Photosynthesis Notes PDF

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Khan Academy – Photosynthesis 

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5 main types of chlorophyll: a o Chlorophyll a o Chlorophyll b o Chlorophyll c o Chlorophyll d Chlorophyll molecules mostly absorb blue and red wavelengths o Include a hydrophobic tail that inserts into the thylakoid membrane o And a porphyrin ring head (circular group of atoms that surround magnesium ion) that absorbs light o All photosynthetic plants, algae, and cyanobacteria contain chlorophyll a, while only plants and green algae contain chlorophyll b along with few types of cyanobacteria o All pigments used in addition to chlorophyll a are known as accessory pigments including other chlorophylls  Other classes of pigments are carotenoids  Use of accessory pigments allows for broader wavelength absorption therefore more energy captured from sunlight Carotenoids o Another key group of pigments that absorb violet and blue-green light o Brightly coloured ones are found in fruit o Help capture light o Important role in getting rid of excess light energy o If energy is not handled properly in can damage the photosynthetic machinery o Dissipate extra energy as heat Pigments o For it to absorb photon of light, it excites the atom from ground state o Only photon with right amount of energy can excite pigment  Explains why pigments absorb different wavelengths than others  Since there are different energy gaps b/w orbitals are different with each pigment which is why different photons of different wavelengths are needed in case to provide an energy boost that matches the gap  Excited pigments are unstable and has options for becoming stable  Can transfer energy or electron to neighbor molecule

Light-Dependent Reactions    

The light energy absorbed is turned to chemical energy Use the light energy to make ATP and NADPH for the next stage of photosynthesis Light reactions take place in the thylakoid membranes Photosystems – large complexes of proteins and pigments (light absorbing molecules) o Photosystem 1 – energy passed from pigment to pigment until it reaches the reaction center and energy is transferred to P680 which excited electron to high energy level and then is passed to an acceptor molecule and replaced with an electron from water. Splitting water and releasing O2 for us to breathe

ATP synthesis – the excited electron goes down ETC and losing energy as goes. Some energy released drives pumping of H+ from stroma into thylakoid interior building a gradient.  The H+ ions from splitting water also add to gradient.  As H+ ion flow down gradient into stroma they pass through ATP synthase driving ATP production in process known as chemiosmosis o Photosystem 2 – electron arrives at photosystem 1 and joins P700 special pair of chlorophylls when light energy is absorbed it excites and get transferred to acceptor where special pairs missing electron is replaced by new electron from PS 11 via ETC o NADPH formation – excited electron travels down short leg of ETC and at end electron is passed to NADP+ (along with a second electron from same pathway) to make NADPH Net effect of steps is to convert light energy into chemical energy in form of ATP and NADPH which are used to make sugars in Calvin Cycle (cyclic photophosphorylation) o Electrons in circular pathway only produce ATP and no NADPH Photosystem o Pigments: chlorophyll a,b and carotenoids are light-harvesting molecules found in thylakoid membrane o Systems are organized with proteins into complexes o Each photosystem has light-harvesting complexes that contain proteins, 300 to 400 chlorophylls, and other pigments. o Pigments in photosystems usually act as an energy funnel, passing energy as a funnel in towards the main reaction center, when that pigment is excited by light it transfers energy to neighboring pigment through direct electromagnetic interactions in a process called resonance energy transfer which can keep passing to neighbors  In these transfers the receiving molecule cannot require more energy for excitation that the donor, may require less (eg. May absorb light of a longer wavelength A unique pair of chlorophyll a molecules is often called special pair and once the energy is passed to it, it will no longer be passed to other pigments through resonance energy transfer o The special pair can lose electrons when excited passing it to the primary electron acceptor and with this transfer the electron starts into the ETC o

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Photosystem 1 vs 2      

2 comes first in pathway Special pairs of 2 are P680nm while PS1 special absorbs best at 700nm (P700) Each special pair passes its electrons to different primary electron acceptors Primary electron acceptor for PS2 is pheophytin, resembles chlorophyll PS1’s is a chlorophyll called A07,8 Once e is lost the PS is replenished by different sources o PS11 – e from water o PS1 – e from the flow down of e in ETC o PS2 passes down e in first part of ETC while PS1 passes it down the second part of ETC o After special pair gives up e it become + and needs new e

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PS2 electron provided from water through process of Manganese center +P680 can pull e off water When manganese center splits molecules it binds 2 at once extracting 4 e and releases 4H+ producing O2 When e leaves PS2 it transfers to small organic molecule (plastoquinone, Pq) then to cytochrome complex (Cyt) and finally to copper containing protein called plastocyanin (Pc) E loses energy as moves some energy is used to pump protons from stroma into thylakoid interior Transfer of H and release of H from water cause proton gradient used to make ATP In PS1, e is transferred to protein (ferredoxin, Fd) then to NADP+ reductase then to NADP+ to make NADPH which travels to Calvin cycle ATP needed in Calvin cycle is provided by light reaction Photophosphorylation = light-driven synthesis of ATP Linear photophosphorylation – e travel in a line from water through PS2 and PS1 to NADPH Cyclic Photophosphorylation – some e break that cycle and instead loop back to first part of ETC repeated cycling through PS1 instead of ending up in NADPH – and does not make NADPH  Switches when ratio is to high for NADPH  Common in photosynthetic cell types with high ATP needs  Eg.) sugar-synthesizing bundle-sheath cells in plants that carry out C4photosynthesis  May play protective role, preventing excess light from damaging photosystem proteins and promoting repair of light-induced damage

Photorespiration     

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A wasteful pathway that happens in the Calvin Cycle when rubisco works on O2 instead of CO2 and can’t make glucose b/c of this Most plants are C3 plants that have no way of fighting photorespiration C4 plants minimize photorespiration by separating the initial CO2 fixation and the Calvin cycle space, processing these steps in different cell types Crassulacean acid metabolism (CAM) they minimize photorespiration and save water by separating these steps in time between day and night Photorespiration is a wasteful metabolic process, it begins when rubisco grabs O2 instead of CO2; uses up carbon, wastes energy, and tends to happen when plants close their stomata(leaf pores) to reduce water loss – high temperatures make it worse Through C4 and CAM pathways, some plants can minimize the worst effects of photorespiration They are adaptations arised from natural selection They work to ensure that Rubisco always encounters high concentrations of CO2, making it unlikely to bind to O2

C4    

In these plants light dependent reactions and the Calvin cycle are physically separated LD occur in mesophyll cells CC occur in special cells around leaf veins – called bundle sheath cells...