Question

What conclusion was drawn from red drop and enhancement experiments in the light reaction of photosynthesis?
A. Two photosystems are present
B. One photosystem is present
C. Two photosystems are present which work at different wavelengths.
D. Two photosystems are present which work at same wavelengths.

Answer 1

Hint:Light reaction of photosynthesis occurs in the gamma region. This is a photochemical step and thus dependent upon light. It takes place in the grana of the chloroplasts.

Complete answer:
Let us analyse the options to find the correct answer.
Rate of photosynthesis is measured as the number of ${O_2}$ molecules released after the light quanta are absorbed.
When plants are exposed to light having wavelength greater than $680nm,$ then only one photosystem is activated. When plants are exposed to the light having wavelength less than 680 nm, the rate of photosynthesis was very low.
Robert Emerson performed an experiment on chlorella and studied photosynthesis at different wavelengths of light $(390 - 760nm)$. Monochromatic light of different wavelengths is used and rate of photosynthesis is measured. Emerson $(1957)$ found a sharp reduction in the rate of photosynthesis when monochromatic light of more than \[680nm\] wavelength (red region of the spectrum) was used alone. It is called Emerson’s red drop.
Robert Emerson concluded that simultaneous application of light of shorter and longer wavelength, increases the rate of photosynthesis. This is called Emerson’s enhancement effect.
The experiment provide evidence for the presence of two photosystems i.e. PS I and PS II which absorb light of different wavelengths. Hence, the correct option is C.

Option A, B and D: As two photosystems are present which absorb light of two different wavelengths, therefore, this is the incorrect option.

Thus, the correct option is (C) Two photosystems are present which work at different wavelengths.

Note:Robert Mayer firstly stated that photosynthesis is the conversion of light energy within a plant. PS I and PS II absorb light of wavelengths $700nm\,\,and\,\,680nm$respectively.