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Chapter 7

Chapter 7- Photosynthesis.docx

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BIOL 1000
Nicole Nivillac

Chapter 7: Photosynthesis Why its Important: o The vast majority of earths energy enters the biosphere through a process called photosynthesis. o In the process of photosynthesis chlorophyll and other related pigments capture photons of light energy, this captured energy is used to convert carbon dioxide into complex energy rich molecules 7.1- Photosynthesis: An Overview o Primary Producers: photosynthetic organisms, also called photoautotrophs. o Primary producers use the energy harnessed from the sun and convert it into chemical energy so that they may assemble simple inorganic molecules into more complex molecular structures o Consumers: the animals that consume plants or other animals as sources of nutrients o Decomposers: use the dead bodies of plants and animals as chemical energy (i.e. bacteria, fungi) The Two Parts of Photosynthesis: o 2 distinct stages of photosynthesis: light-dependent reactions, and light- independent reactions o Light-dependent reactions: also called light reactions. Pigment molecules capture light energy, and then use this energy to synthesize NADPH and ATP. This is executed via the electron transport chain, which uses electrons donated from water. The splitting of water produces a byproduct of oxygen, which is then released into the environment. Formula for Light- dependent: 2H 2O + light energy 4H + + 4e- + 2 o Light- independent reactions: also called Calvin cycle. 2fixation occurs during this stage, C2 is converted from the inorganic to organic form using the electrons carried by NADPH and the energy from ATP. The added electrons, the H+ protons, and the CO 2 molecule combine to create a carbohydrate with the chemical formula (CH 2O)n. The major product of this stage is 3 carbon sugars which can combine to make a 6 carbon sugar (glucose). Formula for Light- independent: CO 2+ H++ e- (CH 2O)n o Overall Equation of Photosynthesis: 6CO 2+12H 20 C 6H 12O6 + 6O2 + 6H2O In Eukaryotes, Photosynthesis Takes Place in Chloroplasts: o Eukaryotes which undergo photosynthesis perform both light- dependent and light- independent processes in the chloroplast o Chloroplast: an organelle with 3 membranes which define 3 distinct sections - Outer membrane: covers the entire surface of the organelle - Intermembrane compartment: compartment between the inner and outer membranes - Innermembrane: lies just inside the outer membrane - Stroma compartment: aqueous environment within the innermembrane. Houses the enzymes responsible for catalyzing the Calvin cycle - Thylakoid membrane: flattened closed sacs which house the molecules that carryout the light reactions of photosynthesis (i.e. pigments, electron transfer carriers, and ATP synthase enzymes) - Thylakoid lumen compartment: space enclosed by the thylakoid 7.2- The Photosynthetic Apparatus: o A sophisticated photochemical apparatus is require to trap light and convert it into chemical energy Electrons in Pigment Molecules Absorb Light: o 2 important points about light and pigment molecules - 1. When a pigment molecule absorbs a photon of light, one electron will move from ground state to an excited state - 2. The difference in energy levels between ground and excited state are equivalent to the energy of the absorbed photon of light. If it does not match the pigment will not be absorbed o 3 possible fates of an excited electron within a pigment molecule - 1. The excited electron may return to its ground state from the excited state, releasing energy as either heat or florescence (emission of light at a lower energy than absorbed) - 2. The excited electron returns to grounds state, and the subsequent energy is transferred to a neighboring pigment molecule, a process called inductive resonance - 3. The high energy, excited state electron is transferred to a near by electron accepting molecule called a primary acceptor Chlorophylls and Carotenoids Cooperate in Light Absorption: o Chlorophyll: the main photosynthetic pigment in plants. The two most dominant types of chlorophyll are chlorophyll a and b (structurally similar) o Carotenoids: second major group of pigments involved in photosynthesis o Chlorophyll a is the most dominant molecule as during photosynthesis only chlorophyll a becomes oxidized and donates an electron the primary electron acceptor. Chlorophyll b and the carotenoids are considered accessory pigments, as after light absorption they donate excitation energy to chlorophyll a via inductive resonance o Absorption spectrum: plot of the absorption of light as a function of wavelength. - Chlorophyll a most readily absorbs blue and red light, and the accessory pigments act to extend to range of wavelengths that can be successfully captured for photosynthesis o Action spectrum: plot of the effectiveness of particular wavelengths in driving photosynthesis, determined by using a suspension of chloroplasts and measuring the amount of oxygen released at different wave lengths o When an action spectrum for a physiological phenomenon matches the absorption spectrum of a pigment it is likely the two are connected Photosynthetic Systems are organized into Photosystems: o Pigment molecules are precisely bound to a number of different proteins, they are not simply floating around the thylakoid membrane. The pigment-protein combinations are called photosystems o Each photosystem is composed of a large antenna complex, and a central reaction center- The antenna complex: also called the light harvesting complex and surrounds the central reaction center - The reaction center: comprises a small number of proteins that bind a pair of specialized chlorophyll a molecules as well as the primary electron acceptor o 2 distinctly different kinds of photosystems: photosystem I and II (also called PSI and PSII) - Photosystem I: at the reaction center of this photosystem is the specialized chlorophyll a called P700 (bc it optimally absorbs light with a 700 nm wavelength) - Photosystem II: at the reaction center of this photosystem is the specialized chlorophyll a P680. - P700 and P680 are structurally identical to other chlorophyll a molecules, their specific absorption comes from interactions with particular proteins in photosynthesis - The function of a photosystem is to trap photons of light and use its energy to oxidize the chlorophyll, with an electron being transferred to the primary acceptor - High rates of oxidation- reduction within reaction centers is caused by large antenna complex of pigments absorbing a wide range of wavelength and efficiently using the energy in reactions, this is done by inductive resonance 7.3- photosynthetic Electron Transport: o Photosystems I and II are two major light trapping components involved in photosynthetic transport in most photoautotroph o The electron transport system consists of non-protein organic g
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