The pyruvate produced in glycolysis undergoes further breakdown through a
process called aerobic respiration in most organisms. This process
requires oxygen and yields much more energy than glycolysis. Aerobic respiration
is divided into two processes: the Krebs cycle, and the Electron Transport
Chain, which produces ATP through chemiosmotic phosphorylation. The energy
conversion is as follows:
C6H12O6 + 6O -> 6CO2 + 6H2O + energy (ATP)
The pyruvate molecules produced during glycolysis
contain a lot of energy in the bonds between their molecules. In order to use
that energy, the cell must convert it into the form of ATP. To do so,
pyruvate molecules are processed through the Kreb Cycle, also known as the citric
1. Prior to entering the Krebs Cycle, pyruvate must be converted into acetyl CoA
(pronounced: acetyl coenzyme A). This is achieved by removing a CO2 molecule
from pyruvate and then removing an electron to reduce an NAD+ into NADH. An enzyme
called coenzyme A is combined with the remaining acetyl to
make acetyl CoA which is then fed into the Krebs Cycle. The steps in the Krebs
Cycle are summarized below:
2. Citrate is formed when the acetyl group from acetyl CoA combines with oxaloacetate from the
previous Krebs cycle..
3. Citrate is converted into its isomer isocitrate..
4. Isocitrate is oxidized to form the 5-carbon α-ketoglutarate. This
step releases one molecule of CO2 and reduces NAD+ to NADH2+.
5. The α-ketoglutarate is oxidized to succinyl CoA, yielding CO2 and NADH2+.
6. Succinyl CoA releases coenzyme A and phosphorylates ADP into ATP.
7. Succinate is oxidized to fumarate, converting FAD to FADH2.
8. Fumarate is hydrolized to form malate.
9. Malate is oxidized to oxaloacetate, reducing NAD+ to NADH2+.
We are now back at the beginning of the Krebs Cycle. Because glycolysis produces two
pyruvate molecules from one glucose, each glucose is processes through the kreb cycle twice.
For each molecule of glucose, six NADH2+, two FADH2, and two ATP.
Electron Transport Chain
What happens to the NADH2+ and FADH2 produced during the Krebs cycle? The
molecules have been reduced, receiving high energy electrons from the pyruvic
acid molecules that were dismantled in the Krebs Cycle. Therefore, they represent
energy available to do work. These carrier molecules transport the high energy
electrons and their accompanying hydrogen protons from the Krebs Cycle to the
electron transport chain in the inner mitochondrial membrane.
In a number of steps utilizing enzymes on the membrane, NADH2+
to NAD+, and FADH2 to FAD. The high energy electrons are transferred to
ubiquinone (Q) and cytochrome c molecules, the electron carriers within the
membrane. The electrons are then passed from molecule to molecule in the inner
membrane of the mitochondron, losing some of their energy at each step. The
final transfer involves the combining of electrons and H2 atoms with oxygen to
form water. The molecules that take part in the transport of these electrons are
referred to as the electron transport chain.
The process can be summarized as follows: the electrons that are delivered to
the electron transport system provide energy to "pump" hydrogen
protons across the inner mitochondrial membrane to the outer compartment. This
high concentration of hydrogen protons produces a free energy potential that can
do work. That is, the hydrogen protons tend to move down the concentration
gradient from the outer compartment to the inner compartment.
However, the only path that the protons have is through enzyme complexes
within the inner membrane. The protons therefore pass through the channel lined
with enzymes. The free energy of the hydrogen protons is used to form ATP by
phosphorylation, bonding phosphate to ADP in an enzymatically-mediated reaction.
Since an electrochemical osmotic gradient supplies the energy, the entire
process is referred to as chemiosmotic phosphorylation.
Once the electrons (originally from the Krebs Cycle) have yielded their
energy, they combine with oxygen to form water. If the oxygen supply is cut off,
the electrons and hydrogen protons cease to flow through the electron transport
system. If this happens, the proton concentration gradient will not be
sufficient to power the synthesis of ATP. This is why we, and other species, are
not able to survive for long without oxygen!