Light Reaction of Photosynthesis | Botany

In this article we will discuss about:- 1. Light Reactions Evidences 2. Four Steps of Light Reaction 3. Products of Light Reaction 4. Robert Emerson’s Work & Red Drop.

Light Reactions Evidences:

1) Hill’s Work with Isolated Chloroplasts:

Hill (1937, 1939) and Scarisbrick (1940).

If Oxidant (i.e. H-acceptor) is present with isolated chloroplast, oxidant is reduced by chloroplast in presence of light. This reaction is called Hill reaction and oxidant is called Hill reagent. The common Hill reagent is Ferricyanide and Benzoquinone.

Robert Hill discovered firstly that chloroplasts use cytochromes for photosynthesis just as mitochondria use them for respiration.

Warburg (in early 1940’s) – Cl^– has stimulating effect on the Hill reaction probably by facilitating the release of O₂ from OH^– ions.

2) D.Arnon’s Work with Isolated Chloroplasts:

i) Arnon (1951) – H-released by splitting of H₂O mol­ecule was accepted by coenzyme II (i.e. TPN⁺, now called NADP⁺⁺).

(NADP – Nicotinamide Adenine Dinucleotide Phos­phate)

ii) Arnon (1954) – In addition to carrying out the Hill reac­tion, the isolated chloroplasts could also synthesize ATP in the light.

iii) Arnon (1959) – Isolated chloroplast also reduced CO₂ in presence of light and this would result in the synthesis of carbohydrate. The conversion of CO₂ into sugar (dark reaction) actually took place in the stroma (the chlorophyll-free portion of the chloroplast) and the Hill reac­tion (Light reaction) took place properly in the grana.

The site of light reaction is grana and dependent on pigment.

Four Steps of Light Reaction:

For better perception, let’s divide the light reaction into four (4) steps:

1st Step:

Photo-excitation of pigment electrons. The effect of photo-excitation is oxidation of pigments.

Emerson’s work discovered the two pigment systems:

It means pigment system works on quality of light i.e., blue and Red.

2nd Step:

3rd Step:

Return of Excited electron from PSII to PSI in ground state.

4th Step:

Transfer of electron from PSI to NADP.

Products of Light Reaction:

i) 1/2 O₂ gas – lost to the atmosphere.

ii) ATP – in 3rd step.

iii) NADPH_Z – a reducing agent in 4th step.

Here ATP and NADPH₂ are the desired products for Dark reaction to reduce CO₂. ATP and NADPH₂ are the assimilatory power because they help in the process of CO₂ assimilation.

There are two pathways of the transfer of electron i.e.:

(a) Non-cyclic, and

(b) Cyclic.

Since ATP is released in the both pathways. Thus photo phosphorylation is the name.

(a) Non-Cyclic Photophosphorylation:

It involves both PSI & PSII systems and occurs in green plant.

(b) Cycle Photophosphorylation:

It involves only PSI and wavelength of light greater than 680 nm.

When green plant treated by Dichloro Dimethyl Urea (DCMU: a selective position which inactivates the PSII); ATP production was continued.

From this experiment two conclusions were drawn:

i. Alternative pathway is present to produce ATP other than non-cyclic.

ii. Alternative pathway was entirely dependent on only PSI. According to Park & Sane (1971) stromal lamellae has PSI whereas granal lamellae has both PSI & PSII.

It means-

Stroma ≡ PSI ≡ cyclic photophosphorylation

Grana ≡ PSII ≡ Non-cyclic photophosphorylation.

Robert Emerson’s Work & Red Drop:

Robert emerson found that 8 quanta of light energy would be required for the reduction of one molecule of CO₂ to carbohydrate.

The average maximum quantum yield in photosynthesis is 12 percent.

Quantum yield may be defined as the number of O₂– molecule released per light quantum absorbed.

Reduction of 1 molecule of CO₂ = transfer of 4 electrons

8 quanta = 4 electrons-transfer

2 quanta of light = one electrons transfer.

Thus for the movement of one electron through the complete system, 2 photons are needed, one at PSI and one at PSII. For the removal of 4e^– from the 4 water molecules, 8 photons are required which generate 1 molecule of O₂; 2 molecules of ATP and NADPH₂. In reductive pentose phosphate pathway (PPP), 3 molecules of ATP are required in the assimilation of 1 molecule of CO₂. An additional 2 photons are sufficient to provide an extra ATP making them to the requirement of 10 photons.

According to Emerson & Arnold (1932) – 2500 chlorophyll molecules (a photosynthetic unit) collaborated together to evolve one molecule of O₂ and 10 quanta of light were needed for that.

Photosynthetic Unit:

It is the smallest group of pigment molecules which collaborate together to cause a photochemical act i.e. the absorption and migration of a light quantum to a trapping centre where it brings about the release of an electron.

Photosynthetic unit = 2500 molecules of chlorophyll.

Example – P₇₀₀ = P₆₈₀

Emerson’s Effect:

Emerson found that-

The quantum yield produced by the mixed light was greater than the total yield got from the two beams of light used separately.

This enhancement of photosynthetic rate is called Emerson’s Enhancement Effect or Emerson’s 2nd Effect.

Explanations of Emerson’s Effect:

In monochromatic light, only one type of pigment is Functional and hence the decrease in the photosynthesis. If plant is exposed to red light, gradually PSII becomes inactive and non- cyclic pathway stops resulting in the decrease in photosynthetic rate.