Light Independent Stage

• Calvin cycle happens at stroma, can occur day or night if there is ATP and NADPH from light dependent reactions available
• CO_2 in atmosphere diffuses through stomata, leaf air spaces, into palisade mesophyll cells through thin cell walls to cell surface membrane to chloroplast envelope into stroma
• 5C sugar intermediate ribulose diphosphate (RuBP); a CO_2 acceptor, is carboxylated, fixes, CO_2 (catalysed by rubisco) producing an unstable 6C intermediate compound
• 6C immediately breaks down into 2 glycerate 3 phosphate (GP) molecules, 3C, (plants where GP is first stable CO_2 fixation product are C3 plants)
• ATP hydrolysed to ADP + P_i (energy source) and NADPH as hydrogen source to reduce GP to triose phosphate (3C), TP, some is converted to amino (need nitrogen source, e.g. nitrates) and fatty acids
• TP does not accumulate: Most regenerates RuBP, completing cycle, requires ATP as energy and phosphate source, 5/6 TP molecules are recycled to make 3 RuBP
• 1/6 converted to a hexose sugar (6C) or glycerol (for lipid synthesis or to combine with fatty acids and form triglycerides); 6 cycle turns produces 1 hexose sugar
• Hexose sugars:

  • Polymerised to cellulose (cell wall synthesis), starch (energy storage), or pentose sugars (for nucleic acid synthesis)
  • Glucose can be isomerised to fructose; these 2 can combine, form sucrose (transport sugar in phloem sieve tubes), and may be used in glycolysis (as respiratory substrate)