BOTANY Higher Secondary Second Year - Textbooks Online

Difference between cyclic and noncyclic electron transport and
photophosphorylation
Cyclic photophosphorylation Noncyclic photophosphorylation
1. It is associated with PS I 1. It is associated with both PS I and
PS II.
2. The electron expelled from 2. The electrons are not cycled back
chlorophyll molecule is cycled but compensated by the electrons
back from photolysis of water.
3. Photolysis of water and 3. Photolysis of water and
evolution of oxygen do not take
place.
evolution of oxygen take place.
4. Photophosphorylation takes 4. Photophosphorylation takes place
place at two places.
5. NADP is not reduced.
only at one place.
5. NADP+ is reduced to NADPH2. carbondioxide in plants during photosynthesis occurs in three stages –
fixation, reduction and regeneration of RuBP.
Fixation
The acceptor molecule of CO 2 is a 5C compound called ribulose-1,5bisphosphate
(RuBP). Fixation of a molecule of CO 2 to RuBP is catalyzed
by the enzyme RuBP carboxylase. The resulting 6C compound is highly
unstable and gets cleaved to form two molecules of 3C compounds called
phosphoglyceric acid (PGA).
Reduction
The two molecules of PGA are further reduced to glyceraldehyde-3phosphates
in two steps. First, two PGA molecules are converted to 1,3 -
bisphosphoglyceric acids by the enzyme PGA kinase. This reaction
consumes two molecules of ATP in the ratio of one ATP for each molecule
of 1,3-bisphosphoglyceric acid formed.
In the second step, the two molecules of 1,3-bisphosphoglyceric acid
are reduced to glyceraldehyde-3-phosphates by the enzyme glyceraldehyde-
3-phosphate dehydrogenase with the help of the light generated reducing
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