Photosynthesis Photosystem II Photosynthesis begins with when light

Photosynthesis: Photosystem II • Photosynthesis begins with when light is absorbed by chlorophyll • Electrons in the pigment get EXCITED! They are elevated to an excited state with potential energy.

Photosystem II

Linear flow of electrons 1. Light is absorbed, electron is excited 2. Electron moves to primary electron acceptor 3. Enzyme catalyzes the splitting of water into 2 e- and 2 H+ and O atom 1. e- replace e- at PEA, H+ gather in thylakoid space, O atom immediately combines with another O atom to form O 2

PSII

Making of ATP • Excited e- passes from PSII to PSI via the Electron Transport chain (ETC) – Series of protein complexes • E- lose energy as they pass, provides energy for ATP to be made in PSII – H+ are pumped into thylakoid space, creating H+ gradient – Chemiosmosis

PSI – making of NADPH • Light excites e-, moves to PEA – Low energy e- from PSII replaces e- • E- passes through ETC membrane • Enzyme NADP+ reductase catalyzes the transfer of 2 e- to reduction to NADPH – Takes H+ from stroma

Light Reactions • Input: Light, H 2 O • Output: ATP, NADPH, O 2 • Takes place along the thylakoid membrane • O 2 released for us to breath • ATP and NADPH in stroma to be used in Calvin cycle

Comparison of Chemiosmosis • similar in the mitochondria and chloroplast – ATP synthase • Differ: – Oxidative phosphorylation in Mitochondria • high energy e- comes from organic molecules, use chemiosmosis to transfer chemical energy from food molecules to make ATP – Photophosphorylation in chloroplast • high energy e- comes from splitting of water, do not need molecules from food, get from light energy. Chloroplasts transform light energy into chemical energy in ATP

Chemiosmosis