Non-Cyclic Photophosphorylation : The electrons lost by P₆₈₀ (PS-II) are taken up by P₇₀₀ (PS-I) and do not get back to P₆₈₀ i.e., unidirectional and hence it is called non- cyclic phosphorylation. It involves both PS I and PS II. In this cycle both ATP and NAD(P)H are formed.

Fig. Non-cyclic photophosphorylation

Action Spectrum. Action spectrum is a curve which shows relative rates of photosynthesis at The graph of the action spectrum indicates that the maximum photosynthesis takes place in the blue and red parts of visible light. 

Wave length of light in nm Graph of Action Spectrum

Absorption Spectrum. Absorption spectrum is a curve which shows the amount of light of different wavelengths absorbed by a substance or pigment. The absorption spectrum of two chlorophyll (chl “a” and chl “b”) and carotene shows that maximum light absorbed is in the blue and red wavelengths. 

Photosystem I [PSI] :

Location : It is present in the membrane of stroma thylakoids and non-appressed parts of grana thylakoids.

Pigment molecules : In PS I the reaction centre chlorophyll a which has an absorption peak at P₇₀₀. 

Reducing agents : The PS I has following reducing agents viz. ferrodoxin, plastoquinone, cytochrome complex, n-plastocyanin.

Function of PS I : In PS I cyclic photophos-phorylation occurs.

Photosystem II [PS II] :

Location : It is present in appressed part of grana thylakoids.

Pigment molecules : It has in the centre special type of chlorophyll ‘a’ molecules called P_680.

Reducing agents : PS II has quencher, plastoquinone, cytochrome complex and plastocyanin reducing agents.

Function : In PS II non-cyclic photophosphorylation occurs, alongwith photolysis of water.

C₃ cycle is also called Calvin cycle. The first stable compound after the fixation CO₂ is a 3-carbon compound. 

                Calvin cycle of photosynthesis