Dark Reaction or C3 Cycle or Biosynthetic Phase or Photosynthetic Carbon Reduction (PCR)Cycle

-

 Dark Reaction or C3 Cycle or Biosynthetic Phase or Photosynthetic Carbon Reduction (PCR)Cycle

Biosynthetic phase of photosynthesis utilises assimilatory powers (ATP and NADPH + Hᶧ) produced during light reaction are used to fix and reduce carbon dioxide into carbohydrates. This reaction does not require light. Therefore, it is named Dark reaction. Ribulose 1,5 bisphosphate (RUBP) act as acceptor molecule of carbon dioxide and fix the CO2 by RUBISCO enzyme. The first product of the pathway is a 3- carbon compound (Phospho Glyceric Acid) and so it is also called as C3 Cycle. It takes place in the stroma of the chloroplast. M. Melvin Calvin, A.A. Benson and their co-workers in the year 1957 found this path way of carbon fixation. Melvin Calvin was awarded Nobel Prize for this in 1961 and this pathway named after the discoverers as Calvin-Benson Cycle. Dark reaction is temperature dependent and so it is also called thermo-chemical reaction. Dark reaction consists of three phases:




1. Carboxylation (fixation)

2. Reduction (Glycolytic Reversal)

3. Regeneration

 Phase 1- Carboxylation (Fixation):

The acceptor molecule Ribulose 1,5 Bisphosphate (RUBP) a 5 carbon compound with the help of RUBP carboxylase oxygenase (RUBISCO) enzyme accepts one molecule of carbon dioxide to form an unstable 6 carbon compound. This 6C compound is broken down into two molecules of 3-carbon compound phospho glyceric acid (PGA).


 Phase 2 – Glycolytic Reversal / Reduction:

 Phospho glyceric acid is phosphorylated by ATP and produces 1,3 bis phospho glyceric acid by PGA kinase. 1,3 bis phospho glyceric acid is reduced to glyceraldehyde 3 Phosphate (G-3-P) by using the reducing power NADPH + Hᶧ. Glyceraldehyde 3  phosphate is converted into its isomeric form di hydroxy acetone phosphate (DHAP).

Phase 3 – Regeneration:

 Regeneration of RUBP involves the formation of several intermediate compounds of 6-carbon, 5-carbon,4-carbon and 7- carbon skeleton. Fixation of one carbon dioxide Rubisco requires 3 ATPs and 2 NADPH + Hᶧ, and for the fixation of 6 CO2 requires 18 ATPs and 12 NADPH + Hᶧ during C3 cycle. One 6 carbon compound is the net gain to form hexose sugar.


Comments

Post a Comment

Popular posts from this blog

Allelic Interactions - dominant and recessive alleles, codominant alleles, lethal alleles, penetrance or expressivity, multiple alleles

-
Image
                                                ALLELIC RELATIONSHIP   Dominant and Recessive Alleles:   Whenever one of a pair of alleles can come to phenotypic expression only in a homozygous genotype, we call that allele a recessive factor. The allele which can phenotypically express itself in the heterozygote as well as in the homozygote is called a dominant factor. Upper- and lower-case letters are commonly used to designate dominant and recessive alleles respectively. Usually, the genetic symbol corresponds to the first letter in the name of the abnormal (or mutant) trait.   (a) Carriers:   Recessive alleles (such as the one for albinism) are often deleterious to those who possess them in duplicate (homozygous recessive genotype). A heterozygote may appear just as normal as th...
Read more

Apomixsis- definition, classification, Agamospermy, vegetative reproduction

-
Image
                                                                                       APOMIXIS   The normal sexual cycle (amphimixis) involves two important processes (a) Meiosis, this transforms a diploid sporophytic cell (megaspore mother cell, MMC) into four haploid gametophytic cells, and (b) Fertilization, In which two haploid gametes of opposite sex fuse re-establishing the diploid sporophytic generation. Thus, in a sexual cycle a diploid generation (sporophytic) alternates with a haploid generation (gametophytic) In angiosperms the gametophytic generations are very short and are represented by embryo sac on the female side and microspores or pollen grains on the male side. In some plants meiosis and syngamy are int...
Read more

Cytokinin and their role

-
Image
                                                                                            Cytokinin Discovery: The presence of cell division inducing substances in plants was first demonstrated by Haberlandt in 1913 in Coconut milk (liquid endosperm of coconut) which contains cell division inducing substances. In 1954, Skoog and Miller discovered that autoclaved DNA from herring sperm stimulated cell division in tobacco pith cells. They called this cell division inducing principle as kinetin (chemical structure: 6-Furfuryl Amino Acid). This does not occur in plants. In 1963, Lethan introduced the term cytokinin. In 1964, Lethan and Miller isolated and identified a new cytokinin called Zeatin from unripe grains ...
Read more