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CELLUL AR
RESPIRATION
CELLULAR RESPIRATION
• The set of metabolic reactions used by cells to
harvest energy from food.
686 kcal/mol
of Glucose
234 kcal/mol is trapped as ATP (34%)
Occurs in a series of small steps.
UNDER AEROBIC CONDITIONS:
IN THE PRESENCE OF O 2
• Glycolysis
–The 6-carbon monosaccharide glucose is converted into two
3-carbon molecules of pyruvate.
• Pyruvate Oxidation
–Two 3-carbon molecules of pyruvate are oxidized to two 2carbon molecules of acetyl CoA and two molecules of CO2
• Citric Acid Cycle 
–Two 2-carbon molecules of acetyl CoA are oxidized to four
molecules of CO2
GLYCOLYSIS
• Takes place in the cytosol
• Involves ten enzyme-catalyzed reactions.
• Releases energy as C-H bonds are oxidized.
• Final product = 2 molecules of Pyruvate
2 molecules of ATP
2 molecules of NADH
• Two Stages
–The initial energy-investing reactions that consume chemical
energy stored in ATP.
Endergonic Reaction
–The energy-harvesting reaction that produce ATP and NADH.
Exergonic Reaction
TWO TYPES OF REACTIONS THAT
OCCUR REPEATEDLY IN GLYCOLYSIS
• Oxidation-reduction:
– Exergonic Reaction
• The energy is trapped via the reductions of NAD+ to NADH.
• Substrate-level phosphorylation:
– Exergonic Reaction
• Less energy is released.
– It is enough to transfer a phosphate from the substrate to ADP, forming
ATP.
• End product of glycolysis is 2 pyruvate molecules.
PYRUVATE OXIDATION
• This is the link between glycolysis and the Krebs Cycle.
• The Oxidation of pyruvate to a 2-carbon acetate molecule
and CO2
– The acetate is then bound to coenzyme A (CoA).
PYRUVATE OXIDATION
•The overall reaction is exergonic
• One molecule of NAD+ is reduced.
• The main role of acetyl CoA is to donate its acetyl
group to the 4-carbon compound oxaloacetate,
forming the 6-carbon molecule citrate.
–This initiates the Kreb’s cycle.
KREB’S CYCLE
CITRIC ACID CYCLE
• Acetyl CoA is the starting point
• 8 reaction pathway
• The completely oxidation of 2-carbon acetyl group to two
molecules of CO2
• The free energy released from these reactions is captured
by ADP and the electron carriers NAD+ and FAD.
• The citric acid cycle operates twice for each glucose
molecule that enters glycolysis.
KREB’S CYCLE
• Starting material oxaloacetate is used then regenerated in the
last step.
–It then accepts another acetate group from Acetyl CoA
• Operates twice for each glucose molecule.
–Once fore each pyruvate molecule made.
• Occurs in the Mitochondria.
• Exergonic Reaction
– Harvests A LOT OF chemical energy.
– NAD+  NADH
– FAD  FADH2
NADH & FADH2
 Krebs cycle produces:
8 NADH
2 FADH2
2 ATP

SO WHY THE KREBS CYCLE?
• If the
yield is only 2 ATP, then why?
–value of NADH & FADH2
• electron carriers
• reduced molecules store energy!
• to be used in the Electron Transport Chain
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!
Why stop here…
LAST STOP AND MOST IMPORTANT!
• Electron Transport Chain
–series of molecules built into inner mitochondrial
membrane
• mostly transport (integral) proteins
–transport of electrons down ETC linked to ATP
synthesis
–yields ~34 ATP from 1 glucose!
–only in presence of O2 (aerobic)
MITOCHONDRIA
• Double membrane
–outer membrane
–inner membrane (ETC here!)
• highly folded cristae*
• fluid-filled space between membranes =
intermembrane space
– Matrix (Kreb’s here!)
• central fluid-filled space
* form fits function!
ELECTRON TRANSPORT CHAIN
ELECTRON TRANSPORT CHAIN
• A series of carriers that pass electrons from one
to the other.
• As electrons pass through the protein complexes
of the respiratory chain, protons are pumped
from the mitochondrial matrix into the
intermembrane space.
• As the protons return to the matrix through ATP
synthase, ATP is formed.
REMEMBER THE NADH?
Kreb’s cycle
Glycolysis
PGAL
8 NADH
2 FADH2
2 NADH
2005-2006
OXIDATIVE
PHOSPHORYLATION
• Occurs in the mitochondria
• NADH (and FADH2) oxidation is used to actively
transport protons (H+ ions) across the inner
mitochondrial membrane, resulting in a proton gradient
across the membrane.
• This drives the synthesis of ATP
• In prokaryotes, this occurs at the cell membrane.
OXIDATIVE
PHOSPHORYLATION
NADH + H+ + ½ O2  NAD+ + H2O
• Does not happen in a single step.
• Respiratory chain
– Series of redox electron carrier proteins
– Embedded in the inner membrane
– Electrons pass from one carrier to the next in a process called the
electron transport.
– Key role of O2 in cells is to act as an electron acceptor and
become reduced.
2H+ + 2e- + ½ O2  H2O
ELECTRON TRANSPORT CHAIN OR CHEMIOSMOSIS
• NADH passes electrons to ETC
–H cleaved off NADH & FADH2
–electrons stripped from H atoms  H+ (H
ions)
–electrons passed from one electron carrier
to next in mitochondrial membrane (ETC)
–transport proteins in membrane pump H+
across inner membrane to intermembrane
space
But what “pulls” the
electrons down the ETC?
ELECTRONS FLOW DOWNHILL
• Electrons move in steps from
carrier to carrier downhill to O2
–each carrier more electronegative
–controlled oxidation
–controlled release of energy
2005-2006
WHY THE BUILD UP
+
H ?
• ATP synthase
– enzyme in inner membrane of mitochondria
ADP + Pi  ATP
– only channel permeable to H+
– H+ flow down concentration gradient = provides energy
for ATP synthesis
• molecular power generator!
• flow like water over water wheel
• flowing H+ cause change in shape of ATP synthase enzyme
• powers bonding of Pi to ADP
• “proton-motive” force
2005-2006
CHEMIOSMOSIS
• The movement of ions across a semipermeable barrier from a region of
higher concentration to a region of lower concentration.
– If the concentration of a substance is greater on one side of a membrane than the
other, the substance will tend to diffuse across the membrane to an area of lower
concentration.
– If a membrane blocks this diffusion, the substance at the higher concentration has
a potential energy , which can be converted to other forms of energy.
– Because the interior of a membrane is nonpolar, protons cannot readily diffuse
across the membrane, but can cross the membrane through the ATP synthase
enzyme. ATP synthase converts potential energy of the proton gradient into the
chemical energy ATP.
ATP YIELD OF
ELECTRON TRANSPORT
CHAIN
About 34 molecules of ATP
are produced per fully
oxidized glucose molecule.
EVOLUTIONARY ADVANTAGE
Those species that could exploit O2
for Oxidative Phosphorylation had a
selective advantage as the
atmosphere filled with from the
ancient photosynthetic
microorganisms.
PYRUVATE IS A BRANCHING POINT
Pyruvate
O2
O2
fermentation
Kreb’s cycle
mitochondria
ANAEROBIC CONDITIONS
•Absence of Oxygen
•Respiratory chain cannot operate.
•Organisms use Fermentation to reoxidize
NADH, thus allowing glycolysis to
continue.
ANAEROBIC ETHANOL FERMENTATION
• Bacteria, yeast
pyruvate  ethanol + CO2
3C
NADH
2C
1C
NAD+
 beer, wine, bread
 at ~12% ethanol, kills yeast
 Animals, some fungi
pyruvate  lactic acid
3C
NADH
3C
NAD+
 cheese, yogurt, anaerobic
exercise (no O2)
2005-2006
FERMENTATION
• Occurs in the cytoplasm.
• Operate to regenerate NAD+
• Consequence of this is that the NADH made during
glycolysis is not available for reoxidation by the
respiratory chain to form ATP.
• Overall yield is restricted to the ATP made in glycolysis.
LACTIC ACID FERMENTATION
ALCOHOLIC FERMENTATION
CELLULAR RESPIRATION SUMMARY