BOTANY Higher Secondary Second Year - Textbooks Online

Cyclic photophosphorylation
Under the conditions of (i) PS I only remains active (ii) photolysis of
water does not take place (iii) requirement of ATP is more and (iv)
nonavailability of NADP
Fig. 5.9 Cyclic photophosphorylation
+ the
cyclic photophosphorylation takes
place. When the molecule in the
PS I is excited, the electrons are
released. The electrons are
captured by ferredoxin through
ferredoxin reducing substrate
(FRS). Due to non-availability of
NADP+, electrons from
ferredoxin fall back to the
molecules of PS I through the
electron carriers - cytochrome b6, cytochrome f and plastocyanin.
These electron carriers facilitate
the down hill transport of
electrons from FRS to PS I.
During this transport of electrons,
two phosphorylations take place
- one between ferredoxin and
cytochrome b6 and the other
between cytochrome b6 and
cytochrome f. Thus, two ATP
molecules are produced in this
cycle.
Dark reactions
The reactions that catalyze
the reduction of CO to 2
carbohydrates with the help of the ATP and NADPH 2 generated by the
light reactions are called the dark reactions. The enzymatic reduction of
CO 2 by these reactions is also known as carbon fixation. These reactions
that result in CO 2 fixation take place in a cyclic way and were discovered
by Melvin Calvin. Hence, the cycle is called Calvin cycle. Fixation of
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