Question

When a plant is exposed to light at a wavelength of 680 nm, more oxygen is generated than the plant is exposed to light at a wavelength of 700 nm. Why? And if you receive the light of 680nm and 700nm together, please explain if the oxygen production is more than receiving each wavelength separately.

Answer 1

(Please go through detailed explanation for better understanding. I hope this answer will help. Please rate the answer)

Summary of answer: As wavelength increases energy decreases. So a huge drop in efficiency has been noticed at 700nm as compared to 680 nm. So more oxygen is generated at 680 nm than &00 nm only. When we give both the frequencies together, Both photosystem 1 and Photosystem 2 works to release more oxygen

Detailed Explanation:

There are two photosystems involved in photosynthesis - PS1 (photosystem 1) and PS2 .

PS1 has a pigment P700 which is a dimer of chlorophyll a. PS2 has another dimeric pigment P680. These pigments are excited by absorbing quanta of light. The numbers 700 and 680 indicate the wavelengths at that are most effective for the particular pigment. The incident light excites an electron in the pigments to a higher energy level. The electron is captured by an electron acceptor, thus oxidizing P700 or P680. These oxidized pigments are very strong oxidising agents and oxidize water during photosynthesis.

We know that the energy of light is directly proportional to its frequency and inversely proportional to its wavelength. As the wavelengths of light incident on the photosystems increases, it's energy and thus capacity to excite the electrons decreases.

So, light of a wavelength longer than around 680 doesn't have enough energy to excite an electron in the second photosystem but it is effective for the first system.

There is a process that occurs during photosynthesis called photophosphorylation. This is the conversion of ADP (adenosine diphosphate) to ATP (adenosine triphosphate) using light energy obtained through the photosystems. There are two types of photophosphorylation - cyclic and non cyclic. Non cyclic photophosphorylation involves both PS1 and PS2. Cyclic photophosphorylation involves only PS1.

An important thing to note here is that while non cyclic photophosphorylation results in the evolution of oxygen, the cyclic process does not. This increase the overall evolution of oxygen thus increasing quantum yield when both wavelengths are used together.