Chapter 6 — Chapter 6 — Bioenergetics

Pigments absorb visible light (400–700 nm). Chlorophyll a (C₅₅H₇₂O₅N₄Mg; –CH₃ group on ring II; blue-green; primary pigment, reaction centre) and chlorophyll b (C₅₅H₇₀O₆N₄Mg; –CHO group instead of –CH₃; yellow-green; accessory). Porphyrin head with Mg²⁺ and phytol tail anchoring in thylakoid membrane. Carotenoids: carotenes (orange) and xanthophylls (yellow) — accessory, photoprotection. Absorption spectrum: graph of light absorbed by a pigment vs wavelength — chlorophylls absorb blue-violet (~430–450 nm) and red (~640–680 nm); chl a peaks ~430 and 662 nm, chl b ~453 and 642 nm. Action spectrum: graph of rate of photosynthesis vs wavelength (Engelmann's experiment with aerobic bacteria and Spirogyra) — closely matches combined absorption. Organization: in thylakoid membranes pigments form photosystems — light-harvesting antenna complex (200–300 chl a, b and carotenoids) funnels energy by resonance to the reaction centre (special pair of chl a: P700 in PS I, P680 in PS II) and primary electron acceptor.