Photosynthesis: Light Reaction & Dark Reaction

Light Reaction

Non-cyclic photophosphorylation : produce ATP & NADPH

• Reaction centre for photosystem II is P680.
• H2O is broken down to provide 2 electrons and O2.
• 2 electrons excited by light energy and achieve high energy potential/energy level, accepted by phaeophytin(primary acceptor).
• 2 electrons flow to/received by plastoquinone, received by cytochrome complex, received by plastocyanin, and received by photosystem I(ADP is used & ATP is produced).
• The flow of electrons provide the energy to pump protons across the thylakoid membrane. This activates ATP synthase to produce ATP.
• From photosystem I, the reaction centre is P700.
• The 2 electron excited by light energy and achieve high energy level, and accepted by FeS.
• 2 electron flow to/received by Ferredoxin and received by NADP reductase.
• Enzyme NADP reductase converts NADP+ to NADPH and H+ ion.

Cyclic photophosphorylation : produce ATP only

• Reaction centre for photosystem I is P700.
• 2 electrons excited by light energy, achieve high energy level, accepted by FeS(primary acceptor).
• The 2 electrons flow to/received by Ferredoxin, received by cytochrome complex, then received by plastocyanin, then received back by photosystem I.(ADP is used & ATP is produced).
• The flow of electrons provide energy to pump protons across thylakoid membrane. This activated ATP synthase to produce ATP.

Dark Reaction

Calvin Cycle

• CO2 combines with 5 carbon compound, RuBP with the help of enzyme rubisco to form unstable 6 carbon compound.------>(CARBON DIOXIDE FIXATION phase)
• 6 carbon compound  split into 2 molecules of 3-phosphoglycerate/phospho-3-glycerate.
• 3-phosphoglycerate undergoes a series of reduction processes to form glycealdehyde-3-phosphate/triose phosphate. ATP & NADPH is used.------>(REDUCTION phase)
• Glyceraldehyde-3-phosphate undergoes regeneration to regenerate RuBP.------>(REGENERATION OF CO2 ACCEPTOR phase)
• Some glyceraldehyde-3-phosphate assimilate to form carbohydrates, lipids, & proteins.------>(PRODUCT SYNTHESIS phase)