Bios 208 1st Edition Lecture 24 Outline of Last Lecture I. The Nature of SunlightII. Photosynthetic Pigments: The Light ReceptorsOutline of Current Lecture I. Excitation of Chlorophyll by LightII. Linear Electron FlowCurrent LectureI. Excitation of Chlorophyll by LightA. When a pigment absorbs light, it goes from a ground state to an excited state, which is unstableB. When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescenceC. If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heatD. A Photosystem: A Reaction-Center ComplexE. Associated with Light-Harvesting ComplexesF. A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexesG. The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction centerH. A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a resultI. Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactionsJ. There are two types of photosystems in the thylakoid membraneK. Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nmL. The reaction-center chlorophyll a of PS II is called P680M. Photosystem I (PS I) is best at absorbing a wavelength of 700 nmN. The reaction-center chlorophyll a of PS I is called P700II. Linear Electron FlowA. During the light reactions, there are two possible routes for electron flow: cyclic and linearThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.B. Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energyC. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680D. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)E. P680+ is a very strong oxidizing agentF. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680G. O2 is released as a by-product of this reactionH. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS II. Energy released by the fall drives the creation of a proton gradient across the thylakoid membraneJ. Diffusion of H+ (protons) across the membrane drives ATP synthesisK. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptorL. P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chainM. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)N. The electrons are then transferred to NADP+ and reduce it to NADPHO. The electrons of NADPH are available for the reactions of the Calvin cycleP. This process also removes an H+ from the
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