Biomedical Engineering Reference
In-Depth Information
NADH
þ
H
þ
2
is used to create
ATP by the release of hydrogen ions through the inner membrane and electrons within
the inner membrane. The energy released by the transfer of each pair of electrons from
NADH
þ
H
þ
The energy stored in these molecules of
ð
Þ
and
FADH
ð
2
is used to pump a pair of hydrogen ions into the intermembrane
space. The transfer of a pair of electrons is through a chain of acceptors from one to another,
with each transfer providing the energy to move another pair of hydrogen ions through the
membrane. At the end of the acceptor chain, the two electrons reduce an oxygen atom to
form an oxygen ion, which is then combined with a pair of hydrogen ions to form
Þ
and
FADH
H
2
O
:
The movement of the hydrogen ions creates a large concentration of positively charged ions
in the intermembrane space and a large concentration of negatively charged ions in the
matrix, which sets up a large electrical potential. This potential is used by the enzyme
ATP synthase to transfer hydrogen ions into the matrix and to create ATP. The ATP pro-
duced in this process is transported out of the mitochondrial matrix through the inner
membrane using carrier facilitated diffusion and diffusion through the outer membrane.
In the following description, we assume all of the hydrogen and electrons are available
from these reactions. In reality, some are lost and not used to create ATP. Other descrip-
tions of the electron transport chain have additional sites and are omitted here for
simplicity.
We first consider the use of
NADH
þ
H
þ
ð
Þ
in the electron transport chain. During the
NADH
þ
H
þ
are transferred to the electron carrier coen-
zyme Q by NADH dehydrogenase (site 1 and Q in Figure 8.26), and using the energy
released, a pair of hydrogen ions are pumped into the intermembrane space.
Next, the coenzyme Q carries the pair of electrons to the cytochrome
first step, a pair of electrons from
1
complex
(site 2 in Figure 8.26). When the pair of electrons are transfered from the cytochrome
bc
bc
1
complex to cytochrome c (site C in Figure 8.26), the energy released is used to pump
another pair of hydrogen ions into the intermembrane space through the cytochrome
bc
1
complex.
In the third step, cytochrome c transfers electrons to the cytochrome c oxidase complex
(site 3 in Figure 8.26), and another pair of hydrogen ions are pumped through the cyto-
chrome c oxidase complex into the intermembrane space. A total of 6 hydrogen ions have
now been pumped into the intermembrane space, which will allow the subseqent creation
of 3 molecules of ATP.
Also occuring in this step, the cytochrome oxidase complex transfers the pair of electrons
within the inner membrane from the cytochrome c to oxygen in the matrix. Oxygen then
combines with a pair of hydrogen ions to form water.
As described previously, the transfer of hydrogen ions into the intermembrane space cre-
ates a large concentration of positive charges and a large concentration of negative charges
in the matrix, creating a large electrical potential across the inner membrane. The energy
from this potential is used in this step by the enzyme ATP synthase (site 4 in Figure 8.26)
to move hydrogen ions in the intermembrane space into the matrix and to synthesize
ATP from ADP and P.
The ATP in the matrix is then transported into the intermembrane space and ADP is
transported into the matrix using a carrier-mediated transport process (site 5 in Figure 8.26).
From the intermembrane space, ATP diffuses through the outer membrane into the cytosol,
and ADP diffuses from the cytosol into the intermembrane space.
