Cellular Respiration Notes
Ch 9
C6H12O6 + 6O2  6CO2 + 6H20 + 36 ATP
The cellular respiration reaction is much like a simple combustion reaction. It burns a hydrocarbon
fuel (glucose) to produce energy, carbon dioxide, and water just like a car burns gasoline. However,
simple combustion of glucose is not feasible in the cell. It would release a bunch of energy all at
once (most of which would go to waste: heat/light).
Therefore, cellular respiration occurs in a series of small steps regulated by enzymes. This allows
energy to be converted into a usable form and into reasonably sized packets (ATP).
Redox Reactions: any reaction in which one molecule gains electrons and another loses them.
 Oxygen is very electronegative, and therefore pulls electrons away from other atoms and
releases energy in the process. It gains electrons and is reduced. The substance losing the
electrons is oxidized. OIL RIG (oxidation is loss, reduction is gain)
 Oxygen is the driving force for cellular respiration because it is needed to pull the electrons
towards it to release the energy. It is the final electron acceptor in the electron transport chain.
Cellular Respiration—A Brief Summary
 Glycolysis: literally “sugar splitting”; results in net production of 2 pyruvate molecules, 2 ATP,
and 2 NADH; does not require oxygen; occurs in cytosol. In reality it makes 4 ATP, but needs
2 ATP to get started, so the net gain is 2 ATP. It is a series of over 10 reaction with several
coenzymes and cofactors.
 LUCAS STEP: This is a made up name (Lucas Enloe currently at West Point is the author)
which describes the break down of the 3C pyruvate into 2 C acetate that binds to Coenzyme A
to become Acetyl CoA. This step releases one molecule of carbon dioxide per pyruvate. The
Acetyl CoA can then enter the mitochondrion.
 Kreb’s Cycle: Further breaking down of carbon molecules to create CO2, ATP, and the electron
carriers/acceptors, NADH and FADH2; does not require oxygen directly, but this cycle will stop
if oxygen is not present. This is because oxygen is necessary to accept the electrons from
NADH and FADH2 when they break down and without it these molecules build up and do not
allow the Kreb’s cycle to go forward. This is an example of feedback inhibition stopping the
cycle. It occurs in the matrix of the mitocondria.
 Electron Transport Chain: Converts NADH and FADH2 into ATP. They do this by transporting
electrons to a series of more and more electronegative cytochrome proteins (electron transport
chain) with oxygen as the final acceptor. As the electron is passed down the chain through a
series of redox reactions, the energy is used to pump hydrogen ions across the membrane
creating a membrane potential. As the H+ ions are coming back across the membrane due to a
concentration gradient, ATP synthase uses this proton motive forces to make ATP. FADH2
gives its electrons to cytochromes further down the chain so it provides only enough energy to
make 2 ATP whereas NADH uses the full chain and makes 3 ATP. This occurs on the inner
membranes of the mitochondria.
Substrate level phosphorylation: the production of ATP molecules directly from the processes of
glycolysis or the kreb’s cycle. Oxygen is NOT used to create the ATP.
Oxidative phosphorylation: the production of ATP from the passing of electrons down the electron
transport chain to oxygen.
Cellular Respiration Notes
Ch 9
Glycolysis—A Closer Look
 Two parts: energy investment phase and energy yielding phase
 Energy investment phase: 2 ATP molecules are put in. This provides glucose with the energy
needed to rearrange so that it can be split into two 3-carbon molecules.
 Energy yielding phase: The two 3-carbon molecules are rearranged to more stable
configurations eventually yielding two 3-carbon molecules of pyruvate. These reactions are
exergonic and therefore release energy—specifically they release 4 ATP molecules. Also, some
electrons get transferred to 2 NAD (nicotinamide adenine dinucleotide) resulting in the release
of 2 NADH molecules.
 The net yield of glycolysis is therefore 2 ATP and 2 NADH.
Kreb’s Cycle—A Closer Look
 The following cycle occurs with each pyruvate molecule. That means it occurs TWICE for each
molecule of glucose.
1. Pyruvate molecule travels from cytoplasm to mitochondrial matrix, where it is converted to
acetyl CoA (a 2-carbon molecule) and CO2.
2. Acetyl CoA binds with 4-carbon compound, Oxaloacetate, forming a 6-carbon compound.
3. Enzyme catalyzed reaction occurs converting 6-carbon compound to a 5-carbon compound and
1 molecule of CO2. 1 NAD molecule is converted to NADH.
4. Another enzymed catalyzed reaction occurs converting the 5-caron compound to a 4-carbon
compound and 1 molecule of CO2. 1 NAD molecule is converted to NADH.
5. 4-carbon compound rearranges several times, 1st releasing ATP, 2nd converting FAD to FADH2,
and 3rd converting NAD to NADH.
6. Final 4-carbon sugar is converted to oxyaloacetate ready to start cycle over again.
NET RESULT: (1 ATP, 4 NADH, 1 FADH2) * 2 = 2 ATP, 8 NADH, 2 FADH2
Electron Transport Chain—A Closer Look
 All NADH and FADH2 travel to electron transport chain. (NADH molecule made in glycolysis
is converted to FADH2 on the way in).
 NADH and FADH2 are electron carriers. They received the electrons earlier in the process of
respiration and they drop them off at the electron transport chain.
 As the electrons get transferred to more and more electronegative molecules (located on the
inner mitochondrial membrane), they release energy. This energy is used to pump protons (H+
molecules) out of the matrix and into the intermembrane space.
 The proton difference across the membrane sets up a concentration gradient (high on one side
and low concentration on the other). This is a form of stored energy because the H+ ions want
to cross back to even out the charge on either side of the membrane. The H+ ions do, in fact,
cross back across the membrane via a transport molecule called ATPase. This is called the
proton motive force (force of protons moving). As they pas through ATPase, the energy the
released is used to make ATP. This process is called chemiosmosis
 NADH molecules carry enough energy to make about 3 ATP molecules; FADH2 molecules
carry enough energy to make 2 ATP molecules.
Accouting:
Glycolysis: 2 ATP, 2NADH (converted to FADH2) = 6 ATP (only 2 each for NADH made here)
Kreb’s/ Electron transport chain: 2ATP, 8NADH, 2FADH2 = 30 ATP
Total: 36 ATP
Cellular Respiration Notes
Ch 9
ANAEROBIC RESPIRATION (FERMENTATION)
When there isn’t enough oxygen to accept electrons from the electron transport chain, the electron
transport chain gets put on hold. This results in a buildup of NADH and FADH2 and a shortage of
NAD and FAD. The excess of these two carriers inhibits the Kreb’s cycle. Thus, the pyruvate
molecules produced by glycolysis go into another reaction. In some organisms, like yeast, the
pyruvate is converted to ethyl alcohol and CO2. In others, like people, the pyruvate is converted to
lactic acid and CO2. In both process the NADH made by glycolysis is converted back to NAD in
the process. This allows glycolysis to continue to make 2 ATP. The problem is that an organism is
burning glucose for 2 ATP instead of 36 ATP and this is wasteful if you have a high metabolism.
Some one cell organisms can afford it, but not many continue for long this way.
Anaerobic respiration = glycolysis + fermentation
Fermentation is used to make bread: The sweet baking smell is the alcohol evaporating and the
rising of the bread is due to the yeast creating carbon dioxide as it undergoes anaerobic
repiration/fermentation. The carbon dioxide is trapped.
Fermentation is used to make wine. This is why the grapes and bacteria must be in an airtight
container. Otherwise the bacteria switch over to aerobic respiration which would make NO alcohol.
Fermentation also makes sauerkraut and pickles.
In us, we do fermentation when our muscle cells our short of oxygen and this builds up lactic acid
which causes the muscle fibers to take longer to return to form after each contraction. This is what
we call fatigue. Eventually they will not be able to “relax” back to form and we will experience a
cramp. This is evolutionarily significant as it lets us know that we are burning sugar too fast for too
little ATP.