Where are the ATP and NADPH Used

BIOLOGY PHOTOSYNTHESIS IN HIGHER PLANTS WHERE ARE THE ATP AND NADPH USED DARK REACTION : The products of light reaction are ATP, NADPH and O2. Of these O2 diffuses out of the chloroplast while ATP and NADPH are used to drive the process leading to synthesis of food, (more accurately sugars). This is biosynthetic phase of photosynthesis. This process does not directly depend on the presence of light but is dependent on the product of the light reaction i.e. ATP and NADPH, besides CO2 and H2O. Q. Does the term dark reaction for biosynthetic phase of photosynthesis, is a misnomer? Ans. Yes, dark reaction term is a misnomer because this reaction not occurs in dark. Instead this reaction is dependent on the product of the light reaction i.e, ATP and NADPH, so dark reaction process also occurs in presence of light. Dark reaction occurs at fluid of stroma and also called Blackman's reaction or Biosynthetic phase. It is a reductive step (CO2 reduced) of photosynthesis. Dark reaction operates in different photosynthetic organisms through three different ways : (A) C3 pathway (B) C4 pathway (C) CAM pathway C3 pathway consists of only C3 cycle or Calvin cycle while C4 pathway and CAM pathway consists of both C3 cycle and C4 cycle. Therefore, Calvin cycle occurs in all photosynthetic plants ; it does not matter wheather they have C3 or C4 (or any other) pathways. (A) The Calvin cycle : The use of radioisotope 14C by Calvin in alga (Chiarella) photosynthesis studies led to the discovery that the first CO2 fixation product is a 3 carbon organic acid called 3 phosphoglyceric acid or in short 3 PGA. It has a 3 carbons thus cycle named C3 cycle. 1 BIOLOGY Melvin Calvin and his co-workers then worked out the whole pathway and showed that the pathway operated in cyclic manner ; the RUBP was regenerated. Calvin cycle can be described under three stages : (1) Carboxylation (2) Reduction (3) Regeneration (1) CARBOXYLATION: It is the most crucial step (because RuBisCO has dual nature and this step will determines wheather Calvin cycle run or photorespiration). In this step CO2 is utilised for the carboxylation of RUBP. This reaction is catalysed. by the enzyme RuBisCO which results in the formation of two molecules of 3-PGA. RuBisCO • Its full name is Ribulose 1,5-bisphosphate carboxylase oxygenase enzyme, Its former name, was carboxydismutase. • RuBisCO is considered as a most abundant protein on the earth. • It has dual nature, thus capable to bind with CO2 (carboxylase) as well as O2 (oxygenase). Although the enzyme has more affinity with CO2. • Binding of CO2 or O2 is competitive with active site. It is the relative concentration of O2 and CO2 in the stroma of chloroplast that determines which of the two will bind to the enzymes. • Mg and Red light are necessary for activation of enzyme RuBisCO. 2 BIOLOGY Fig : The Calvin cycle proceeds in three stages: (1) carboxylation, during which CO2 combines with ribulose-1,5-bisphosphate; (2) reduction during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and (3) regeneration during which the CO2 acceptor ribulose- 1,5-bisphosphate is formed again so that the cycle continues (2) REDUCTION : These are a series of reactions that lead to the formation of glucose. (glycolytic reversal) The fixation of six molecules of CO2 and for this fixation 6 turns of the Calvin cycle are required for removal of one molecule of glucose from the pathway. (3) REGENERATION : Regeneration of the CO2 acceptor molecule RUBP is crucial if the cycle is to continue uninterrupted. 3 BIOLOGY Fig. : Calvin cycle Steps of calvin cycle Number of ATP and NADPH required per CO2 Carboxylation Zero ATP and Zero NADPH + H+ Reduction Two ATP and two NADPH + H+ Regeneration One ATP and Zero NADPH + H+ 4 BIOLOGY Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. Input Output 6 CO2 C6H12O6 18 ATP 18 ADP 12 NADPH 12 NADP+ 5