922
UNIT 4
Maintenance of the Body
24
example, some of the proteins, the
flavins
, contain flavin mono-
nucleotide (FMN) derived from the vitamin riboflavin, and oth-
ers contain both sulfur (S) and iron (Fe). Most of these proteins,
however, are brightly colored iron-containing pigments called
cytochromes
(si
9
to-krōmz;
cyto
5
cell,
chrom
5
color), includ-
ing complexes III and IV depicted in Figure 24.8. Neighboring
carriers are clustered together to form four
respiratory enzyme
complexes
that are alternately reduced and oxidized as they
pick up electrons and pass them on to the next complex in the
sequence.
e-
e-
e-
e-
50
40
30
20
10
0
FMN
F
e•S
Q
F
e•S
Cyt b
F
e•S
Cyt c
1
Cyt c
Cyt a
Cyt a
3
O
2
Free energy relative to O
2
(kcal/mol)
Glycolysis
Krebs
cycle
1
2
ATP
Enzyme
Complex I
Enzyme
Complex III
Enzyme
Complex IV
Enzyme
Complex II
Electron trans-
port chain
and oxidative
phosphorylation
NADH+H
+
FADH
2
ATP
ATP
Figure 24.9
Electronic energy gradient in the electron
transport chain.
Each member of the chain (respiratory enzyme
complex) alternates between a reduced state and an oxidized state.
A complex becomes reduced by accepting electrons from its “uphill”
neighbor and then reverts to its oxidized form as it passes electrons
to its “downhill” neighbor. The electron transport chain breaks up
the overall energy drop into smaller steps.
As Figure 24.8 shows, the first such complex accepts hydro-
gen atoms from NADH
1
H
1
, oxidizing it to NAD
1
. FADH
2
transfers its hydrogen atoms slightly farther along the chain
to the small complex II. Te hydrogen atoms that the reduced
coenzymes deliver to the electron transport chain are quickly
split into protons (H
1
) plus electrons. Te electrons are shuttled
along the inner mitochondrial membrane from one complex
to the next, losing energy with each transfer. Te protons es-
cape into the watery matrix, only to be picked up and “pumped”
across the inner mitochondrial membrane into the intermem-
brane space by one of the three major respiratory enzyme com-
plexes (I, III, or IV).
Ultimately the electron pairs are delivered to half a molecule
of O
2
(in other words, to an oxygen atom), creating oxygen ions
(O
2
) that strongly attract H
1
and form water, as indicated by
the reaction
2H
1
1
2e
2
1
1
2
O
2
S
H
2
O
Virtually all the water resulting from glucose oxidation is
formed during oxidative phosphorylation. Because NADH
1
H
1
and FADH
2
are oxidized as they release their burden of
picked-up hydrogen atoms, the net reaction for the electron
transport chain is
Coenzyme-2H
1
1
2
O
2
S
coenzyme
1
H
2
O
reduced
oxidized
coenzyme
coenzyme
Te transfer of electrons from NADH
1
H
1
to oxygen re-
leases large amounts of energy. If hydrogen combined directly
with molecular oxygen, the energy would be released in one big
burst and most of it would be lost to the environment as heat.
Instead, energy is released in many small steps as the electrons
stream from one electron acceptor to the next.
Each successive carrier has a greater affinity for electrons
than those preceding it. For this reason, the electrons cascade
“downhill” from NADH
1
H
1
to progressively lower energy
levels until they are finally delivered to oxygen, which has the
greatest affinity of all for electrons. You could say that oxygen
“pulls” the electrons down the chain
(Figure 24.9)
.
Te electron transport chain functions as an energy con-
verter by using the stepwise release of electronic energy to pump
protons from the matrix into the intermembrane space. Because
the inner mitochondrial membrane is nearly impermeable to
H
1
, this chemiosmotic process creates an
electrochemical pro-
ton (H
1
) gradient
across that membrane, a gradient that has
potential energy and the capacity to do work.
Te proton gradient (1) creates a pH gradient, with the H
1
concentration in the matrix much lower than that in the inter-
membrane space; and (2) generates a voltage across the mem-
brane that is negative on the matrix side and positive between
the mitochondrial membranes. Both conditions strongly attract
H
1
back into the matrix. But how can they get there?
Te only areas of the membrane freely permeable to H
1
are
large enzyme-protein complexes (complex V) called
ATP syn-
thases
. Tese complexes, which populate the inner mitochon-
drial membrane
(Figure 24.10)
, lay claim to being nature’s
previous page 956 Human Anatomy and Physiology (9th ed ) 2012 read online next page 958 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off