DARK REACTION OR CALVIN – BENSON CYCLE OR C3 CYCLE

The series of reactions in which carbon is fixed in the synthesis of

sugar is called Calvin — Benson cycle. It takes place in the stroma of chloroplast.

Discovery of Calvin Benson cycle

Melvin Calvin, Benson and coworkers determined the detail of Calvin cycle. They used radioactive isotope of carbon (C”) in CO,. “[hey performed experiments on unicellular algae Chlorella. They introduced the radioactive CO, to the algae. Photosynthesis took place. They used alcohol to stop the process of photosynthesis at different intervals. Then they removed various radioactive compounds and determined their chemical nature. They found follow Mg compounds:

I. After 1-2 seconds, radioactive CI‘ appeared in phosphoglyceric acid (PGA)

  1. After 30 seconds, radioactive CH was found in triose phosphate, fructose 1-6 phosphate and glucose phosphate.
  2. After 30 minutes, radioactive CH appeared in six carbon sugars like alucoses, sucrose etc.
  3. After I hour, C” was found in some amino acids.

Mechanism of Calvin cycle

The Calvin cycle is composed of four steps.

  1. Carbon fixation

It ibulose I .5-bisphoshate (RuBP) is already pre ent in the stroma of the chloroplast. CO2 reacts with RuBP to form a six carbon compound. Thi, six carbon compound is unstable. It at once breaks into two molecules ii 3-phosphoglycerate (PGA). The rdaction is catalyzed by enzy me Ribulose bisphosphate earboxylase. 1 his enzyme is common!) called rubiseo.

  1. Reduction

The ATP and NADPII2 are used during this phase of reaction. [he L0011 group of 3-PGA is phosphorvlated by ATP. It gives I.3- bisphosphoglvcerate. The I.3-bisphosphogly cerate react xx ith NADPI bit is reduced into 3-phosopgleeraldehyle tPGAI.i. This reaction is controlled by enzyme triose phosphate dehydrogenase. .11ns reaction can he viritten as:

PGA + ATP + NADPIli ______________ • PGAI. 4 NA DP 1 ADP +Pi

  1. Regeneration of RuBP

ROB’ is the starting point of (’02. It must be regenerate for its reuse in the coming reaction. It is regenerated in following reactions.

(i)      PGAl. is converted into dihydroxyaeetone phosphate.

(ii)     Two molecules of dihydroxyacetone phosphate condense (aldol condensation) to give fructose I,6-bisphosphate. It is hydrolyzed into fructose 6-phosphate.

(iii)    Fructose 6-phosphate (6C) combines with a molecule of PGAL (3C). ‘[hey combine to give two molecules of erythrose 4 — phosphate (4C) and xylulose, 5- phosphate (5C).

(iv)    Erythrose 4 — phosphate (4C) combine with dihydrokyacetone phosphate (3C) to form sedoheptulose 7- phosphate (7C).

(v)      Sedoheptulose 7- phosphate (7C) combine with PGAL (3C) to give two molecules of xylulose 5-phosphate (5C).


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(vi)    Two molecules of xylulose 5-phosphate arrange with each other to form two molecules of Ribulose 5-phosphate.

(vii)  Ribulose 5-phosphate is phosphorylated by ATP. It gives Ribulose-1,5-bisphospate (RuBP).

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  1. Synthesis of final products

Three molecules of CO, and three molecules of RuBP are used during Calvin cycle. It produces 6 molecules of PGAL Only one molecules of PGAL come out of cycle. It is used for synthesizing glucose, starch etc. the remaining five molecules of PGAL are used in the regeneration of RuBP. PGAL of many cycles combine to form following compounds:

(i)      Carbohydrates: Triose sugar combines to form glucose and fructose. Glucose and fructose combine to form sucrose. Many molecules of glucose’s combine to form starch and cellulose.

(ii)    Lipids: Some triose phosphates are converted into acetyl CoA. It is then converted into fatty acids.

(iii)   Proteins: Some Hose phosphate is converted into a-ketoglutrate in Krebs cycle. The u-ketoglutrate combines  with ammonium ion to give different amino acids.

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