Chapter 10 outline - Ch.10 Photosynthesis Ch.10...

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Unformatted text preview: Ch.10 Photosynthesis Ch.10 Photosynthesis Sarah Burton and Lauren Thompson Plants and other Plants and other autotrophs are the producers of the biosphere •Autotrophs­ organisms that obtain organic matter without eating other organisms or substances derived from organisms. •Photoautotrophs­ organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide. •Chemoautotrophs­ organism that needs only carbon dioxide as a carbon source but that obtains energy from oxidizing inorganic substances. •Hertotrophs­ organism that obtains organic food molecules by eating other organisms or their by­products. Chloroplasts are the Chloroplasts are the sites of photosynthesis in plants Chlorophyll­ green pigment located within chloroplasts of plants. Parts of the Leaf •Mesophyll­ ground tissue of a leaf, sandwiched between upper & lower epidermis & specialized for photosynthesis. •Vascular bundle­ part of the transport system which exists and either the xylem and phloem. •Stomata­ microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange. Chloroplasts­ organelle found Chloroplasts­ organelle found only in plants & photosynthetic protists that absorbs sunlight & uses it to drive the synthesis of organic compounds from carbon dioxide & water. •Stroma­ fluid in the chloroplast surrounding the thylakoid membrane; involved in synthesis of organic molecules from carbon dioxide and water. •Thylakoids­ flattened membrane sac inside the chloroplasts, used to convert light energy to chemical energy. •Grana­ stacked proton of the thylakoid membrane in the chloroplast Photosynthesis Equation Photosynthesis Equation Photophosphorylation­ process of generating ATP from ADP and phosphate by means of proton motive force motivated by the thylakoid membrane of chloroplast during light reactions of photosythesis. NADPH2­ acceptor that temporarily stores energized electrons produced during the light reactions. Carbon fixation­ incorporation of carbon from CO2 into an organic compound by an autotrophic organism. Calvin Cycle Calvin Cycle The Calvin cycle uses ATP and NADPH from the light­dependent reactions to convert CO2 into sugar that the plant can use. CO2 is obtained from the outside environment though gas­ exchanging organs on the plant’s surface known as stomata. The process of carbon fixation incorporates the CO2 into organic molecules. The incorporation of CO2 is possible because of the energy­rich enzyme rubisco (ribulosebiphosphatecarboxylase, or RuBP), a protein made during the light­dependent reactions of photosynthesis and abundant in plant leaves. A CO2 molecule binds to RuBP. The molecule then splits into two 3­ carbon molecules of PGA (3­phosphoglycerate). A series of reactions occur to convert the PGA into the 3­carbon sugar molecule glyceraldehyde 3­phosphate. This 3­carbon sugar molecule can then be used to make other sugars, including glucose and sucrose. The production of a single 3­carbon sugar molecule requires 3 CO2, 9 ATP, and 6 NADPH. Wavelike properties of light Wavelike properties of light Electromagnetic spectrum­ entire spectrum of radiation ranging in wavelength for less than a nanometer to more than a kilometer. Visible light­ portion of electromagnetic spectrum detected as various colors by the human eye, ranging in wavelength from about 380 nm to about 750 nm. Photosynthetic Pigments: Photosynthetic Pigments: Light Receptors Pigments­ material that changes the color of light it reflects as the result of selective color absorption Absorption spectrum­ shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. Action spectrum­ rate of a physiological activity plotted against wavelength of light. Accessory Pigments Chlorophyll a­ type of blue green photosynthetic pigment that participates directly in light reactions Chlorophyll b­ type of yellow green accessory photosynthetic pigment that transfers energy to chlorophyll a. Carotenoids­ either yellow or orange; in the chloroplast of plants; broaden the spectrum of colors that can drive photosynthesis. Photosystems: Light­Harvesting Photosystems: Light­Harvesting Complexes of the Thylakoid Membrane The light­harvesting (or antenna) complex of plants is an array of protein and chlorophyll molecules embedded in the thylakoid membrane which transfer light energy to one chlorophyll a molecule at the reaction center of a photosystem. The function of the reaction center chlorophyll is to use the energy absorbed by and transferred to it from the other chlorophyll pigments in the photosystemto undergo a charge separation, a specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates called an electron transport chain. Primary electron acceptor – a specialized molecule sharing the reaction center with chlorophyll a molecule; it accepts an electron from chlorophyll a molecule. Photosystem I­ one of the two light harvesting units of a chloroplast’s thylakoid membrane; used the P700 reaction center chlorophyll. Photosystem II­ one of the two light harvesting units of a chloroplast’s thylakoid membrane; used P680 reaction center chlorophyll. How a photosystem harvests How a photosystem harvests light When a photon strikes a pigment molecule, the energy is passed from molecule to molecule until it reaches the reaction center. At the reaction center, an excited electron from the reaction­center chlorophyll is captured by a specialized molecule called the primary electron acceptor. Steps in creating NADPH Steps in creating NADPH Photosystem II absorbs solar energy in the form of light. The solar energy excites electrons in the reaction center of photosystem II, which then enter an electron transport chain. These electrons originate from the splitting of water, which produces free electrons and O2. As electrons pass down the electron transport chain, protons are pumped into the thylakoid membrane space of the chloroplast. Protons diffuse out of the thylakoid membrane space through an ATP synthase protein, creating ATP. PhotosystemI accepts electrons from the electron transport chain and uses light energy to excite the electrons further. Noncyclic Electron Flow Noncyclic Electron Flow Noncyclic electron flow – a route of electron flow during the light reactions of photosynthesis that involves both photosystems and produces ATP, NADPH, and oxygen. The net electron flow is from water to NADP+ Noncyclicphotophosphorolation – the production of ATP by noncyclic electron flow Cyclic Electron Flow Cyclic Electron Flow Cyclic electron flow ­ a route of electron flow during the light reactions of photosynthesis that involves onlyphotosystem I and that produces ATP but not NADPH or oxygen Cyclic photophosphorolation ­ the generation of ATP by cyclic electron flow Alternative Mechanisms of Carbon Alternative Mechanisms of Carbon Fixation Have Evolved in Hot, Arid Climates Plants are divided into three different categories depending on their method of carrying out photosynthesis: the C3 pathway, the CAM pathway, and the C4 pathway. C3 Plants Photorespiration presents a major problem for C3 plants because they have no special adaptations to reduce the process. The problem is exacerbated in hot, arid climates, where the rate of photorespiration increases as the temperature goes up. Consequently, C3 plants are rarely found in these climates. Most plants, including wheat, barley, and sugar beet, are C3 plants. CAM Plants CAM Plants CAM (crassulacean acid metabolism) plants reduce photorespiration and conserve water by opening their stomata only at night. CO2 enters through the stomata and is fixed into organic acids, which are then stored in the cell’s vacuole. During the day, the acids break down to yield high levels of CO2 for use in the Calvin cycle. Through the periodic opening and closing of the stomata, CAM plants maintain a high CO2 to O2 ratio, minimizing the rate of photorespiration. CAM plants, such as cacti and pineapple, are most common in dry environments. C4 Plants C4 plants use the enzyme PEP carboxylase to fix CO2 in the mesophyll cells of their chloroplasts. The fixed CO2 is then shuttled to specialized structures known as the bundle­sheath cells, where it is released and incorporated into the Calvin cycle. This process is energetically expensive, but it limits photorespiration by allowing high concentrations of CO2 to build up in the bundle­sheath cells. C4 plants, such as corn and sugar cane, are common in warm environments. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. Light Reactions: Light Reactions: Are carried out by molecules in the thylakoid membranes. Convert light energy to the chemical energy of ATP and NADPH. Split H2O and release O2 to the atmosphere Calvin Cycle Reactions: Take place in the stroma Use ATP and NADPH to convert CO2 to the sugar G3P Return ADP, inorganic phosphate, and NADP+ to the light reactions ...
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This note was uploaded on 12/02/2009 for the course SCI 90210 taught by Professor Durkka during the Spring '09 term at École Normale Supérieure.

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