Botany

Calvin-Benson Cycle and its Stages

Calvin, Benson and their coworkers determined the detail of this reaction. Experiments were performed on unicellular Algae “Chlorella”. Radioactive isotopes of carbon (6C14) in CO2. Alcohol was used to stop the photosynthesis. Chemical nature of compounds (produced during intervals) was determined. Following products were found; After 1-2 seconds, radioactive C14 was found in phosphoglyceric acid (PGA).

After 30 seconds, C14 in triose phosphate, fructose 1,6 phosphate and glucose phosphate. After 30 minutes, C14 in six carbon sugars like glucose, sucrose etc. After 60 minutes, C14 in some amino acids.

STAGES OF CALVIN CYCLE

Calvin cycle has three stages

  1. Carboxylation/Carbon fixation
  2. Reduction
  3. Regeneration of (RUBP)

1. Carboxylation/Carbon fixation:

RUBP (5C) already present in the stroma of Chloroplast. RUBP reacts with CO2 to form six carbon compound. Two molecules of 3-Phosphoglyceric acid (3 PGA) from unstable six carbon compound. 3-PGA is three carbon Compound. This cycle is C3 cycle and plants are called C3 plants. This reaction catalyzed by Ribulose bisphosphate carboxylase or Rubisco.

2. Reduction:

ATP and NADPH2 molecules are used. 3-PGA is phosphorylated by ATP and 1,3 bisphosphoglycerate is formed. This reaction is catalyzed by phosophoglycerate kinase. 1,3 bisphosphoglycerate is reduced into glyceraldehyde 3-phosphate (G3P) by NADPH2. This reaction is catalyzed by glyceralde 3-phosphate dehydrogense. Six molecules of G3P are produced.

3. Regeneration of (RUBP):

RUBP is acceptor of CO2, so it must be regenerated. Five molecules of G3P (3C) are used for RUBP regeneration. Two molecules of G3P (3C) are converted into dihydroxy acetone 3-phosphate (3C). Two molecules of dihydroxy acetone 3-Phosphate (3C) condense to form one molecules of fructose 1,6 bisphostae (6C).

Fructose 1,6-bisphostae is hydrolyzed into fructose 6-phosphate (6C). Fructose 6-phosphate (6C) combines with a molecules of 3PG (3C) to form Erythrose 4- phosphate (4C) and Xylulose 5-phosphate (5C). Erythrose 4-phosphate combines with a molecules of Dihydroxy acetone 3-Phosphate (3C) to form Sedoheptulose 7-phosphate (7C). Sedoheptulose 7-phosphate (7C) combines with a molecules of G3P(3C) to form two molecules of Xylulose 5-phosphate (5C).

Three molecules of Xylulose 5-phosphate arrange with each other to form thee molecules of Ribulose 5-phosphate. Ribulose 5-phosphate is phosphorylated by three ATPs to form Ribulose 1,5-bisphosphate (RUBP).

SYNTHESIS OF FINAL PRODUCTS

Six molecules of G3P are produced. Fives molecules of G3P are used in regeneration of RUBP . One molecules of G3P comes out of cycle and used in synthesis of glucose, starch etc.

Calvin cycle proceeds in three stages
Fig. The Calvin cycle proceeds in three stages. (Adapted from Plant Physiology bt Taiz & Zeiger 5th Edition)

PHOTORESPIRATION

Respiration in the presence of light is called photorespiration. Instead of generating energy it rather utilizes the energy. It occur in C3 plants which live in the dry and hot conditions. When temperature increases stomata closes and as a result gaseous exchange stop. This will lead to the high concentraion of O in the leaves and low concentration of CO2.

RUBISCO will bring about the fixation of oxygen instead of CO2 in such conditions. RUBP will combine O and from one molecule of 3PG and one molecule of glycolate. This glycolate will enter the peroxisome and combine with amine(NH2) to form glycine(2C). Two molecules of glycine combine to form one molecule of serine(3C).  This serine molecule will then split into glycolate and one molecule of CO2.

TYPES OF CO2 ASSIMILATION PATHWAYS:

During photosynthesis, there are three types of CO2 assimilation pathways:

  • C3 pathway
  • C4 pathway
  • CAM pathway

C3 pathway:

The pathway during which first product after the carbon assimilation is 3-carbon molecule is called C3 pathway. The plants in which this pathway occurs are called C3 plants.

C4 pathway:

C4 pathway

The pathway during first product after the carbon assimilation is 4-carbon molecule is called C4 pathway. It is also called as Hatch and Slack pathway. The plants in which this pathway occurs are called C4 plants.

CAM pathway:

CAM-pathway

The pathway during which first product after the carbon assimilation is also 4-carbon molecule Energy from sunlight is absorbed during day time. Carbon fixation occurs at night time. It is called as crassulacean acid metabolism (CAM). CAM pathway is a kind of adaptation in plants. The plants in which this pathway occurs are called CAM plants.

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