Chapter 6 textbook sec 6.4

How Photosystems Convert Light to Chemical Energy

Noncyclic Photophosphorylation

• Learning Objective 6.4.1: Understand the role of the electron transport system in noncyclic photophosphorylation.

• Plants utilize two photosystems in sequence to synthesize both ATP and NADPH.

• This two-step process is termed noncyclic photophosphorylation since the transferred electrons do not return to their original source; instead, they are incorporated into NADPH.

• Replenishing Electrons: Electrons lost from photosystems are replaced through the splitting of water (H2O).

Photosystem II

• Structure: Photosystem II includes a reaction center with over 10 transmembrane protein subunits.

• Antenna Complex: Composed of about 250 chlorophyll a molecules and accessory pigments, this structure captures photon energy and directs it to the reaction center.

• The reaction center transfers high-energy electrons to a primary electron acceptor in the electron transport system after capturing light energy.

• The departure of energized electrons creates vacancies that are filled by electrons extracted from water molecules.

• Water Splitting: The enzyme associated with photosystem II splits water molecules, releasing oxygen and supplying electrons needed to refill the reaction center's emptied orbital.

Electron Transport System

• Function: Electron transport involves a primary electron acceptor passing energized electrons to a series of electron carriers embedded in the thylakoid membrane.

• One of the proteins acts as a proton pump, moving protons from the stroma into the thylakoid space, thereby establishing a proton gradient.

• After losing energy, the electrons are transferred to photosystem I.

Making ATP: Chemiosmosis

• Protons pumped into the thylakoid create a concentration gradient, allowing them to diffuse back to the stroma through ATP synthases.

• ATP Synthase: This enzyme synthesizes ATP from ADP as protons flow through it, a process termed chemiosmosis due to its reliance on the diffusion of protons.

Photosystem I

• Photosystem I comprises at least 13 protein subunits, receiving electrons from the electron transport system.

• The antenna complex for photosystem I encompasses 130 chlorophyll a molecules and accessory pigments.

• An additional photon absorption raises the energy level of the electron significantly.

Making NADPH

• Photosystem I passes the high-energy electrons to its own electron transport system, where they ultimately reduce NADP+ to form NADPH by transferring protons and electrons from the stroma.

• This process further contributes to the proton gradient established earlier in photosynthesis.

Products of the Light-Dependent Reactions

• The light-dependent reactions primarily yield ATP and NADPH, essential for the Calvin cycle, and release oxygen as a by-product.

• ATP is used to fuel the reactions that produce carbohydrates, while NADPH serves as the reducing agent providing electrons and hydrogen for carbohydrate synthesis.