Download 9/29/2015 Chapter 9: CELLULAR RESPIRATION & FERMENTATION

9/29/2015
Chapter 9:
CELLULAR RESPIRATION
& FERMENTATION
1. Overview of Respiration
2. Glycolysis
3. The Citric Acid Cycle
4. Oxidative Phosphorylation
5. Fermentation
1. Overview of Respiration
Chapter Reading – pp. 163-167
Cellular Respiration
Cellular
Respiration is
essentially the
reverse of
photosynthesis
Light
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2  H2O
Catabolism of
energy rich
organic
molecules is
coupled to
the endergonic
synthesis of ATP
Organic
 O2
molecules
Cellular respiration
in mitochondria
ATP
ATP powers
most cellular work
Heat
energy
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Important Concepts
Before examining the process of Cellular
Respiration it is important to review some
important concepts that are central to the process:
• mitochondrial structure
• oxidation/reduction reactions
• the central role of HYDROGEN and its
electrons (e–), and electron carriers
• substrate-level phosphorylation vs
oxidative phosphorylation
• gradual or incremental release of energy
Mitochondrial Structure
Mitochondria have 2 membranes (inner
and outer) and 2 distinct compartments:
• intermembrane space
• matrix
inner membrane is folded into cristae to increase surface area
Oxidation/Reduction
There are 2 ways in which are said to be
oxidized or reduced:
1
OXIDATION = the loss of electrons
REDUCTION = the gain of electrons
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
becomes oxidized
becomes reduced
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2
OXIDATION = the partial loss of electrons
REDUCTION = the partial gain of electrons
Products
Reactants
becomes oxidized
Energy
becomes reduced
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
Oxidation/Reduction & Hydrogen
The electrons associated with hydrogen atoms are
key to the oxidation states of organic molecules in
biochemical processes:
• electrons (e–) are transferred as part of a hydrogen atom
in most biochemical reactions (i.e., with a proton)
• when a molecule gains hydrogen it is “reduced” and
when a molecule loses hydrogen it is “oxidized”
becomes oxidized
becomes reduced
When e– of H are transferred from a less electronegative
atom to a more electronegative atom, energy is released!!
Electron Carriers
Hydrogens and their e– will be captured by electron
carriers such as NADH and delivered elsewhere:
NAD
NADH
Dehydrogenase
Reduction of NAD
(from food)
Oxidation of NADH
Nicotinamide
(oxidized form)
Nicotinamide
(reduced form)
Dehydrogenase
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Substrate-level Phosphorylation
Enzyme
Enzyme
ADP
P
Substrate
ATP
Product
A phosphate group from an organic molecule is
transferred directly to ADP to make ATP
• an enzyme called a “kinase” catalyzes the reaction
involving the transfer of a phosphate from one substrate
to another, ADP
Oxidative Phosphorylation
“Oxidative Phosphorylation” refers to the
phosphorylation of ADP to make ATP by a
process that depends on oxidation-reduction
reactions (“oxidative”).
As we shall see this is more complex than it
sounds and involves two key processes:
1) A series of oxidation/reduction reactions involving
the “Electron Transport Chain” which will produce an
electrochemical gradient of H+ ions
2) Coupling the energy stored in this electrochemical
gradient of H+ to the endergonic synthesis of ATP
Respiration Liberates Energy Gradually
H2  1/2 O2

2H
1/
2
O2
(from food via NADH)
Controlled
release of
energy for
synthesis of
ATP
Explosive
release of
heat and light
energy
Free energy, G
Free energy, G
2 H+  2 e
ATP
ATP
ATP
2
e
1/
2 H+
H2O
(a) Uncontrolled reaction
2
O2
H2O
(b) Cellular respiration
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Cellular Respiration at a Glance
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Pyruvate
oxidation
Glycolysis
Glucose
Pyruvate
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Citric
acid
cycle
Acetyl CoA
MITOCHONDRION
CYTOSOL
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
2. Glycolysis
Chapter Reading – pp. 168-169
What is Glycolysis?
Glycolysis is a metabolic pathway occurring
in the cytoplasm that is essentially “phase 1”
of the catabolism of glucose and other
monosaccharides.
• technically glycolysis is NOT part of cellular
respiration, though two of the products of this
pathway are used in cellular respiration:
2 ATP
2 NADH*
2 pyruvate*
*used in cellular respiration
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Glycolysis uses ATP to make ATP
Energy Investment Phase
• 2 ATP must be
“consumed”
during the
catabolism of
glucose in
order to
produce 4 ATP
Glucose
2 ADP  2 P
2 ATP used
Energy Payoff Phase
4 ADP  4 P
4 ATP formed
2 NAD+  4 e  4 H+
2 NADH  2 H+
2 Pyruvate  2 H2O
Net
net yield of
2 ATP
2 Pyruvate  2 H2O
Glucose
4 ATP formed  2 ATP used
2 ATP
2 NADH  2 H+
2 NAD+  4 e  4 H+
The Energy Investment Reactions
Glycolysis: Energy Investment Phase
Glucose
ATP
ATP
Glucose 6-phosphate
Fructose 1,6-bisphosphate
Fructose 6-phosphate
ADP
ADP
Hexokinase
Phosphoglucoisomerase
Phosphofructokinase
2
3
1
Aldolase
Dihydroxyacetone
phosphate
4
Glyceraldehyde
3-phosphate
Isomerase
To
step 6
5
The initial reactions of glycolysis require the hydrolysis
(i.e., “consumption” or “loss”) of 2 ATP in order to add
2 phosphates to intermediates of the pathway thus
increasing their potential energy (PE).
The Energy Payoff Reactions
In the subsequent steps of glycolysis, the PE invested is
used to accomplish:
• the addition of 2 more phosphates
• the capture of 2 pairs of energy-rich e– associated with
hydrogen (reduction of 2 NAD+ to NADH)
• the synthesis of 4 ATP (substrate-level phosphorylation)
Glycolysis: Energy Payoff Phase
2 ATP
2 ATP
2 H2O
2 NADH
2 NAD
Triose
phosphate
dehydrogenase
6
+ 2 H
2 ADP
2
2
Phosphoglycerokinase
2Pi
2 ADP
2
Enolase
Phosphoglyceromutase
Pyruvate
kinase
9
1,3-Bisphosphoglycerate
7
3-Phosphoglycerate
8
2-Phosphoglycerate
Phosphoenolpyruvate (PEP)
10
Pyruvate
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3. The Citric Acid Cycle
Chapter Reading – pp. 169-171
What is the Citric Acid Cycle?
The Citric Acid Cycle (CAC) is a metabolic
pathway occurring in the mitochondrial matrix
that is essentially “phase 2” of the catabolism
of glucose:
• pyruvate from glycolysis is first catabolized to
acetyl-Coenzyme A before entering the CAC
• all carbons from the original glucose will be
completely oxidized to waste CO2
• more energy-rich e– in hydrogens will be
captured by electron carriers
• 2 more ATP by substrate-level phosphorylation
Oxidation of Pyruvate to Acetyl-CoA
MITOCHONDRION
CYTOSOL
CO2
Coenzyme A
3
1
2
Pyruvate
NAD
NADH + H
Acetyl CoA
Transport protein
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Acetyl CoA
Pyruvate
CO2
NAD
CoA
NADH
+ H
Acetyl CoA derived
from pyruvate (or the
catabolism of fatty
acids and other
organic molecules)
will directly enter the
Citric Acid Cycle
Acetyl CoA
CoA
CoA
Citric
acid
cycle
2 CO2
3 NAD
FADH2
3 NADH
FAD
+ 3 H
ADP + P i
ATP
Acetyl CoA is
combined with
the end product
of the pathway
(oxaloacetate)
to produce citric
acid to begin
the cycle again
Acetyl CoA
CoA-SH
NADH
+ H
Citric Acid
Cycle
H2O
1
NAD
8
Oxaloacetate
2
Malate
Citrate
Isocitrate
NAD
NADH
3
+ H
7
Per acetyl group,
the cycle yields:
H2O
CO2
Fumarate
CoA-SH
-Ketoglutarate
4
6
1 ATP
3 NADH
1 FADH2
2 CO2
CoA-SH
5
FADH2
NAD
FAD
Succinate
GTP GDP
CO2
NADH
Pi
Succinyl
CoA
+ H
ADP
ATP
Keeping Score
Up to this point, the original molecule of
glucose has yielded the following:
GLYCOLYSIS – 2 ATP 2 NADH
OXIDATION of PYRUVATE – 2 NADH 2 CO2
CITRIC ACID CYCLE – 2 ATP 6 NADH
2 FADH2 4 CO2
TOTAL – 4 ATP 10 NADH
2 FADH2 6 CO2
• NADH & FADH2 provide energy-rich e– for the synthesis
of many more ATP by oxidative phosphorylation…
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4. Oxidative Phosphorylation
Chapter Reading – pp. 172-176, 180-182
Overview of
Oxidative Phosphorylation
Oxidative Phosphorylation involves 2 distinct
processes:
1) Electron Transport
• e- from NADH and FADH2 are passed along the electron
transport chain (ETC) via oxidation-reduction reactions
• convert energy from e- into energy stored in H+ gradient
2) Chemiosmosis
• energy from H+ gradient harnessed to make ATP
both processes occur across the inner mitochondrial membrane
Electron Transport & Chemiosmosis
H
H
Protein
complex
of electron
carriers
H
H
Cyt c
Q
I
IV
III
II
FADH2 FAD
NADH
2 H + 1/2O2
NAD
ATP
synthase
H 2O
ADP  P i
(carrying electrons
from food)
ATP
H
1 Electron transport chain
2 Chemiosmosis
Oxidative phosphorylation
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Basics of Electron Transport
Occurs within the inner mitochondrial membrane
High energy electrons supplied by:
• NADH & FADH2
Electron Transport generates H+ gradient between
intermembrane space & matrix
• provides proton motive force for ATP synthesis
Requires O2 as the final electron acceptor
• anaerobic respiration involves other final
electron acceptors
Electron Transport Chain
H
INTERMEMBRANE
SPACE
H
H
Cyt c
Protein complex
of electron
carriers
IV
Q
III
I
II
2 H + 1/2 O2
FADH2 FAD
NADH
NAD
H2O
MITOCHONDRIAL MATRIX
(carrying electrons from food)
Basics of Chemiosmosis
“The flow of a H+ from high to low concentration”
Energy derived from the flow of H+ is used to
synthesize ATP (from ADP & Pi)
• H+ flows “down” concentration gradient through
ATP synthase in the inner mitochondrial membrane
• ATP synthase = enzyme complex that catalyzes:
ADP + Pi
H+ flow
ATP
Yields up to 28 ATP per glucose molecule!
• in addition to 2 ATP in glycolysis, 2 ATP in Krebs cycle
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INTERMEMBRANE SPACE
H
ATP Synthase
Stator
The enzyme complex
known as ATP synthase
couples the energy released
by chemiosmosis to the
synthesis of ATP
Rotor
• as H+ ions flow through ATP
synthase, the rotor portion of
the enzyme complex and the
internal rod actually rotate
Internal
rod
Catalytic
knob
ADP
+
Pi
• rotation of the internal rod
provides the force needed to
to get ADP & Pi bound by the
catalytic knob close enough to
react and form ATP
ATP
MITOCHONDRIAL MATRIX
Summary of Cellular Respiration
Electron shuttles
span membrane
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
Glycolysis
Glucose
2 Pyruvate
2 NADH
Pyruvate oxidation
2 Acetyl CoA
6 NADH
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Citric
acid
cycle
 about 26 or 28 ATP
 2 ATP
 2 ATP
Maximum per glucose:
About
30 or 32 ATP
CYTOSOL
Respiration
& other
Organic
Molecules
In addition to
glucose, other
organic molecules
“feed” into the
process of
respiration at
various stages
Proteins
Carbohydrates
Amino
acids
Sugars
Fats
Glycerol
Fatty
acids
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
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Glucose
AMP
Glycolysis
Fructose 6-phosphate
Stimulates

Phosphofructokinase


Regulation of
Respiration
Fructose 1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Allosteric regulation of
key enzymes such as
phosphofructokinase
helps keep the entire
process in balance.
Citric
acid
cycle
Oxidative
phosphorylation
5. Fermentation
Chapter Reading – pp. 177-180
The Purpose of Fermentation
In the absence of O2,
glycolysis is the only
source of ATP
NAD+
glycolysis
fermentation
NADH
• fermentation ensures sufficient NAD+ is available for glycolysis
2 ADP  2 P
Glucose
2 ADP  2 P
2 ATP
i
Glycolysis
Glucose
2 ATP
i
Glycolysis
2 Pyruvate
2 NAD 
2 NADH
 2 H
2 NAD 
2 CO2
2 NADH
 2 H
2 Pyruvate
2 Ethanol
Yeast
(a) Alcohol fermentation
2 Acetaldehyde
2 Lactate
Animals
(b) Lactic acid fermentation
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Fermentation or Respiration?
Glucose
Glycolysis
CYTOSOL
Pyruvate
O2 present:
Aerobic cellular
respiration
No O2 present:
Fermentation
MITOCHONDRION
Ethanol,
lactate, or
other products
Acetyl CoA
Citric
acid
cycle
If O2 is
present,
respiration is
preferred
(more ATP!),
however
without O2
fermentation
is the only
option to
produce ATP
Variety in Fermentation Products
Glucose
NAD+
NADH
Pyruvate
Propionibacterium
Aspergillus
Lactobacillus
Streptococcus
Saccharomyces
Clostridium
Escherichia
Acetobacter
NADH
Acetic acid
NAD+
Fermentation
CO2, propionic acid
Lactic acid
CO2, ethanol
Acetone, isopropanol
Fermentation
products
Swiss cheese
Cheddar cheese,
yogurt, soy sauce
Wine, beer
Nail polish remover,
rubbing alcohol
Vinegar
• different organisms produce different fermentation products
Key Terms for Chapter 9
• glycolysis, fermentation
• Citric Acid Cycle
• electron carriers (NADH, FADH2)
• oxidation vs reduction
Relevant
Chapter
Questions
1-10, 13
• substrate-level vs oxidative phosphorylation,
• electron transport chain (ETC), chemiosmosis
• ATP synthase
• mitochondria: inner & outer membranes, matrix,
cristae
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