PHOTOSYNTHESIS REACTIONS - LIGHT REACTION AND DARK REACTION

PHOTOSYNTHESIS REACTIONS

• Photosynthesis takes place in two stages.
• In the First stage, called the Light reactions, the light energy is used to convert ADP and Phosphates to ATP. In addition, in the predominant form of the light-dependent reactions, the electron carrier NADP⁺ is reduced to NADPH. The coenzyme NADPH, like NADH, is an energy- rich carrier of electrons.
• In the Second stage, the light-independent (Dark) reactions, these electrons are used along with energy from ATP to reduce CO₂ to Carbohydrate sugar.

LIGHT REACTION (or) LIGHT DEPENDENT REACTION (or) PHOTOPHOSPHORYLATION

• Photophosphorylation is one of the three ways ATP is formed, and it occurs only in photosynthetic cells.
• In this mechanism, Light energy is absorbed by Chlorophyll molecules in the photosynthetic cell, exciting some of the molecules electrons.
• The Chlorophyll is located in the membranous Thylakoids of Chloroplasts in Algae and Green plants.
• In Cyanobacteria, Thylakoids are found in the photosynthetic structures.
• Bacteria use Bacteriochlorophylls to carry out the Light reaction.

Cyclic photophosphorylation (Photosystem – I)

• Cyclic photophosphorylation is observed in all the photosynthetic microorganisms.
• The excited electrons jump from the Chlorophyll to the first of a series of carrier molecules, an electron transport chain similar to that used in respiration. As electrons are passed along the series of carriers, protons are pumped across the membrane, and ADP is converted to ATP by Chemiosmosis (Electron Transport Chain).
• In Cyclic photophosphorylation, the electrons eventually return to Chlorophyll.

Cyclic photophosphorylation

Non – cyclic photophosphorylation (Photosystem – II)

• Non-cyclic photophosphorylation is observed in all the photosynthetic microorganisms except Photosynthetic bacteria.
• In Non - cyclic photophosphorylation, which is used in oxygenic organisms, the electrons released from chlorophyll do not return to chlorophyll but become incorporated in to NADPH. The electrons lost from chlorophyll are replaced by electrons from H₂O.
• The products of Non- cyclic photophosphorylation are ATP (formed by Chemiosmosis using energy released in an Electron Transport Chain), O₂ (from water molecules) and NADPH (in which the hydrogen electrons and protons were derived ultimately from water).

 

Non - cyclic photophosphorylation

DARK REACTION (or) LIGHT INDEPENDENT REACTION (or) CALVIN – BENSON CYCLE

• The Light - independent (Dark) reactions are so named because they require no light directly. They include a complex cyclic pathway called the Calvin-Benson cycle, in which CO₂ is "fixed"- that is, used to synthesize Carbohydrate sugars.
• The Calvin cycle is also called the “Reductive Pentose Phosphate Cycle” because it is essentially the reverse of the Pentose Phosphate Pathway.
• The Calvin cycle is present in Purple bacteria, Cyanobacteria, algae, green plants, most Chemolithotrophic Bacteria, and a few Archaea.
• The reactions of the Calvin cycle occur in the Chloroplast stroma of eukaryotic microbial autotrophs. In Cyanobacteria, some nitrifying bacteria, and Thiobacillus sp. (sulfur oxidizing Chemolithotrophs), the Calvin cycle is associated with Inclusion bodies called Carboxysomes. These are polyhedral structures that contain the enzyme critical to the Calvin cycle and may be the site of CO₂ fixation.
• The cycle requires CO₂ and a CO₂ - acceptor molecule (NADPH, ATP and two key enzymes viz., Ribulose bisphosphate carboxylase and Phosphoribulokinase).

Phases of Calvin – Benson Cycle

The Calvin cycle is divided into three phases: (i) Carboxylation phase, (ii) Reduction phase and (iii) Regeneration phase.

(i) Carboxylation phase (Carbon fixation)

During the first phase or Carboxylation phase, the enzyme Ribulose- 1,5-bisphosphate carboxylase, also called Ribulose bisphosphate carboxylase/oxygenase (RubisCo), catalyzes the addition of CO₂ to the 5-carbon molecule Ribulose-1,5-bisphosphate (RuBP), forming a six-carbon intermediate that rapidly and spontaneously splits into two molecules of 3 - Phosphoglycerate (PGA). The PGA is an intermediate of Glycolysis.

(ii) Reduction Phase

The PGA is then phosphorylated and reduced to a key intermediate of Glycolysis, Glyceraldehyde-3-phosphate. From this, glucose can be formed by reversal of the early steps in Glycolysis.

This phase produces carbohydrates such as Glyceraldehyde-3-phosphate, Fructose-6-phosphate and Glucose-6-phosphate, all of which are precursor metabolites. In this phase, the biosynthesis of carbohydrate sugars takes place.

(iii) Regeneration Phase

Finally, in the Regeneration phase, RuBP is regenerated from the Ribulose – 5 – phosphate with the help of an enzyme Phosphoribulokinase, so that the cycle can repeat.

Calvin – Benson Cycle