C1.3.13 Reduction of NADP by photosystem I (HL) Notes

Photosystem I Reduces NADP⁺ to Form NADPH, an Energy Carrier

1. In the light-dependent reactions of photosynthesis, Photosystem I (PSI) plays a critical role in the reduction of NADP⁺ to form NADPH, a key molecule required for the light-independent reactions (Calvin cycle).
2. This reduction involves the transfer of two electrons from PSI and the addition of a hydrogen ion (H⁺) sourced from the stroma.
3. Together, these processes convert NADP⁺ into NADPH, a stable energy carrier and reducing agent.
4. The reduction of NADP is a critical step in photosynthesis, where light energy is transformed into chemical energy.

NADP Acts as an Electron Carrier, Shuttling Energy in Photosynthesis

1. NADP (nicotinamide adenine dinucleotide phosphate) acts as an electron carrier in photosynthesis.
2. It exists in two forms:
   1. NADP^+^: The oxidized form, which can accept electrons.
   2. NADPH: The reduced form, which carries high-energy electrons and hydrogen ions.

How Photosystem I Reduces NADP

The reduction of NADP involves several key steps:

1. Absorption of Light by Photosystem I

1. Photosystem I is located in the thylakoid membrane of chloroplasts.
2. It contains a reaction center with a special pair of chlorophyll molecules called P700.
3. When light energy is absorbed by PSI, it excites an electron in the P700 chlorophyll, raising it to a higher energy level.

2. Transfer of Electrons

1. The excited electron is transferred from P700 to a series of electron carriers.
2. These carriers form a short electron transport chain, which includes molecules like ferredoxin.

3. Reduction of NADP

1. The electron is ultimately transferred to the enzyme NADP reductase, located on the stroma side of the thylakoid membrane.
2. NADP reductase catalyzes the reduction of NADP^+^ by adding two electrons and a hydrogen ion ($H^+$), forming NADPH.

NADPH is Essential for Driving the Calvin Cycle and Glucose Synthesis

1. The reduction of NADP is crucial because it provides the reducing power needed for the Calvin cycle.
2. Without NADPH, the light-independent reactions of photosynthesis could not proceed, and glucose would not be synthesized.

Photosystem I and Photosystem II(PSII) Work Together To Generate ATP and NADPH

1. PSII absorbs light and uses the energy to split water molecules in a process called photolysis, releasing electrons, protons, and oxygen.
2. The electrons from PSII are passed through an electron transport chain, generating ATP and eventually reaching PSI.
3. PSI then re-excites these electrons with additional light energy, allowing them to reduce $NADP^+ $ to NADPH.

Proton Gradient Drives ATP Synthesis Through ATP Synthase

1. Proton pumping by the electron transport chain, driven by the energy from electrons excited in Photosystem II (PSII) and PSI, creates a proton gradient across the thylakoid membrane.
2. This proton gradient drives ATP synthesis through ATP synthase, which is a membrane-bound enzyme that synthesizes ATP from ADP and inorganic phosphate (Pi) as protons flow through it.

Cyclic Photophosphorylation Increases ATP Production Without Making NADPH

Sometimes, the chloroplast needs more ATP than NADPH. In this case, electrons from PSI can take a cyclic path:

1. Instead of reducing $NADP^+$, the electrons are redirected back to the electron transport chain, passing through cytochrome b6f and plastocyanin.
2. This process pumps additional protons into the thylakoid lumen, enhancing ATP production through chemiosmosis.
3. No NADPH or oxygen is produced in this cyclic pathway.