Biosynthetic Phases Of Photosynthesis

Dark Or Biosynthetic Phase

Biosynthetic Phase Definition: The part of photosynthesis that takes place, even in the absence of light, within the stroma of the chloroplast and during which glucose is synthesised, is called the dark phase.

Biosynthetic Phase Site of occurrence: It takes place in the stroma of the chloroplast.

Biosynthetic Phase Components: Atmospheric carbon dioxide, ATP and NADPH are produced during the light phase and RuBP is present in the cell.

Stages of dark phase: The path of carbon in the dark phase was traced by Melvin Calvin. He traced the path by using radioisotope C¹⁴, a technique called autoradiography. Hence, the dark phase is also known as the Calvin cycle.

The Calvin cycle is divided into three phases or stages. These are—

1. Carboxylation,
2. Reduction phase,
3. Synthesis phase,
4. Regeneration phase. These are discussed below in separate heads.

Carboxylation or carbon assimilation phase

In this pathway, CO₂ is introduced into the cycle by the carboxylation of ribulose-l,5-bisphosphate (RuBP) to form 3-phosphoglyceric acid (3PGA). CO₂ combines with the phosphorylated 5-carbon sugar ribulose bisphosphate (RuBP) in the presence of RuBisCO (ribulose-l,5-bisphosphate carboxylase/oxygenase) enzyme. The product generated is two molecules of 3-phosphoglyceric acid (PGA) in the presence of water.

This initial product, 3-phosphoglyceric acid (3PGA), is a 3C compound. Hence, this pathway is also known as the C₃ pathway. Since CO₂ attaches itself to RuBP present in the cell, this phase is also known as the carbon assimilation phase. Thus RuBP acts as the carbon acceptor compound.

6 cycles of the Calvin cycle involve 6 molecules of CO₂ reacting with 6 molecules of RuBP, to produce 12 molecules of PGA

Biosynthetic Phase Reduction phase

The PGA molecules, produced by the carbon assimilation phase, are further phosphorylated and reduced (by NADPH and H⁺) to form 3-phosphoglyceraldehyde (PGAId) or glyceraldehyde-3-phosphate (GAP). NADPH and ATP produced during the light phase, provide protons and energy, respectively, for this process. Because of this cycle, the C₃ cycle is also known as the photosynthetic carbon reduction cycle (PCR cycle).

This phase consists of two reactions. The first reaction involves the phosphorylation of 3PGA by ATP to form 1, 3-bisphosphoglycerate. The reaction is catalysed by the enzyme 3-phosphoglycerate kinase.

The second reaction involves the reduction of 1, 3-BPGA by NADPH in the presence of glyceraldehyde-3-phosphate (GAP) dehydrogenase.

Synthesis phase

1. Out of 5 GAP molecules produced during the reduction phase, 2 are isomerised to dihydroxyacetone phosphate (DHAP) by the enzyme triose phosphate isomerase.

2. 1 molecule of GAP and 1 molecule of DHAP react to produce the 6-carbon compound fructose-1, 6-bisphosphate (FBP) by the enzyme fructose bisphosphate aldolase.

3. Fructose bisphosphate (FBP) is then converted to fructose 6-phosphate by the enzyme fructose bisphosphatase (FBPase)

4. Fructose-6-phosphate (F6P) is further converted to glucose-6-phosphate (G6P) by fructose-6-phosphate isomerase.

5. Finally, glucose-6-phosphate is converted to glucose, in the presence of enzyme phosphatase.

Regeneration phase

To keep the C₃ cycle going, RuBP must be continuously replaced within the cell. During this phase, 10 molecules of GAP (produced during the synthesis phase) form 6 molecules of the 5-carbon compound, Ribulose monophosphate (RuMP) in the presence of different enzymes. RuMP then gets converted into RuBP by the enzyme phosphoribulose kinase.

The reactions for the above processes are as follows—

1. 2 molecules of fructose-6-phosphate and 2 molecules of GAP, in the presence of transketolase, produce erythrose 4-phosphate (E4P) and xylulose 5-phosphate (Xu5P).

2. The next step is the second aldolase reaction in which 2 molecules of sedoheptulose-l,7-bisphosphate is formed by 2 molecules of E4P and 2 molecules of DHAP. The enzyme used in this reaction is sedoheptulose bisphosphate aldolase or transaldolase.

3. Sedoheptulose bisphosphate is then hydrolysed to sedoheptulose-7-phosphate catalysed by sedoheptulose bisphosphatase.

4. This is followed by the second transketolase reaction in which xylulose-5-phosphate (Xu5P) and ribose-5-phosphate (R5P) are produced from sedoheptulose-7-phosphate and GAP.

5. 2 molecules of ribose-5-phosphate get converted into 2 molecules of ribulose-5-phosphate (Ru5P), by the enzyme ribulose phosphate isomerase. Xylulose 5-phosphate (Xu5P) is also converted to ribulose 5-phosphate by the enzyme ribulose phosphate epimerase. This produces 4 molecules of Ru5P. A total of 6 molecules of Ru5P are produced.

6. The regeneration phase is completed by the phosphorylation of ribulose 5-phosphate to ribulose-1, 5-bisphosphate. Thus the C02 acceptor RuBP is regenerated. This step is catalyzed by the enzyme phosphoribulokinase in which ATP acts as a phosphate donor and the enzyme is activated by light.

If the cycle occurs once, 3 molecules of CO₂ are fixed and a triose sugar (GAP) is formed. For the formation of one molecule of glucose, the cycle occurs twice and 12 molecules of CO₂ are fixed.

Significance Of Dark Phase (Calvin cycle)

1. Carbohydrates are synthesised from PGAId or GAP in the Calvin cycle.
2. RuBP is generated in the Calvin cycle. This is required for the continuation of the dark phase.
3. Several intermediate compounds obtained within the cycle are used in other metabolic processes.
4. Since CO₂ gets absorbed during this phase, therefore it helps to maintain the CO₂– O₂
5. balance in nature.

Interdependence Of Light And Dark Phase

The products obtained during the light phase are ATP and NADPH₂, which are required during the dark phase. They take part in the dark phase and get converted into products that are required during the light phase.

Dependence of dark phase on light phase

ATP, generated during photophosphorylation in the light phase, takes part in the dark phase, in the following manner—

⇒ \(\mathrm{PGA}+\mathrm{ATP} \longrightarrow \mathrm{BPGA}+\mathrm{ADP}\)

2. NADPH₂, generated during photophosphorylation in the light phase, takes part in the dark phase, in the following manner—

⇒ \(\mathrm{BPGA}+\mathrm{NADPH} \mathrm{N}_2 \longrightarrow \text { PGAld + NADP }\)

Dependence of light phase on dark phase

1. ADP, obtained during the dark phase, takes part in the photophosphorylation stage of the light phase, in the following manner—

⇒ \(\mathrm{ADP}+\mathrm{Pi} \longrightarrow \mathrm{ATP}\)

2. NADP⁺ obtained during the dark phase gets reduced to NADPH₂ in the light phase, in the following manner—

⇒ \(\mathrm{NADP}+2 \mathrm{H}^{+}+2 \mathrm{e} \longrightarrow \mathrm{NADPH}_2\)

Without ADP and NADP⁺ formation again, the energy of the electron could not have been used. As a result, chlorophyll would not have released electrons, thereby getting destroyed. This, in turn, would have stopped photosynthesis.

Photosynthetic quotient or PQ

During photosynthesis, solar energy is converted to chemical energy. Water is oxidised and CO₂ is reduced to produce glucose and O₂. The ratio of O₂ released and CO₂ absorbed is known as the photosynthetic quotient (PQ). It can be expressed as follows—

⇒ \(\begin{aligned} \mathrm{PQ} & =\frac{\text { Amount of } \mathrm{O}_2 \text { released during photosynthesis }}{\text { Amount of } \mathrm{CO}_2 \text { absorbed during photosynthesis }} \\ & =\frac{60_2}{6 \mathrm{CO}_2}=1 \end{aligned}\)

When PQ increases, the rate of photosynthesis is said to have increased.