Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.6 Dark Phase of Photosynthesis. C3 Pathway of Photosynthesis (Calvin Cycle)

The products of the light reactions of Photosynthesis, ATP and NADPH • H2, are utilized in the dark reactions to reduce CO2 to the level of 80

CARBOHYDRATES. Currently, the C₃ and C₄ pathways of CO2 fixation, Crassulacean Acid METABOLISM (CAM photosynthesis), and Photorespiration are known.

The C₃ pathway of photosynthesis, or The Calvin Cycle, is a CO2 assimilation pathway found in all photosynthetic plants. It was named the Calvin cycle in honor of the American biochemist M. Calvin, who, along with his coworkers, discovered and studied it between 1946 and 1956 using the radioactive tracer method and Chromatography. Plants were fed ¹⁴СО₂ for varying periods of time, and their extracts were chromatographed. Radioactive isotopes do not differ in chemical properties from stable ones. Participating in reactions, they label the compounds they become incorporated into. The cycle consists of three stages: carboxylation, reduction, regeneration of the primary CO2 acceptor, and synthesis of the final photosynthetic product (Fig. 4.5).

1. Carboxylation. Phosphoribulokinase phosphorylates ribulose-5-phosphate in the presence of ATP to yield ADP and ribulose-1,5-diphosphate. The latter serves as a CO₂ acceptor and, under the action of ribulose diphosphate carboxylase, fixes CO₂. This results in The formation of two molecules of 3-phosphoglyceric acid (3-PGA).

2. Reduction. Phosphoglycerate kinase phosphorylates 3-PGA using ATP, and the resulting 1,3-diphosphoglyceric acid is reduced by NADPH and phosphoglyceraldehyde dehydrogenase to 3-phosphoglyceraldehyde (3-PGALD).

3. Regeneration. Following the fixation of three CO2 molecules and the formation of six 3-PGALD molecules, five of them are used to synthesize ribulose-5-phosphate, while one 3-PGALD molecule is utilized for glucose production.

Triosephosphate isomerase converts 3-PGALD into phosphodihydroxyacetone. Then, aldolase combines 3-PGALD and phosphodihydroxyacetone to form fructose-1,6-diphosphate. It loses one phosphoric acid residue under the action of fructose-1,6-diphosphatase and is converted into fructose-6-phosphate. Transketolase transfers a glycolaldehyde group from fructose-6-phosphate to 3-PGALD, yielding erythrose-4-phosphate and xylulose-5-phosphate. Aldolase adds phosphodihydroxyacetone to erythrose-4-

phosphate to form sedoheptulose-1,7-diphosphate. It is dephosphorylated

by phosphatase and, through the action of transketolase, condenses with 3-PGALD.

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Figure 4.5 - The Calvin cycle (C₃ pathway of photosynthesis): 1 - phosphoribulokinase,

2 - ribulose diphosphate carboxylase, 3 - phosphoglycerate kinase,

4 - triosephosphate dehydrogenase, 5 - triosephosphate isomerase, 6 - aldolase, 7 - phosphatase, 8 - transketolase, 9 - aldolase, 10 - phosphatase, 11 - transketolase,

12 - ribosephosphate isomerase, 13 - phosphoketopentose epimerase (after V. V. Polevoy)

The products of this reaction are xylulose-5-phosphate and ribose-5-phosphate. Two molecules of xylulose-5-phosphate, with the participation of ribulose phosphate epimerase, and one molecule of ribose-5-phosphate, under the action of ribose phosphate isomerase, are converted into three molecules of ribulose-5-phosphate.

The sixth 3-PGALD molecule is used to synthesize fructose-1,6-diphosphate during the repetition of the cycle. From two molecules of fructose-1,6-diphosphate, fructose-6-phosphate and glucose-1-phosphate are formed. The latter, interacting with uridine triphosphate, yields uridine diphosphate glucose. This compound and fructose-6-phosphate form sucrose phosphate, which upon dephosphorylation is converted into sucrose. Thus, the formation of a single sucrose molecule requires four turns of the Calvin cycle. Starch is synthesized from uridine diphosphate glucose through the action of amylosynthetase.

Amino Acids have also been detected among the products of photosynthesis. In case of an NADPH shortage, 3-PGA is converted not into 3-PGALD, but into pyruvic acid. By attaching ammonia, it forms Alanine. From pyruvic acid, organic acids are produced in the Krebs cycle, which subsequently yield amino acids via amination and Transamination reactions.

The overall equation of the Calvin cycle:

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Thus, for every three molecules of CO2 entering the cycle, one molecule of PGALD is produced.