HL 1 Biology

8.2b Photosynthesis light-indepedent reactions

Carbon Fixation and carbohydrate synthesis

  • Carbon fixation reaction: converted into another carbon compound: MOST IMPORTANT
  • plants/algae = occurs in stroma
  • Product = three carbon compound: glycerate 3-phosphate
  • carbon dioxide reacts with a five-carbon compound: ribulose biphosphate (RuBP) = to produce  2 molecules of glycerate 3-phosphate
  • stroma contains large amounts of rubisco (enzyme which catalyses reaction) to maximize carbon fixation

Regeneration of RuBP

  • triose phophate: first carbohydrate produced by the light-independent reactions
  • hexose phosphate produced with two triose phosphate
  • starch produced with hexose phosphate combined by condensation reaction
  • Calvin cycle: a cycle formed by the reaction of RuBP being consumed and produced in the light-independent reaction s of photosynthesis


Identifying the First Products of Carbon Fixation (p.112)

1) Glycerate 3-phosphate is the first product of carbon dioxide fixation because looking at the graph, the glycerate 3-phosphate showed a high amount in the first few seconds yet later, the amount has decreased through time. Also, all the other phosphates and molecules use the glycerate 3-phosphate in order to synthesize increasing gradually the percentage of radioactivity.  Since carbon fixation reaction is being experimented, the product of glycerate 3-phosphate.

2) The conversion of glycerate-3-phosphate to triose phosphate is evident since the concentration of triose phosphate increases as the concentration of the glycerate-3-phophate decreases with time, according to the graph. This shows how the glycerate-3-phophate is being used to produce more of the triose phosphate. Thus, the glycerate-3-phosphate is an essential material in order for the conversion to the triose phosphate.  The conversion of glycerate-3-phosphate to sugar phosphates are also evident as its production rises as the glycerate-3-phosphate falls as well. Hydrogen is added to the glycerate-3-phosphate in order to perform a reduction reaction to produce the sugar and triose phosphate.


One thought on “8.2b Photosynthesis light-indepedent reactions

  1. Grade 6 A consistent and thorough understanding of the required knowledge and skills, and the ability to apply them in a wide variety of situations.

    DBQ Identifying the 1st Products of Photosynthesis

    • The highest amount of radioactivity was in the G3P initially, and then fell with time
    • Other molecules being synthesized depend on the radioactive G3P as a raw material

    • As concentration of G3P falls with time
    • concentration of TP rises.
    • G3P is a raw material for TP production

    • Because radioactive carbon appears in G3P in 1-2 seconds after its introduction
    • CO2 must be able to diffuse into cells within 1-2 seconds
    • and Rubisco must be able to fix carbon within 1-2 seconds

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