Chapter 20, Problem 1P

Summary Introduction

To determine: The reason that why illumination by a combination of light of 680 nm and 700 nm gives a higher rate of photosynthesis than light of either wavelength alone.

Introduction:

The photosynthesis is a process in which the green plants and algae obtain their nutrients. This process takes place in the chloroplast where chlorophyll molecules are present. The chloroplast contains thylakoid molecules where light reaction takes place. The end product of the light reaction is the formation of ATP and NADPH which is an energy storing molecule that together helps in driving the Calvin cycle in stroma of chloroplast.

Explanation of Solution

Explanation:

Photosynthesis is a process in which the solar energy that is sunlight is used for the formation of chemical energy that is glucose from the carbon dioxide and water. During photosynthesis, oxygen gas is evolved. This process takes place in the thylakoid membrane present in the chloroplast. This thylakoid membrane contains two photosystem units: photosystem I (PS I) and photosystem II (PS II).

PS I absorbs 700 nm light and PS II which absorb 680 nm of light. PS I absorb 700 nm of light and start catalyzing NADP reduction and cyclic photophosphorylation. PS II absorb 680 nm of light and splits the water molecules into hydrogen ion and oxygen ion. The splitting of water molecules results in the generation of oxygen molecules and releases proton. The released proton then is transferred to PS I for the production of NADPH molecules.

Thus, 680 nm of light absorption by PS II promotes the splitting of water molecules and production of oxygen (O₂) for driving photosynthesis reaction, but the maximum photosynthesis rate in plants are observed only when both wavelengths of light are illuminated simultaneously. Thus, operating of both photosystem units (PS I and PS II) simultaneously increases the efficiency of photosynthesis.