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PHOTOSYNTHESIS DETAILS BEGINS Hi there kids. I’m here to talk about some of the details about photosynthesis now that you have a good understanding of the pathway and how it works. Before I start, I would like to help to clarify some chemical names we have or will been using in photosynthesis and show how they are the same (reused) to those in metabolism. PGAL is the same as G-3-P or glyceraldehyde3phosphate of glycolysis PGA is the same as 3-phosphoglyceric acid of glycolysis PEP is phosphoenolpyruvate from glycolysis NADPH is made up of the same subunits as NADH used in metabolism. NADPH simply has a phosphate added onto the end. Both are used as electron acceptors. Please tell me you recall these from my lesson. My job is on the line! Multiply each plant photosynthetic cell by the amount of rubisco and you get the most numerous protein on Earth! From the previous work of photosynthesis, you probably remember that I am the enzyme responsible for attaching carbon dioxide to the RuBP. I am extremely important as I am the only enzyme capable of completing this operation. It’s like my job is guaranteed. Oh, and did I also mention that I’m lazy as far an enzymes go. Most enzymes work about 1000 substrates a second while I only work about 3 a second. That means you need a lot of me to accomplish carbon fixation in a plant! Senor, remember how the enzymes work. LIGHT INDEPENDENT REACTION PGA 1. RuDP CO2 O2 is a competitive inhibitor of rubisco PGA That is what I do. I hook these two together, but consider this. I have a little problem, well actually big problem. You see I can’t really distinguish between oxygen and carbon dioxide. So my active site can bring 1.carbon dioxide into position to attach RuDP or it could bring 2.oxygen in to position to attach to RuDP PHOTORESPIRATION 2. O2 3 carbon sugar (remains in Calvin cycle) RuDP GLYCOLIC ACID (useless and released as CO2) When I hook O2 to RuBP it is called photorespiration. When I do this the five carbon RuBP splits into a useable three carbon molecule and a useless two carbon compound (glycolic acid). Glycolic acid exits the Calvin cycle and the chloroplast altogether where it is broken into CO2. Glycolic acid leaves the chloroplast and enters a peroxisome. It is then broken down into two carbon dioxide units without any ATP energy produced. This is therefore very unproductive for the photosynthetic process. This drains away some of the carbon already fixed by the Calvin cycle. In the soybean plant about 50% of fixed carbon is lost this way. Photorespiration occurs only in light, consumes oxygen, produces carbon dioxide and produces no ATP energy! Organisms rely on plants to provide food as reduced carbons in sugars. Photorespiration is very wasteful destroying work already done in carbon fixation! C3 plants soybeans geranium trees cucumber tomato Most plants •C3 plants make up the majority of plant species. • Photorespiration occurs in C3 plants.(they only have Rubisco) •C3 plants are so named because the first stable intermediate formed by the dark reaction is a three carbon compound called PGA. Now given conditions where the weather is cool moist and cloudy, I will work fairly well. I will attach carbon dioxide as long as oxygen concentrations do not get too high. Now you might wonder, how do oxygen concentrations get too high? The atmospheric oxygen is 21% whether it is cool or not. Well let me explain: Stoma The process of carbon dioxide fixation actually occurs in the leaf. At the bottom of the leaf are small holes called stoma that can open or close to allow more or less air into the interior of the leaf for carbon fixation to occur. If it is cool and moist the stoma remain open; thus O2 levels remain the same. If, however it is hot and dry the stoma close and O2 levels inside the leaf increase. This is due to CO2 levels that drop (CO2 used by Calvin cycle) and lack of air entering through the leaf stoma. During cool moist and cloudy conditions, the stoma are wide open allowing the maximum amount of air into the leaf and thus the carbon dioxide levels remain relatively high and oxygen levels do not rise above 21%. So in this situation, oxygen levels are low. Open stoma allows O2 out and CO2 in Cloudy, Cool Moist conditions I’m good! Open Stoma Low Oxygen Little photorespiration! That means higher photosynthetic productivity! During hot dry and sunny conditions, the stoma close and only allow the minimum amount of air into the leaf. The carbon dioxide is used up by the Calvin cycle and oxygen levels rise above 21%. So in this situation, oxygen levels are high. Closed stoma decreases CO2 and thus increases the O2 content. Rubisco will begin photorespiration. Sunny hot and dry weather Closed Stoma High Oxygen I’m bad I turn traitor, and instead of attaching CO2 to RuBP I attach O2. This results in lots of photorespiration and thus lower photosynthetic productivity! PHOTORESPIRATION!! In order to reduce the loss as a result of my poor performance in hot sunny and dry conditions, some plants have evolved different pathway s. There are both C4 and CAM plants. Hello there. I’m Hello there. I’m Mrs. señor cactus. Maize and I am a C4 That gringo they plant. I can’t begin to call Rubisco, if tell you how you try to make disappointed I was with him work in the that Rubisco fellow. My hot desert he is species and all other useless . I’m a members of the C4 member of the group, have evolved a CAM plant group pathway to circumvent and we have his bad habit of developed a photorespiration. I’ll better method of tell you my secret in a carbon fixation. minute. We C4 plants use PEP carboxylase and PEP instead of using Rubisco to hook CO2. PEP carboxylase is the enzyme that attaches CO2 to the 3 carbon sugar, PEP (phosphoenolpyruvate). I’m so glad we found PEP carboxylase who is vastly superior to Rubisco because of his higher affinity to attract CO2 and his loyalty. He will not turn traitor and bond O2. I don’t know what I would do without him as I live in hot dry and sunny environments. So we C4 plants stop photorespiration by using PEP! Fix carbon at night Photosynthesis during day We CAM plants are also called succulent or water storing plants. We only open our stoma at night and then we attach CO2 to a variety of organic acids. Since the only time evaporation is low in the desert is at night, this works well for us. We store the fixed carbon in the organic acids until daylight when we can use the light reaction to power up the Calvin cycle and reduce the carbons to sugars. Thus water loss is minimized and photorespiration is stopped. Did you know? Bring a blanket—the desert is not always hot ---At night the desert is cold Colder temperatures mean less evaporation and more moisture in the air. Less water loss from stoma if they are open. We are three from the East. We travel by night and follow the star So in summary and scientific lingo Sugar cane is a C4 plant 1. C4 plants preface the Calvin cycle with reactions that incorporate CO2 into four carbon compounds. Instead of using Rubisco and RuBP to hook CO2, they use PEP (phosphoenolpyruvate) and the enzyme PEP carboxylase. PEP carboxylase is vastly superior to rubisco because is has a higher affinity to attract CO2 and it has no affinity to bond O2. So these plants do not allow photorespiration and rubisco cannot turn traitor! 2. CAM plants are also called succulent or water storing plants. These plants only open their stoma at night and the CO2 is incorporated into a variety of organic acids. Thus water loss is minimized and photorespiration is stopped. Cactus is a CAM plant So do you think I got fired from my job by the C4 and CAM plants? Think again! We C4 plants still have to do the Calvin cycle, so we still need Rubisco to fix carbon the RuBP. We accomplish this by locking Rubisco up in a place where there is no access to oxygen. Rats, these bundle sheath cells make it so I can’t get any oxygen . C4 plants have a special anatomy. The mesophyll cells have access to the air and they incorporate our hero PEP who will reduce CO2. Bundle sheath cells do not have access to the air and are located beneath the mesophyll cells. PEP can pass the CO2 to Rubisco who, without oxygen, behaves himself and conducts the Calvin cycle without any photorespiration. The sugars produced then enter the vascular tissue. This is my C4 or Hatch Slack pathway. Notice how air is only accessible to mesophyll cells. The bundle sheath cells contain Rubisco and receive CO2 from Malate. Air here Carbon dioxide Oxygen Carbon dioxide Oxygen PEP carboxylase Mesophyll cell plasmodesmata Bundle Sheath Cell Mesophyll cell Bundle Sheath Cell Rubisco Rubisco Sugar Sugar No oxygen Vascular tissue contains sugar water No oxygen The C4 pathway is not free. CO2 is actually fixed twice and that costs energy in the form of more ATP. The C3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C4 pathway requires 30 ATP. But if you use me, you will lose about 50% of your fixed carbon to photorespiration in hot dry and sunny environments !! Minimizing the loss of fixed carbon makes me more efficient than C3 plants in hot dry environments even though additional energy ATP is required. I you hear about the Krantz Anatomy, it is just talking about my bundle sheath cells and the fact that the thylakoids are not arranged in grana. The grana are arranged in a wreath like appearnace and likely concentrate the CO2 around Rubisco to enhance uptake. Since the mesophyll cells do have grana they made up a special word to describe this chloroplast arrangement its called dimorphic Today, C4 plants, which grow primarily in the Tropics (Latitude 450 and less) represent about 5% of Earth's plant biomass. C4 plants are only 1% of the total plant species, yet they account for around 30% of terrestrial carbon fixation. Pretty good work huh? Before you go any further, think about evolution of plant photosynthesis. Photosystem I probably evolved first to produce ATP followed by Photosystem II and the Calvin cycle to provide the ability to conduct biosynthesis of chemicals to build cells. Rubisco is an ancient enzyme likely from a time when the Earth did not possess oxygen in the atmosphere. Given those conditions, the inability of the enzyme’s active site to distinguish CO2 from O2 would have not made a difference. About 2.5 billion years ago, the plants increased atmospheric oxygen content and then Rubisco could combine oxygen resulting in photorespiration. Since the Calvin cycle pathway had already been established for billions of years of plant photosynthesis, plants retained the existing pathway and simply put methods in place to minimize the photorespiration. It looks like the C4 pathway evolved several times in different plant species. This is called convergent evolution. This is a similar pattern to cells retaining the ancient glycolysis pathway and merely extracting energy more efficiently from the end product via Krebs and ETC. I’m telling you, there’s really a lot of important and relevant information in my lesson. Skip ahead, your teacher won’t mind. Look for the lesson First Cells in the evolution section Now, señor, let me tell you about my CAM pathway Can you remember the Krebs cycle chemicals like malate and oxaloacetate ? •CAM stands for Crassulacean acid metabolism named after one of my plant families. • I close my stoma during the day to conserve water. • At night I open my stoma to collect CO2 and attach it to organic acids from the Krebs cycle and store them in vacuoles. •When the sun rises and photosynthesis starts, I use the ATP and NADPH generated to supply the Calvin cycle with energy. The organic acids release CO2 to Rubisco to supply the Calvin cycle. Hey, what category plant is this Agave stuff? C3, C4 or CAM? Agave is a CAM plant. Pineapple also is a CAM plant. Jade is a CAM and actually a member of the Crassulaceae family. Sound familiar? How about this Jade plant? How about this pineapple? Now there are two colours on these graphs. Which do you think belongs to C4 plants like me and which to C3 plants? C3 is the yellow line C4 is the green line Now can you explain this graph next to me? Please utilize the information you now know. In temperate regions, the lower light intensity, and cooler temperatures act to the disadvantage of C4 plants. C3 plants have an advantage due to their low rate of photorespiration and because they need no energy for the preliminary fixation of CO2 by PEP. In tropical regions approaching the equator, the higher light intensity and hot temperatures set the C4 plants at an advantage reflected by the increase in net yield of fixed carbon sugars. C3 plants suffer from increasing amounts of photorespiration. So long! Good Bye Adios END PHOTOSYNTHESIS DETAILS Photosynthesis – working everyday to provide you with food.