During photosynthesis in plants, the NADPH required for the Calvin cycle (light independent reactions) is mainly generated by reactions that are initiated in which photosystem?

Introduction / Context:
Photosynthesis involves two major stages: the light dependent reactions and the light independent reactions, also known as the Calvin cycle. The Calvin cycle requires both ATP and reducing power in the form of NADPH to fix carbon dioxide into sugars. Understanding where this NADPH comes from inside the chloroplast is a key concept in plant physiology and is frequently tested in multiple choice questions.

Given Data / Assumptions:

• The focus is on NADPH required for the Calvin cycle.
• The question asks which system or pathway produces this NADPH.
• The options include photosystem I, the citric acid cycle, photosystem II, and glycolysis.
• We assume a standard description of the Z scheme of photosynthetic electron transport.

Concept / Approach:
In the light dependent reactions of photosynthesis, electrons move through a chain involving two photosystems: photosystem II (PS II) and photosystem I (PS I). PS II absorbs light energy and drives the splitting of water, releasing oxygen and providing electrons. These electrons flow through an electron transport chain to PS I. When PS I absorbs light, it further energises the electrons, which are then used by an enzyme complex to reduce NADP+ to NADPH. This NADPH is subsequently used in the Calvin cycle to reduce carbon dioxide into carbohydrates. Therefore, NADPH production is directly associated with reactions initiated in photosystem I.

Step-by-Step Solution:
Step 1: Recall that the Calvin cycle takes place in the stroma of the chloroplast and requires ATP and NADPH. Step 2: In the thylakoid membranes, photosystem II absorbs light and drives the initial splitting of water, generating electrons, protons, and oxygen. Step 3: These electrons move through an electron transport chain, helping to generate a proton gradient used to make ATP. Step 4: The electrons then reach photosystem I, which absorbs light and raises the electrons to an even higher energy level. Step 5: The high energy electrons from photosystem I are transferred to NADP+ via an enzyme system, forming NADPH. Step 6: This NADPH moves to the stroma and donates reducing power to the reactions of the Calvin cycle, enabling carbon dioxide fixation.

Verification / Alternative check:
Diagrams of the Z scheme in photosynthesis clearly show that NADP+ is reduced to NADPH after electrons pass through photosystem I. Textbooks describe photosystem II as associated mainly with water splitting and the start of electron flow, while photosystem I is associated with generating the high energy electrons needed to reduce NADP+. Mitochondrial citric acid cycle and glycolysis are cellular respiration pathways, not photosynthetic light reactions, and they produce NADH or FADH2 rather than NADPH destined for the Calvin cycle. This confirms that reactions initiated in photosystem I are responsible for the NADPH used in the Calvin cycle.

Why Other Options Are Wrong:
Reactions of the citric acid cycle in mitochondria are part of aerobic respiration and produce NADH and FADH2, not the chloroplast NADPH used in photosynthesis, so option B is incorrect. Reactions initiated in photosystem II that directly generate NADPH is wrong because PS II mainly drives water splitting and initial electron flow; the reduction of NADP+ to NADPH is specifically linked to PS I. Reactions of glycolysis in the cytoplasm generate ATP and NADH as part of glucose breakdown, not NADPH for the Calvin cycle, so option D is also incorrect.

Common Pitfalls:
Students sometimes memorise that photosystem II comes first and wrongly assume it must also be responsible for producing all energy carriers, including NADPH. Another common confusion is mixing up NADH and NADPH, which are different coenzymes used in different pathways. To avoid these mistakes, remember that the Calvin cycle needs NADPH and that this reducing power comes from photosystem I, while ATP is generated partly due to the proton gradient created between photosystem II and photosystem I. The association PS I to NADPH is a helpful memory aid.

Final Answer:
The correct choice is Reactions initiated in photosystem I that lead to NADPH formation, because NADPH for the Calvin cycle is generated in the light reactions after electrons are excited in photosystem I.