Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The synthesis of each glucose molecule involves 18 molecules of ATP, so the answer is (c).

Step 1: Understanding the Question

The problem asks us to determine how many molecules of ATP are involved in the synthesis of one molecule of glucose during the process of photosynthesis. This will involve knowledge of the Calvin Cycle, which is where ATP is used in photosynthesis.

Step 2: Calvin Cycle Overview

Photosynthesis primarily occurs in the chloroplasts and consists of light-dependent reactions and the Calvin Cycle. The Calvin Cycle, also known as the light-independent reactions, uses ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into glucose.

Step 3: ATP Use in the Calvin Cycle

The Calvin Cycle requires energy, which comes in the form of ATP. Specifically, the cycle uses ATP for the conversion of 3-phosphoglycerate (3-PGA) molecules into glyceraldehyde-3-phosphate (G3P), as well as for the regeneration of ribulose-1,5-bisphosphate (RuBP), the molecule necessary for the fixation of carbon dioxide.

Step 4: Calculating ATP Requirement

In the Calvin Cycle, to synthesize one molecule of glucose, 6 carbon dioxide molecules are fixed, requiring 6 turns of the cycle. Each turn of the cycle requires 3 ATP molecules, for a total of 18 ATP molecules to form one glucose molecule.

Step 5: Conclusion

Based on this understanding, for the synthesis of each glucose molecule during photosynthesis, 18 ATP molecules are required. This corresponds to option (c).

Key Concepts

ATP Synthesis

ATP, or Adenosine Triphosphate, is often referred to as the energy currency of the cell. It plays a crucial role in photosynthesis, especially in the Calvin Cycle, where energy is needed to convert molecules into sugar. During photosynthesis, ATP is generated in the light-dependent reactions, which take place in the thylakoid membranes of chloroplasts.

In these reactions, sunlight is absorbed by chlorophyll, driving a chain of processes that produce ATP and NADPH. The process begins when light energy excites electrons in the chlorophyll molecules. These high-energy electrons pass through the electron transport chain (ETC), releasing energy at each step.

This released energy is used to pump hydrogen ions across the thylakoid membrane, creating an electrochemical gradient. The enzyme ATP synthase then utilizes this gradient. As hydrogen ions flow back through ATP synthase, ATP is produced from ADP and inorganic phosphate. This ATP is then transported to the Calvin Cycle, where it provides the energy needed for the conversion processes that build glucose molecules.

Calvin Cycle

The Calvin Cycle, also known as light-independent reactions or the dark reactions, occurs in the stroma of chloroplasts. Unlike the light-dependent reactions, the Calvin Cycle does not require light. Instead, it uses ATP and NADPH generated from the light-dependent reactions to convert carbon dioxide into glucose.

The cycle begins with carbon fixation, where CO₂ is attached to a five-carbon sugar named ribulose bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO and results in an unstable six-carbon compound which splits into two molecules of 3-phosphoglycerate (3-PGA).

In the reduction phase, ATP and NADPH are utilized to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. It takes three turns of the Calvin Cycle to produce one G3P molecule, and thus six turns are needed to produce a single glucose molecule. The remaining G3P molecules are used to regenerate RuBP, allowing the cycle to continue. This requiration of ATP and NADPH ensures that the Calvin Cycle contributes significantly to the synthesis of glucose.

Glucose Formation

Glucose formation in photosynthesis is the ultimate goal of the Calvin Cycle. Glucose is a simple sugar that serves as a primary energy source for plants and, subsequently, the broader ecosystem. The formation of glucose from carbon dioxide and water can be represented by the overall photosynthesis equation:

\[6 \, \text{CO}_2 + 6 \, \text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6 \, \text{O}_2\]
In the Calvin Cycle, ATP and NADPH generated in the light-dependent reactions are consumed to drive the synthesis of G3P, a precursor to glucose.

Once G3P is produced, it can be further processed in several ways:

• Some G3P molecules combine to form glucose and other carbohydrates. This step involves multiple enzymatic reactions where G3P is converted into fructose-6-phosphate, which can then join together to form glucose.
• Glucose can be stored in plants as starch, providing an energy reservoir.
• It can also be used to produce cellulose, which is vital for plant structure.

The process requires 18 molecules of ATP and 12 molecules of NADPH for the formation of a single glucose molecule. Understanding how ATP and other molecules are consumed in this process highlights the intricate nature of photosynthesis and the essential role of energy conversion in plant life.