BIO 201 with Todd Hennessey Week Eight Notes
BIO 201 with Todd Hennessey Week Eight Notes BIO 201
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This 16 page Class Notes was uploaded by ChiWai Fan on Wednesday March 23, 2016. The Class Notes belongs to BIO 201 at University at Buffalo taught by TODD HENNESSEY in Spring2015. Since its upload, it has received 111 views. For similar materials see CELL BIOLOGY in Biology at University at Buffalo.
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Date Created: 03/23/16
Cell Biology on March 21, 23, 25 (all images taken from Professor Todd Hennessey’s slides)—ChiWai Fan An Overview of Photosynthesis (lightcarbohydrate) all inside chloroplast and stroma Light Reactions o Convert light energy into chemical energy stored in NADPH and ATP o Water is the initial e- donor to the electron transport chain. Then a proton gradient is set up. o Water gets oxidized to O2 in chloroplast o Then you get NADPH and ATP into the Calvin Cycle (dark reactions) Dark Reactions 1. The NADPH and ATP made in the light reactions are used to make carbohydrates from CO 2 The Light Reactions The Electromagnetic Spectrum Wavelength determines color Shorter wavelength= higher frequency = more energy = violet has more energy than red How do thylakoid membranes harvest light to make NADPH and ATP? Five different things that can happen to light after it hits a pigment Not all light shined on plants are harvested and used 1. Light can be absorbed, a little bit can be emitted as heat 2. Emitted as fluorescence--at a longer wavelength than it is absorbed 3. Transmitted—no change in wavelength, goes right on by 4. The energy of light is absorbed and passed along to another energy carrier as electron transport energy (we want this step) 5. Reflected—green is not very well absorbed. Green means reflected light. Why isolated pigments (like chlorophyll) fluorescent but chloroplasts aren’t? As a chloroplast starts to die, it becomes fluorescent. The electron transport chain starts to back up because nothing to receive the electron. So it will be emitted as fluorescent Absorption Spectrum Which wavelengths are absorbed best by the pigments? For example: Chlorophyll a absorbs 435nm the best, Chlorophyll b absorbs 470nm the best. You can tell which chlorophyll it is because they have different preference for wavelength If you shine only green light on some green plants, they will die. Why? It can’t absorb it. A green plant prefers white light. Ex. Beta carotene does not absorb orange light because they look orange. Energy Transfer and Electron Transport—we want to get to the exciting state Two light harvesting centers: Light harvesting center 1: This happens first P680 in Photosystem II (prefers 680nm) (PSII) Light harvesting center 2: This happens second P700 in Photosystem I (prefers 700nm) (PSI) Photosystem embedded in thylakoid membrane Using light energy to increase the reducing power in the excited state 1. The ground state photopigments don’t have enough reducing power (ground state) 2. Light energy is absorbed and transferred to the photopigments (excited state) 3. Photopigments in their excited state now have enough energy to reduce the next electron carrier 4. -ΔG when electrons go from excited state to ground state It takes energy (+ΔG) to get from ground state to excited state If you want to go upstairs, you have to take elevator (which takes energy). But if you want to go down, you can take the stairs. (there’s only stairs going down) The z-scheme Kick up energy, it goes back down, kick up again, goes down again 1. Ground state P680 does not have enough reducing power to reduce PQ 2. Light provides the energy to do this by exciting P680 3. The passage of electrons from excited P680 to ground state P700 provides energy to set up the H+ gradient (pmf) 4. Excited P700 can reduce ferredoxin (FD) so it can reduce NADP+ to NADPH Falling ball analogy—This is what plants do by setting up potential energy stores 1. The yellow ball cannot run uphill from 1 to 2 (+ΔG) 2. Light provides the energy to boost it up (light energy is –ΔG) 3. As it rolls downhill from 2 to 3, energy is produced (-ΔG). Now the yellow ball cannot roll uphill from 3 to 4 because it used up most of the energy 4. Light energy is used to boost it up. Now it has high potential energy. From PSII use some energy then kick it up some more to PSI We have not made any ATP or sugar yet. All we’re doing is setting up proton gradient to make ATP and providing NADPH for the process. FYI clarification: ΔG = − n (23.062 kcal) (ΔE) Redox Potential (or reduction potential) is E Electrons can only be passed to something with a higher reduction potential Making ΔE more positive will make ΔG more negative Main steps of the light reactions 1. Light energy is absorbed and does two things: (photolysis) A. Split water into H+, O2and e- in the lumen, this takes energy (light) B. Increases the reducing power of p680 and p700 in the thylakoids 2. Electron transport does two things: (in chloroplast) A. Provides energy for H+ transport into the lumen, loading up lumen with protons B. Provides electrons to reduce NADP+ to make NADPH in the stroma 3. The energy of the H+ gradient (pmf) is used to make ATP in the stroma What do you get from the Light Reactions? NADPH and ATP. Light Reactions of Photosynthesis Electron transport starts with water then to NADP+ Light is not the first electron donor, it only helps to split water to give us electron 1. The energy from the sun is used to set up a H+ gradient with high H+ inside thylakoids 2. When H+ flow out (through the CFo/CF1), ATP is made in the stroma The initial e- donor is 2 O and the final e- acceptor is NADP+ Photophosphorylation produces NADPH and ATP. Why? To give to the Dark Reactions to help make carbohydrates PQ is plastoquinone. It is a lipid PC is Plastocyanin. It is a peripheral protein Stages of e- flow in photosynthesis: 1. Photolysis. PSII uses light energy to split water in the lumen. This produces three important products: a. 2. Pass e- from PSII to PSI. The energy generated helps to increase the [H+] in the lumen 3. Pass e- from PSI to NADP+. Produces NADPH Where do the electrons come from to start this e- transport? water Summary of Light Reactions 1. PSII (P680) uses light energy to split water by photolysis:H2O H+ + O 2 + e- A. The H+ contribute to the H+ gradient in the lumen B. The electrons are passed to the e- transport chain and provide energy for more H+ pumping 2. Electrons passed to PQ and H+ go to the lumen to increase [H+] in lumen 3. Cytb6/f complex accepts e- (GER) and pumps some more H+ into the lumen 4. Electrons passed to PC and PSI (P700) 5. Electrons passed to NADP+ to make NADPH 6. The energy of the H+ gradient is used to make ATP in the stroma Why do plants need water, light and CO t2 grow? Some inhibitors of photosynthesis 1. DCMU. Block electron transport from PSII to PQ 2. Atrazine (herbicide): Blocks e- transport from PSII to PQ. Why doesn’t it kill us? Because we don’t have PQ or PSII 3. DCIP: Artificial electron acceptor that “steals” electrons from PQ. We will use this in our labs too. 4. Paraquat: “Steals” electrons from PSI so that NADP+ doesn’t get reduced to NADPH. It is another herbicide 5. You can have herbicide targeted to one kind of plant and just kill the plant and not us. The Source of the Oxygen Produced by Photosynthesis Give plants radioactive oxygen from radioactive water Hot = radioactive March 23, 2016 The Dark Reactions Three steps in the Calvin cycle What goes in? CO & 2TP & NADPH What comes out? Mostly Glyceraldehyde-3-phosphate (G3P) 1. Carbon fixation by Rubisco (CO2 joining with Ribulose-1,5-bisphosphate carboxylase) (taking CO2 from atmosphere to make covalently bigger carbon compound) 2. Reduction of 3PG to G3P (the purpose of Calvin Cycle is getting G3P) 3. Regeneration of RuBP to keep cycle going This is also called reductive biosynthesis and carbon fixation Note: The Calvin cycle does not make glucose. It makes the G3P that can be used to make glucose Calvin cycle summary 1. You have to have enough RuBP to start this cycle 2. CO2is added to RuBP to make a transient 6 carbon compound. This splits into 2 three carbon compounds (3PG) 3. 3PG is converted to G3P in stroma. This uses ATP and NADPH from light reaction in stroma 4. Some of the G3P is used to regenerate RuBP and the rest goes to make sugars. This requires ATP. Where are the ATP and NADPH needed? It takes 1 ATP and 1 NADPH to convert 1 mole of 3PG to G3P It takes 1 ATP to charge up RuMP to RuBP RuBP Is the Carbon Dioxide Acceptor What’s the point? This is carbon fixation It yields two 3PGs then it turns into G3P 1. What happens to the 3PG? 3PG is converted to G3P 2. 5/6 of the G3P is recycled into RuBP 3. Some is converted to glucose in the cytoplasm for the plant to use 4. Some is converted into sucrose (disaccharide) 5. Some is used to make glucose inside the chloroplast to be stored as starch (polysaccharide) Where is everything? Calvin cycle does not directly make glucose! G3P isn’t the only thing that is produced. Starch granules-polymer of glucose Leaf Anatomy of C an3 C Plan4s C3 plants rely on Rubisco in mesophyll cells to fix CO but 2his enzyme also reacts with oxygen, lowering the rate of CO fixa2ion They are called C3 plants because their first product has 3 carbons (3PG) Mesophyll cells of C4 plants have PEP carboxylase The first product of C4 plants is a 4 carbon compound (oxaloacetate) Specialized bundle sheath cells concentrate CO for Rubisco 2 The blueish cells have Rubisco C3 C 4arbon Fixation PEP (phosphoenolpyruvate) is 3C. This picks up CO in mes2phyll cells to make 4C This 4C compound can diffuse into the bundle sheath cells and release CO . 2 It takes the energy of ATP to regenerate PEP in mesophyll cells Why do this? To concentrate CO for rubi2co in the Calvin cycle C4 (more expensive) The main difference in CAM plants is that they fix CO in t2e mesophyll cells primarily at night (when it is cooler), then ship the 4C compounds to the bundle sheath cells for the Calvin cycle during the day CAM plants: In hot regions when they might lose water (the first electron donor). They fix carbon at night. Metabolic Interactions in a Plant Cell Note that all three processes (Calvin cycle, glycolysis and citric acid cycle) also produce compounds for other metabolic pathways. They are not just used to make ATP and sugars March 25, 2016 Main difference between C4 and CAM plants Some examples of C3 plants: Most small seeded cereal crops such as rice, wheat, barley, rye and oat Also soybean, peanut, cotton, sugar beets, tobacco, spinach and potato Most trees like pine and beech and lawn grasses such as rye, fescue, and Kentucky bluegrass. Some examples of C4 plants: Corn, sugarcane, sorghum, millets, switch grass Also weeds such as the nutgrass, Bermuda grass, barnyard grass, goose grass, crabgrass, pigweed and tumbleweed Some examples of CAM plants: Succulents, such as cactus and agaves Also orchids, bromeliads and pineapple How efficient is photosynthesis in capturing light energy?