Download I. Cellular Energy • ATP: a) When the terminal phosphate is removed

I. Cellular Energy
• ATP:
a) When the terminal phosphate is removed, the remaining molecule is ADP. This is an exergonic reaction having a
relatively large -∆G (-7.6 kcal/mol). The liberated phosphate is then donated to another molecule which supplies
the free energy necessary to drive vital endergonic reactions.
Figure 1: ATP
•
Cellular Respiration:
a) Aerobic Respiration: occurs in the presence of O2 & is capable of producing large quantities of ATP. Practiced by
some unicellular & ALL multicellular organisms.
b) Anaerobic Respiration: does not utilize oxygen, but rather some other electronegative atom or molecule to drive
ATP synthesis. Is not as efficient as aerobic respiration. Practiced by unicellular organisms (prokaryotes).
c) Fermentation: involves a single aspect of the aerobic / anaerobic pathways that does not utilize oxygen. Is the
least efficient in terms of ATP output. Practiced by unicellular organisms (yeast).
II. Chemistry of Aerobic Respiration
Redox Reactions
• Oxidation:
•
Reduction:
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Figure 2: Redox Reaction
Hydrogen Acceptors
• Redox reactions in cells usually involve the transfer of a H atom & its associated electron rather than just an
electron. The enzymes responsible for this transfer are called Dehydrogenases.
The electron, along with its energy, is then transferred to a coenzyme that acts as a Hydrogen Acceptor. Coenzymes
associated with the dehydrogenases active during respiration include cofactors include:
a) NAD+: stores 2 electrons (1 hydrogen atom) from an oxidized substrate to become reduced to NADH.
b) FAD+: stores 2 electrons (2 hydrogen atoms) from an oxidized substrate to become reduced to FADH2.
•
Figure 3: Redox of NAD+
Chemical Equation of Aerobic Respiration
C6H12O6 + 6O2 à 6CO2 + 6H2O + 36-38ATP (∆G = -686 kcal/mol)
Glucose (raw material): derived from food. Completely oxidized (loses hydrogen along w/e-) to 6 CO2 (waste –
excreted through exhalation, incorporated into urea, etc).
• Oxygen (raw material): derived from breathing. Reduced as it accepts hydrogen /e- from the glucose molecule to
form 6 H2 O (waste –excreted in sweat & urine).
• 36-38 ATP: main product. Used to phosphorylate /energize molecules to power endergonic metabolic reactions.
•
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Phases of Aerobic Respiration
• Most of the reactions of aerobic cellular respiration occur within the mitochondria. These reactions can be
summarized in 2 general steps:
a) Anaerobic Phase: Glycolysis
b) Aerobic Phase: Acetyl Coenzyme A (aCoA) Formation, Krebs/Citric Acid Cycle & Electron Transport Chain
Anaerobic Phase: Glycolysis
•
Glycolysis:
a) The rxns of glycolysis can be divided into an energy investment & energy payoff phase.
•
Glycolysis (Energy Investment Phase):
Figure 4: Energy Investment Phase
*As a result of the splitting of fructose 1,6 biphosphate, all subsequent products of cellular respiration are DOUBLED!
•
Glycolysis (Energy Payoff Phase):
•
Substrate Level Phosphorylation:
End Products: For every 1 molecule of glucose oxidized during glycolysis, 2 pyruvic acid, 4 ATP (2net), & 2 NADH’s
are produced.
•
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Figure 5: Energy Payoff Phase
aCoA Formation
• Upon entering the mitochondria, the carboxyl group of the 2 pyruvic acid molecules is removed & given off as 2CO 2.
The remaining 2-carbon fragment is oxidized to form acetate.
An enzyme transfers the extracted electrons to NAD+, reducing it to 2NADH. Finally, Coenzyme A is attached to the
acetate by an unstable bond that makes the acetyl group very reactive. The product of this reaction is 2 Acetyl
Coenzyme A.
•
Figure 6: Pyruvate to Acetyl CoA
Aerobic Phase: Krebs Cycle
• Krebs /Citric Acid Cycle:
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Figure 6.1: Kerbs (Citric Acid) Cycle
End Products: For every Acetyl coA molecule entering the cycle, 3 NADH, 1 FADH2, & 2 CO2 molecules are evolved,
along with 1 ATP. Since 2 acetyl coenzyme A molecules enter the cycle for every glucose molecule oxidized, a total of
8 NADH (6 + 2 from acetyl CoA formation), 2 FADH2, 2 ATP, & 6 CO2 are produced.
•
Aerobic Phase: Electron Transport Chain
•
Electron Transport Chain:
a) Each protein in the chain is more electronegative than the one preceding it, causing electrons to cascade “down”
the chain. The last protein of the chain passes its electrons to oxygen (most electronegative), which also picks up
a pair of H+ from the aqueous solution to form water.
b) FADH2 adds its electrons to the chain at a lower energy level than NADH. Thus, the electron transport chain provides
about 1/3 less energy for ATP synthesis when the electron donor is FADH2 rather than NADH. Upon donating their
electrons to the proteins of the electron transport chain, NADH & FADH2, are oxidized back to NAD+& FAD+.
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Figure 7: Electron Transport Chain
Chemiosmosis
•
Chemiosmotic Model:
a) As e- cascade down the chain, the exergonic redox reactions liberate free energy that is harnessed by the transport
proteins to actively pump protons from the matrix into the intermembrane space. This establishes a proton
gradient that stores energy.
b) Since membranes are impermeable to ions, the only way for the protons to re-enter the matrix is through
specialized enzymes embedded in the inner membrane called ATP Synthases. Each ATP synthase consists of a
cylindrical component embedded w/in the membrane, a protruding knob, & stalk connecting the two. As protons
rush back into the matrix via ATP synthases, energy is released & harnessed by these enzymes to drive the
Oxidative Phosphorylation of ADP to ATP.
•
Oxidative Phosphorylation:
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Figure 8: Chemiosmosis
ATP Accounting
For each NADH contributing electrons to the electron transport chain, 3 ATP molecules are evolved & 2 ATP molecules
for every molecule of FADH2.
a) 10 NADH (8 Krebs; 2 glycolysis) =
ATP
b) 2 FADH2 =
ATP
c) 4 ATP (2 Krebs; 2 glycolysis)
d) Grand Total =
ATP (net)
•
Figure 9: Summary –Aerobic Cellular Respiration
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Mitochondrial Shuttle Systems
•
Inner Mitochondrial membrane
a) Liver, kidney, & heart cells: shuttle system transfers the electrons from NADH through the inner mitochondrial
membrane to an NAD+ in the matrix. These electrons are transferred to the electron transport chain to yield 3
ATP/electron pair (38 ATP’s/glucose molecule).
b) Skeletal muscle, brain cells: another shuttle system transfers electrons from NADH through the inner mitochondrial
membrane to ubiquinone to yield only 2 ATP’s/electron pair (36 ATP’s/glucose molecule).
Regulating Cellular Respiration
Cellular respiration is controlled by feedback inhibition in which the process speeds up when ATP is in demand slows
down when it is abundant, thus sparing valuable organic molecules for other functions.
•
Phosphofructokinase has an active site which binds both ATP & fructose-6-phosphate, but has allosteric inhibitor &
activator sites for ATP, ADP, AMP & citrate.
•
As ATP levels accumulate, it binds to an allosteric site on phosphofructokinase, changing the enzyme’s conformation
& decreases its activity & lowers the rate of respiration. The enzyme becomes active again as cellular work converts
ATP to ADP. At high levels, ADP binds to an allosteric activator site on phosphofructokinase to stimulate its activity &
increase the rate of respiration (ATP binds to its active site along w/F-6-P).
•
High citrate concentrations from the Krebs cycle can also inhibit phosphofructokinase. This helps to synchronize the
rates of glycolysis & the Krebs cycle. As citrate accumulates, glycolysis slows down & the supply of acetate to the
Krebs cycle decreases. If citrate consumption speeds up, glycolysis accelerates to meet the demand.
•
Figure 9: Regulating Cellular Respiration
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III. Chemistry of Fermentation
•
Fermentation:
Lactic Acid Fermentation
Figure 10: Lactic Acid Fermentation
In order to have a ready supply of NAD+, the 2 NADH molecules evolved during glycolysis donate hydrogens & electrons
to pyruvic acid, which becomes the final electron acceptor instead of oxygen. In doing so, pyruvic acid is reduced to
lactic acid.
•
•
In bacteria, lactic acid fermentation can lead to the souring of milk as lactose is converted to lactic acid via lactase.
Lactic acid fermentation in skeletal muscle cells can lead to fatigue. During strenuous exercise, the oxidation of
glucose occurs faster than oxygen can be supplied from the blood. As a result, cells generate ATP anaerobically via
lactic acid fermentation:
a) The increased acidity resulting from the accumulation of lactic acid w/in the cell hinders important contractile &
metabolic functions.
b) Once a sufficient oxygen supply is restored, lactic acid may be used directly for energy or be carried away by the
blood to the liver, where it is converted back to pyruvic acid.
•
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Alcohol Fermentation
Figure 11: Alcohol Fermentation
In order to have a ready supply of NAD+, the 2 NADH molecules evolved during glycolysis donate hydrogens & electrons
to 2 acetyl aldehyde, which becomes the final electron acceptor instead of oxygen. In doing so, it is reduced to ethyl
alcohol (ethanol).
•
Due to the ethyl alcohol by-product evolved by fermentation in yeast, they are very important in the brewing process.
The carbon dioxide evolved enables baked goods to rise during the baking process.
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Both lactic acid & alcohol fermentation cannot synthesize nearly as much ATP as aerobic respiration because glucose is
not completely broken down –much of its energy remains (98%) in either ethyl alcohol or lactic acid, which is not
utilized by the cell to produce ATP.
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