Photochemical and Biosynthetic Phase of Photosynthesis

In this article we will discuss about the Photochemical and Biosynthetic Phase of Photosynthesis.

Photochemical Phase (Light or Hill Reaction):

It occurs inside the thylakoids, especially those of grana region. Photochemical step is dependent upon light. The function of this phase is to produce assimilatory power consist­ing of reduced coenzyme NADPH and energy rich ATP molecules.

Photochemical phase involves the following reactions:

(a) Photolysis of Water:

The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplasts is called photolysis or photo catalytic splitting of water. Light energy, an oxygen evolving complex (OEC) and an electron carrier Y_z are required. Oxygen evolving complex was formerly called Z-enzyme.

It is attached to the inner surface of thylakoid membrane. The enzyme has four Mn ions. Light energized changes in Mn (Mn²⁺, Mn³⁺, Mn⁴⁺) removes electrons from OH^– component of water forming oxygen. Liberation of O₂ also requires two other ions, Ca²⁺ and СГ. Electron carrier Y_z transfers the released electrons to P₆₈₀ (It is a component of a protein connected with P₆₈₀ centre).

(b) Production of Assimilatory Power (NADPH and ATP):

The electrons released during photolysis of water are picked up by P₆₈₀ photo Centre of photosystem II. On receiv­ing a photon of light energy the photo Centre expels an electron with a gain of energy (23 kcal/ mole). It is the primary reaction of photosynthesis which involves the conversion of light energy into chemical form. The phenomenon is also known as quantum conversion.

The electron extruded by the photo Centre of photosystem II is picked up by the quencher phaeophytin. From here the electron passes over a series of carriers in a downhill journey losing its energy at every step. The major carriers are plastoquinone (PQ), cytochrome b — f complex and plastocyanin (PC).

While passing over cytochrome complex, the electron loses sufficient energy for the creation of proton gradient and synthesis of ATP from ADP and inorganic phosphate. The process is called photophosphorylation (noncyclic).

From plastocyanin the electron is picked up by the trap centre P₇₀₀ of photosystem I. On absorbing a photon of light energy, P₇qo pushes out the electron with a gain of energy. The electron passes over carriers X (a special chlorophyll molecule), FeS, ferredoxin and NADP-reductase. The latter gives electrons to NADP⁺ for combining with H⁺ ions to pro­duce NADPH.

NADPH is a strong reducing agent. It constitutes the reducing power which also contains a large amount of chemical energy.

Biosynthetic Phase (Dark or Blackman’s Reaction):

It catalyses assimilation of CO₂ to carbohydrates. The reactions are called carbon reactions. They occur in stroma or matrix of chloroplasts. The reactions do not require light. Instead assimilatory power (ATP and NADPH) produced during photochemical phase is used in fixation and reduction of carbon dioxide. All the enzymes required for the process are present in the matrix or stroma of the chloroplast.

There are two main pathways for the biosynthetic or dark phase— Calvin cycle and C₄ di-carboxylic acid cycle. A third pathway is CAM metabolism which is intermediate between the two.

The plants exhibiting Calvin cycle and C₄ dicarboxylic acid cycle are respectively called C₃ and C₄ plants. There is no taxonomic importance of the two pathways. Thus Euphorbia corollata is a C₃ plant while Euphorbia maculata is C₄ plant. Grass Alloteropsis semialata has both C₃ and C₄ ecotypes (ecological variants).