Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
H + H +
H + H +
intermembrane space
respiratory complex
inner
mitochondrial
membrane
e − e − ADP + P i ATP
H +
reduced
NAD
NAD
reduced
FAD
FAD
e − e − e −
ATP
synthase
matrix
H +
oxygen
water
Figure 12.10 Oxidative phosphorylation: the electron transport chain.
274
Reduced NAD and reduced FAD are passed to the
electron transport chain. Here, the hydrogens are removed
from the two hydrogen carriers and each is split into its
constituent proton (H + ) and electron (e − ). The energetic
electron is transferred to the first of a series of
electron carriers.
Most of the carriers are associated with membrane
proteins, of which there are four types. A functional unit,
called a respiratory complex, consists of one of each of
these proteins, arranged in such a way that electrons can be
passed from one to another down an energy gradient.
As an electron moves from one carrier at a higher
energy level to another one at a lower level, energy is
released. Some of this energy is used to move protons
from the matrix of the mitochondrion (Figure 12.11)
into the space between the inner and outer membranes
of the mitochondrial envelope. This produces a higher
concentration of protons in the intermembrane space than
in the matrix, setting up a concentration gradient.
Now, protons pass back into the mitochondrial matrix
through protein channels in the inner membrane, moving
down their concentration gradient. Associated with each
channel is the enzyme ATP synthase. As the protons pass
through the channel, their electrical potential energy is
used to synthesise ATP in the process called chemiosmosis
(Figure 12.5, page 271).
Finally, oxygen has a role to play as the final electron
acceptor. In the mitochondrial matrix, an electron and a
proton are transferred to oxygen, reducing it to water. The
process of aerobic respiration is complete.
The sequence of events in respiration and their sites
are shown in Figure 12.11. The balance sheet of ATP used
and synthesised for each molecule of glucose entering the
respiration pathway is shown in Table 12.1.
Theoretically, three molecules of ATP can be produced
from each molecule of reduced NAD, and two molecules of
ATP from each molecule of reduced FAD. However, this yield
cannot be achieved unless ADP and P i
are available inside the
mitochondrion. About 25% of the total energy yield of electron
transfer is used to transport ADP into the mitochondrion
and ATP into the cytoplasm. Hence, each reduced NAD
ATP used ATP made
Net gain in
ATP
glycolysis −2 4 +2
link reaction 0 0 0
Krebs cycle 0 2 +2
oxidative
phosphorylation
0 28 +28
Total −2 34 +32
Table 12.1 Balance sheet of ATP use and synthesis for each
molecule of glucose entering respiration.