💡 Light Reactions

Cyclic, Non-Cyclic Photophosphorylation, Emerson's Effect

For better understanding, 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:
- 1 mμ = 1 milli-micron = 10^-3 x 10^-6 meter = 10^-9 m
- 1 nm = 1 nano-metre = 10^-9 m
- 1 A° = 1 Angstrom = 10^-10 m

👉🏻 Effect

Direct: Reduction of PS-I
Indirect: Photophosphorylation
- i.e. Production of ATP by the involvement of photons (light)
- i.e. Energy stored in the form of ATP

Products of Light reaction

Primary function of light reaction is production of ATP and NADPH2
1/2 O₂ gas — lost to the atmosphere
ATP: in 3rd step
NADPH₂: 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 above pathway. Thus photo phosphorylation is the name.

When green plant treated by Dichloro Dimethyl Urea (DCMU): a selective poision which inactivates the PS-II; ATP production was continued.
From this experiment two conclusions were drawn:
- Alternative pathway is present to produce ATP other than non-cyclic.
- Alternative pathway was entirely dependent on only PS-I.

According to Park & Sane (1971) stromal lamellae has PS-I whereas granal lamellae has both PSI & PSII. It means
- Stroma Lamellae → PSI → Cyclic photophosphotylation
- Grana → PSI & PSII → Non-cyclic photophosphorylation

Robert Emerson found that 8 quanta of light energy would be required for the reduction of one molecule of CO₂ to carbohydrate.
8 quanta of photons (i.e. light energy) = For reduction of 1 molecule of CO₂ to carbohydrate = i.e. Production of 1 molecule of O₂.

8 quanta = 1 molecule of CO₂ (reduction) = 1 molecule of O₂ (production)

Thus quantum yield or yield per quantum = 1/8 = 12 %
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
Two 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 4 e- 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 election.

The decrease in the quantum yield or photosynthetic rate after using the monochromatic light i.e. red, for the long duration is called Red Drop.

Emersion found that C > (A + B)
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.

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 PS-II becomes inactive and non-cyclic pathway stops resulting in the decrease in photosynthetic rate.