
2 ADP
10
Pyruvate kinase
2 ATP
2
O–
C
O
C
O
CH3
Pyruvate
The Citric Acid Cycle
• The citric acid cycle completes the energy-yielding oxidation of
organic molecules
• The citric acid cycle
– Takes place in the matrix of the mitochondrion
• Before the citric acid cycle can begin
– Pyruvate must first be converted to acetyl CoA, which links the
cycle to glycolysis
Pyruvate
(from glycolysis,
2 molecules per glucose)
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidative
phosphorylation
ATP
CO2
CoA
NADH
+ 3 H+
Acetyle CoA
CoA
CoA
Citric
acid
cycle
2 CO2
3 NAD+
FADH2
FAD
3 NADH
+ 3 H+
ADP + P i
ATP
Oxidative Phosphorylation
•
•
During oxidative phosphorylation, chemiosmosis couples electron transport
to ATP synthesis
NADH and FADH2
– Donate electrons to the electron transport chain, which powers ATP
synthesis via oxidative phosphorylation
The Pathway of Electron Transport
•
•
•
•
In the electron transport chain
– Electrons from NADH and FADH2 lose energy in several steps
ATP synthase
– Is the enzyme that actually makes ATP
At certain steps along the electron transport chain
– Electron transfer causes protein complexes to pump H+ from the mitochondrial
matrix to the intermembrane space
The resulting H+ gradient
– Stores energy
– Drives chemiosmosis in ATP synthase
– Is referred to as a proton-motive force
INTERMEMBRANE SPACE
H+
H+
A rotor within the
membrane spins
clockwise when
H+ flows past
it down the H+
gradient.
H+
H+
H+
H+
H+
A stator anchored
in the membrane
holds the knob
stationary.
A rod (for “stalk”)
extending into
the knob also
spins, activating
catalytic sites in
the knob.
H+
ADP
+
Pi
MITOCHONDRIAL MATRIX
ATP
Three catalytic
sites in the
stationary knob
join inorganic
Phosphate to ADP
to make ATP.
Chemiosmosis
•
Chemiosmosis
– Is an energy-coupling mechanism that uses energy in the form of a H+ gradient
across a membrane to drive cellular work
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane
ATP
ATP
H+
H+
H+
Protein complex
Intermembrane of electron
space
carners
Q
I
Inner
mitochondrial
membrane
Mitochondrial
matrix
H+
Cyt c
IV
III
II
FADH2
NADH+
NAD+
(Carrying electrons
from, food)
FAD+
2 H+ + 1/2 O2
ATP
synthase
H2O
ADP +
ATP
Pi
H+
Chemiosmosis
Electron transport chain
+
ATP
synthesis
powered by the flow
Electron transport and pumping of protons (H ),
+
+
Of H back across the membrane
which create an H gradient across the membrane
Oxidative phosphorylation
ATP Production in Respiration
•
•
During respiration, most energy flows in this sequence
– Glucose to NADH to electron transport chain to proton-motive force to
ATP
About 40% of the energy in a glucose molecule
– Is transferred to ATP during cellular respiration, making approximately
38 ATP
Electron shuttles
span membrane
CYTOSOL
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
2 NADH
Glycolysis
Glucose
2
Pyruvate
2
Acetyl
CoA
6 NADH
Citric
acid
cycle
+ 2 ATP
+ 2 ATP
by substrate-level
phosphorylation
by substrate-level
phosphorylation
Maximum per glucose:
About
36 or 38 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 32 or 34 ATP
by oxidative phosphorylation, depending
on which shuttle transports electrons
from NADH in cytosol
Fermentation
•
•
•
•
•
Fermentation enables some cells to produce ATP without the use of oxygen
Cellular respiration
– Relies on oxygen to produce ATP
In the absence of oxygen
– Cells can still produce ATP through fermentation
In alcohol fermentation
– Pyruvate is converted to ethanol in two steps, one of which releases
CO2
During lactic acid fermentation
– Pyruvate is reduced directly to NADH to form lactate as a waste product
– Both fermentation and cellular respiration
– Use glycolysis to oxidize glucose and other organic fuels to
pyruvate
2 ADP + 2
Glucose
P1
2 ATP
Glycolysis
O–
C
O
C
O
CH3
2 Pyruvate
2 NADH
2 NAD+
H
H
2 CO2
H
C
C
OH
CH3
O
CH3
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation
2 ADP + 2
Glucose
P1
O–
Glycolysis
2
NAD+
O
H
2 ATP
C
O
C
OH
CH3
2 Lactate
(b) Lactic acid fermentation
2 NADH
C
O
C
O
CH3
Proteins
Amino
acids
Fats
Carbohydrates
Sugars
Glycolysis
Glucose
Glyceraldehyde-3- P
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Glycerol
Cellular respiration
Is controlled by allosteric enzymes at key points in
glycolysis and the citric acid cycle
Fatty
acids