Objectives for Lecture 11 BOT 3015 Summer 2013 You should be able to write a chemical equation that describes photosynthesis know that it occurs in two separate reactions the light rxn and the Calvin cycle The light rxn takes place on the thylakoid membranes and light captured by chlorophyll leads to electron transport and finally to the formation of ATP from ADP and the reduction of NADP to NADPH The Calvin cycle takes place in the stroma of the chloroplast and fixes CO2 into carbohydrates using ATP and NADPH Reaction CO2 H2O Light Energy CH2O O2 Light Reaction Takes place in thylakoid membranes or grana stacks of thylakoid membranes The energy transduction reaction Light energy is used to form ATP from ADP and inorganic phosphate and to reduce the electron carrier molecules the coenzyme NADP to NADPH NADPH is used to provide energy for biosynthetic pathways including the synthesis of sugars during photosynthesis In the light reaction water molecules are split O2 is liberated and the e that are released are used to reduce NADP to NADPH The NADPH is then used to provide reducing power for the carbon fixation reactions during photosynthesis Two Photosystems are involved in Light Reactions Photosystem II comes before I they were named in the order of their discovery In chloroplast the pigment molecules are embedded in the thylakoids in discrete units of organization called photosystems Each photosystem includes about 250 400 pigment molecules and consists of two closely linked components antenna complex and reaction center Antenna complex consists of pigment molecules that gather light and funnel it into the reaction center The reaction center is made up of a complex of proteins and chlorophyll molecules that enable light energy to be converted into chemical energy Water is split into O2 and H Within the photosystems the chlorophyll molecules are bound to specific chlorophyll binding membrane proteins and held in place to allow efficient capture of light energy All the pigments within a photosystem are capable of absorbing photons but only one special pair of chlorophyll a molecules per reaction center can actually use the energy in the photochemical reaction Picture caption Energy transfer during photosynthesis The diagram shows a portion of the antenna complex which occurs in the thylakoid membrane Light energy is absorbed by a pigment molecule anywhere in the antenna complex passes by reasonance energy transfer from one pigment molecule to another until it reaches one of the two special chlorophyll a molecules at the reaction center When this chlorophyll a molecule absorbs the energy one of its e is boosted to a higher energy level and is transferred to an e acceptor molecule Figure tracks the electrons released from water as it passes through the two photosystems which are required to convert NADP to NADPH Also in this process ATP is made The two photosystems are linked together by an e transport chain In photosystem I the special pair of chlorophyll a molecules of the reaction center are P700 P stands for pigment abd 700 designates the optimal absorption peak in nanometers The reaction center in photosystem II also has a special pair of chlorophyll a molecules Its optimal peak is 680 nanometers and it s called P680 Starting in Photosystem II light energy is absorbed by the antenna complex by acessory pigments which is transferred into the reaction center of Photosystem II Water is then split releasing 2 e 2H and O2 Light absorbed is used to raise the energy level of the 2 e These 2e are passed through a series of cofactors and cytochrome complex e transport chain and delivered to the reaction center of photosytem I In Photoystem I light energy is absorbed by acessory pigments and this light energy is used to riase the energy of the e and they are ultimately used to reduce NADP to NADPH H ions accumulate and is used to make ATP Get increase in H ions which are transported into the lumen of thylakoid membrane which forms the proton gradient have more H on one end than the other H used by ATP synthase complex which is how ADP is converted into ATP Overall product of light reactions Get NADPH and ATP which are the two forms of energy required for the Calvin Cycle Know the difference between and action spectrum and an absorption spectrum and what the roles of chlorophyll b and carotenoids are in photosynthesis What we see as white light can actually be separated into different colors by passing it through a prism Isaac Newton found out All radiation in spectrums have wavelength Shorter the wavelength more energy Particles of light or light energy are photons Absorption Spectrum vs Action Spectrum In order for light to be used by photosynthetic org light has to be ABSORBED o Pigment a substance that absorbs light o Most pigments absorb only certain wavelengths of light and transmit or reflect the wavelength they do not absorb Absorption spectrum shows which wavelengths are absorbed in each individual type of chlorophyll The Action spectrum is the range of light that a specific light requiring process occurs So for photosynthesis the action shown in the figure occurs in the blue and red light spectrum You can couple the two action and absorption which is how we found out that chlorophyll a functions in photosynthesis because pathway of action spectrum of photo looks similar to the absorption spectrum so photosynthesis is driven by red and blue light The main photosynthesis pigments are chlorophyll and carotenoids Chlorophyll do not absorb green light they reflect it which gives plants their green color The most important molecule in the light reaction is chlorophyll A because it functions in the photosynthetic reactions Chlorophyll A occurs in all photosynthetic eukaryotes and in cyanobacteria Chlorophyll B is an accessory pigment not directly involved in photosynthetic Gathers the light and transfer it to A which serves to broaden the range of light that can be used in photosynthesis Carotenoids pigments which protect the chloroplast from photo bleaching Function as an anti oxidant preventing oxidative damage to the chlorophyll molecules of light Carotenoids are also accessory pigments Chlorophyll B and carotenoids cannot substitute chlorophyll A in photosynthesis Pigment molecules Chlorophyll A and B and carotenoids are embedded on the thylakoids membranes in units of organizations known as Photosystems or photoreactions Know that the enzyme that carries out CO2 fixation is
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