light-independent stage

• the light-independent stage of photosynthesis takes place in the stroma of chloroplasts

  • although it does not use light energy, it uses the products of the light-dependent stage

  →the plant is not illuminated, the light-independent stage soon cease, because ATP + hydrogen are not available to reduce carbon dioxide + synthesise large complex organic molecules
• The role of carbon dioxide

  carbon dioxide is the source of carbon dioxide for the production of all organic molecules found in all the carbon-based life forms on Earth

  • these organic molecules may be used as structures (e.g. cell membranes, antigens, enzymes, muscle proteins, cellulose cell walls) or act as energy stores (starch + glycogen)
  • carbon dioxide in air enters the leaf through the stomata + diffuses through the spongy mesophyll layer to the palisade layer, into the palisade cells, through their thin cellulose + then through the chloroplast envelope into the stroma
• the role of carbon dioxide 2

  • the fixation of carbon dioxide in the stroma maintains a concentration gradient that aids this diffusion

  →carbon dioxide that is a by-product of respiration in plant cells may also be used for this stage of photosynthesis

  the series of reactions whereby carbon dioxide is converted to organic molecules is called the Calvin Cycle
• The Calvin Cycle

  1. Carbon dioxide combines with a carbon dioxide acceptor, a five-carbon compound called ribulose biphosphate (RuBP) - reaction is catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase)
  2. RuBP, by accepting the carboxyl group, becomes carboxylated, forming an unstable intermediate six-carbon compound that immediately breaks down
  3. the product of this reaction is two molecules of a three-carbon compound, GP (glycerate-3-phosphate) + carbon dioxide has now been fixed

  
• Calvin Cycle 2

  4. GP is then reduced, using hydrogen from the reduced NADP made during the light-dependent stage, is used at this stage at the rate of two molecules of ATP for every molecule of carbon dioxide fixed during stage 3

  5. in 10 of every 12 TP molecules, the atoms are rearranged to regenerate six molecules of RuBP + this process requires phosphate groups

  →Chloroplasts contain only low levels of RuBP, as it is continually being converted to GP, but is also continually being regenerated; the remaining two of the 12 molecules of TP are the product
• The Calvin cycle only runs during daylight

  ATP and reduced NADP are continuously needed for Calvin Cycle to run

  • during the light-dependent stage, hydrogen ions are pumped from the stroma into the thylakoids spaces, so the concentration of free protons in the stroma falls, raising pH to around 8, which is optimum for the enzyme RuBisCO (is also activated by presence of extra ATP in the stroma)
• The Calvin Cycle only runs during daylight 2

  • in daylight, the concentration of magnesium ions increase in the stroma

  →these ions attach to the active site of RuBisCO, acting as cofactors ro activate it

  • the ferredoxin that is reduced by electrons from PSI activates enzymes involved in the reaction of Calvin Cycle
• The uses of triose phosphate (TP)

  some TP molecules are used to synthesise organic compounds e.g.

  • some glucose is converted to sucrose, some to starch + some to cellulose
  • some TP is used to synthesise amino acids, fatty acids + glycerol
  • rest of TP is recycled to regenerate the supply of RuBP

  → 5 molecules of the 3-carbon compound TP interact to form 3 molecules of the 5-carbon compound RuBP