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Transcript
Chapter 7: Photosynthesis I. Photosynthesis consists of a complex set of reactions A. Overall balanced equation: 1. chlorophyll 6CO2 + 6H20 + light C6H12O6 + 6O2 enzymes 2. H2O is oxidized and CO2 is reduced via H atom transfer 3. endergonic – requires a net input of energy from sunlight (products are at a higher energy than reactants) B. Significance 1. all food directly or indirectly comes from plants (primary producers – first link in food chains) 2. autotrophs (self feeding – plants make their own food) vs heterotrophs (other feeding – animals and fungi) 3. In the process of photosynthesis, plants also manufacture O2 4. Other photosynthetic organisms include bacteria: blue-green bacteria, protists (seaweed and phytoplankton) C. Light energy 1. part of the electromagnetic spectrum – travels in waves 2. visible light (ROYGBIV or VIBGYOR) 3. 400-700nm = wavelength of visible light 4. plants (primary producers) differentially absorb various wavelengths of light with different “pigments” D. Plant pigments – capture sun light and convert it into chemical energy to “power” photosynthesis 1. each pigment has a unique chemical structure sensitive to specific wavelengths of light 2. main light trapping pigment = chlorophyll a (also chlorophyll b and c – in various primary producers) 3. other accessory pigments include: carotenoids, phycobilins, phycocyanins which capture different wavelengths of light than chlorophyll a,b,and c and “protect” chlorophyll from bright light E. Structure of chloroplasts, leaf, and chlorophyll 1. chloroplasts = stacks of internal membranes (thylakoids) and spaces inbetween them (stroma) 2. chlorophyll molecules (chlorophyll a) are organized into two photosystems (I – P700 and II – P680) within the thylakoid membranes 3. Each chlorophyll molecule has an “antennae” porphyrin ring at one end with Mg atom in the center. This “antennae” region is sensitive to light. 4. Leaf = main photosynthetic organ of the plant a. stomata = openings through which gases diffuse (CO2, O2, H2O) b. veins (xylem and phloem) conduct sugars (phloem) from the leaves and minerals (xylem) from the roots c. palisade layer and spongy layer d. “transpirational pull” – when stomata are open, water vapor molecules leave the stomata creating a “negative pressure” which pulls up water and minerals through the xylem from the roots. Capillary action (cohesion and adhesion) assists in water motion up from the roots (root pressure – positive pushing force). e. Most leaves are covered with a waxy layer to prevent water loss. II. Chemical Reactions of Photosynthesis (light vs dark reactions) A. Photochemical reactions (light) include: light absorption (#1-2), electron transport (#3-5) and chemiosmotic ATP synthesis (#6-7) 1. light is absorbed by chlorophyll a molecules in PSII and PSI. Electron is boosted to a higher energy level in the reaction center. 2. two water molecules are “split” releasing 4H+ ions, O2, and 4 electrons which replace the electrons lost by PSII. 3. electrons lost by PSII and PSI are passed on to protein carriers that are part of the electron transport system in the thylakoid membrane 4. electrons fromPSII P680 reaction center are passed on to proteins in the electron transport system embedded in the thylakoid membrane (plastoquinone, cytochromes b-f, and plastocyanin), eventually reaching PSI P700. 5. electrons leaving PSI P700 are picked up by ferredoxin (another electron transport protein) and passed on to NADP reductase complex where NADP+ combines with H+ to form NADPH – the final electron pair acceptor 6. H+ ions are built up inside the thylakoid membrane through decomposition of water and active transport by the electron transport system (thylakoid membrane space inside serves as a hydrogen ion reservoir) 7. H+ ions are used to make ATP through chemiosmosis. a. H+ concentration builds to 1000x inside the thylakoid b. Chemical gates in the ATP synthase complex open when this concentration is reached, allowing H+ ions to rush through it c. Energy transition is used to bond P to ADP to make ATP 8. non-cyclic (generates ATP and NADPH) vs cyclic (generates ATP only – found in photosynthetic bacteria) electron transport B. Carbon fixation or Calvin cycle (C3 cycle) – stroma (dark reactions or light independent reactions) 1.carbon dioxide is attached to a 5 carbon compound (ribulose biphosphate) 2.RuBP is immediately broken down into 2PGA (phosphoglycerate) 3.a series of intermediate molecules are then formed, ultimately resulting in the formation of hexose (glucose) and more complex sugars 4.RuBP is regenerated 5.NADPH and ATP from the light reactions are used to change PGA into PGAL (phosphoglyceraldehyde) 6.3CO2 molecules enter the cycle to produce one PGAL that leaves the cycle while regenerating RuBP 7.2PGAL form glucose, fructose or can be combined to form sucrose, starch, and cellulose 8.PGAL can also be used to synthesize amino acids and fatty acids – precursors of other biomolecules III. Factors affecting the rate of photosynthesis A. Light 1. intensity or brightness – as intensity increases, the rate increases up to a point (limit) 2. wavelength – each pigment absorbs specific wavelengths of light (chlorophyll a – blue and violet as well as red – 2 peaks) 3. longer periods of light increase photosynthesis (summer vs winter) = photoperiods B. Temperature 1. warmer temperatures increase the rate of photosynthesis up to a point C. Concentrations of reactants and products 1. Increasing the concentration of reactants (CO2 and H2O) increases photosynthesis up to a point 2. Removing the products (C6H12O6 and O2) increases photosynthesis up to a point IV. Variations in Photosynthesis A. C4 plants 1. have extra metabolic steps preceding the C3 pathway 2. have very high rates of photosynthesis 3. examples: crab grass, sugar cane, corn 4. a high concentration of CO2 is built up in mesophyll cells surrounding bundle sheath cells and then changed into intermediate compounds (phosphophenolpyruvate, oxaloacetate, and malate = C4) before entering the C3 cycle 5. net effect is to increase the rate of photosynthesis (C3 cycle) while decreasing photorespiration – thus leading to faster growth and reproduction in these plants B. CAM (Crassulacean Acid Metabolism) 1. succulents and cacti 2. stomata must often be closed during very hot days to prevent water loss, however, this prevents entrance of CO2 3. during cooler nights, stomata open, carbon dioxide diffuses in, and is fixed and stored in organic acids – crassulacean acid 4. during the day, carbon dioxide is removed from the organic acids and enters the C3 cycle C. Sun vs Shade Plants (variations in the C3 pathway) 1. Sun plants – prefer bright light and achieve fast rates of photosynthesis (soybeans, cotton, tomatoes) 2. Shade plants – prefer dim or “filtered sunlight” to achieve their maximum rates of photosynthesis (ferns, orchids, African violets) V. Photorespiration = oxidizes organic compounds using oxygen and ATP while releasing CO2 A. Conditions 1. occurs in the presence of light 2. does not involve electron transport 3. does not produce ATP 4. occurs because CO2 and O2 can both bind to RuBP carboxylase 5. if O2 binds with RuBP carboxylase along with RuBP one molecule of PGA and a 2 carbon compound (which later releases CO2) are produced and leave the chloroplast instead of two PGA (normal C3 cycle) B. Significance 1. half of the carbon dioxide fixed in photosynthesis may be “lost” and not converted into glucose 2. may have a benefit in removing high concentrations of O2 which can be destructive to living tissue