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.