In green plants, which photosystem absorbs photons to excite electrons in the reaction center?
The correct answer and explanation is:
Correct Answer: Photosystem II (PSII)
Explanation:
In green plants, Photosystem II (PSII) is the first photosystem in the sequence of light-dependent reactions of photosynthesis. It plays a critical role in converting light energy into chemical energy. PSII is located in the thylakoid membranes of chloroplasts and is responsible for the initial absorption of light (photons) to excite electrons in its reaction center, which contains a special chlorophyll a molecule known as P680 (named for its peak light absorption at 680 nm).
When light strikes PSII, photons are absorbed by antenna pigments, and the energy is transferred to P680. This excites an electron in the P680 molecule to a higher energy level. The excited electron is then transferred to the primary electron acceptor, initiating the electron transport chain (ETC). The lost electron in P680 is replaced by electrons obtained through the photolysis (splitting) of water molecules, which generates oxygen (O₂) as a by-product.
The electron then travels down the electron transport chain to Photosystem I (PSI), during which energy is used to pump protons (H⁺) into the thylakoid lumen, contributing to a proton gradient that drives ATP synthesis via ATP synthase.
Photosystem I (PSI), with its chlorophyll P700, also absorbs light but works after PSII in the chain and is mainly responsible for the production of NADPH.
Thus, while both photosystems absorb photons, Photosystem II is the first to absorb light and excite electrons, playing the crucial initiating role in the light-dependent reactions of photosynthesis.
Summary:
- Photosystem II (PSII) absorbs light first.
- Excites electrons in P680 chlorophyll.
- Electrons begin the electron transport chain.
- Water is split to replace lost electrons.
- Leads to ATP and NADPH formation for the Calvin cycle.