light independant reaction

• where does it take place?

  • stroma of chloroplast
• reactants

  • reduced NAPD
  • ATP
• products

  • NADP
  • ADP+PI
  • complex organic molecules: starch(storage), sucrose(transport), cellulose(cell walls)
  • immediate products: GP and GALP
• 3 main stages

  1. carbon fixation
  2. reduction of GP
  3. regeneration of RuBP
• carbon fixation

  • CO2(1C) and RuBP(5c) in a reaction catalysed by enzyme RUBISCO
  • resulting in a 6C unstable compound
  • carbon dioxide has become fixed-removed from external environment and become part of a molecule inside a plant
• reduction of GP


  unstable 6C compound forms 2xGP(3C) molecules

  1. ATP and reduced NADP are used to reduce 2 GP molcules to form 2 GALP(3C) molecules- energy from ATP and hydrogen from NADPH
  2. 1/6 carbons of GALP go towards the formation of inorganic molecules eg glucose- glucose is a 6C compound so 6 turns of calvin cycle produce 1 glucose molecule.
• regeneration of RuBP

  • 5/6 carbons of GALP molecules are used to regenerate RuBP
  • required energy from ATP=ADP
• Explain why increase in C02 concentration affects the growth of plants in glasshouses (3)

  -       Carbon dioxide is the limiting factor 

  -       Carbon is fixed with RuBP to produce GP and then into GALP

  -       Thus, more carbon dioxide means more glucose is being produced and amino acids 

  -       Which would lead to greater rate of growth
• Describe the roles of the products of the light dependant reactions in the Calvin cycle (3) 

  -       ATP provides energy and reduced NAPH provides hydrogens for the reduction of GP into GALP 

  -       ATP also used in the regeneration of RuBP- converting GALP to RUBP
• Explain the importance of RUBISCO to the productivity of an ecosystem (2) 

  -       Fixes inorganic carbon/ CO2 (combines RuBP with CO2) 

  -       Allowing formation of organic molecules by the carbon cycle e.g., glucose

  -       These organic molecules allow transfer of energy to next trophic level
• A decrease in the activity of rubisco limits the rate of photosynthesis, why?  (2) 

  -       Less RuBP and C02 are combining 

  -       Less GP and GALP produced so no regeneration of RuBP
• Explain what is meant by fixation of carbon dioxide (3) 

  -       Removal of carbon dioxide from the air 

  -       Carbon atoms from carbon dioxide are incorporated into organic material 

  -       Catalysed by enzyme rubisco

  -       To build up plants biomass/cells/tissues