The Light Independent Reaction

Light Independent Rxn  Occurs in stroma of chloroplast  Referred to as ‘Calvin cycle’  raw materials are CO 2 -obtained from the atmosphere by diffusion through the stomata, ATP and NADPH both obtained from the light dependent stage  produces organic compounds such as glucose, sucrose, amino acids, fatty acids and glycerol (building block of other carbohydrates and plant oils)

Light Independent Rxn  CO2 converted to carbohydrate  Does not require light (dark rxn)

Light Independent Rxn  Occurs in a cycle of 4 processes: (1) Carboxylation -CO 2 fixation (2) Reduction of glycerate-3-phosphate (G- 3-P) (3) Regeneration of ribulose bisphosphate (RuBP) (4) Product synthesis -metabolism of TP to carb, a.a., lipids

CO 2 FIXATION  CO 2 is combined with a 5-carbon (pentose) sugar called ribulose bisphosphate (RuBP) in the presence of Rubisco  involves one of the most abundant enzyme in nature-ribulose bisphosphate carboxylase, RuBP carboxylase or Rubisco  RuBP-CO2 acceptor  Addition of CO2-carboxylation

CO 2 FIXATION  produces an unstabe 6C short-lived intermediate temporarily attached to Rubisco  As Rubisco releases the intermediate it immediately disintegrates into 2 molecules of a 3C compound called glycerate-3-phosphate (G-3-P)  First product of photosynthesis

CO 2 FIXATION  RuBP + CO2 + H2O Rubisco 2GP (5C sugar) (3C acid)

REDUCTION ATP ADP + Pi  GP TP (3C acid) NADPH+H NADP+ H2O (3C sugar) GP-glycerate phosphate is a 3C acid, contains a carboxyl gp-COOH (acidic) TP-triose phosphate is a 3C sugar, conains an aldehyde gp-CHO with a phosphate gp attached. Contains more chemical energy than GP

REDUCTION OF (G-3-P)  involves the conversion of G-3-P to a triose phosphate (TP) by a reduction reaction in the presence of ATP and NADPH  TP is the first carbohydrate made in photosynthesis  ATP provides the energy for the reaction and NADPH provides the hydrogen for the reduction reaction

REDUCTION  Reducing power of reduced NADP and energy from ATP used to remove O from GP (reduction)  Rxn takes place in 2 stages 1. Using some of the ATP produced in light dep rxn 2. Using ALL the reduced NADP

REGENERATION OF RIBULOSE BISPHOSPHATE (RuBP)  5/6 of the total TP produced is used to regenerate the starting RuBP molecule  Remaining ATP used to convert RuP to RuBP  NB- 6 mol CO2 used to make 2 mol of TP (3C sugar)

PRODUCT SYNTHESIS  TP does not accumulate-immediately converted to glucose phosphate, glucose  Sugar translocated as sucrose-disacch  Glucose phosphates starch- stored in stems, roots  Glycerol + fatty acids lipids-glycerol formed from TP, fatty acids from the accumulation of acetyl coenzyme A