Human Physiology, 14th edition (2016)

114 Chapter 5

H^1 that drives the phosphorylation of ADP to ATP ( fig.  5.9 ).

In this way, phosphorylation (ADP  1  P i → ATP) is coupled to

oxidation (the removal and transport of electrons) in the pro-

cess of oxidative phosphorylation.

Function of Oxygen

If the last cytochrome remained in a reduced state, it would be

unable to accept more electrons. Electron transport would then

progress only to the next-to-last cytochrome. This process would

continue until all of the elements of the electron-transport chain

remained in the reduced state. At this point, the electron-transport

system would stop functioning and no ATP could be produced

in the mitochondria. With the electron-transport system incapaci-

tated, NADH and FADH 2 could not become oxidized by donat-

ing their electrons to the chain and, through inhibition of citric

acid cycle enzymes, no more NADH and FADH 2 could be pro-

duced in the mitochondria. The citric acid cycle would stop and

only anaerobic metabolism could occur.

Oxygen, from the air we breathe, allows electron trans-

port to continue by functioning as the final electron acceptor

of the electron-transport chain. This oxidizes cytochrome a 3 ,

allowing electron transport and oxidative phosphorylation to

Figure 5.8 The electron transport system. Each element in the electron-transport chain alternately becomes reduced and oxidized
as it transports electrons to the next member of the chain. This process provides energy for the pumping of protons into the intermembranous
space of the mitochondrion, and the proton gradient is used to produce ATP (as shown in fig. 5.9 ). At the end of the electron-transport chain,
the electrons are donated to oxygen, which becomes reduced (by the addition of 2 hydrogen atoms) to water.

NADH

FADH 2

FAD

Oxidized

CoQ

Reduced

Fe2+

Cytochrome b

Fe3+

Fe2+

Cytochrome

c 1 and c

Fe3+

Fe3+

Cytochrome a

Fe2+

Fe2+

Cytochrome a 3

Fe3+

NAD

FMN

FMNH 2

2 H+

Electron energy

2 e–

2 e–

2 e–

2 e–

2 H+

+ O 2

H 2 O

(^1) –
2
CLINICAL APPLICATION
Mitochondrial molecules, including those of the respira-
tory chain, are coded by the 37 genes in mitochondrial
DNA and by genes in nuclear DNA. Mitochondrial DNA is
inherited from the oocyte, and so is passed from mother
to child. Mutations in mitochondrial DNA may occur in
some mitochondria but not others. This produces vari-
able symptoms in mitochondrial diseases, although the
brain and skeletal muscles are generally affected. There
is a deficiency in ATP production, and most mitochon-
drial diseases are associated with excessive lactic acid
production. One such mitochondrial disease is MELAS —
mitochondrial encephalomyopathy, lactic acidosis, and
strokelike episodes.
A superoxide radical is an oxygen molecule with an
extra, unpaired electron that can be produced within mito-
chondria by leakage of electrons from the electron transport
system. The superoxide and other free radicals are believed
to contribute to many diseases by exerting an oxidative
stress on the body. Antioxidants are molecules that scav-
enge free radicals (chapter 19, section 19.1).