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Chapter 7

BIOL 1000 Chapter 7 - Photosynthesis.docx

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York University
BIOL 1000
Jennifer Steeves

Photosynthesis • Plants are INCREDIBLY diverse • Natural Selection – Those that had traits that were useful for their situation survived and reproduced. • There are so many different stressors that these plants are exposed to; how do they deal with them?  Frost Damage, Heat Damage, Salt; road runoff & Irrigation, evaporation/drought • Cold Stressor Research: Glycine betaine is a molecule that moderates stresses in plants  Focused on Tomato Plant – will they be able to deal with the stress of cold better?  Exposed two groups of plants to the cold for 7 days, and then took them in to examine  It increased the resistance to cold stressors in tomato plants 7.1 – Photosynthesis: An Overview • Photosynthesis – The use of light energy to convert carbon dioxide into an organic form • Photoautotrophs – Use the energy of sunlight o drive synthesis of organic molecules form simple inorganic molecules such as CO 2  Primary producers of the planet  The organic molecules are used by the photosynthetic organisms themselves as fuels  Also form the primary energy source for animals, fungi, and other heterotrophs (consumers and decomposers) • Photosynthesis can be divided into the Light Reactions and the Calvin Cycle  In Eukaryotes, both stages take place in the chloroplasts • The Light Reactions – Use the energy of the light to drive the synthesis of NADPH and ATP  Consumed by the Calvin Cycle  Occurs in the Thylakoid Membrane • Calvin Cycle – Fixes CO into carbohydrates 2  Reduction reaction with electrons (and protons) being added to CO2 + -  Common Reaction: CO + H + e2 (CH O) 2 n • In most photosynthetic organisms the source of electrons required for fixation is water; oxygenic photosynthesis + -  2H O 2 light energy  4H + 4e + O 2 • 6CO +212H O 2C H O6+ 12 6 6H O 2 2 7.2 – The Photosynthetic Apparatus • The absorption of light energy by pigment molecules results in electrons within the pigment being raised to a higher-energy (excited) state • Three Fates of Excited State: 1. Energy Loss 2. Transfer of the Energy 3. Oxidation of the Pigment (loss of the electron) • Chlorophylls & Carotenoids are the photon-absorbing pigments in eukaryotes & cyanobacteria  Absorb light energy at a range of wavelengths, enabling a wide spectrum of light to be used • Photosynthetic pigments are organized into Two Types of Photosystems 1. Photosystem II 2. Photosystem I • Each photosystem consists of a reaction centre surrounded by an antenna complex • Energy trapped by pigments in the antenna is funnelled to the reaction centre • It is then used to oxidize a special reaction centre for chlorophyll  Denoted by P680 for Photosystem II and P700 for Photosystem  Coupled to the reduction of the primary electron acceptors within each reaction centre • Photosystem II:  Light energy is funnelled to the reaction centre where it excites P680, forming P680*  Oxidation of P680* forms P680 ; reduced back to P680 by donation of an electron from water  O is2released as a by-product and is released into the atmosphere 7.3 – The Light Reactions • The Photosynthetic ETC (Light Reactions) – Uses the energy of light absorbed by Photosystem II and Pho
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