What do you understand by Dark Reaction? Explain Calvin Cycle-/c3 pathway and Crassulacean acid metabolism?

The Dark Reaction

The dark reaction of photosynthesis reduces carbon dioxide to glucose.  These reactions take place in the stroma of chloroplast.  The energy required for the reduction of carbon dioxide is derived from ATP and hydrogen from NADPH2 formed in the light reaction of photosynthesis.  Hence, dark reaction requires the assimilatory power (that is 18 ATP and 12 NADPH2).

Melvin Calvin and his co-worker (1946-1953) worked out the mechanism of dark reaction in chlorella by using (C14 O2), which is popularly known as C3 pathway or Calvin cycle.

Calvin cycle can be described under 3 stages:

  1. Carboxylation.
  2. Reduction.
  3. Regeneration.

CARBOXYLATION

Addition of carbon dioxide to a compound is called carboxylation.  In this step of dark reaction, ribulose 1, 5 diphosphate, (also known as ribulose biphosphate).  A phosphorylated 5 carbon sugar which acts as carbon dioxide acceptor, combines with carbon dioxide.  This reaction is catalyzed by the enzyme, RUBISCO, which results in the formation of 2 molecules of 3-phosphoglyceric acid. 

REDUCTION

In this phase of dark reaction, phosphoglyceric acid forms phosphoglyceral dehyde  by utilizing ATP molecules.  Phosphoglyceral dehyde is eventually converted into hexose sugar.  Thus, the entire process is just a reverse of glycolysis in which hexose sugars are first oxidized into phosphoglyceral dehyde and then to carbon dioxide and water, releasing ATP.  The step involves utilization of 2 molecules of ATP for phosphorylation and 2 molecules of NADPH for reduction per carbon dioxide molecule fixed.  The fixation of 6 molecules of carbon dioxide and 6 turns of the cycles are required for the removal of 1 molecule of glucose from the pathway.

REGENERATION

In the first step of Calvin cycle, carbon dioxide is accepted by ribulose diphosphate and it thus enters the cycle.  Later, once glucose is formed, this ribulose 5 diphosphate must be regenerated so that it is once again available to accept carbon dioxide to manufacture hexose sugar.

The regeneration steps require ATP for phosphorylation to form RuBP.  Hence for every carbon dioxide entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required.  To make 1 molecule of glucose, 6 turns of the cycle are required.

6 molecules of CO2+18 ATP+12 NADPH→1 molecule of glucose+18 ADP+12 NADP

 

Calvin Cycle

Calvin cycle

Calvin Cycle

THE VARIOUS STEPS INVOLVED IN CALVIN CYCLE/ C3 PATHWAY ARE AS FOLLOWS: 

Step 1:  6 molecules of ribulose 1-5 disphosphate combines with 6 molecules of carbon dioxide in the presence of enzyme, carboxydismutase to form an unstable 6 carbon compound β-keto acid which immediately splits into 12 molecules of 3 carbon compound 3-phosphoglyceric acid.

Step 2:  12 molecules of 3-phosphoglyceric acid reacts with 12 molecules of ATP to produce 12 molecules 1, 3-diphosphoglyceric acid in the presence of enzyme, phosphoglycerokinase.

Step 3:  12 molecules 1-3 diphosphoglyceric acid are reduced to 3-phosphoglyceraldehyde by 12 molecules of NADPH2 and 12 molecules of H2O in the presence of enzyme, phosphoglyceraldehyde dehydrogenase.

Step 4:  3-phosphoglyceraldehyde participates in 4 kinds of reaction, out of 12 molecules, 5 molecules of 3-phosphoglyceraldehyde are converted into dihydroxyacetone phosphate in the presence of triosephosphate isomerase.

Step 5:  Out of 5 molecules of dihydroxyacetone, 3 molecules condense with 3 molecules of 3-phosphoglyceraldehyde to form a 6 carbon compound fructose 1-6 diphosphate.

Step6:  Fructose 1-6 diphosphate is then converted into fructose-6-phosphate by losing a molecule of phosphate in the form of H3PO4 in the presence of enzyme, phosphatase and water.

Step 7:  Out of 3 molecules of fructose 6 phosphate, 1 molecule converts into fructose or glucose and is released as final product of photosynthesis.

Step 8:  2 molecules of fructose 6 phosphate combines with 2 molecules of 3-phosphoglyceraldehyde to give 2 molecules of a 4 carbon compound.  Erythrose 4-phosphate and 2 molecules of 5 carbon compound xylulose 5-phohsphate in the presence of enzyme, transketolase.

Step 9:  2 molecules of dihydroxyacetone phosphate reacts with 2 molecules of erythrose 4-phosphate in the presence of transaldolase to form 2 molecules of a 7 carbon compound, sedoheptulose-1, 7 diphosphate.

Step 10:  Sedoheptulose-1, 7 diphosphate is then converted into sedoheptulose 7-phosphate in the presence of enzyme, phosphatase and water.

Step 11:  2 molecules of sedoheptulose 7-phosphate reacts with remaining 2 molecules of 3 -phosphoglyceraldehyde to produce 2 molecules of xylulose 5-phosphate and 2 molecules of ribose 5-phosphate in the presence of transketolase.

Step 12:  2 molecules of ribose 5-phosphate are isomerised to 2 molecules of ribulose 5-phosphate in the presence of enzyme, phosphoribose isomerase.

Step 13:  All the 4 molecules of xylulose 5-phosphate are isomerised to ribulose 5-phosphate in the presence of phosphoribulose epimerase

Step 14:  All the 6 molecules of ribulose 5-phosphate are converted into ribulose 1, 5-diphosphate in the presence of ATP and enzyme phosphoribulokinase.

CRASSULACEAN ACID METABOLISM (CAM)

Some members of the family Crassulaceae like Bryophyllum, sedum, kalanchoe and some other plants like cactus, orchid etc., display diurnal pattern of organic acid formation, i.e., accumulation of organic acid occurs in the leaves at night and decreases during the day.  These plants are also known as CAM plants.  These plants are usually succulents which grow under extremely xeric condition.  The stomata of these plants remain closed during daytime and open at night.  In darkness, stored carbohydrate is converted to phosphoenol pyruvic acid which reacts with carbon dioxide to form oxaloacetic acid.  It is converted to malic acid as that of C4 pathway.

During daytime, the malic acid is decarboxylated to produce pyruvic acid.  The carbon dioxide released during this reaction is accepted by ribulose 1, 5-disphophate and fix carbon dioxide through Calvin cycle.  The pyruvic acid may enter into Krebs cycle or it may be reconverted to phosphoenol pyruvic acid.