Question

Red drop discovered by Emerson is due to disruption of photochemical activity of
A. Carotenoid
B. PS I
C. PS II
D. Both B and C

Answer 1

Hint: Inefficiency in the process of photosynthesis occurs due to availability of light of wavelength greater than 680nm.

Complete answer:
Red drop effect discovered by Emerson is due to disruption of photochemical activity of PS II. Wavelengths beyond 700nm are useless for the process of photosynthesis, so a huge drop of inefficiency is noticed at wavelength 700nm. This phenomenon is called the red drop effect.
Number of oxygen molecules released per light quanta is called quantum yield. So in simple words, the quantum yield decreases after 680nm. This decrease in quantum yield is seen in the red part of the spectrum. Since PS II functions at a wavelength of 680nm, it’s disruption would mean that only PS I is functional, whose optimum wavelength is 700nm. So the Red Drop effect is seen due to the disruption of photochemical activity of PS II.

Additional Information:
- The pigments required for photosynthesis are organised into two light harvesting complexes, PS1 ( photosystem 1) and PSII ( photosystem 2).
- Each photosystem has all the pigments, except one molecule of chlorophyll forming a light harvesting complex called antennae.
- The single missing Chlorophyll a molecule forms the reaction centre.
- In PSI the reaction centre has an absorption peak at 700nm.
- In PSII the absorption peak of the reaction centre is at 680nm.
- The entire system together carries out the Light Reaction.

Note:
- Red drop effect is seen in the red part of the spectrum and leads to a drop in efficiency of photosynthesis, leading to the given name.
- The Emerson effect is the increase in the rate of photosynthesis after chloroplasts are exposed to light of wavelength 670 nm (red light) and 700 nm (far red light). When simultaneously exposed to light of both wavelengths, the rate of photosynthesis is far higher than the sum of the red light and far red light photosynthesis rates.
- These effects are an evidence of the existence of 2 Photosystems.