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Photosynthesis I

by: MaKena Betler

Photosynthesis I BIO 105 Cr.4

MaKena Betler
UW - L

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Chapter 7 Section 1: Photosynthesis I
General Biology
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This 4 page Class Notes was uploaded by MaKena Betler on Saturday October 31, 2015. The Class Notes belongs to BIO 105 Cr.4 at University of Wisconsin - La Crosse taught by a professor in Fall 2015. Since its upload, it has received 18 views. For similar materials see General Biology in Biology at University of Wisconsin - La Crosse.


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Date Created: 10/31/15
Lecture 21: Photosynthesis I  Key Concept #1: Photosynthesis is the source of all the reduced carbon in the biosphere and all the O in the atmosphere. 2 o Producers bring energy and matter into the biosphere by fixing carbon ( CO 2 ) into carbohydrates (sugars, starch) o Autotrophs: produce complex organic molecules (glucose, starch) from simple inorganic molecules ( CO 2 H 2 ) o Photoautotrophs: transform radiant energy (sunlight) into chemical energy (ATP) to produce complex organic molecules (glucose, starch) o Producers store potential energy in the covalent bonds of the sugars, starch cellulose, etc. o Humans (heterotrophs) eat apples for the energy they contain in their starch molecules (through respiration) o Photosynthesis net reaction: 6CO 26H 0+2unlight→C H O6+612 6 2 o Overview of photosynthesis:  Light excites electrons in chlorophyll, which are passed to NADP+ to make NADPH H 0  Electrons from 2 reduce chlorophyll back to its original form, making O2  In these steps, an electron transport mechanism like that of the mitochondria generates an H+ gradient across the thylakoid membrane, and ATP synthase (like in the mitochondria) harvests the gradient to make ATP  Light independent reactions:  Calvin Cycle: A metabolic pathway that uses ATP energy and electrons from NADPH to reduce CO 2 and produce sugars like glucose  Occurs in the stoma  Key Concept #2: The light reactions of photosynthesis generate ATP using a proton gradient and ATP synthase, just as in mitochondrial ATP synthesis. o If one molecule is being reduced: NADP+ to NADPH, another is being oxidized o The chlorophyll at the reaction center donated an electron and needs to get one back o H2O gets oxidized and serves as the electron donor: +¿+O 2 ¿ −¿+4H ¿ 2H O24e  Respiration and catabolic pathways generally use NAD+/NADH  Photosynthesis and anabolic pathways, generally use NADP+/NADPH o Glucose is the “start” of respiration metabolism and the “end” of photosynthesis o Respiration and photosynthesis are superficially reverse processes o Photosynthesis occurs within chloroplasts of mesophyll cells  Stacks of thylakoid membranes within the chloroplast carry membrane protein complexes that carry onto photosynthesis  Chloroplasts: endosymbiosis analogues to mitochondria  Photosynthesis consists of:  Light-driven reactions to generate ebergy  Calvin Cycle to generate organic compounds o The light reactions  The photosystem II: Chlorophyll electrons ( −¿ ) excited e by light reduce electron carrier PQ, −¿ from H O re- e 2 reduce chlorophyll, resulting in O2 −¿  As e¿ pass through cytochrome (Cyt.) and plastocyanin (Pc) to photosystem I, H+ pumped in cytochrome pumps H+ exactly like complex III in mitochondrial −¿ transport e  Light re-excites −¿¿ in chlorophyll of photosystem I, e making them energetic enough reduce ferredoxin (Fd) and then NADP+ to make NADPH  ATP synthase works just like the mitochondria o Chlorophyll is a natural pigment that absorbs photons of light:  “True” chlorophylls are found only in cyanobacteria (and plastids)  Bacteriochlorophylls found in proteobacteria (purple sulfur bacteria)  Why do spectra differ?  Small changes to the shape of the ring = large changes in the wavelengths of light absorbed. o Electromagnetic radiation is composed of photons whose energy is inversely proportional to wavelength o Phototrophic organisms are really “eating light”  Just like with any ecosystem where multiple organisms are competing for the same resource evolution selects for those organisms that specialize a particular kind of “prey” o Carotenoids in leaves become apparent in fall o Photosynthetic life began in the seas, where light is sometimes hard to come by o Chlorophyll molecules transmit energy from excited electrons in the antenna complex to a reaction center  ~300 chlorophyll molecules per complex  Larger surface area than single chlorophyll  Accessory pigments expand wavelengths absorbed  Key Concept #3: The ~ sole source of energy that enables the low entropy state in all biology is light captured by photosystems to excite electrons so they can reduce other molecules o A single photon does not have enough energy to make NADPH o Photosystem II is a starting point for an electron transport chain – PS1 gives the ETC a “boost” of energy in order to reduce NADPH  Key Concept #4: The light reactions of photosynthesis generate ATP using a proton gradient and ATP synthase just as in mitochondrial ATP synthesis o Ferrodoxin (Fd) can go on to reduce NADP+ (non-cyclic flow) or go “backwards” to make more ATP (cyclic flow)  Non-cyclic flow is used to create NADPH and a proton gradient (and thus ATP), while cyclic flow is only used to make a larger proton gradient (and thus more ATP). Depending on the needs of the cell, it can adjust to either make more ATP or more NADPH o Summation of the “Light Reactions” so far: Non-cyclic Electron Flow 1  PS2: H 2+2 photons+PQ→ O +PQH 2 2 2 +¿gradient  Cyt. b6f : ¿ PQH+2 Pc →ox+Pc +H ¿  PS1: 2Pc ¿2Fd +2oxhotons→2Pc +2Fd ox ¿  NDR: +¿→2Fd +NoxPH 2Fd +NADP+H ¿ ¿ +¿gradient 1 ¿  Total: +¿→ O +NA2PH+H 2 H2O+4 photons+NADPH+H ¿ o Summation of the “Light Reactions” so far: Cyclic Electron Flow:  PS1: 2Pc ¿2Fd +2oxhotons→2Pc +2Fd ox ¿  FPO: +¿→2 Fd +Pox 2 2 Fd¿+PQ+H ¿  Cyt. B f : +¿gradient ¿ 6 PQH +2Pc →PQoxPc +bigger ox  Total: +¿gradient ¿ 2 photons→bigger H


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