Download Chapter 9: Cellular Respiration, Harvesting Chemical Energy

Chapter 9: Cellular Respiration, Harvesting Chemical Energy
Overview
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Energy stored in organic molecules of food ultimately comes from the sun
o Energy comes in as sunlight, leaves as heat
Everything else that is essential to life is recycled
o Photosynthesis creates > Mitochondria use , create C and H2O > Photosynthesis
uses C and H2O to create more
How Cells harvest the chemical energy stored in organic molecules and use it to generate
ATP, the molecule that drives most cellular work
o Glycolysis, The Citric Acid Cycle, and Oxidative Phosphorylation
Concept 9.1: Catabolic Pathways Yield Energy by Oxidizing Organic Fuels
Catabolic Pathways and Production of ATP
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Catabolic Pathways- Metabolic pathways that release stored energy by breaking down
complex molecules, that are rich in potential energy, into smaller waste products, that
have less energy
o Fermentation- Partial degradation of sugars without the use of O2
o Cellular Respiration- O2 is consumed as a reactant along with the organic fuel
 Most prevalent and efficient catabolic pathway (mostly the breakdown of
sugar)
 Can be thought of as burning gasoline in a car, The breakdown of gasoline
(sugar) creates energy to make the car move (organism) which then throws
away the unused reactants (exhaust)
 The breakdown of glucose is exergonic (releases energy) and is
spontaneous (negative delta G, can occur without an input of energy)
 Depend on ATP
Redox Reactions: Oxidations and Reduction
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Relocation of e- releases the energy stored in the bonds of organic molecules
Energy is released when an electron moves from a lesser electronegative bond, to a
greater one
o Energy is released as a ball is pushed down a hill
In general, organic molecules with an abundance of hydrogen are excellent fuels
o More “hill top” e- (more electrons waiting to move to a greater electronegative
bond, meaning, more energy released when they pair up with a highly
electronegative atom
Activation energy serves as a barrier to keep glucose from bonding to (and releasing
energy)
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o Enzymes lower activation energy needed in order to start reductions
Energy stored in organic molecules are broken down in steps, each one catalyzed by an
enzyme
o Exploding the gas tank of a car won’t make it go far
At key steps, e- are stripped from the molecule, each e- travels with a proton (=hydrogen
atom)
o Hydrogen atoms are NOT directly transferred to Oxygen
o Are first passed onto NAD+, a coenzyme that functions as an oxidizing agent
(receives e-)
Dehydrogenases remove a pair of Hydrogen atoms from the substrate, 2 protons and 2 eo 2 e- and 1 proton are then added to NAD+, other proton is released into
surrounding
 NADH has now been created, serves as a source to make ATP
o e lost very little potential energy when transferred from food to NAD+
H2O is a byproduct of cellular respiration
o Organic molecules provide the Hydrogen atoms
Respiration uses an electron transport chain in order to ensure that instead of one big
explosive reaction occurring between H2 and O2, a series of smaller reactions occurs
o This transport chain is made up of mostly proteins and is built into the inner
membrane of a mitochondrion
o Electrons removed from food by NADH are moved to the higher energy part of
the electron transport chain
o Electrons drop down a step in the electron transport chain by way of carrier
molecules
 They lose a bit of energy every time that they drop a step
 Each downhill carrier is more electronegative than its uphill neighbor
o Oxygen captures the electrons at the lower energy part of the electron transport
chain
The Stages of Cellular Respiration: A Preview
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Respiration is the CUMULATIVE function of THREE metabolic stages:
1. Glycolysis
2. The Citric Acid Cycle
3. Oxidative Phosphorylation
Only the Citric Acid Cycle and Oxidative Phosphorylation require the use of O2
Glycolysis and the Citric Acid Cycle are catabolic pathways that decompose glucose into
fuel
Glycolysis beings the degradation process by breaking glucose into two molecules of
pyruvate
o This occurs in the cytosol
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Citric Acid Cycle completes breakdown of glucose by oxidizing a derivative of pyruvate
to CO2
o This occurs in the mitochondrial matrix
Oxidative Phosphorylation uses the energy released by the electron transport chain to
create ATP, H2O is released
o This occurs in the inner membrane of the mitochondrion
o Electron transport and chemiosmosis together constitute oxidative
phosphorylation
Oxidative Phosphorylation accounts for almost 90% of the ATP generated by respiration
o The other 10% is formed directly in a few reactions of glycolysis and the citric
acid cycle by a mechanisms called substrate-level phosphorylation
Substrate-level phosphorylation occurs when an enzymes transfers a phosphate group
from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as
in oxidative phosphorylation
For each molecule of glucose degraded into carbon dioxide and water, 38 molecules of
ATP, each with 7.3 kcal/mol of free energy
Concept 9.2: Glycolysis Harvests Chemical Energy by Oxidizing Glucose to Pyruvate
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During glycolysis, glucose (6 carbons) is broken down into two 3 carbon sugars
o These smaller sugars are then oxidized and their remaining atoms rearranged into
two molecules of pyruvate
The 10 stages of the pathway of glycolysis can be broken down into two stages
o The Energy Investment Phase:
Hexokinase phosphorylates sugar, now more reactive and has a charge (can’t pass
membrane)
Phosphoglucoisomerase converts glucose-6-phosphate to its isomer
a. Fructose-6-phosphate now created
Phosphofructokinase phosphorylates fructose-6-phosphate, now allowing it to split in half
a. Key step in regulation, phosphofructokinase allosterically regulated by ATP and
products
Aldolase cleaves the sugar into two three carbon sugars
a. Dihydroxyacetone phosphate and Glyceraldehyde-3-phosphate
Isomerase catalyzes the reversible conversion between both three carbon sugars
a. Not really in equilibrium, ONLY Glyceraldehyde-3-phosphate being used in next
step
i. This means that more Glyceraldehyde-3-phosphate is created
o The Energy Payoff Phase:
6. Triose phosphate dehydrogenase catalyzes two sequential reactions as it holds
glyceraldehyde-3-phosphate in its active side
1. Sugar is oxidized by electron transfer from H+ to NAD+ to form NADH
i. Very exergonic reaction
2. Energy used from previous step to phosphorylate the oxidized substrate (sugar)
i. Gives the substrate a lot of potential energy
7. Phosphoglycerokinase takes the phosphate group added in step 6.2 and adds it to ADP
a. 3-phosphoglycerate now created (not a sugar, now an acid, -CO to –COO-)
8. Phosphoglyceromutase relocates the last phosphate group remaining
a. 2-phosphoglycerate now created
9. Enolase causes a double bond and makes an unstable molecule by removing water
a. Phosphoenolpyruvate (PEP) now created
10. Pyruvate kinase transfers a phosphate group from PEP to ADP
a. Pyruvate now created
Concept 9.3: The Citric Acid Cycle Completes the Energy-Yielding Oxidation of Organic
Molecules
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Glycolysis releases less than a quarter of the energy stored in a glucose molecule
o The rest is stockpiled in the pyruvates
Upon entering the mitochondrion, Pyruvate is converted into Acetyl Coenzyme A (CoA)
 Pyruvate enters the mitochondrion with the help of a transport protein
because it is charged and won’t be allowed to pass through the membrane
o The Carboxyl group (-COO-) is already fully oxidized and has little energy
 Given off as CO2
o The remaining two-carbon fragment is oxidized, forming acetate
 An enzyme transfers the extracted electrons from NAD+, making NADH
o Coenzyme A, a sulfur containing compound derived from a B vitamin, is attached
to the acetate by an unstable bond
 This creates an acetyl group that is VERY reactive
Citric Acid Cycle = Krebs’s Cycle = Tricarboxyl Acid Cycle
The CAC functions as a metabolic furnace that oxidizes organic fuel derived from
pyruvate (Acetyl CoA)
o The Cycle generates 1 ATP per turn by substrate-level phosphorylation
o Most of the chemical energy is transferred to NAD+ and the related coenzyme
FAD (create NADH and FADH2)
The CAC is broken down into 8 steps, each catalyzed by a specific enzyme
o The CAC is a cycle because it both starts and ends with Oxaloacetate
1. Acetyl CoA adds its two-carbon acetyl group oxaloacetate, producing citrate
2. Citrate is converted to its isomer, isocitrate, by removal of one water molecule and the
addition of another water molecule
3. Citrate loses a CO2 molecule, and the resulting compound is oxidized, reducing NAD+ to
NADH
4. Another CO2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH.
The remaining molecule is then attached to coenzyme A by an unstable bond.
5. CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, and
then to ADP, forming ATP (substrate-level phosphorylation)
6. Two hydrogens are transferred to FAD, forming FADH2 and oxidizing succinate
7. Addition of a water molecule rearranges bonds in the substrate
8. The substrate is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate
Concept 9.4: During Oxidative Phosphorylation, Chemiosmosis Couples Electron
Transport to ATP Synthesis
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Most of the ATP created in cellular respiration comes from oxidative phosphorylation
Oxidative Phosphorylation uses the energy created from the electron transport chain in
order to synthesize ATP
The Pathway of Electron Transport
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Electron transport chain is a collection of molecules embedded into the inner membrane
of the mitochondrion
o Most components of the chain are proteins which exist in multiprotein complexes
o Prosthetic groups, nonprotein components essential for the catalytic functions of
certain enzymes, tightly bound to multiprotein complexes
During electron transport along the chain, electron carriers alternate between reduced and
oxidized states
o Reduced as it accepts an electron from its uphill neighbor, oxidized as it passes it
down
Ubiquinone- Small hydrophobic molecule that is the only nonprotein of the electron
transport chain
o Cytochromes- Protein carriers between ubiquinone and oxygen
NADH provides more energy than FADH2 because FADH2 is added later on in the chain
The purpose of the electron transport chain is to control the release of energy of fuel
o ATP is not directly created by the electron transport chain
The mitochondrion uses the energy released by the electron transport in order to create
ATP by a process called Chemiosmosis
Chemiosmosis: The Energy-Coupling Mechanism
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ATP synthase is a protein complex that populates the inner membrane of the
mitochondrion
o It uses the movement of H+ ions in order to fuel the synthesis of ATP
ATP Synthase is composed of four parts, each made up of multiple polypeptide
o A rotor, knob, internal rob, and stator.
Hydrogen ions flow down a space between the stator and rotor and causes the rotor and
its attached rod to rotate
The spinning rod causes changes in the stationary knob, activating three catalytic sites in
the subunits that make up the knob
o This causes an ADP and an inorganic phosphate to combine and make ATP
Proton-motive force- the H+ gradient created by the release of hydrogen ions into the
intermembrane space of a mitochondrion by the electron transport chain
Chemiosmosis in an energy-coupling mechanism that uses energy stored in the form of
an H+ gradient across a membrane to drive cellular work
Chloroplasts use chemiosmosis to generate ATP during photosynthesis, however, light
energy is used, instead of chemical energy
Prokaryotes lack both mitochondria and chloroplasts, they generate H+ gradients across
their plasma membranes
o They then tap the proton-motive forces to create ATP, pump nutrients, pump
wastes, and move their flagella
An Accounting of ATP Production by Cellular Respiration
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We cannot state an exact number of ATP molecules generated by the breakdown of
glucose
1. There is not a whole number ratio of the amount of ATP generated by NADH or
FADH2
a. NADH and FADH2 move H+ across the membrane, different amount of
H+ then creates ATP
2. ATP yield varies slightly depending on the type of shuttle used to transport
electrons from the cytosol into the mitochondrion (From NADH created by
glycolysis)
a. NADH moves electrons to NAD+ or FAD
3. The use of the proton-motive force generated by the redox reactions of respiration
to drive other kinds of work
a. Some proton-motive force can be used to take pyruvate from the cytosol
Concept 9.5: Fermentation Enables Some Cells to Produce ATP Without the use of Oxygen
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Provides a form of energy creation that does not require oxygen in order to function
Glycolysis produces 2 ATP regardless of Oxygen because NAD+ is the oxidizing agent
o The creation of ATP by glycolysis would stop if NADH weren’t recycled to
NAD+
 Aerobic: It is recycled by the electron transport chain
 Anaerobic: It is recycled by the transfer of electrons from NADH to
pyruvate
Types of Fermentation
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Alcohol Fermentation: Pyruvate is converted to Ethanol, CO2 is released
1. Pyruvate is converted to the two-carbon compound acetaldehyde
2. Acetaldehyde is reduced by NADH to ethanol
Lactic Acid Fermentation: Pyruvate is reduced directly by NADH to form lactate
Fermentation and Cellular Respiration Compared
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Aerobic vs. Anaerobic
Both produce 2 ATP molecules by substrate phosphorylation
Oxygen vs. Acetaldehyde or Pyruvate as NAD+ recyclers
Full release of pyruvate energy (Krebs’s cycle) vs. No release of pyruvate energy
Facultative Anaerobes- species that can make enough ATP to survive using either
fermentation or respiration
The Evolutionary Significance of Glycolysis
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Glycolysis most likely present in ancient prokaryotes
o Do not require the use of Oxygen (no O2 in Earth’s atmosphere)
o Occurs in the cytosol (no membrane bound organelles)
Concept 9.6: Glycolysis and the Citric Acid Cycle Connect to many other Metabolic
Pathways
The Versatility of Catabolism
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Glycolysis can be used to broken down more than just sugar
o Fats, proteins, sucrose, starch
Proteins must be broken down into constituent amino acids
Beta oxidation- breaks down fatty acids to 2-carbon fragments and enters the CAC as
Ace. CoA.
Biosynthesis (Anabolic Pathways)
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We can take the products of cellular respiration and build on top of them to make things
that the cell may need
o Pyruvate can be made back into sugar, fatty acids can be made from Acetyl CoA
Regulation of Cellular Respiration via Feedback Mechanisms
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Cells change the processes that they are carrying out based upon the needs of the
products
The entire rate of the catabolic process can be changed based upon phosphofructokinase
o Works depending on the amount of ATP vs. ADP/AMP
o Works depending on the amount of citrate in the cell
Energy that keeps us alive is released, not produced, by cellular respiration