Photosynthesis: Light-independent reaction

Where does the light-independent reaction occur?

• The light-independent reaction occurs in the stroma of chloroplasts

What are the reactants and products of the LIR

• The light-independent reaction uses carbon dioxide, as well as ATP and NADPH from the light-dependent reaction

• The light-independent reaction produces glucose and other organic molecules

Stage 1 of LIR (carbon fixation)

• Carbon dioxide enters the leaf through the stomata and diffuses into the stroma of the chloroplast. Here the carbon reacts with ribulose bisphosphate (RuBP), which is a 5-carbon compound to form a 6 carbon compound (carbon dioxide already has 1 so 1+5 = 6) 

  • This is called carbon fixation because the carbon from carbon dioxide is ‘fixed’ into an organic molecule

  • The reaction is catalysed by the enzyme rubisco

Stage 2 of LIR (making GP)

• RuBP then immediately breaks down to form 2 molecules of glycerate 3-phosphate or GP (has 3 carbons each)

Stage 3 of LIR (triose phosphate)

• GP is then reduced to triose phosphate

  • The energy for this reaction comes from ATP (from the light-dependent reaction)

    • The ADP and Pi will return to the light-dependent reaction

  • Also requires proteins and electrons from NADPH, which is oxidised to NADP

    • The NADP that is reformed returns to the light-dependent reaction to be reduced again

  • 2 ATP molecules and 2 NADPH molecules are used as there are 2 triose phosphate molecules produced

• Some triose phosphate is used to regenerate RuBP using ATP and some triose phosphate is converted to useful organic substances such as cellulose, amino acids, glucose etc.

How many turns of the LIR to produce 6 molecules of triose phosphate?

• Three turns of the cycle produces six molecules of triose phosphate, because two molecules of triose phosphate are made for every one Carbon dioxide molecule used

How many out of the 6 molecules of triose phosphate are used to regenerate RuBP

• Five out of six of these triose phosphate molecules are used to regenerate RuBP

  • This means that for three turns of the cycle only one triose phosphate is produced which is used to make a hexose sugar

How many times must the LIR turn to form a hexose sugar such as glucose?

• A hexose sugar such as glucose has six carbons, so two triose phosphate molecules are needed to form one hexose sugar

• This means the cycle must turn six times to produce two molecules of triose phosphate that can be used to make one hexose sugar

• Six turns of the cycle need 18ATP and 12 NADPH from the light-dependent reaction