Topic 4 Photosynthesis: Photosynthesis steps

Question 1

• Light dependent reaction occurring as follows: ADP + Pi + light -> ATP
  a. Electrons are trapped by P680 in PSII and energized by light
  b. 2 excited electrons passed to primary electron acceptor from PSII
  c. Excited electrons enter ETC which consists of PSII containing cytochrome proteins and FE2+. It is analogous to oxidative phosphorylation
  d. 2 electrons move down ETC and lose energy which is used to phosphorylate 1.5 ATP
  e. ETC terminates with PSI/P700 where electrons are reenergized by sunlight and passed to a primary electron acceptor. Electrons can now enter cyclic or non-cyclic path
  f. If non-cyclic path: 2 electrons pass down ETC with proteins to combine NADP+ +H+ + 2 electrons = NADPH
  g. Electrons lost in step 2 are replaced when water splits to form 2 electrons, 2 H+ and one oxygen. H+ is used for NADPH formation and oxygen released as gas

Answer 1

1. Non-cyclic photophosphorylation

Question 2

• Alternate path replenishing ATP when Calvin consumes it
  a. 2 excited electrons from PSI join with protein carriers in first ETC and create 1 ATP
  b. These two electrons are recycled into PSI and can enter cyclic or non-cyclic path

Answer 2

2. Cyclic Photophosphorylation

Question 3

• Fixes 6 CO2 molecules
• Synthesizes glucose
  a. Carboxylation: ^ CO2 + 6 RuBP join forming 12 PGA and is catalyzes by enzyme rubisco or RuBP
  b. Reduction: 12 ATP + 12 NADPH convert 12 PGA -> 12 G3P or 12 PGAL
• Energy is incorporated; by-products NADP+ and ADP go into non-cyclic
  c. Regeneration: 6 ATP convert 10 G3P -> 6 RuBP allowing Calvin to repeat
  d. Carbohydrate synthesis: 2 remaining G3P are used to build glucose

Answer 3

3. Calvin Cycle