Photosynthesis : Photochemical or Light Reaction

Photosynthesis : Photochemical or Light Reaction

Synthesis of carbohydrate by autotrophic plants from carbondioxide and water, using energy from sunlight inside green plastid chloroplast is termed as photosynthesis.

Following equation represents photosynthesis:

Photosynthesis consists of two reactions,

1. Light reaction, also known as photochemical reaction or Hill’s reaction and
2. Dark reaction, also known as biochemical reaction or Blackman’s reaction

Light reaction takes place inside the thylakoids of grana and dark reaction takes place inside the stroma of chloroplast.

Light Reaction of Photosynthesis

The primary photochemical act of photosynthesis involves breakdown of H2O into H⁺, and OH^– ion in the presence of light.  It takes place inside the grana of chlorophyll.  The OH^– ion reunite to form water and molecular oxygen.  Besides oxygen, light reaction gives rise to a reducing agent, NADPH₂ and an energy-rich compound ATP.  The complex mechanism of light reaction can be discussed under the following subheadings:

Hill’s reaction:

Robert Hill in 1937 and Scarisbrick (1940) found that isolated chloroplast when illuminates in presence of water produces oxygen if some hydrogen acceptors are present in the medium.

Arnon’s work:

Arnon while working on chloroplast in 1951 discovers that the splitting of water molecules releases H⁺ ion and coenzymes NADP accept the H⁺ ion .

Another discovery of Arnon in the year 1954, was that apart from carrying out Hill’s reaction, the chloroplast also synthesizes ATP in presence of light from ADP and inorganic phosphate.

Emerson’s experiment:

The exact participation of various pigment molecules and the existence of 2 pigment systems in photosynthesis came from the study made by Robert Emerson and his co-workers.

Emerson’s first experiment (red drop):

Emerson and Lewis determine that the quantum yield of photosynthesis under different wavelength of light and found that there was a sharp drop in the region above 680 nm.  The fall in photo yield beyond red region of spectrum is termed as “red drop.”

Emerson’s second experiment (Emerson effect):

Emerson and his co-worker further extended the previous experiment by supplying additional shorter wavelength of light along with far red light.  The monochromatic far red light which was insufficient, when supplied with shorter wavelength of light enhance the photosynthetic yield and recovered the red drop.  The enhancement in yield due to combination of shorter wavelength of light with far red light is termed as”Emerson effect.”

According to them, simultaneous or quick alternate giving of 2 wavelengths surprisingly gave a photosynthetic rate, higher than total rate got from 2 beams of light used separately.

The discovery of Emerson’s effect has clearly shown the existence of two pigment system.

Photosynthetic Units (PSU)

A photosynthetic unit is the smallest group of pigment molecules which take part in a photochemical act or conversion of light energy into chemical energy.  It has a photocentre or rection centre which is fed by about 200 harvesting pigment molecules.  The photocentre consist of a special chlorophyll a molecule, P₇₀₀ or P₆₈₀.  Reaction centre absorbs light energy at longer wavelengths.  The harvesting molecules are of two types, antenna molecules and core molecules.

The antenna molecules absorb light of various wavelengths but shorter than that of photocentre.  On absorption of light energy the antenna molecules get excited.  In the excited state an electron is pushed to an outer orbital.  The excited antenna molecules hand over their energy to core molecules by resonance and come to the ground state.  The energy picked up by the core molecules is given to the photocentre.  On absorption of energy the photocentre gets excited and extrudes an electron after which it comes to ground state to repeat the cycle.

Pigment system I (PSI) or Photosystem I

It mainly consists of pigment absorbing longer wavelength of light.  It comprises of chlorophyll b, chlorophyll a ₆₇₀, chlorophyll a ₆₈₀, chlorophyll a ₆₉₅, carotenoids, one single molecule of chlorophyll a _{700 (P-700)}, one cytochrome f, one plastocyanin, two cytochrome b, 563, one molecule of iron protein sulfur complex A (FeS), Ferredoxin-reducing substance (FRS) and 1 or 2 molecules of membrane-bound ferredoxin.

It produces a strong reductant, which reduces NADP to NADPH₂  (NADP⁺→NADPH+H⁺).

Pigment system II (PSI )or Photosystem II

It mainly consists of pigment absorbing shorter wavelength of light.  It comprises of phycobillins, xanthophylls, chlorophyll b ₆₆₀, chlorophyll ₆₇₀ and a molecule of chlorophyll _{680 (P-680)} and an unidentified electron donor “Z”, a primary electron acceptor quinone (Q), plastoquinone (PQ), 4 plastoquinone equivalent, 3 manganese  molecules, chlorides and 2 cytochrome b 559.

It releases oxygen and generates a strong oxidant and a weak reductant.

Photophosphorylation

The process in which the interaction of weak oxidant of pigment system I (PSI) and weak reductant of pigment system II(PSII) results in the formation of ATP from ADP and inorganic phosphate.  There are two types of photophosphorylation, cyclic and noncyclic.

Non-cyclic Photophosphorylation

Process of synthesis of ATP and NADPH₂ from ADP and NADP in the presence of light during noncyclic transfer of electrons is termed as non cyclic photophosphorylation.

It involves pigment system I (PSI) and pigment system II (PSII).  Pigment system II absorbs light and becomes excited.  The accessory pigments absorb light and transfer their energy to reaction centre P-680.  Electron from P-680 comes out due to excitation .  Quinone absorbs the electron and the chlorophyll becomes oxidized.

In the meantime, water diassociates into hydrogen and OH¯ in presence of Mn⁺⁺ Cl^–.  The OH¯ uses its electron to z (unknown electron donor) and becomes hydroxide radical.  Transfer of electron from z to P₆₈₀ takes place. Hydroxide radical gives rise to hydrogen peroxide, which breaks into H20 and O2.

The reduced quinone transfers its electron to plastoquinone.  The electron then moves through a series of electron acceptor like cytochrome b, cytochrome f, plastocyanin (Pc).  During the transfer of electron from cytochrome b to cytochrome f, there is synthesis of one molecule of ATP from ADP and inorganic phosphate.

Simultaneously, PSI gets excited and the reaction centre P₇₀₀ejects electrons, which goes to reduce ferredoxin-reducing substance (FRS) through a compound A (FeS).  The excited P₇₀₀ immediately takes up electron from PSII through plastocyanin.

The reduced FRS transfers its electron to ferredoxin (Fd).  Electron then passes to NADP⁺  through Fd NADP reductase.  Reduction of NADP⁺ to NADPH⁺ + H⁺ takes place by utilizing elctrons coming from ferredoxin and H⁺ ions coming from water.

Significance of non cyclic photophosphorylation

1. Reduction of NADP⁺ takes place due to electron coming from pigment system I (PSI).
2. Reduction of pigment system I (PSI)  takes place due to electron coming from pigment system II (PSII).
3. Production of ATP molecules takes place during electron transport from cytochrome b to cytochrome f.
4. Reduction of pigment system I (PSI) occurs due to electron coming from water.
5. Photooxidation of water takes place  releasing molecular oxygen.
6. Flow of electron is unidirectional.

Cyclic photophosphorylation

It is a process of synthesis of ATP from ADP in presence of light during cyclic transfer of electron.  Absorption of light by pigment system I (PSI) results in the excitation of its reaction centre P₇₀₀, which emits electron.  Transfer of  electrons to ferredoxin-reducing substance (FRS) through A (FeS) takes place.  Reduced FRS transfers its electron to ferredoxin and gets oxidized.

If the concentration of NADP⁺  is low, then FRS instead of transferring its electron to Fd, transfers electron to cytochrome b. The electron transfers to cytochrome f, then from cytochrome f to plastocyanin.  From plastocyanin, electron then returns back to pigment system I and chlorophyll gets reduced.

During the transfer of electron from Fd to cytochrome b and from cytochrome b to cytochrome f, synthesis of two molecules of ATP from ADP and inorganic phosphate takes place.

Significance of cyclic photophosphorylation

1. During cyclic photophosphorylation, the electron returns or cycles back to its original position in the chlorophyll molecules.
2. It produces two molecules of ATP.

So in light reaction, there is evolution of molecular oxygen, reduction of NADP⁺ , and production of ATP.  The end product of light reaction i.e.,  ATP and NADPH_2, are termed as assimilatory powers. The plants use  the assimilatory power in fixation of carbon dioxide in dark reaction.