Page 9 - LN-PHOTOSYNTHESIS IN PLANTS

membrane.
These electrons thus, obtained by the splitting of water are needed to replace those
electrons which are removed from the photosytems-I thus, are provided by
photosystem-II.
2H 2O —> 4H +O 2 + 4e
+
–
While all the electrons formed, are replaced, the protons gets accumulated in the
lumen of the thylakoid and the oxygen gets into the atmosphere.
Photophosphorylation
Photophosphorylation is the process through which, ATP is synthesised from ADP
and inorganic phosphate (P) by the cell organelles (like mitochondria and
chloroplasts) with the help of energy from solar radiation.
Photophosphorylation in mitochondria is not light dependent, but it uses the energy
by oxidation of nutrients to produce ATP, hence it is called oxidative
phosphorylation.
The process of photophosphorylation is of two types
i. Non-cyclic Photophosphorylation
Non-cyclic photophosphorylation is type of photophosphorylarion in which both the
photosystems (PS-I and PS-II) cooperate in light driven synthesis of ATP. During this
cycle, the electron released from PS-II does not return to it hence, it is known as non-
cyclic photophosphorylation. Both NADPH and ATP are formed during this reaction.
ii. Cyclic Photophosphorylation
It is the type of photophosphorylation in which only PS-I is taking part and the
electron released from the reaction centre P 700 returns to it after passing through a
series of carrier i.e., circulation takes within the photosystem and the
phosphorylation occurs due to cyclic flow of electrons.
iii. When non-eyeiie form of photophosphorylation gets stopped under certain
conditions, the cyclic photophosphorylation occurs.
The cyclic photophosphorylation takes place in the stromal lamellae of the
chloroplast. This happens because the stromal lamellae does not possess enzyme
NADP reductase (essential for reducing NADP to NADPH) and PS-II. Thus, the
+
excited electrons in the cyclic photophosphoiylation does not pass on to
NADP instead it gets cycled back to the PS-I complex.
+

Hence, through the cyclic flow only the synthesis of ATP takes place.
Chemiosynthetic Hypothesis