218 ^|

A Handbook of Biology



The reduced ubiquinone

(ubiquinol or UQH ) is then

oxidized with the transfer

of electrons to cytochrome

c

via

cytochrome

bc1

complex

(complex

III).

Cytochrome c is a small

protein attached to the

outer surface of the inner

membrane. It acts as a

mobile carrier of electrons

between complex III and

IV.



Complex

IV

refers

to

cytochrome

c

oxidase

complex

containing

cytochromes a & a3, and 2

copper centers.



When the electrons pass

from one carrier to another

via complex I to IV, they are coupled to ATP synthase (complex V) for the

ATP production.



In aerobic respiration, the role of oxygen is limited to the terminal

stage. Yet, oxygen is vital since it drives the whole process by removing

hydrogen from the system. Oxygen acts as the final hydrogen acceptor.



In respiration, energy of oxidation-reduction is utilized for the

phosphorylation. So this process is called oxidative phosphorylation while

in photophosphorylation where, light energy is utilized for the production

of proton gradient for phosphorylation. Mechanism of membrane linked

ATP synthesis is explained by chemiosmotic hypothesis.



The energy released during the ETS is utilized to synthesize ATP by ATP

synthase (complex V).



ATP synthase has two major components: F1 & F0.

ADP

F0

F1

ATP

Outer

side

2H⁺

Pi

Matrix

Inner

mitochondrial

membrane

F0

F0

(integral

membrane

protein complex): It creates

a pathway for protons to pass

through the inner membrane.

The transport of protons is

coupled to the F1 component’s

catalytic site for ATP synthesis.

F1

F1 headpiece (peripheral

membrane

protein

complex):

Site

for

ATP

production

from

ADP

and

inorganic phosphate.

Inter-membrane

space

Inner Mitochondrial

membrane

Matrix

4H⁺

4H⁺

2H⁺

2H⁺

Electrochemical

gradient

NADH + H⁺

NAD⁺

Complex I

(NADH dehydrogenase)

Complex III

(Cytochrome bc!)

Complex II

(Succinate dehydrogenase)

Succinate

Fumarate

Cyt c

Cyt c

H2O

Complex IV

(Cytochrome c oxidase)

ADP + Pi

ATP

H⁺

Cu"

CuB

Cyta

Cyta#

IV

F1

F0

ATP

synthase

(Fe-s)

FAD

UQH$

UQH$

Cyt C1

Fe-S

Cyt b

e^-

(Fe-S)

FMN

2e^-

e^-

e^-

e^-

e^-

e^-