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Phenomenal Photosynthesis Chapter 10 notes Photosynthesis Capturing light energy from the sun and converting it to chemical energy stored in sugar and organic molecules Where do you get your Energy? Autotrophs – make organic molecules from inorganic raw material (don’t need to eat) = PRODUCERS!! Photoautotrophs – use light as an energy source to make organic molecules ex. plants, algae, some protists, some prokaryotes Chemoautotrophs – Use inorganic substances (sulfur, ammonia) to make organic compounds (no light) ex. Bacteria only Where do you get your Energy? Heterotrophs – live on compounds produced by other organisms (must eat to live) = CONSUMERS!!! ex. animals, decomposers, etc Depend on photosynthesis for both food and oxygen The Perfect Setting Leaf Structure Chloroplasts – contained in any green part of a plant but photosynthesis happens mainly in the leaves Chlorophyll – pigment in chloroplasts that absorbs light from the sun Mesophyll – tissue on interior of leaf that contains chloroplasts The Perfect Setting Leaf Structure Stomata – microscopic pores in leaves that allow CO2 in and O2 out Veins – transport water from roots to leaves and sugar from leaves to the rest of the plant An Overview of Photosynthesis Easy Equations 6CO2 + 12 H20 + Light C6H12O6 + 6O2 + 6H20 Simplified 6CO2 + 6 H20 + Light C6H12O6 + 6O2 REALLY SIMPLIFIED CO2 + H20 CH2O + O2 An Overview of Photosynthesis Mechanisms A series of experiments have been done to determine how photosynthesis works. Read about them in your book An Overview of Photosynthesis The Bottom Line 1. The oxygen that is released comes from the splitting of WATER (not from CO2) 2. Photosynthesis is a redox process (like respiration). Water is oxidized and CO2 is reduced to sugar An Overview of Photosynthesis 3. Light Reactions ** convert solar energy to chemical energy Inputs – H20, NADP+, Light, ADP Outputs – O2, NADPH, ATP ATP is made by photophosphorylation Happens in the THYLAKOIDS of chloroplasts An Overview of Photosynthesis 4. Calvin cycle (“dark reaction”) Carbon fixation – incorporates CO2 from air into organic molecules using ATP and NADPH from the light reaction Inputs – CO2, ATP, NADPH Outputs – Glucose, NADP+, ADP Does not require light, but normally occurs during the day Happens in the STROMA of chloroplast The Sunlight and Color Sunlight – electromagnetic energy Visible light – The wavelengths of light used in photosynthesis – detectable by the human eye Photons – particles of light that have a certain amount of energy The Sunlight and Color Pigments – Substances that absorb visible light Different pigments can absorb different wavelengths of light Wavelengths that are absorbed disappear Example – leaf appears green because chlorophyll absorbs red and blue light but transmits and reflects green light (red and blue disappear and green shows up The Sunlight and Color Blue and Red light are the most effective wavelengths for photosynthesis and green light is the least effective Plant Pigments Chlorophyll A – The main pigment in plants. Participates directly in the photosynthesis light reaction. This is a GREEN pigment Plant Pigments Chlorophyll B – An accessory pigment. It doesn’t participate directly in the light reaction, but it can transfer energy to chlorophyll a. This is a YELLOWGREEN pigment Plant Pigments Carotenoids – Yellow and Orange hydrocarbons that are found in the thylakoid membrane of chlorophyll a and b Fun Photosystems The three plant pigments are arranged into photosystems within the thylakoid membrane. Each photosystem is made of 3 parts 1. Antenna complex – made of several hundred chlorophyll a, b and carotenoid molecules. This complex gathers light and absorbs photons and then passes the photons from one molecule to the next Fun Photosystems 2. Reaction Center - Only ONE chlorophyll a molecule can actually transfer an excited electron into the light reaction. This chlorophyll a molecule is located in the reaction center Fun Photosystems 3. Primary electron acceptor – Near the reaction center. It traps the excited electron that comes from the reaction center. This is the first step of the light reaction. The energy from this trapped electron will be used to make ATP and NADPH to be used later Electrifying Light Reaction Photosystem II (PSII) 1. Light Strikes!!! 2. An electron in the chlorophyll P680 is excited to a higher level…Weeee!!! Electrifying Light Reaction 3. Water is split by an Enzyme. Electrons from the water are given to P680 to replace their long lost electron from step 2. 4. Oxygen is Released – Yeah – we can breathe!!! Electrifying Light Reaction 5. The excited electron passes from Photosystem II (PSII) to Photosystem I (PSI) by an Electron Transport Chain. 6. As the electron falls down the chain, their fall drives the reaction ADP + Pi ATP (Chemiosmosis). Remember, the energy actually pumps those H+ out. As they come back in through ATP synthase, ADP gets phosphorylated to make ATP. Electrifying Light Reaction Photosystem I 7. When the electron gets to the bottom of the chain, it fills a hole in P700. 8. More photons of sunlight come in and excite the electron in P700 to a higher energy level. Electrifying Light Reaction 9. The electrons falls down another electron transport chain. NADP+ reductase (another enzyme) transfers the electron from Fd to NADP+ to make NADPH (our energy storage molecule). 10. NADPH and ATP move on to the Calvin Cycle!!! Let’s go make some sugar! Kickin’ Calvin Cycle Setting… Stroma of chloroplast ***CO2 in , Sugar Out ***Use ATP and NADPH (from light reaction) ***Produces Glyceraldehyde-3biphosphate (G3P) ***Needs to go around 6 times to make a glucose Kickin’ Calvin Cycle 1. Carbon Fixation CO2 enters through stomata Each CO2 is combined with ribulose bisphosphate (RuBP) by an enzyme called Rubisco It produces a 6 carbon molecule. It’s not stable so it immediately breaks in half. At the end you’re left with 2, 3-carbon phosphoglycerates (PGA) Kickin’ Calvin Cycle 2. Reduction ATP phosphorylates each PGA NADPH reduces it to G3P (by donating electrons) – this also recycles NADP+ 1 molecule of G3P is released to form sugar Kickin’ Calvin Cycle 3. Regeneration of CO2 acceptor (RuBP) Carbon skeletons of 5 molecules of G3P are rearranged into RuBP in a series of FUN chemical reactions!! Kickin’ Calvin Cycle Fun facts In all, Calvin cycle uses 18 ATP and 12 NADPH to get just ONE glucose The Skinny on Rubisco, the friendly Enzyme Rubisco is the most common protein on Earth because it is the enzyme that “fixes” CO2 in plants. Rubisco can also “fix” oxygen because it fits in its active site. This is called photorespiration. OH NO!!! Competitive Inhibition!! The Skinny on Rubisco, the friendly Enzyme Photorespiration doesn’t produce useful products. The products have to be broken down by peroxisomes. Photorespiration slows down photosynthesis and wastes valuable resources. Ugh!! Fun Photosynthesis, Final details Non-cyclic Photophosphorylation produces ATP, NADPH and O2 by using both Photosystem I and II (like we just learned) Problem – there is not enough ATP produced to run the Calvin cycle Cyclic Photophosphorylation A simpler pathway that involved only Photosystem I Makes ATP without producing NADPH or O2 Cyclic Photophosphorylation Here’s how it works 1. Photons of light are absorbed by Photosystem I and P700 releases the electrons to the primary electron acceptor 2. From here, the electrons travel to Ferredoxin (Fd) and then to the cytochrome of PSII and end up back at PSI again. 3. During this electron transport chain, ATP is made by chemiosmosis and it is called cyclic photophosphorylation. C4 Photosynthesis Used in plants that live in Hot, Arid (dry) conditions. These hot dry conditions favor photorespiration (bad) over photosynthesis (good). C4 plants take in CO2 and convert it to a 4 carbon sugar BEFORE the Calvin cycle C4 Photosynthesis C4 plants have a special leaf structure with two types of photosynthetic cells 1. Bundle sheath cells – have tightly packed sheaths (insulators) around the veins of the leaf. Their thylakoids are not stacked into grana THE CALVIN CYCLE IS DONE ONLY IN CHLOROPLASTS OF THE BUNDLE SHEATH CELLS IN C4 PLANTS C4 Photosynthesis 2. Mesophyll cells are found between the bundle sheath and the leaf surface (near the stomata). CO2 enters the plant through the mesophyll cells. C4 Photosynthesis Here’s how it works 1. CO2 enters through the stomata and goes into the mesophyll cells. 2. CO2 is add to PEP to form oxaloacetate (a 4 carbon product). This reaction is catalyzed by PEP carboxylase (another enzyme). PEP carboxylase is much more efficient than Rubisco. Oxygen does not fit in its active site, so there is NO PHOTORESPIRATION!! C4 Photosynthesis This carbon fixation is useful in hot dry areas because the stomata on the leaves close so that the plant doesn’t dry out. When the stomata are closed, CO2 from the air can’t get in, and O2 from the light reactions can’t get out. Levels of CO2 decrease and O2 levels increase inside the leaf. C4 Photosynthesis 3. After CO2 has been fixed, Oxaloacetate is converted to another 4 carbon compound (malate). 4. The malate moves into the Bundle sheath cells (by the wonderful plasmodesmata bridges) 5. In the bundle sheath cells, the malate releases CO2 which is then fixed by rubisco for the Calvin Cycle. C4 Photosynthesis Overall No Oxygen is allowed into the bundle sheath cells, making rubisco fix ONLY CO2 and not Oxygen. NO Photorespiration. HOORAY!!! This type of photosynthesis uses a SEPARATION OF SPACE to make it more efficient CAM Photosynthesis Used in plants that open their stomata mostly at NIGHT, because it is too hot in the day and they would lose too much water. (desert plants like cactus) Problem – by keeping stomata closed, water is conserved, but CO2 can’t get in for photosynthesis. CAM Photosynthesis At night, when stomata are open, CO2 enters the plant and is incorporated into organic acids. This carbon fixation is called Crassulacean Acid Metabolism (CAM) photosynthesis Organic acids are stored in the vacuoles of the mesophyll cells until morning when stomata close. CAM Photosynthesis During the daytime, light reactions supply the ATP and NADPH for the Calvin cycle. At this time, CO2 from the organic acids is released and made into sugar in the chloroplasts. CAM photosynthesis uses a SEPARATION OF TIME to make photosynthesis more efficient. End Thoughts on Photosynthesis Overall, photosynthesis is the most important chemical reaction that happens on Earth. Without it, we wouldn’t be here. It produces both Food and Oxygen. End Thoughts on Photosynthesis Plants use about 50% of the products of photosynthesis to drive respiration (for ATP) and for growth. The main sugar produced is glucose (monomer) which combines to for sucrose and cellulose (cell walls). The other 50% is stored as starch in roots, tubers, seeds and fruits (YUM!!!)