Essential Cell Biology 5th edition

Molecular Mechanisms of Electron Transport and Proton Pumping
469
electrons to the NADH dehydrogenase complex—the first complex in the
chain. As the electrons pass from one enzyme complex to the next, they
promote the pumping of protons across the inner mitochondrial membrane.
In this way, each NADH molecule provides enough net energy to
generate about 2.5 molecules of ATP (see Question 14–5 and its answer).
FADH 2 molecules, on the other hand, bypass the NADH dehydrogenase
complex and pass their electrons to the membrane-embedded mobile
carrier ubiquinone (see Figure 14–14). Because these electrons enter
further down the respiratory chain than those donated by NADH, they
promote the pumping of fewer protons: each molecule of FADH 2 thus
produces only 1.5 molecules of ATP. Table 14−1 provides a full accounting
of the ATP produced by the complete oxidation of glucose.
Although the biological oxidation of glucose to CO 2 and H 2 O consists
of many interdependent steps, the overall process—known as cell
respiration—is remarkably efficient. Almost 50% of the total energy that
could be released by burning sugars or fats is captured and stored in
the phosphate bonds of ATP during cell respiration. That might not seem
impressive, but it is considerably better than most nonbiological energyconversion
devices. Electric motors and gasoline engines operate at about
10–20% efficiency. If cells operated at this efficiency, an organism would
have to eat voraciously just to maintain itself. Moreover, because the
wasted energy is liberated as heat, large organisms (including humans)
would need far better mechanisms for cooling themselves. It is hard to
imagine how animals could have evolved without the elaborate yet economical
mechanisms that allow cells to extract a maximum amount of
energy from food.
MOLECULAR MECHANISMS OF ELECTRON
TRANSPORT AND PROTON PUMPING
For many years, biochemists struggled to understand why electrontransport
chains had to be embedded in membranes to function in ATP
production. The puzzle was essentially solved in the 1960s, when it was
discovered that transmembrane proton gradients drive the process. The
concept of chemiosmotic coupling was so novel, however, that it was not
widely accepted until more than a decade later, when experiments with
artificial energy-generating systems put the power of proton gradients to
the test, as described in How We Know (pp. 476–477).
Although investigators are still unraveling some of the details of chemiosmotic
coupling at the atomic level, the fundamentals are now clear. In
this section, we examine the basic principles that underlie the movement
of electrons, and we explain in molecular detail how electron transport
can drive the generation of a proton gradient. Because very similar mechanisms
are used by mitochondria, chloroplasts, and prokaryotes, these
principles apply to nearly all living things.
Protons Are Readily Moved by the Transfer of Electrons
Although protons resemble other positive ions such as Na + and K + in the
way they move across membranes, in some respects they are unique.
Hydrogen atoms are by far the most abundant atom in living organisms:
they are plentiful not only in all carbon-containing biological molecules
but also in the water molecules that surround them. The protons in water
are highly mobile: by rapidly dissociating from one water molecule and
then associating with its neighbor, they can quickly flit through a hydrogen-bonded
network of water molecules (see Figure 2−15B). Thus water,
which is everywhere in cells, serves as a ready reservoir for the donation
TABLE 14–1 PRODUCT YIELDS
FROM GLUCOSE OXIDATION
Process
Glycolysis
Pyruvate
oxidation
to acetyl
CoA
(two per
glucose)
Complete
oxidation
of the
acetyl
group
of acetyl
CoA
(two per
glucose)
Direct
Product
2 NADH
(cytosolic)
Final ATP
Yield per
Glucose
3*
2 ATP 2
2 NADH
(mitochondrial
matrix)
6 NADH
(mitochondrial
matrix)
5
15
2 FADH 2 3
2 GTP 2
TOTAL 30
*NADH produced in the cytosol
yields fewer ATP molecules than
NADH produced in the mitochondrial
matrix because the mitochondrial
inner membrane is impermeable to
NADH. Transporting NADH into the
mitochondrial matrix—where it can pass
electrons to NADH dehydrogenase—
thus requires energy.
QUESTION 14–5
Calculate the number of usable
ATP molecules produced per pair
of electrons transferred from
NADH to oxygen if (i) five protons
are pumped across the inner
mitochondrial membrane for each
electron passed through the three
respiratory enzyme complexes,
(ii) three protons must pass through
the ATP synthase for each ATP
molecule that it produces from ADP
and inorganic phosphate inside the
mitochondrion, and (iii) one proton
is used to produce the voltage
gradient needed to transport
each ATP molecule out of the
mitochondrion to the cytosol
where it is used.