Essential Cell Biology 5th edition

462 CHAPTER 14 Energy Generation in Mitochondria and Chloroplasts
plasma membrane
polysaccharides sugars glucose
pyruvate pyruvate
acetyl CoA
fats fatty acids
fatty acids fatty acids
MITOCHONDRION
CYTOSOL
Figure 14–9 Acetyl CoA is produced in
the mitochondria. In animal cells and other
eukaryotes, pyruvate produced during
glycolysis and fatty acids derived from the
breakdown of fats enter the mitochondrion
from the cytosol. Once inside the
mitochondrial matrix, both of these foodderived
molecules are converted to acetyl
CoA and then oxidized to CO 2 .
The citric acid cycle gets the fuel it needs to produce these activated carriers
from food-derived molecules that make their way into mitochondria
from the cytosol. Both the pyruvate produced by glycolysis and the fatty
acids derived from the breakdown of fats (see Figure 13−3) can enter
the mitochondrial intermembrane space through the porins in the outer
mitochondrial membrane. These fuel molecules are then transported
across the inner mitochondrial membrane into the matrix, where they
are converted into the crucial metabolic intermediate, acetyl CoA (Figure
14−9). The acetyl groups in acetyl CoA are then oxidized to CO 2 via the
citric acid cycle (see Figure 13−12). Some of the energy derived from this
oxidation is saved in the form of high-energy electrons, held by the activated
carriers ECB5 NADH e14.09/14.09 and FADH 2 . These two activated carriers can then
donate their electrons to the electron-transport chain in the inner mitochondrial
membrane (Figure 14–10).
The Movement of Electrons Is Coupled to the Pumping
of Protons
The chemiosmotic generation of energy begins when the activated carriers
NADH and FADH 2 donate their electrons to the electron-transport
chain in the inner mitochondrial membrane, becoming oxidized to NAD +
and FAD, respectively, in the process (see Figure 14–10). The electrons are
quickly passed along the chain to molecular oxygen (O 2 ) to form water
(H 2 O). The stepwise movement of these electrons through the components
of the electron-transport chain releases energy that can then be
used to pump protons across the inner mitochondrial membrane (Figure
14–11). The resulting proton gradient, in turn, is used to drive the synthesis
of ATP. The full sequence of reactions is shown in Figure 14–12. The
inner mitochondrial membrane thus serves as a device that converts the
energy contained in the high-energy electrons of NADH (and FADH 2 ) into
the phosphate bond of ATP molecules (Figure 14–13). This chemiosmotic
Figure 14–10 NADH donates its
“high-energy” electrons to an electrontransport
chain. A hydride ion (a hydrogen
atom with two electrons, red ) is removed
from NADH and is converted into a proton
and two electrons (blue). Only the part
of NADH that carries these high-energy
electrons is shown; for the complete
structure and the conversion of NAD + back
to NADH, see the structure of the closely
related NADPH in Figure 3–34. Electrons
are also carried in a similar way by FADH 2 ,
whose structure is shown in Figure 13−13B.
two high-energy
electrons from
sugar oxidation
H H O
H
H
C C C
H
C NH ELECTRON
BOND
2
C C C
DONATION REARRANGEMENT
C C
C C
H N
H
H N
hydride ion H –
NADH NAD +
H + 2 e –
O
C
H
two electrons passed to electrontransport
chain in inner membrane
NH 2