RUBISCO - RUBP carboxylase oxygenase enzyme is the most abundant protein found an earth and forms 16% of chloroplast protein. It acts as carboxylase in presence of CO₂ and in the absence of CO_2, it functions as oxygenase.
1) In C₃ phtosynthesis it facilitates fixation of carbon di oxide molecule from the air by Ribulose 1,5 Bisphosphate (RUBP) to form a six carbon compound. In C₄ cycle RUBISCO operates in the Bundle sheath cells in the second phase.
2) Photorespiration is the excess respiration taking place in photosynthetic cells due to absence of CO₂ and and increase of oxygen. This condition changes the carboxylase role of RUBISCO into oxygenase. Photo respiration or C₂ cycle takes place in chloroplast, Peroxisome and Mitochondria. RUBP is converted into PGA and a 2C compound Phosphoglycolate by Rubisco enzyme in chloroplast. The C2 cycle does not yield any free energy in the form of ATP. Under certain conditions 50% of the photosynthetic potential is lost because of photo respiration. 
3) Thus RUBISCO can interfere with C₃ and C₂ cycles based on availability of carbondioxide and oxygen in the atmosphere.

The green wave length of light has low energy levels and is not useful for photosynthesis. So if plants get response of green light for long, They may not carry out photosynthesis that might lead plant cell to starvation and death.

Biosynthetic Phase or Photosynthetic Carbon Reduction Cycle (Per) :
1) Biosynthetic Phase of Photosynthesis utilises assimilatory powers (ATP and NADPH + H⁺) produced during light reaction. They are used to fix and reduce carbon dioxide into carbohydrates.
2) This reaction does not require light. Therefore, it is named Dark reaction. Ribulose 1,5 bisphosphate (RUBP) act as acceptor molecule of carbon dioxide and fix the CO2 by RUBISCO enzyme.
3) The first product of the pathway is a 3- carbon compound (Phospho Glyceric Acid) and so it is also called as C₃ Cycle or Calvin Cycle. It takes place in the stroma of the Chloroplast. It was discovered Melvin Calvin.
4) Dark reaction is temperature dependent and so it is also called Thermo-Chemical Reaction. It consists of three phases.
a) Phase 1- Carboxllation (Fixation)
i) The acceptor molecule Ribulose 1,5 Bisphosphate (RUBP) a 5 carbon compound with the help of RUBP carboxylase oxygenase (RUBISCO) enzyme accepts one molecule of carbon dioxide to form an unstable 6 carbon compound.
ii) This 6C compound is broken down into two molecules of 3-carbon compound phospho glyceric acid (PGA).
RUBP + CO₂\(\xrightarrow{Rubisco}\) 2 molecules PGA
b) Phase 2 - Glycolytic Reversal/Reduction
i) Phospho glyceric acid is phosphorylated by ATP and produces 1,3 bis phospho glyceric acid by PGA kinase.
ii) 1,3 bis phospho glyceric acid is reduced to glyceraldehyde 3 Phosphate (G-3-P) by using the reducing power NADPH + H⁺.
iii) Glyceraldehyde 3 phosphate is converted into its isomeric form dihydroxy acetone phosphate (DHAP).

c) Phase 3 - Regeneration
i) Regeneration of RUBP involves the formation of several intermediate
compounds of 6-carbon , 5-carbon, 4-carbon and 7- carbon skeleton.
ii) Fixation of one carbon dioxide requires 3 ATPs and 2 NADPH + H⁺ , and for the fixation of 6 CO₂ requires 18 ATPs and 12 NADPH + H⁺ during C₃ cycle. One 6 carbon compound is the net gain to form hexose sugar.

Overall equation for dark reaction:
6C0₂ + 18ATP + 12NADPH + H⁺\(\rightarrow\) C₆ H₁₂ 0₆ + 6H₂0 + 18ADP+ 18Pi + 12NADP⁺

Chloroplast are the main site of photosynthesis.
1) It is a double wall membrane bounded organelles, discoid or lens shaped, 4-10 Mm is diameter and 1-33 Mm is thickness.
2) The proteinaceous matrix is called stroma.
3) A sac like membranous system called thylakoid is present in stroma and are arranged one above the other like a stack called granum. 40-80 grana occur in each chloroplast. 5-30 thylakoids occur is each granum.
4) Fret membrane connects grana.
5) PSI is located is outer thylakoid membrane facing stroma and PSII is located on inner membrane towards the lumen of thylakoid.
6) Chloroplast contains 30-35 prteins 20-30% phospholipids 5-10% chlorphyll caroteroids, 70s Ribosomes, circular DNA and starch grain.
7) Inner surface of lamellar membrane consists of small spherical structers called quantosomes.
8) Due to presence to 70s Ribosome and DNA they are considered to be semi autonomous organelles.
9) Thylakoids contain pigment systems which help in light reaction of Photosynthesis.
10) Stroma contains enzymes for dark reaction.

Photorespiration Dark Respiration 1. It takes place in Photosynthetic green cells It takes place in all living cells 2. It takes place only in the presence of light It takes place all the time 3. It involves chloroplast, peroxisome, and mitochondria It involves only mitochondria 4. It does not involve Glycolysis, Kreb's Cycle, and ETS It involves glycolysis, Kreb's Cycle, and ETS 5. The substrate is Glycolic acid Substrate is Carbohydrates, protein or fats 6. It is not essential for survival Essential for survival 7. No phosphorylation and yield of ATP Phosphorylation produces ATP energy 8. NADH₂ is oxidized to NAD⁺ NAD⁺ is a reduction to NADH₂ 9. Hydrogen peroxide is produced Hydrogen peroxide is not produced  10. End products are CO₂ and PGA End products are CO₂ and water