Dictionary of Chemistry [6th Ed.]

tions vary enormously. These rates

can be explained in terms of the way

in which molecules of the solvent

solvating the reactants rearrange so

as to solvate the products in the case

of the outer-sphere mechanism. In

the case of the inner-sphere (ligand-

bridged) reactions the rate of the re-

action depends on the intermediate

and the way in which the electron is

transferred.

electron transport chain (elec-

tron transport system) A sequence of

biochemical reduction–oxidation re-

actions that effects the transfer of

electrons through a series of carriers.

An electron transport chain, also

known as the respiratory chain,

forms theÜnal stage of aerobic respi-

ration. It results in the transfer of

electrons or hydrogen atoms derived

from the *Krebs cycle to molecular

oxygen, with the formation of water.

At the same time it conserves energy

from food or light in the form of

*ATP. The chain comprises a series of

carrier molecules that undergo re-

versible reduction–oxidation reac-

tions, accepting electrons and then

donating them to the next carrier in

the chain – a process known as elec-

tron Ûow. In the mitochondria,

NADH and FADH 2 , generated by the

Krebs cycle, transfer their electrons

to a chain including coenzyme Q (see

ubiquinone) and a series of *cy-

tochromes. This process is coupled to

the formation of ATP at three sites

along the chain. The ATP is then car-

ried across the mitochondrial mem-

brane in exchange for ADP. An

electron transport chain also occurs

in *photosynthesis.

electronvolt Symbol eV. A unit of

energy equal to the work done on an

electron in moving it through a po-

tential difference of one volt. It is

used as a measure of particle ener-

gies although it is not an *SI unit. 1

eV = 1.602 × 10 –19joule.

electroorganic reaction An or-

ganic reaction produced in an elec-

trolytic cell. Electroorganic reactions

are used to synthesize compounds

that are difÜcult to produce by con-

ventional techniques. An example of

an electroorganic reaction is *Kolbe’s

method of synthesizing alkanes.

electroosmosis The movement of

a polar liquid through a membrane

under the inÛuence of an applied

electricÜeld. The linear velocity of

Ûow divided by theÜeld strength is

the electroosmotic mobility. Elelec-

troosmosis was formerly called elec-

troendosmosis.

electroosmotic mobility See elec-

troosmosis.

electropherogram See capillary

electrophoresis.

electrophile An ion or molecule

that is electron deÜcient and can ac-

cept electrons. Electrophiles are

often reducing agents and Lewis

electron transport chain 200

e

inner mitochondrial

membrane

H+

NADH

dehydrogenase

NADH

+H+

NAD+

2e– 2e– 2e–

H+ H+

H 2 O

2H+ + ½O 2

ubiquinone cytochromeb-c

1 complex

cytochrome

oxidase

Electron transport chain