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Transcript
PHOTOSYNTHESIS BEGINS Hello there, I’m Dr. Melvin Calvin speaking from my famous lab at the University of California, Berkeley. I will be presenting the process of photosynthesis to you. You probably already know I hold the 1961 Nobel Prize for chemistry and thus am quite qualified to present this lesson to you. I am particularly famous for my work on the Light independent (Calvin-Benson reaction). So hold on for a wild ride on the photosynthesis pathway! CYCLIC ELECTRON FLOW On the next slide, you will see how the plant can conduct photosynthesis to generate ATP. Thus plants can provide ATP utilizing light energy. The method utilizes an electron that is boosted to a higher energy level by the photon. A model of the reaction center of a photosystem is shown below. Photon energy captured by electron Makes ATP No OXYGEN produced No NADPH produced Electrons flow back to origin at chlorophyll a Reaction center p700 Photosystem I group of chlorophyll called antenna assembly p700 ee Enzyme 1 e Enzyme 4 Cytochrome ADP P ATP Lets start with CYCLIC PHOSPHORYLATION or Photosystem I and watch the electron. The black colour represents the low energy electron and the red colour the high energy electron. One more time for CYCLIC PHOSPHORYLATION or Photosystem I p700 ee Enzyme 1 e Enzyme 4 Cytochrome ADP P ATP Notice how the photon hits P700 boosting the electron to the higher orbital. It is then carried by enzyme 1 to the cytochrome where it’s energy is used to convert ADP into the higher energy ATP. The electron then becomes low energy (black) and is carried by enzyme 4 back to p700 where it will repeat the process again. Remember the sugars we built together. We used carbon and hydrogen plus some oxygen to make glucose! Now it is your responsibility to memorize and understand the cyclic photophosphorylation pathway. This pathway will generate ATP energy for cells but it does not provide the most important function of the photosynthetic process .. THE ABILITY TO BIOSYNTHESIZE! Plants must be able to make biomass like sugars that can be turned into cellulose for their cell walls or other important chemicals. In order to do this they need a carbon source and a source of hydrogens to attach to the carbons . The next slide describes a process essential to getting photosystem II started. Photosystem II is the pathway for generating the hydrogens and creating the reducing power needed to conduct biosynthesis reactions. Recall from your earlier work on acid base chemistry the ionization of water shown by the equilibrium equation below. H2O In Photosynthesis these H+ ions are removed by p680 which has a great affinity for electrons. An enzyme called the Z protein speeds up the reaction. Since p680 strongly attracts the hydrogen, and removes it from the equation , more water splits in order to replace the disappearing hydrogen ions. This results in the continuous splitting of water molecules called photolysis. H+ + OH This happens in normal water as discussed in acid base chemistry such that the H+ concentration is 10-7. This value remains constant unless something disrupts the equilibrium. Lets take a closer look at how this water is split by PHOTOLYSIS. H2O H+ + OH H Remove 4 H+ to be used for reduction. H H H Then 4 OHremain. What happens to these? O H O H O H O H These form one oxygen gas molecule O2 These form 2 water molecules Powering this whole process is the p680 which strongly attracts the low energy electrons, causing the water to continually split apart H2O e p680 Therefore splitting the water molecule results in the following: 4H2O 4H+ O2 2H2O Low energy electrons Now we have the starting point for photosystem II. This cycle starts with the splitting of water and the p680 chlorophyll molecule located on the far right hand side of our photosynthesis diagram. The electrons in the photosynthetic cycles actually travel as electron pairs (two electrons). For simplicity, the model only shows one electron. LIGHT REACTION p700 e Now lets follow the pathway for Photosystem II or Noncyclic Photophosphorylation Enzyme 1 Enzyme 2 O2 Split H2O H2 O e Enzyme 4 e Enzyme 3 Cytochrome ADP P ATP p680 NADP+ Notice that the low energy electron starts at p680, is hit by the photon and becomes high energy. It then is carried by enzyme 3 to the cytochromes where its energy is converted into ATP and the electron becomes low energy. It is passed to enzyme 4 and then to p700 where it is hit by another photon and turns high energy. It then progresses through enzyme 1 and enzyme 2 to be used as energy to attach the hydrogen to NADP which will be used to reduce carbon later in the Light independent Reaction (Calvin cycle). It then is low energy and drops below the dotted line, entering the Calvin cycle e e H NADPH LIGHT REACTION p700 e Lets go through the cycle once again Enzyme 1 Enzyme 2 O2 Split H2O H2 O e Enzyme 4 e Enzyme 3 Cytochrome ADP P ATP p680 NADP+ Notice that the low energy electron starts at p680, is hit by the photon and becomes high energy. It then is carried by enzyme 3 to the cytochromes where its energy is converted into ATP and the electron becomes low energy. It is passed to enzyme 4 and then to p700 where it is hit by another photon and turns high energy. It then progresses through enzyme 1 and enzyme 2 to be used as energy to attach the hydrogen to NADP which will be used to reduce carbon later in the Light independent Reaction (Calvin cycle). It then is low energy and drops below the dotted line, entering the Calvin cycle e e H NADPH At this point the light reaction has generated ATP for energy and NADPH which has vast reducing power. These two products will be used in the Calvin cycle in the biosynthesis of the organic molecules. See the complete cycle in the next lesson!! End Photosystem 1 and 2 Lesson continues with the Calvin cycle