ch22 biochem

• photosynthesis converts light energy into chemical energy in the form of glucose
• photosynthesis takes place in chloroplasts
• stroma is the site of dark reactions: synthesis of glucose from CO2 and H2O using ATP and NADPH formed in the light reactions
• Thylakoid membranes are the location of the light reactions of photosynthesis
• When a photon of the appropriate energy is absorbed by a pigment, an electron in the pigment molecule jumps to a higher energy state.

  thee excited electron may fall to its original state, releasing the energy as light or heat
• Electron transfer: the excited electron moves to a nearby molecule that has a lower excited state

  • results in photoinduced charge separation at the reaction center
• Light energy is absorbed and passed on by accessory chlorophyll molecules or other pigments until it reaches the reaction centre
• Photosystem I generates biosynthetic reducing power in the form of NADPH.
• Photosystem II replenishes the electrons of photosystem I while generating a proton gradient that is used to synthesize ATP
• Photosystem 1 uses light energy to generate reduced ferredoxin, an iron-sulfer protein to accept excited electrons
• Photosystem 2 transfers electrons from water to plastoquinone to generate a proton gradient
• Reaction center in PS1: P700

  Reaction center in PS2: P680
• Ferredoxin-NADP+ reductase converts NADP+ to NADPH
• Upon excitation of P680, electrons flow to pheophytin, then to plastoquinone, and finally to reduce a mobile plastoquinone (QH2)
• Cytochrome bf links PS1 and PS2
• P680+ is a strong oxidant that removes electrons from water. This reaction, the photolysis of water, occurs at the water-oxidizing complex (aka manganese center) of photosystem II.
• CF1-CF0: Newly synthesized ATP is released into the stroma, where it is used in carbohydrate synthesis.
• cyclic electron flow through photosystem 1 leads to the production of ATP instead of NADPH
• The absorption of 8 photons yields one O2, 2 NADPH, and 3 ATP