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Cell Respiration
What is Glycolysis?
• Breaking down glucose: “glyco – lysis” (splitting sugar)
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Most ancient form of energy capture.
Starting point for all cellular respiration.
Inefficient: generates only 2 ATP for every 1 glucose.
Happens in the cytosol. Why does that make evolutionary sense?
Evolutionary perspective
• Life on Earth first evolved without free oxygen (O2) in atmosphere.
• Energy had to be captured from organic molecules in absence of O2
• Organisms that evolved Glycolysis are ancestors of all modern life
• All organisms still utilize glycolysis.
Overview
• 10 reactions
• Convert 6C glucose to two 3C pyruvate
• Produce 2 ATP & 2 NADH
Glycolysis Summary
Endergonic
• Invest some ATP.
Exergonic
• Harvest a little ATP & a
little NADH.
Overall
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Glycolysis Reaction Overview
• 10 Reactions Total
• 9 Different Enzymes needed.
Priming reactions.
Glycolysis begins
with the addition of
energy. Two high
energy phosphates
from two molecules
of ATP are added to
the six-carbon
molecule glucose,
producing a sixcarbon molecule with
two phosphates.
Cleavage reactions
Then, the six-carbon
molecule with two
phosphates is split in
two, forming two
three-carbon sugar
phosphates.
Energy-harvesting
reactions.
Finally, in a series of
reactions, each of the
two three-carbon
sugar phosphates is
converted to
pyruvate. In the
process, an energyrich hydrogen is
harvested as NADH,
and two ATP
molecules are
formed.
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Substrate-level Phosphorylation
• In the last step of glycolysis, where did the P come from to make ATP?
• P is transferred from PEP to ADP
o kinase enzyme
o ADP → ATP
Is that all there is?
• Not a lot of energy…
• For 1 billon years+ this is how life on Earth survived
• Only harvest 3.5% of energy stored in glucose
• Slow growth, slow reproduction
We can’t stop there…
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Going to run out of NAD+.
How is NADH recycled to NAD+?
Without regenerating NAD+, energy production would stop so another
molecule must accept H from NADH.
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The answer? Fermentation.
• In Bacteria & Yeast.
o beer, wine, bread
o at ~12% ethanol, kills yeast
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In Animals & some Fungi.
o cheese, yogurt
o anaerobic exercise (no O2), this is why your legs burn when running.
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Glycolysis is only the start
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Glycolysis
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Pyruvate has more energy to yield
3 more C to strip off (to oxidize)
If O2 is available, pyruvate enters mitochondria
Enzymes of Krebs cycle complete oxidation of sugar to CO2
What is the point? TO MAKE ATP!!!
Oxidation of pyruvate
• Pyruvate enters mitochondria
o 3 step oxidation process
o Releases 1 CO2 (count the carbons!)
o Reduces NAD → NADH (stores energy)
o Produces acetyl CoA
• Acetyl CoA enters Krebs cycle
o Where does CO2 go?
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Pyruvate oxidized to Acetyl CoA
Yield = 2C sugar + CO2 + NADH
Krebs cycle
• Aka Citric Acid Cycle
o in mitochondrial matrix
o 8 step pathway
each catalyzed by specific enzyme
step-wise catabolism of 6C citrate molecule
• Evolved later than glycolysis
o Does that make evolutionary sense?
bacteria →3.5 billion years ago (glycolysis)
free O2 →2.7 billion years ago (photosynthesis)
eukaryotes →1.5 billion years ago (aerobic respiration
(organelles)
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So we fully
oxidized
glucose:
C6H12O6
↓
CO2
& ended
up with 4
ATP!
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So why is the Citric Acid
Cycle so great?
Krebs cycle produces large
quantities of electron carriers
• NADH
• FADH2
• Stored energy!
• go to ETC (Electron
Transport Chain)
Energy accounting of Krebs Cycle
Net gain = 2 ATP
= 8 NADH + 2 FADH2
So why the Krebs cycle?
• If the yield is only 2 ATP, then why?
o Value of NADH & FADH2
Electron carriers
Reduced molecules store energy!
To be used in the Electron Transport Chain
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ATP accounting so far…
• Glycolysis → 2 ATP
• Kreb’s cycle → 2 ATP
• Life takes a lot of energy to run, need to extract more energy than 4 ATP!
• There’s got to be a better way!
There is a better way!
• Electron Transport Chain
o Series of molecules built into inner mitochondrial membrane
o Mostly transport proteins
o Transport of electrons down ETC linked to ATP synthesis
o Yields ~34 ATP from only 1 glucose!
o Only in presence of O2 (aerobic)
Mitochondria (Form fits function!)
Double membrane
o outer membrane
o inner membrane
highly folded cristae*
fluid-filled space between membranes = intermembrane space
o matrix
central fluid-filled space
Electron Transport Chain
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Remember the NADH?
Glycolysis = 4 NADH
Kreb’s Cycle = 8 NADH, 2 FADH2
lectron Transport Chain
NADH passes electrons to ETC
Electron Transport Chain
NADH passes electrons to ETC
o H cleaved off NADH &
FADH2
o electrons stripped from H
atoms → H+ (H ions)
o electrons passed from one
electron carrier to next in
mitochondrial membrane
(ETC)
o transport proteins in
membrane pump H+ across
inner membrane to
intermembrane space
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But what “pulls” the electrons down the ETC?
Electrons flow downhill
Electrons move in steps from
carrier to carrier downhill to
O2
o each carrier more
electronegative
o controlled oxidation
o controlled release of
energy
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Why the build up H+?
ATP synthase
o enzyme in inner membrane of
mitochondria
ADP + Pi → ATP
only channel permeable to H+
H+ flow down concentration gradient provides
energy for ATP synthesis
o molecular power generator!
o flow like water over water wheel
o flowing H+ cause change in
o shape of ATP synthase enzyme
ATP Synthesis
Chemiosmosis couples ETC to ATP synthesis
build up of H+ gradient just so H+ could flow through ATP synthase enzyme to build ATP
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Cellular respiration
Summary of cellular respiration
Where did the glucose come from?
Where did the O2 come from?
Where did the CO2 come from?
Where did the H2O come from?
Where did the ATP come from?
What else is produced that is not listed in this equation?
Why do we breathe?
Taking it beyond…
What is the final electron acceptor in electron transport chain?
O2
So what happens if O2 unavailable?
o ETC backs up
o ATP production ceases
o Cells run out of energy and you die!
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Beyond glucose: Other carbohydrates
• Glycolysis accepts a wide range of carbohydrates fuels
o Polysaccharides → → → glucose
(hydrolysis)
ex. starch, glycogen
o Other 6C sugars → → → glucose
(modified)
ex. galactose, fructose
Beyond glucose: Proteins
• Proteins → → → → → amino acids
(Hydrolysis)
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Fats → → → → → glycerol & fatty acids
(Hydrolysis)
o Glycerol (3C) → → PGAL → → glycolysis
o Fatty acids → 2C acetyl groups → acetyl coA →Krebs cycle
Carbohydrates vs. Fats
• Fat generates 2x ATP vs. carbohydrate
o More C in gram of fat
o More O in gram of carbohydrate so it’s already partly oxidized.
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Respond to cell’s needs
• Key points of control
o Phosphofructokinase
Allosteric regulation of enzyme
• “can’t turn back” step
before splitting glucose
AMP & ADP stimulate
ATP inhibits
citrate inhibits
Why is this regulation important?
• Balancing act: Availability of raw materials vs.
Energy demands vs. Synthesis.
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ATP Accounting (From Glucose to ATP)
= 2 Pyruvate
4 ATP
2 NADH
= ~34 ATP
= 4 CO2
2 ATP
2 FADH2
6 NADH
= 2 Acetyl CoA
2 CO2
2 NADH
But why does the NADH from
Glycolysis make less ATP per
molecule than the NADH from the
Kreb’s Cycle during the ETC?
This yields approximately 38 Total
ATP from 1 glucose molecule.
(~3 ATP per NADH & ~2 ATP per
FADH2)
10 NADH +
2 FADH2
ETC
2 Acetyl CoA
Kreb’s Cycle
2 Pyruvate
Oxidation of Pyruvate
Glucose + 2 ATP
Glycolysis