BCHM 307 1nd Edition Lecture 38 Outline of Last Lecture I Oxidative Phosphorylation Inhibition II Hibernation A 2 4 Dinitrophenol III Heterotrophs vs Phototrophs Outline of Current Lecture I Photosynthetic Energy II Light Energy into Chemical Energy A Three Conditions III Chloroplasts IV Absorbance Spectrum Current Lecture This lecture continues on with the topic of photosynthesis Greater than 10 17 kcal of free energy are stored by photosynthesis on Earth per year This is the same as 68 billion barrels of oil This energy also corresponds to the assimilation of 1010 tons of carbohydrates The purpose of photosynthesis is to capture light energy This light energy is then used for the reduction of carbon dioxide into carbohydrates This can be seen by the following equation 6CO2 6 H2O light energy yields C6 H12 O6 6O2 The equilibrium constant for this reaction is 10 496 at 27 degrees Celsius This value means that energy must be put into this reaction in order to drive it forward This reaction can also be thought of as the reversal of glycolysis and mitochondrial respiration The next question is how does the light energy become chemical energy This transduction depends upon three things 1 There must be sequential oxidation reduction reactions that pass electrons from water to NADPH 2 These reactions must be compartmentalized in the chloroplast 3 A proton gradient must be generated from the above Photosynthesis takes place in the chloroplasts of plant cells Chloroplasts are a type of organelle found in plant cells A part of the chloroplast called the stroma can be thought of as the equivalent to the mitochondrial matrix The compartmentalization take place in the form of discs called thylakoids Light energy is absorbed during photosynthesis through the use of photosynthetic pigments This pigments have conjugated double bond systems These pigments include the well known chlorophyll When looking at the absorbance spectrum of different photosynthetic pigments it can be seen that they absorb blue and red light This is in the range of 400 500 and 600 respectively Accessory pigments can absorb outside these numbers and effectively extend the action spectrum for photosynthesis Green light in the 500 600 range is never absorbed This is why chloroplasts appear green it is the only color not absorbed
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