The dark reaction of photosynthesis can define as the phase, which doesn’t need light energy or photons to complete the cycle, and hence also called as “light-independent reactions”. Dark reaction or Calvin cycle was first explained by a scientist named Melvin Calvin. It occurs at the site of the chloroplast within the fluid surrounding the thylakoid that refers to the stroma. In dark reaction, the photosynthetic organisms make glucose that is utilized by the cells as chemical energy to make other essential components.
The energy (ATP and NADPH) obtained from the light reactions is harnessed by the plant cells to drive chemical energy. Therefore, the dark reaction does not require light energy directly, but it somehow dependent upon the by-products of light-dependent reactions, i.e. ATP and NADPH, to facilitate the production of glyceraldehyde 3-phosphate.
Content: Dark Reaction of Photosynthesis
Definition of Dark Reaction
The dark reaction can define as the bio-synthetic phase or carbon reduction phase, which fix the atmospheric carbon dioxide into useful compounds like glucose. These reactions occur in the stroma of chloroplast that encircles the thylakoid sacs. The Calvin cycle uses ATP and NADPH from the light reaction to form sugars. The conversion of CO2 into sugar requires three phases:
- Carbon fixation
- Reduction reaction
- Ribulose 1,5-bisphosphate (RuBP) regeneration
Occurrence of Dark Reaction
It occurs in the stroma. The light reaction of photosynthesis produces ATP and NADPH that collectively initiate Calvin’s cycle, and releases ADP, Pi and NADP+ that is again utilized by the cells to form high energy molecules in the light reaction.
Melvin Calvin, with his co-workers in the University of California, used the radioactive isotope of carbon (carbon14) to trace the pathway. In his research, Calvin suspended chlorella first in water and then subjected to sunlight, and finally labelled it with carbon-14 (a radioactive carbon). Then he took individual cells of chlorella and killed them in hot methanol and examined the series of compounds containing the radioactive carbon at different stages of its photosynthetic pathway.
Melvin Calvin described the whole process by publishing a book named “The Path of Carbon in Photosynthesis” in the year 1957 and The Photosynthesis of Carbon Compounds in the year 1962. Calvin concluded that the plants transform light energy to chemical energy via electron excitation by the array of light-harvesting and accessary pigments. Later his research on carbon compounds formed during the photosynthesis was known after his name (Calvin’s cycle).
Steps of Dark Reaction
The dark reaction of photosynthesis or Calvin cycle includes three consecutive stages:
It is the initial stage of dark reaction, which starts when the atmospheric carbon dioxide goes into the plant cells. Then this atmospheric CO2 is fixed via an enzyme (rubisco) that binds it to a (RuBP) ribulose-1, 5-bisphosphate (5-C compound). The association of carbon dioxide and ribulose-1, 5-bisphosphate forms a 6-carbon compound (3-keto-2-carboxyarabinitol 1, 5-bisphosphate).
The unstable 3-keto-2-carboxyarabinitol 1, 5-bisphosphate eventually splits into two 3-carbon compound (3-phosphoglyceric acid). Therefore, the conversion of inorganic carbon into organic compounds like sugar simply refers as carbon fixation.
It is the second and the most crucial stage of Calvin cycle that makes the use of energy molecules (ATP and NADPH) coming from the light-dependent phase of photosynthesis and the specific enzyme system. The 3-Phosphoglyceric acid goes through reduction reaction via an enzyme phosphoglycerate kinase, which phosphorylates 3-PGA by using ATP to form glycerate-1, 3-bisphosphate and ADP and Pi as by-products.
After that, glycerate-1, 3-bisphosphate is reduced via an enzyme glyceraldehyde 3-phosphate dehydrogenase by using NADPH to form a 3-C sugar (glyceraldehyde 3-phosphate). During this stage, the NADPH oxidizes into NADP+.
The glyceraldehyde-3-phosphate is used up by the plants during cellular respiration to form glucose and other carbohydrate sources. Glucose being a 6-C compound requires two G3P molecules produce one glucose molecule.
It is the last stage of the Calvin cycle, which involves the regeneration of RuBP or allows the cycle to continue. Six molecules of carbon dioxide produce twelve molecules of glyceraldehyde 3-phosphate.
Out of twelve, ten molecules are required to synthesize ribulose bisphosphate by utilizing six molecules of ATP that can bind incoming carbon dioxide and restart the cycle. The hydrolysis of ATP produces six molecules each of ADP and inorganic phosphate. The remaining two molecules of G3P are necessary for the synthesis of glucose.
A Calvin cycle produces 2 glyceraldehyde-3-phosphate (G3P), 3 ADP, and 2 NADP+ as the products in one turn.
- This ADP and NADP+ formed in the dark reaction are used for the RuBP regeneration to continue the cycle. Later, it can also be used in the light-dependent reactions.
- A G3P molecule is a 3-carbon compound that is required for the Calvin cycle to continue or regenerates RuBP (ribulose 1, 5-bisphosphate). G3P formed in Calvin cycle is utilized by plant cells to form carbohydrates such as glucose, starch, sucrose, and cellulose, depending on what the plant needs.
Therefore Calvin cycle is the second phase of photosynthesis that mediates the synthesis of bioorganic compounds, necessary for cell function and metabolism. Dark reaction utilizes ATP and NADPH energy molecules coming from the light reaction of photosynthesis for the regeneration of RuBP and carbohydrate synthesis.