Question

Reduction of CO2 to form one mole of triose phosphate requires 9 moles of ATP and 6 moles of NADPH and produces 9 moles of ADP and 8 moles of inorganic phosphate.

(a) What is the source of NADPH used in the reduction of CO2

(b) Account for the 9 moles ADP and 8 moles inorganic phosphate formed by the syntheiss of triose phosphate

Answer 1

a) Source of NADPH used in the reduction of CO₂

​Answer:

During the last step of the electron chain in the light reactions of photosynthesis (i.e., in the last reaction of the photosystem I) NADPH is produced from NADP⁺ by ferredoxin-NADP⁺ reductase.  

This is the source of NADPH used in the reduction of CO₂

b) Account for the 9 moles ADP and 8 moles inorganic phosphate formed by the syntheiss of triose phosphate

Answer:

Calvin cycle involves three stages

Stage I Carbon Fixation

Stage II Reduction

Stage III Regenaration of starting materials

During the reduction stage (Stage II in the calvins cycle) 6 moles of ADP is produced from 6 moles of ATP and 6 moles of NADP⁺ and 6 moles of inorganic phosphate is produced from 6 moles of NADPH.

During the regenaration of starting molecules 2 moles of inorganic phosphate and 3 moles of ADP is produced.

6 moles of ADP from stage II and 3 moles of ADP from stage III, so totally (6+3) 9 moles of ADP is produced in the calvin cycle

6 moles of inorganic phosphate (Pi) from stage II and 2 moles of inorganic phosphate (Pi) from stage III, so totally (6+2) 8 moles of inorganic phosphate is produced in the calvin cycle

Three stages involved in the syntheiss of triose phosphate is shown in the below diagram

3 PGA Glycerate 3-phosphate

ATP Adenosine Triphosphate                

ADP Adenosine diphosphate

NADPH Nicotinamide adenine dinucleotide phosphate reduced form

NADP+                 Nicotinamide adenine dinucleotide phosphate

Pi Inorganic phosphate

G3P Glyceraldehyde 3-phosphate

RuBP Ribulose 1,5-bisphosphate