Question
Give schematic representation of Calvin Cycle.
✓
Answer
$1.$ The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co$-$worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin$-$Benson cycle. Since the first stable product formed is a $3-$carbon compound, it is also called as $C_3$ pathway and the plants are called $C_{14}$ plants.
$2.$ Calvin carried out experiments on unicellular green algae $($Chlorella$)$, using radioactive isotope of carbon, $C_{14}$ as a tracer. It is also called synthesis phase or second phase of photosynthesis.
The cycle is divided into the following phases:
$1.$ Carboxylation phase:
$a.$ Carbon dioxide reduction starts with a five$-$carbon sugar ribulose$-l,5-$bisphosphate $(RuBP).$ It is a $5-$ carbon sugar with two phosphate groups attached to it.
$b. \ RuBP$ reacts with $CO_2$ to produce an unstable $6$ carbon intermediate in the presence of Rubisco.
$c.$ It immediately splits into $3$ carbon compounds called $3-$phosphoglyceric acid.
$d. \ RuBisCO$ is a large protein molecule and comprises $16\%$ of the chloroplast proteins.
$2.$ Glycolytic reversal:
$a. \ 3-$phosphoglyceric acid form $1,3-$diphosphoglyceric acid by utilizing $\text{ATP}$ molecule.
$b.$ These are then reduced to glyceraldehyde$-3-$phosphate $(3-PGA)$ by $\text{NADPH}$ supplied by the light reactions of photosynthesis.
$c.$ In order to keep Calvin cycle continuously running there must be sufficient number of $RuBP$ and regular supply of $\text{ATP}$ and $\text{NADPH}.$
$d.$ Out of $12$ molecules of $3-$phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule.
$3.$ Regeneration of $RuBP:$
$a. \ 10$ molecules of $3-$phosphoglyceraldehyde are used for the regeneration of $6$ molecules of $RuBP$ at the cost of $6 \ \text{ATP}$.
$b.$ Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.
$2.$ Calvin carried out experiments on unicellular green algae $($Chlorella$)$, using radioactive isotope of carbon, $C_{14}$ as a tracer. It is also called synthesis phase or second phase of photosynthesis.
The cycle is divided into the following phases:
$1.$ Carboxylation phase:
$a.$ Carbon dioxide reduction starts with a five$-$carbon sugar ribulose$-l,5-$bisphosphate $(RuBP).$ It is a $5-$ carbon sugar with two phosphate groups attached to it.
$b. \ RuBP$ reacts with $CO_2$ to produce an unstable $6$ carbon intermediate in the presence of Rubisco.
$c.$ It immediately splits into $3$ carbon compounds called $3-$phosphoglyceric acid.
$d. \ RuBisCO$ is a large protein molecule and comprises $16\%$ of the chloroplast proteins.
$2.$ Glycolytic reversal:
$a. \ 3-$phosphoglyceric acid form $1,3-$diphosphoglyceric acid by utilizing $\text{ATP}$ molecule.
$b.$ These are then reduced to glyceraldehyde$-3-$phosphate $(3-PGA)$ by $\text{NADPH}$ supplied by the light reactions of photosynthesis.
$c.$ In order to keep Calvin cycle continuously running there must be sufficient number of $RuBP$ and regular supply of $\text{ATP}$ and $\text{NADPH}.$
$d.$ Out of $12$ molecules of $3-$phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule.
$3.$ Regeneration of $RuBP:$
$a. \ 10$ molecules of $3-$phosphoglyceraldehyde are used for the regeneration of $6$ molecules of $RuBP$ at the cost of $6 \ \text{ATP}$.
$b.$ Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.
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