AEROBIC
RESPIRATION Chapter 8
AEROBIC RESPIRATION Aerobic respiration is the next step after Glycolysis if the cell can
obtain oxygen.
We won’t need it until the last step…but we still need it.
Remember that the final product of Glycolysis is pyruvate.
Aerobic respiration takes place in the mitochondria
The first step is an intermediate reaction to prepare the pyruvate for
the citric acid cycle
(The intermediate step is only one step, and it’s not technically part
of Glycolysis OR aerobic respiration. It’s just a preparatory step.)
The pyruvate gives off a carbon as CO2, donates a H to form NADH, and the
final molecule is an acetyl-CoA molecule
CITRIC ACID CYCLE
The citric acid cycle takes place in the matrix of the
mitochondria
**Remember: each step in aerobic respiration happens
TWICE—once for each PGAL formed in Glycolysis
Step 1
Acetyl CoA (2-carbon molecule) bonds with an oxaloacetate (4-
carbon molecule) to form citric acid (6-carbon molecule)
CITRIC ACID CYCLE
Step 2
Citric acid gives off a CO2
The molecule now contains 5-carbons
CO2 is not needed by the cell, so it is expelled out into the blood
stream.
Citric acid donates a hydrogen to an NADH
Citric acid reforms to an alpha-ketoglutarate
CITRIC ACID CYCLE
Step 3
Alpha-ketogluterate gives off a CO2, a hydrogen for an
NADH, and a phosphate for ATP
The molecule then forms a succinate
The molecule is now back to the 4-carbon molecule that the
cycle started with
CITRIC ACID CYCLE
Step 4
Succinate donates a hydrogen for an FADH2 molecule.
The succinate then rearranges to form a molecule of
fumerate
CITRIC ACID CYCLE
Step 5
The fumerate rearranges to form a molecule called
malate
CITRIC ACID CYCLE
Step 6
The malate donates a hydrogen to form NADH
The malate then reforms to the original
oxaloacetate molecule
The oxaloacetate begins the cycle over again.
CITRIC ACID CYCLE SUMMARY
Inputs…per cycle, (per glucose)
Acetyl CoA…1 (2)
Outputs
ATP… 1 (2)
FADH2… 1 (2)
CO2… 2 (4)
NADH… 3 (6)
ELECTRON CARRIERS
A hydrogen is simply one proton and one electron. So, when a
“hydrogen” is donated, it is also appropriate to say an “electron” is
donated
Electron carriers are molecules that transport a hydrogen from one
location to another
Typically, the electron of the hydrogen will be used as a cofactor for an
enzyme
NADH and FADH2 have been synthesized multiple times so far in
cellular respiration.
All will finally be used in the electron transport chain as reactants
ELECTRON TRANSPORT CHAIN
The electron transport chain follows glycolysis and the citric
acid cycle.
It is taking place at the same time as glycolysis and citric acid cycle,
but it uses the products of glycolysis and the citric acid cycle as
reactants
The ETC takes place in the inner membrane of the
mitochondria.
The ETC is powered thanks to the concept of diffusion and
equilibrium
Important fact to remember: diffusion and osmosis naturally occur
in the universe, which means that these processes happen for free.
ELECTRON TRANSPORT CHAIN The ETC is a series of protein channels embedded in the cristae
(inner mitochondrial membrane).
The NADH and FADH2 give off their electron, which powers each
protein channel in sequence.*
The NAD+ and FAD+ then return to pick up another electron
*REMEMBER: If we can’t do this step, then the cell has to do fermentation instead.
These proteins move hydrogen atoms from inside the membrane to
outside the membrane, against the concentration gradient.
The energy for this comes from the NADH and FADH2 electrons.
The hydrogen that cross the membrane are already present. They never leave.
This creates an unequal ratio of hydrogen atoms along the membrane
(more are outside than inside). The membrane is NOT in equilibrium
ATP SYNTHASE The only way for the hydrogen atoms to get back across
the membrane (and reach equilibrium) is through a specific
channel enzyme called ATP synthase.
ATP synthase looks like an upside-down light bulb.
As the hydrogen atoms pass through the ATP synthase
from the outside of the membrane to the inside, they
provide kinetic energy to the enzyme.
With this energy, ATP synthase attaches phosphates to
ADP molecules in the “bulb” part, building an ATP
molecule.
ATP PRODUCTION Each molecule of NADH powers the ETC enough to build 3
molecules of ATP
FADH gives a little less power and can build only 2 ATP
This means the ETC can produce a total of 32-34 ATP per glucose
molecule.
Add that to the four ATP already produced in glycolysis and the citric
acid cycle, you have a maximum-possible net gain of 36-38 ATP
molecules from 1 molecule of glucose.
With fermentation: it’s two.
To remove the electron from the ETC, the cell bonds it with a
molecule of oxygen and forms H2O
This is why you need to breathe. This is what the oxygen is used
for.
ETC SUMMARY
Inputs (per molecule of glucose)
10 NADH
2 FADH2
O2
Outputs
28-30 ATP from NADH
4 ATP from FADH2
NAD+
FAD+
H2O
CELL RESPIRATION SUMMARY
C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy
C6H12O6 : For glycolysis
6 O2 : To collect the electron in the ETC
6 CO2 : Given off in intermediate step and Citric Acid Cycle
6 H2O : Given off in the ETC
Energy : In the form of ATP