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Summary 4-Photosynthesis.doc

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Biology 1202B
Richard Gardiner

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Photosynthesis Summary Spectrum • Light is a form of energy (electromagnetic) and travels in waves o Wavelength = distance between crests Light • Photons – particles of light (fixed energy quantity which is inversely proportional to wavelength) o Shorter wavelength = greater amount of energy (violet light = ~2x energy of red light) • Light can be reflected absorbed or transmitted o The colour we see is the colour that is reflected (white = all reflected, black = all absorbed) Types of Organisms • Autotroph – self feeding, don’t eat other organisms, energy source is light, photoautotroph • Heterotroph – eat other organisms as food source Photosynthesis History • Van Helmont grew a 5 lb. willow to 200 lb., soil lost 2 oz. (Conclusion: weight came from water) • Joseph Preisstley – noted that if plant was in a container with no oxygen, O 2ould be replenished • Ingen-Housz – showed Priestley’s result only occurred if plant was illuminated Pigments • Broadens colour spectrum that drives photosynthesis/acts as photoprotection for excess light • Plants, algae; chlorophylls (green) – a, b, c, d; carotenoids – carotene (red), xanthophylls (orange/yellow) o Chlorophylls: a (all photosynthetic organisms except bacteria), b (all higher plants + green algae with few exceptions), c (brown algae), d (red algae) o Carotenoids: often the major pigments in fruits and flowers (red tomato/orange carrot) Absorption Spectrum • Measures % absorption of a particular pigment/compound vs. particular wavelength of light o Measured by spectrophotometer in the lab (proteins absorb 280, nucleic acids = 260) Action Spectrum • Plots some measure of photosynthetic rate against wavelength of light o Not identical to absorption spectrum of chlorophyll since there are other pigments Tissue Localization • Leaves have evolved to have large surface area for absorption • Thylakoid membrane – double membrane organelle with stacked grana (stacking increases SA) • Stroma – liquid part of chloroplast which contains ribosomes and DNA • Adaptations: waxy cuticle reduces water loss but also makes it difficult to transport material Photosynthesis Summary o Stomata are opening sued to bring in CO and2remove O 2 Light Dependent Reactions (thylakoid membrane) • When excited, an electron has more potential energy and when it falls it releases this as heat/light • In chloroplasts, the electrons are not allowed to fall because they need to use the potential energy NADPH – Nicotinamide-adenine-dinucleotide • Electron acceptor in enzymatic removal of hydrogen ions from specific substrates Light-Harvesting Complexes • Reaction center pigments – chlorophyll a  P680 and P700 complexes\ o 680 and 700 refers to wavelength of light they absorb • Accessory pigments – chlorophylls b, c, d, carotenoids  pass energy to reaction center o Increase amount of different light absorbed (increase efficiency) • Antenna Pigments – 300 chlorophyll a (1 reaction center) ad other accessory pigments • Reaction Center pigments – chlorophyll a P700 and P680 Photosystems • Chlorophyll, proteins and small organic molecule are organized in the thylakoid membrane o Acts like a light-gathering “antenna complex” • Membrane is studded with photosystems (structure has evolved to capture most light possible) • Antenna molecule absorbs a photon which is transmitted from molecule to molecule until it reaches the reaction center (a particular chlorophyll a molecule) o Primary electron acceptor removes the excited electron from the reaction center • Each photosystem has a reaction center chlorophyll and primary electron acceptor surrounded by an antenna complex  this makes up the light-harvesting unit • Photosystem I has a P700 center and Photosystem II has a P680 center (work together) o Difference is in the proteins associated with each reaction center Noncyclic Electron Flow (predominant route that produces ATP and NADPH) 1. Photosystem II absorbs light, excited electron is captured by primary electron acceptor, reaction center is left oxidized 2. Enzyme extracts electrons from water and supplies them to oxidized reaction center,(reaction involves splitting water to 2 hydrogen ions and an oxygen atom which combines with another) 3. Photoexcited electrons pass along the ETC by redox reactions (energy is harnessed here to produce ATP) ending up at the oxidized photosystem I reaction center 4. Electrons fill the “hole” in photosystem I which is created when excited electrons are captured by the second primary electron acceptor and transmitted along a second, shorter ETC 5. Electrons are passed to NADP+ forming NADPH (carries reducing power for Calvin Cycle) • Hydrogens (from H O 2plitting) are pumped into thylakoid membrane to regenerate
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