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BIO 1107, week 9 notes

by: Monica Notetaker

BIO 1107, week 9 notes BIO 1107

Marketplace > Kennesaw State University > Biology > BIO 1107 > BIO 1107 week 9 notes
Monica Notetaker
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These notes cover the basis of photosynthesis, the reactions that occur, and important molecules involved in the process. This information will be on the next test.
Intro to Biology
Dr. Brookshire
Class Notes
Photosynthesis, Biology




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This 4 page Class Notes was uploaded by Monica Notetaker on Friday October 7, 2016. The Class Notes belongs to BIO 1107 at Kennesaw State University taught by Dr. Brookshire in Fall 2016. Since its upload, it has received 13 views. For similar materials see Intro to Biology in Biology at Kennesaw State University.

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Date Created: 10/07/16
Photosynthesis Notes Background Information  Plants and other photosynthetic organisms convert sunlight into usable chemical energy stored in sugar and other organic molecules through a process called photosynthesis.  Photosynthesis occurs in the organelle called the chloroplast, and the majority of photosynthesis takes place in the leaves of a plant (although every green part of a plant contains chloroplasts).  About 50% of the fuel generated from photosynthesis is used in the mitochondria for cellular respiration in plants.  Necessary for production of cellulose, major component in plant cell walls and contributes immensely to their structure.  Plants are autotrophs, which means they produce their own food for energy and supplement. Consumers that feed on other organisms are called heterotrophs, and they are not able to make their own food so they get their energy from other organisms.  There are several important structures to note. Component Structure Function Chloroplast Double membrane bound Harvests light energy to convert organelle, containing some of into chemical energy its own DNA, enzymes, and pigments. Mesophyll Type of tissue found in the Mesophyll cells contain interior of a leaf chloroplasts Stomata Microscopic pores in the leaf CO2 and O2 enters and leaves cells the leave via these pores Stroma Dense fluid encased around the Suspends substances in the membrane of the chloroplast chloroplast; site of Calvin cycle Thylakoids Membranous sacs in chloroplast Serves as a third membrane system; separates stroma from thylakoid space; site of light reactions Chlorophyll Pigment protein in the Transfers electrons by chloroplast absorbing light and reflecting it to generate energy; gives plants green color  Equation for photosynthesis: 6 CO2 + 6 H2O -> C6H12O6 + 6 O2  Photosynthesis is a redox reaction, where CO2 is reduced and H2O is oxidized.  Electrons increase in potential energy as they move from water to sugar, so this process is an endergonic process that requires energy input to occur.  There are essentially two phases of photosynthesis: the light reactions and the Calvin Cycle.  The light reactions are the photo part of photosynthesis, converting solar energy in the form of visible light to chemical energy, while the Calvin cycle synthesizes the organic molecules using the energy retrieved by the light reactions. Light Reactions  The light reactions occur in the thylakoids in the chloroplast.  The pigments in the chloroplast absorb and reflect light based on their electromagnetic spectrum, and each pigment has a different ability to absorb certain wavelengths.  Chlorophyll a is the main pigment directly involved in the light reactions. It absorbs red and violet-blue light, while reflecting green light. It is responsible for receiving photons that send an electron into an excited state that can be used in the light reactions.  Chlorophyll b is an accessory pigment that has a slightly different composition than chlorophyll a, causing it to absorb light a little differently. This accessory pigment assists in the transfer of excited electrons to chlorophyll a, providing more energy to be utilized and transferred into the light reactions.  Carotenoids are other accessory pigments which absorb light at different wavelengths, and is responsible for photoprotection by absorbing and dismissing excess energy from the chloroplast to keep from damage to the cell.  Photosystems are reaction-center complexes surrounded by light harvesting complexes (chlorophyll a, chlorophyll b, and carotenoids attached to proteins), where light energy is absorbed and transferred from pigment to pigment until it reaches the reaction-center complex.  The reaction-center complex has a primary electron acceptor that accepts the electron and is reduced.  There are two types of photosystems in the chloroplast: Photosystem II (PS II) and Photosystem I (PS I)  Photosystem II occurs first in the light reactions, and has a special chlorophyll a molecule called P680, since it is best at absorbing light with wavelengths of 680nm. Photosystem I comes next, with special chlorophyll a molecules called P700 because it is best at absorbing wavelengths of 700nm.  P680+ is actually the strongest biological oxidizing agent known  The key to energy transfer in the light reactions between photosystems is the linear electron flow which generates ATP and NADH through a series of steps. 1. Photon of light strikes a pigment molecule in PS II, exciting an electron. The electron excitement stimulates the excitement of an electron in a neighboring pigment molecule as the initial electron falls back to ground state. 2. This continues until the electron reaches P680 chlorophyll a molecules, which transfers the electron to the primary electron acceptor, turning P680 to P680 . + 3. An enzyme then catalyzes the splitting of a water molecule into 2 electrons, 2 H+ ions, and an oxygen atom. The electrons, one by one, are supplied to P680+ to replace the electrons taken by the primary electron acceptor. H+ is released into thylakoid space, and oxygen binds with another oxygen to form O2. 4. Each photoexcited electron is passed from PS II to PS I via an electron transport chain. 5. The “fall” of each electron down the chain provides energy for ATP synthesis, and the H+ released at the cytochrome protein (electron acceptor in the ETC) in the ETC to form the energy gradient utilized through chemiosmosis. 6. Light energy has been harvested from pigments in PS I, where the P700 transfers the electron to the primary electron acceptor, making it P700+. The P700+ can accept electrons from the ETC connected to PS II. 7. Electrons from primary acceptor in PS I go down another ETC through the protein ferredoxin, but does not create a proton gradient, thus not generating ATP. 8. Enzyme NADP+ reductase catalyzes transfer of electrons from ferredoxin to NADP+, using 2 electrons to reduce it to NADPH.  In certain cases, electrons can take a different path only using PS I called cyclic electron flow. The electrons cycle back from ferredoxin to the cytochrome complex, then back to P700. It only generates ATP, nothing else. Calvin Cycle  The Calvin cycle is an anabolic cycle that builds carbohydrates by consuming energy.  The carbohydrate produced from the cycle is a 3-carbon sugar called glyceraldehyde 3- phosphate (G3P).  There are three phases of the Calvin cycle, keeping in mind that 3 CO2 molecules are followed through the reactions. 1. Carbon Fixation - Each CO2 molecule, one at a time, is attached to a 5-carbon sugar called ribulose bisphosphate (RuBP). Rubisco is the enzyme that catalyzes this reaction, forming a 6- carbon sugar intermediate that is so unstable that it almost immediately splits off into 2 molecules of 3-phosphoglycerate for each CO2 fixated. 2. Reduction - Each 3-phosphoglycerate molecule receives an addition phosphate group from ATP, becoming 1,3-bisphosphoglycerate. - Electron pair donated from NADPH to the 1,3-bisphosphoglycerate, removing a phosphate group and turning it into G3P. - For every 3 molecules of CO2 that enters the cycle, 6 molecules of G3P are formed. Only one molecule exists the cycle to be used by the cell, and the other 5 are joined back into the cycle to regenerate RuBP. 3. Regeneration of RuBP (CO2 acceptor) - Through a series of reactions, the carbon skeletons of the 5 G3P molecules are rearranged into 3 RuBP molecules in the last steps of the Calvin Cycle. - The cell spends 3 ATP on this process, and the cycle then continues.  The Calvin cycle consumes 9 ATP molecules and 6 NADPH molecules through one cycle  G3P is used to make glucose and other molecules. Alternative Carbon Fixation Methods  C3 Plants use the Calvin cycle as the initial step in incorporating CO2 into organic material, forming a 3-carbon sugar as an intermediate.  On hot, dry days when C3 plant’s stoma close, sugar production is low and access to CO2 is hard to come by, and O2 builds up in the cell. To account for this, plants use photorespiration when rubisco adds O2 to the cycle instead of CO2, generating a 2-carbon compound that leaves the chloroplast to be modified by peroxisomes and mitochondria and releases CO2.  Photorespiration uses ATP, but produces no sugar  C4 Plants endure preceding reactions to the Calvin cycle which incorporate CO2 in generating a 4-carbon sugar, and the end product supplies CO2 for the cycle.  There are 2 distinct photosynthetic cells in C4 plants: mesophyll and bundle sheath cells, which are arranged in tightly packed sheaths around the veins of the plant. The Calvin cycle is preceded by a series of steps occurring in mesophyll cells. 1. PEP Carboxylase enzyme adds CO2 to PEP molecules, forming 4-carbon product oxaloacetate. PEP carboxylase has a high affinity for CO2 and can fit effectively, even when stomas are closed and CO2 is low in the cell. 2. Exports 4-carbon product to bundle sheath cells via plasmodesmata. 3. In the bundle sheath cells, the 4-carbon compounds release CO2, which is reused to make organic materials by rubisco and the Calvin cycle. Bundle-sheath cells only contain PS I, so their only way of generating ATP to run this process is through cyclic electron flow.  The mesophyll cells pump CO2 into the bundle-sheath cells in order to keep the concentration of CO2 high enough so rubisco binds to CO2 and not O2.  C4 plants spend ATP to reduce photorespiration and produce more sugar output.  CAM Plants are adapted to arid areas, and utilize a form of photosynthesis where they open their stoma at night to obtain CO2 to be incorporated into organic acids, and release CO2 from organic compounds during the day for use in the Calvin cycle. These plants consist of succulents like cacti.  Similar to C4 in that the CO2 is incorporated into intermediates before entering the Calvin Cycle.


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