Unit 5 - #6 Photosynthesis

Overall Goal Of Light Reactions

• Light energy is transferred to ATP and NADPH

• Both play a critical role in second phase of photosynthesis: Carbon Fixation

Light Independent Reactions

• “Synthesis” part of photosynthesis 

• A.K.A. C3 photosynthesis because first compound produces three carbon molecule (PGA) 

• Carbon fixation (converts carbon dioxide into carbohydrates) occurs in stroma 

Where do the three reactants come from?

• ATP

  • From light reaction that gives energy to Calvin Cycle 

• NADPH 

  • From light reaction that give H+ and e- to Calvin Cycle 

• CO2 

  • From air that delivers carbon and oxygen atoms to cycle

What are the three phases of the Calvin Cycle?

• Carbon Fixation

• Reduction Reactions

• RuBP Regeneration

Carbon Fixation

• First reaction of Calvin Cycle

• 3 molecules of carbon dioxide adds onto 3 molecules of RUBP (5-C) forming 3, 6-C intermediates 

• Each unstable intermediate immediately splits into two 3-C molecules called PGA (3-phosphoglycerate) (6 PGA total) 

• Catalyzed by enzyme Rubisco

Rubisco

• Most important enzyme for life 

• Slow acting (processes three substances per second vs. 1000 per second with other plant enzymes) 

• Plant needs many rubisco molecules to work efficiently 

• Most abundant protein on Earth

Reduction Reactions

• 6 PGA molecules are phosphorylated by 6 ATP molecules to form 6 molecules of 1,3-bisphosphoglycerate (1,3-BPG) 

• A pair of electrons from each of the 6 NADPH molecules reduce the 6 molecules of 1,3-BPG to make 6 molecules of glyceraldehyde 3-phosphate (G3P) 

• G3P is a sugar 

• 1 molecule of G3P exits the cycle as a product. The rest continue to phase 3 

RuBP (ribulose 1-5, bisphosphate) Regeneration

• A series of enzyme catalyzed reactions, along with an input of H2O’s and 3 ATP’s convert remaining 5 G3P molecules (5 x 3-Carbons) into 3 molecules of RUBP (3 x 5-Carbons) 

• This completes the cycle 

• Aside: 

  • Three CO2 must be fixed for one G3P to be removed from the cycle

G3P

• Production of G3P is goal of photosynthesis 

• A total of 9 ATP and 6 NADPH are needed to make 1 G3P’s which exits the cycle

• 2 G3P’s must exit the cycle to produce 1 glucose