Chapter 9, Problem 4STP

Summary Introduction

Introduction:

In living organisms, chemical energy is stored in organic compounds and this energy is converted to other forms of energy. Photosynthesis is an anabolic pathway in which light energy from the Sun is converted to chemical energy for use by the cell.Photosynthesis occurs in two phases; light dependent reaction which occurs in grana of thylakoids in chloroplasts and light independent reactions which occurs in stroma of chloroplasts. The breakdown of water is essential for photosynthesis to occur.

Answer to Problem 4STP

Correct answer :

The correct answer is option A. action of ATP synthase on ADP.

Explanation of Solution

Explanation/justification for the correct answer:

Option A. action of ATP synthase on ADP−During light reactions, ATP is produced in conjunction with electron transport by the process of chemiosmosis. The H+ ions produced by splitting of water molecules accumulate in the interior of thylakoid. Due to difference in concentration of H+ ions in the interior of thylakoid and stroma, the H+ ions diffuse down the concentration gradient through ion channels. ATP synthases help in diffusing of H+ ions. ATP synthase is an enzyme used during light reaction of photosynthesis to generate ATP. As a result of this movement, ATP is formed in the stroma. Hence this is the correct option.

Explanation for incorrect answer:

Option B. conversion of GAP molecules into RuBP−In the final step of the Calvin cycle, Rubisco, an enzyme converts ten G3P(glyceraldehyde 3-phosphate) molecules into 5- carbon molecules called ribulose 1,5- bisphosphates (RuBP). In this reaction water is not required.Hence this is not the correct option.

Option C. conversion of NADP+ to NADPH- During light reaction, photosystem I transfers electrons to a protein called ferrodoxin which in turn transfers electron to NADP⁺, forming NADPH. Hence this is not the correct option.

Option D. transfer of chemical energy to form GAP molecules- Chemical energy stored in ATP and NADPH is transferred to the 3-PGA molecules to form high energy molecules of G3P. This process also does not require water. Hence this is not the correct option.