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Biology for Advanced Level Secondary Schools

Cyclic photophosphorylation original position in photosystem I (Figure
Task 5.5 5.3). As electrons travel back, they release
Search for information from internet energy, which is utilised to bind inorganic
the simulations/video showing cyclic phosphate to ADP, forming ATP with the
photophosphorylation. Note down what aid of ATP synthase. In this cycle, only ATP
you have observed. is produced, but NADPH and oxygen are
FOR ONLINE READING ONLY
not produced.
Cyclic photophosphorylation involves
production of ATP, which uses photosystem The amount of ATP required in Calvin cycle
I but not photosystem II. This begins with is higher than that produced in the non-
photo activation of photosystem I through cyclic photophosphorylation. This makes
which the electrons are excited and pass on to the cyclic photophosphorylation important
a chain of electron carriers. These electrons to balance the ATP deficit without increasing
travel back along a chain of carriers to their the amount of NADPH.
Primary
Electron
Acceptor
2e -
2e - ADP+Pi
Light rays Electron carrier
systems
ATP

2e -

Photosystem I

Figure 5.3: Cyclic photophosphorylation

Table 5.1: Differences between cyclic and non-cyclic photophosphorylation
Cyclic photophosphorylation Non-cyclic photophosphorylation
Excited electrons always return to the Excited electrons do not return to the original
original position position but they are handed over to reduce
NADP to NADPH
+
2
It involves only one photosystem (P700) It involves two photosystems (P700 and P680)
Photolysis of water does not occur Photolysis of water occurs
Reduced compounds are not formed; the Reduced compounds, especially NADPH are
only product is ATP formed together with ATP
The final destination of electrons is photo The final destination of electrons is NADP
system I
The first source of electrons is photosystem I The first source of electrons is water
Oxygen is not produced, instead only ATP is Oxygen is produced
formed

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