Question

Which of the following best describes light reaction?

• A. $H_2+\text{Light energy}\to 2H+\text{ATP}+\text{NADPH}$
• B. $C{O}_2+\text{Light energy}\to O_2+\text{ATP}+\text{NADPH}$
• C. $H_{2}0+\text{Light energy}\to \text{ATP}+\text{NADPH}+O_2$
• D. $O_2+\text{Light energy}\to C{O}_2+\text{ADP}+\text{NADPH}$

Answer 1

The correct option is C $H_{2}0+\text{Light energy}\to \text{ATP}+\text{NADPH}+O_2$

Photosynthesis is a unique process in which green plants synthesise carbohydrates by utilising water and $C{O}_2$ with the help of chlorophyll in presence of sunlight. Apart from synthesising food, oxygen is also released during this process.

Photosynthesis includes light reaction and light independent reaction.

During light reaction, there is synthesis of assimilatory powers, ATP and NADPH, by utilising light energy. Also, there is splitting of water which leads to the evolution of oxygen.

Option a is incorrect as H${}_2$0 is hydrogen donor in green plants during photosynthesis and it gets oxidised to release oxygen.

Option b is incorrect as the oxygen released during photosynthesis comes from water but not from carbon dioxide. Moreover, carbon dioxide fixation occurs during light independent reaction.

Option d is incorrect as oxygen is a product of photosynthesis but not the raw material required for photosynthesis.

Hence, option c correctly describes the light reaction in which plants utilise light energy and synthesise the assimilatory powers ATP and NADPH. Water plays a major role in light reaction. The splitting of water releases electrons, protons and oxygen. The electrons are accepted by the reaction centre of PS-II. The protons accumulate in the lumen of thylakoid and create a proton gradient which is required to synthesise ATP.