Cyclic photophosphorylation: 

a. Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it. 

b. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation. 

c. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.

Non-cyclic photophosphorylation:: 

a. It involves both photosystems- PS-I and PSII. 

b. In this case, electron transport chain starts with the release of electrons from PS-II. 

c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH. 

d. The reduced NADP⁺ (NADPH) is utilized for the reduction of CO₂ in the dark reaction. 

e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.

f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP⁺ whereas oxygen is released. 

g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.

Interdependence of light and dark reactions: 

1. The light reaction gives rise to two important products, a reducing agent NADPH₂ and an energy rich compound ATP. Both these are utilized in the dark phase of photosynthesis. 

2. ATP and NADPH₂ molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction CO₂ is reduced to carbohydrate. 

3. During dark reaction, ATP and NADPH₂ are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.

Calvin cycle: 

The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C₃ pathway and the plants are called C₁₄ plants. 

Calvin carried out experiments on unicellular green algae (Chlorella), using radioactive isotope of carbon, C₁₄ as a tracer. It is also called synthesis phase or second phase of photosynthesis. 

The cycle is divided into the following phases: 

1. Carboxylation phase: 

a. Carbon dioxide reduction starts with a fivecarbon sugar ribulose-l,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.

b. RuBP reacts with CO₂ to produce an unstable 6 carbon intermediate in the presence of Rubisco. 

c. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid. 

d. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.

2. Glycolytic reversal: 

a. 3-phosphoglyceric acid form 1,3- diphosphoglyceric acid by utilizing ATP molecule. 

b. These are then reduced to glyceraldehyde3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis. 

c. In order to keep Calvin cycle continuously running there must be sufficient number of RuBP and regular supply of ATP and NADPH. 

d. Out of 12 molecules of 3- phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule. 

3. Regeneration of RuBP: 

a. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP. 

b. Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.

Photorespiration: Mechanism: 

1. Photorespiration involves three organelles chloroplast, peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle. (Photosynthetic Carbon Cycle) 

2. Enzyme Rubisco acts as oxygenase at higher concentration of O₂ and photorespiration begins. 

3. When RuBP reacts with O₂ rather than CO₂ to form a 3-carbon compound (PGA) and 2- carbon compound phosphoglycolate. 

4. Phosphoglycolate is then converted to glycolate which is shuttled out of the chloroplast into the peroxisomes. 

5. In Peroxisomes, glycolate is converted into glyoxylate by enzyme glycolate oxidase. 

6. Glyoxylate is further converted into amino acid glycine by transamination. 

7. In mitochondria, two molecules of glycine are converted into serine (amino acid) and CO₂ is given out. 

8. Thus, it loses 25% of photosynthetically fixed carbon. 

9. Serine is transported back to peroxisomes and converted into glycerate. 

10. It is shuttled back to chloroplast to undergo phosphorylation and utilized in formation of 3-PGA, which get utilized in C₃ pathway. 

Hatch-Slack pathway:

M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis. It occurs in tropical and sub-tropical grasses and some dicotyledons. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C₄ pathway and plants are called C₄ plants. 

Mechanism:

1. CO₂ taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase. 

2. It is converted to another 4-C compound, malic acid. 

3. It is transported to the chloroplasts of bundle sheath cells. 

4. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of CO₂ in the cytoplasm. 

5. Thus, concentration of CO₂ increases in the bundle sheath cells. 

6. Chloroplasts of these cells contain enzymes of Calvin cycle. 

7. Because of high concentration of CO₂ , RuBP carboxylase participates in Calvin cycle and not photorespiration.

8. Sugar formed in Calvin cycle is transported into the phloem. 

9. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP. 

10. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. 

11. xi. Thus, C₄ pathway needs 12 additional ATP. 

12. The C₃ pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C₄ pathway requires 30 ATP. Thus, C₄ plants are better photo synthesizers as compared to C₃ plants as there is no photorespiration in these plants.

13. CAM Pathway:

In CAM plants, malic acid accumulates during night, which is formed from Oxaloacetic acid in presence of the enzyme malate dehydrogenase.