Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Synthesis of each molecule of glucose in photosynthesis involves : [Main 2013] (a) 18 molecules of ATP (b) 10 molecules of ATP (c) 8 molecules of ATP (d) 6 molecules of ATP

Step-by-Step Solution

Answer

(a) 18 molecules of ATP.

Step 1: Understand Photosynthesis Process

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. It involves the conversion of carbon dioxide and water into glucose and oxygen using sunlight.

Step 2: Identify the Stages of Photosynthesis

Photosynthesis consists of two main stages: the light-dependent reactions (occur in the thylakoid membranes) and the Calvin cycle, also known as light-independent reactions (occur in the stroma of chloroplasts).

Step 3: Energy Requirement in the Calvin Cycle

The Calvin cycle uses ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into glucose. Specifically, for one molecule of glucose, the Calvin cycle requires 18 ATP and 12 NADPH molecules.

Step 4: Determine the Answer Based on ATP Requirement

Using the information that each glucose molecule synthesis in the Calvin cycle requires 18 molecules of ATP, option (a) 18 molecules of ATP is identified as the correct answer.

Key Concepts

Calvin Cycle

The Calvin Cycle is an essential part of photosynthesis, sometimes referred to as the light-independent reactions or the dark phase. Although it does not require light directly, it still heavily relies on the products of the light-dependent reactions. It takes place in the stroma of chloroplasts, where it uses energy in the form of ATP and electrons provided by NADPH to synthesize glucose from carbon dioxide.

• The Calvin Cycle consists of three main phases: Carbon Fixation, Reduction Phase, and Regeneration of RuBP (Ribulose Bisphosphate).
• During the Carbon Fixation phase, carbon dioxide is attached to a five-carbon sugar named RuBP, which is catalyzed by an enzyme called RuBisCO.
• In the Reduction Phase, ATP and NADPH are used to convert 3-phosphoglycerate (PGA) into glyceraldehyde-3-phosphate (G3P), a sugar molecule.
• Finally, the Regeneration Phase ensures that RuBP is regenerated, allowing the cycle to process additional carbon dioxide. For regeneration, ATP is essential.

Additionally, for the production of one glucose molecule, this cycle must turn six times. It consumes 18 ATP and 12 NADPH molecules per glucose molecule, showing the significant energy input required for carbon fixation and regeneration.

ATP in Photosynthesis

ATP, or adenosine triphosphate, plays a vital role in photosynthesis, acting as the principal energy currency for cells. In photosynthesis, ATP is mainly generated during the light-dependent reactions and then used in the Calvin Cycle.

• ATP is produced in the thylakoid membranes during the light-dependent reactions, through a process known as photophosphorylation.
• It provides the necessary energy for the biosynthesis processes in the Calvin Cycle, specifically utilized in the reduction of 3-phosphoglycerate into glyceraldehyde-3-phosphate.
• The energy from ATP is crucial for the Calvin Cycle's reduction and regeneration phases.

Each molecule of glucose synthesized through the Calvin cycle requires 18 molecules of ATP. This shows its critical role in fixing carbon dioxide and in the conversion processes involved to form glucose.

Light-dependent reactions

Light-dependent reactions are the initial phase of photosynthesis which depend directly on sunlight. These reactions occur in the thylakoid membranes of chloroplasts.

• They involve the capture of light energy by chlorophyll, the green pigment in plants, which energizes electrons.
• These high-energy electrons pass through an electron transport chain, leading to the production of ATP and NADPH.
• Photolysis is another key process here, where water is split to release oxygen as a byproduct, and provides protons and electrons for further processes.

The primary purpose of the light-dependent reactions is to convert solar energy into chemical energy in the form of ATP and NADPH. This energy is then utilized in the Calvin Cycle to produce glucose from carbon dioxide, illustrating the importance of light-dependent reactions in the overall photosynthetic process.