Biology 101, Week 10 Notes
Biology 101, Week 10 Notes BIO 101 (Biology, Dr. Dhameja, Principals of Biology)
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This 6 page Class Notes was uploaded by Eleanor Costello on Sunday October 16, 2016. The Class Notes belongs to BIO 101 (Biology, Dr. Dhameja, Principals of Biology) at University of South Carolina taught by Milan Dhameja in Fall 2016. Since its upload, it has received 44 views. For similar materials see General Biology in Biology at University of South Carolina.
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Date Created: 10/16/16
Dr. Dhameja – Biology 101 Chapter 10: Photosynthesis Vocabulary (all will be defined throughout the notes) : 1. Photosynthesis 18. Visible light 2. Autotrophs 19. Photons 3. Photoautotrophs 20. Pigments 4. Heterotrophs 21. Absorbance spectrum 5. Chlorophyll 22. Action spectrum 6. Stomata 23. Fluorescence 7. Mesophyll 24. Photosystem 8. Thylakoids 25. Reaction-center 9. Grana 26. Light-harvesting complexes 10. Stroma 27. Photosystem II 11. Light reactions 28. Photosystem I 12. Calvin Cycle 29. Linear electron flow 13. Phosphorylation 30. Cyclic electron flow 14. Electromagnetic radiation 31. Dehydration 15. Wavelength 32. Photorespiration 16. Electromagnetic spectrum 33. C4plants 17. Pigments 34. CAM plants Subtopic1: Photosynthesis Autotrophs are “self-sustaining” organisms, that sustain themselves without eating or consuming anything derived from other organisms. Autotrophs = producers of the biosphere o Producing organic molecules from CO and o2her inorganic molecules All plants are autotrophs, more specifically they can be classified as photoautotrophs. o Photoautotrophs use the energy from sunlight to make organic molecules from H 2 and CO 2 Heterotrophs, unlike autotrophs, obtain their organic material from other organisms. Heterotrophs = consumers of the biosphere Almost all heterotrophs depend on photoautotrophs for food and O 2 Humans = Heterotrophs Photosynthesis is the process by which solar energy is converted to chemical energy. *Whether directly or indirectly, photosynthesis nourishes (almost) the entire world Photosynthesis occurs in plants, algae, certain protists and some prokaryotes. The structural organization of cells allows for the chemical reactions in photosynthesis. o Leaves are the major location for photosynthesis, although it can occur in other locations. The green color of leaves comes from chlorophyll, which is defined as the green pigment in chloroplasts. CO 2nters the leaf and O e2its, this occurs through microscopic pores, called stomata. CO 2 2 o Light energy absorbed by the chlorophyll drives the synthesis of organic molecules in the chloroplast. Chloroplasts are mainly found in the cells of the mesophyll, the interior tissue of the leaf. (30-40 chloroplasts per 1 mesophyll) o Chlorophyll is found in the membrane of thylakoids (connected sacs) Thylakoids may be found stacked in columns called grana. o Chloroplasts also contain stroma, a dense fluid. o Chloroplasts split H O into hydrogen and oxygen, incorporating the electrons of 2 the hydrogen into sugar molecules. Equation of Photosynthesis: 6CO +212H O +2Light energy C H O +66 +12 O6 2 2 (Note that in this equation glucose is represented but in reality photosynthesis produces a 3-carbon sugar.) Photosynthesis is a redox reaction in which H 2 is oxidized and CO is2reduced. 6CO +212H O 2C H O +6O 12H 6 2 2 Photosynthesis is made up of two parts: o Light Reactions (photo part) o The Calvin Cycle (synthesis part) Simplistic break down of photosynthesis: Light Reactions (in the thylakoids) o Split H O2 o Release O 2 o Reduce NADP to NADPH o Generate ATP from ADP by phosphorylation The Calvin Cycle (in the stroma) forms sugar from CO ,us2ng ATP and NADPH o Carbon fixation o Reduction o Regeneration Subtopic2: The conversions ofLight Reactions Chloroplasts are solar-powered chemical factories. The thylakoids of chloroplast transform light energy into the chemical energy of ATP and NADPH. Light is presented in the form of electromagnetic energy, also called electromagnetic radiation. Like many other electromagnetic energy, light travels in rhythmic waves. o Wavelength is the distance between the crests (peaks) of waves. Wavelength determines the type of electromagnetic energy. The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation. Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors that the human eye can see. Light behaves as though it consists of discrete particles, called photons. Visible light is from 380nm to 750nm Shorter wavelength Longer wavelength Higher energy Lower energy Subtopic3: The Light Receptors(Photosynthetic Pigments) Pigments are substances that absorb visible light. Different pigments absorb different wavelengths. There are three different types of pigments: o Chlorophyll a o Chlorophyll b o Carotenoids Wavelengths not absorbs by the pigments are reflected or transmitted. o Leaves appear green because chlorophyll reflects and transmits green light. When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable. o The falling of excited electron, back to ground state, causes photons to be given off, causing an afterglow called fluorescence. If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat. An absorbance spectrum is a graph plotting a pigment’s light absorbance versus it’s wavelength. Chlorophyll a: violet-blue and red light work best for photosynthesis An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process. Chlorophyll a is the main photosynthetic pigment. Chlorophyll b, an accessory pigment, helps to broaden the spectrum used for photosynthesis. Carotenoids, another accessory pigment, absorbs excessive light that would damage chlorophyll. Subtopic4: Part 1 ofPhotosynthesis – Light reactions take place in photosystems in the thylakoid membrane A photosystem consists of a reaction-center complex surrounded by light-harvesting complexes. The primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a. The light-harvesting complexes funnel the energy of photons to the reaction center. There are two types of photosystems in the thylakoid membrane: o Photosystem II o Photosystem I (these are listed in the order of occurrence, but they are named for the order of discovery) Photosystem II (PSII) functions first and is best at absorbing a wavelength of 680nm. The reaction-center chlorophyll a of PSII is called p680 Photosystem I(PSI) functions second and is best at absorbing a wavelength of 700nm. The reaction-center chlorophyll a of PSI is called p700. During light reactions there are two possible routes for electron flow: 1. Linear electron flow 2. Cyclic electron flow Linear electron flow, the primary pathways for electrons, involves both photosystems and produces ATP and NADPH using light energy. Step-by-Step of Linear Electron Flow 1. A photon hits a pigment and the energy is passed until it excites P680 2. The excited electron from P680 is transferred to the primary electron acceptor; resulting in P680 a very strong oxidizing agent. 3. H 2 is split by enzymes, and the electrons are transferred from the H to the P680 , thus reducing it to P680; O2is released as a byproduct of this reaction. 4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PSII to PSI 5. Energy released by the fall eventually drives the creation of a proton gradient across the thylakoid membrane. Diffusion of H (protons) across the membrane drives ATP synthesis. 6. In PSI (like PSII), transferred light excites P700 which loses an electron to an electron + acceptor; P700 accepts an electron passed down from PSII via the electron transport chain 7. Each electron “falls” down an electron transport chain from the primary electron acceptor of PSI to the protein ferredoxin (d ) 8. The electrons are then transferred to NADP and reduce it to NADPH; the electrons of NADPH are available for the reactions of the Calvin cycle. Note: After step 4 and 5 the electron flow goes from Photosystem II to Photosystem I P680 is the strongest biological oxidizing agent Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH. Cyclic electron flow generate ATP, satisfying the higher demand in the Calvin cycle. *the cyclic electron flow is thought to have evolved before the linear electron flow. Linear Electron Flow Cyclic Electron Flow 1. Uses both photosystems 1. Uses only photosystem I 2. Produces ATP and NADPH 2. Produces a surplus of ATP Subtopic5: Part 2 ofPhotosynthesis – The Calvin Cycle uses ATP and NADPH to convert CO to 2ugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. The Calvin cycle has three phases: o Carbon Fixation (catalyzed by rubisco) o Reduction o Regeneration of the CO acce2tor (RuBP) Carbon enters the cycle as CO an2 leaves as a sugar named glyceraldehyde-3-phosphate (G3P) o For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2 Subtopic6: Alternative mechanisms ofcarbonfixation that evolved dueto hot, arid climates Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis. On hot, dry days, plant close their stomata, which in turn conserves H O2but also limits photosynthesis. o The closing of stomata reduces access to CO and 2auses O build 2p. Hot, dry conditions favor a seemingly wasteful process called photorespiration. (In most plants, C plants, intitial fixation of CO via rubisco, forms a three-carbon compound.) 3 2 In photorespiration, rubisco adds O 2nstead of CO in2the Calvin cycle. Consumes O and2organic fuel and releases CO witho2t producing ATP or sugar. Limits damaging products of light reactions that build up in the absence of the Calvin cycle. In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle. Subtopic7: Two alternative mechanisms ofcarbonfixation – C and CAM 4 C 4lants minimize the cost of photorespiration by incorporating CO into2four-carbon compounds in mesophyll cells. This require the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO t2an rubisco does, it can fix CO ev2n when CO co2centrations are low. Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO int2 organic acids Stomata close during the day, and CO is2released from organic acids and used in the Calvin cycle.
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