BOTANY Higher Secondary Second Year - Textbooks Online

electrons are transported through electron transport chain of plastoquinone,
cytochrome b 6, cytochrome f and plastocyanin. The electrons released
from PS II phosphorylate ADP to ATP. This process of ATP formation
from ADP in the presence of light in chloroplast is called
photophosphorylation.
Now, the PS II is in oxidised state. It creates a potential to split water
molecules to protons, electrons and oxygen. This light dependent splitting
of water molecules is called photolysis of water. Manganese, calcium
and chloride ions play prominent roles in the photolysis of water. The
electrons thus released are used in the reduction of PS II. Similar to PS II,
PS I is excited by absorbing photons of light and gets oxidised. This
oxidised state of the PS I draws electrons from PS II and gets reduced.
The electrons released to PS I are transported through electron transport
chain of ferredoxin reducing substrate, ferredoxin and ferredoxin NADP
reductase to reduce NADP + to NADPH 2 .
5.2.5. Cyclic and noncyclic photophosphorylation
In chloroplasts, phosphorylation occurs in two ways – noncyclic
photophosphorylation and cyclic photophosphorylation.
Noncyclic photophosphorylation
When the molecules in the PS I are excited the electrons are released.
So, an electron deficiency or a hole is made in the PS I. This electron is
now transferred to ferredoxin to reduce NADP + . When the molecules in
the PS II get excited, electrons are released. They are transferred to fill
the hole in PS I through plastoquinone, cytochrome b6, cytochrome f and
plastocyanin. When the electron is transported between plastoquinone and
cytochrome f, ADP is phosphorylated to ATP.
The ‘hole’ in the PS I has been filled by the electron from PS II. Then
the electrons are transferred from PS I to NADP + for reduction. Therefore,
this electron transport is called noncyclic electron transport and the
accompanying phosphorylation as noncyclic photophosphorylation. The
noncyclic electron transport takes place in the form of ‘Z’. Hence, it is
also called Z-scheme.
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