Photosynthesis and the LIR

• The Calvin cycle (LIR): takes place in the stroma o the chloroplast.
• The Calvin cycle makes:

  1. a molecule called triose phosphate from CO2
  2. ribulose bisphosphate (a 5 carbon compound)
• Triose phosphate can be used to make glucose and other useful organic substances.
• There are many steps in the cycle and it needs ATP and H+ ions to keep it going - the reactions are linked in a cycle which means the starting compound, ribulose bisphosphate, is regenerated
• 1: Formation of glycerate 3-phosphate

  CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. Here it is combined with ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme rubisco. This gives an unstable 6-carbon compound, which quickly breaks down into 2 molecules of a 3-carbon compound called glycerate 3-phosphate
• 2: Formation of triose phosphate

  The hydrolysis of ATP (from the LDR) provides energy to reduce the 3-carbon compound, GP, to a different 3 carbon-compound called triose phosphate (TP). This reaction also requires H+ ions, which come from reduced NADP (also from the LDR). Reduced NADP is recycled to NADP. Some triose phosphate is then converted into useful organic compounds (e.g. glucose) and some continues in the calvin cycle to regenerate RuBP
• 3: Regeneration of ribulose bisphosphate

  5 out of every 6 molecules of TP produced in the cycle aren't used to make useful organic compounds, but to generate RuBP. Regenerating RuBP used the rest of the ATP produced by the LDR
• Hexose sugars are simple 6-carbon sugars e.g. glucose. One hexose sugar is made by joining 2 molecules of triose phosphate together. Hexose sugars can be used to make larger carbohydrates.
• The Calvin cycle needs to turn 6 times to make 1 hexose sugar

  1. Three turns of the cycle produces 6 molecules of TP (2 molecules are made for every 1 CO2 use)
  2. 5/6 of these TP molecules are used to regenerate ribulose bisphosphate
  3. This means that for 3 turns of the cycle, only one TP is produced that is used to make a hexose sugar
  4. A hexose sugar has 6 carbons - 2 TP molecules are needed to form one hexose sugar
  5. The cycle must turn 6 times to produce 2 molecules of TP that can be used to make one hexose sugar
• 6 turns of the cycle need 18 ATP and 12 NADPH from the LDR
• Seems inefficient - keeps the cycle going and makes sure that there is always enough RuBP ready to combine with CO2 taken in from the atmosphere
• Calvin cycle is the starting point for making all organic substances a plant needs. TP (triose phosphate) and GP (Glycerate 3-phosphate) molecules are used to make carbohydrates, lipids and amino acids
• Carbohydrates: hexose sugars are made from two TP molecules and larger carbohydrates (e.g. sucrose, cellulose, starch) are made by joining hexose sugars together in different ways
• Lipids - these are made using glycerol, which is synthesised from TP and fatty acids, which are synthesised from glycerate 3-phosphate
• Amino acids - some amino acids are made from glycerate 3-phosphate