• The light-independent reactions of photosynthesis take place in the?stroma?of the chloroplasts
• The stroma is within the double membrane and is a thick protein rich environment containing the?enzymes?needed for the light-independent reactions

• The light-independent reactions of photosynthesis are also known as the?Calvin cycle
• Carbon dioxide?is converted into carbohydrates, namely?glucose, during the cycle in a series of?anabolic?reactions
  • Anabolic reactions?require energy in order to build large complex molecules from smaller simpler ones
• The Calvin cycle relies on the?products of the light-dependent reactions?namely ATP and reduced NADP
• During the cycle?endergonic?reactions take place that involve the hydrolysis of ATP and oxidation of reduced NADP
  • An?endergonic reaction?requires energy to be absorbed before the reaction can proceed
• There are three main steps within the Calvin cycle:
  • The enzyme?rubisco?catalyses the?fixation of carbon dioxide?by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound
  • GP is reduced?to triose phosphate (TP) in a reaction involving reduced NADP and ATP
  • RuBP is regenerated?from TP in reactions that use ATP

Carbon fixation

• Carbon dioxide is the source of carbon for all organisms that carry out photosynthesis
• Carbon fixation involves carbon dioxide (1C) being removed from the external environment and becoming part of the plant, and is then said to be “fixed”
  • It is transformed into a three-carbon compound (3C) called?glycerate-3-phosphate?(sometimes shortened to as GP)
• During the fixation step of the Calvin cycle?carbon dioxide?is combined with a five-carbon compound (5C) called?ribulose bisphosphate (RuBP)?to make an?unstable?six-carbon (6C) compound that splits into?two?molecules of?glycerate-3-phosphate
• This reaction is catalysed by the enzyme?rubisco
• Glycerate-3-phosphate is then used in the next step of the cycle

• Energy?from ATP and?hydrogen?from reduced NADP (from the light-dependent reactions) are used to?reduce?glycerate-3-phosphate?to a phosphorylated three-carbon molecule called?triose phosphate?(sometimes shortened to TP)
• After the reduction step?one sixth?of the triose phosphate is converted into?usable products?for the plant:
  • Hexose phosphates which can be used to produce carbohydrates such?starch, sucrose or cellulose
  • Glycerol?and?fatty acids?which join to form cell membranes
  • Production of?amino acids?for protein synthesis
• It is important that not all the triose phosphate is converted to alternative compounds for the plant, or the supplies of ribulose bisphosphate would run out
• The remaining triose phosphate is used to regenerate RuBP

• The remaining five sixths of triose phosphate are used to?regenerate?the four-carbon compound ribulose bisphosphate?(RuBP)
• This process requires?ATP?(from the light-dependent reaction)
• Once RuBP been has regenerated it can go on to fix further carbon dioxide and the cycle can begin again