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Photosynthetic Process FREE ENERGY THE SUN: MAIN SOURCE OF (available for work) ENERGY FOR LIFE ON EARTH vs. HEAT (not available for work) THE BASICS OF PHOTOSYNTHESIS • Almost all plants are photosynthetic autotrophs (self producing), as are some bacteria and prtozoas – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in the form of chemical bonds (a) Mosses, ferns, and flowering plants (c) Euglena (d) Cyanobacteria Light Energy Harvested by Plants & Other Photosynthetic Autotrophs 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2 WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids). THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED • Chloroplasts absorb light Reflected Light light energy and convert it to chemical energy Absorbed light Transmitted Chloroplast light Photosynthesis occurs in chloroplasts • In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts • A chloroplast contains: – stroma, a fluid – grana, stacks of thylakoids • The thylakoids contain chlorophyll – Chlorophyll is the green pigment that captures light for photosynthesis • The location and structure of chloroplasts Chloroplast LEAF CROSS SECTION MESOPHYLL CELL LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Thylakoid Stroma Thylakoid compartment Thylakoid Thylakoid Membrane Thylakoid Space Granum Chloroplast Pigments • Chloroplasts contain several pigments – Chlorophyll a – Chlorophyll b (Chlorophyll a (alpha) absorbs well at a wavelength of about 450 nm but its primary absorption is at 675nm in the long red wavelengths. Chlorophyll b (beat) absorbs most effectively at blue 470 but also with shorter peaks at 430 and 640nm) – Carotenoids – Xanthophyll Fall Colors • During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments. • Carotenoids are pigments that are either red or yellow. Chlorophyll Molecules • Located in the thylakoid membranes. • Chlorophyll have Mg+ in the center. • Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red-660 nm are most important). • Plants are green because the green wavelength is reflected, not absorbed. Chlorophyll a & b •Chl a has a methyl group •Chl b has a carbonyl group Porphyrin ring delocalized e- Phytol tail Absorption of Chlorophyll Absorption violet blue green yellow orange red wavelength Different pigments absorb light differently AN OVERVIEW OF PHOTOSYNTHESIS • Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbon Water Glucose Oxygen dioxide gas PHOTOSYNTHESIS AN OVERVIEW OF PHOTOSYNTHESIS • The light reactions Light convert solar Chloroplast energy to chemical energy NADP – Produce ATP & NADPH ADP +P Calvin • The Calvin cycle makes Light reactions cycle sugar from carbon dioxide – ATP generated by the light reactions provides the energy for sugar synthesis – The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose Steps of Photosynthesis • Light hits reaction centers of chlorophyll, found in chloroplasts • Chlorophyll vibrates and causes water to break apart. • Oxygen is released into air • Hydrogen remains in chloroplast attached to NADPH • “THE LIGHT REACTION” Steps of Photosynthesis • The DARK Reactions= Calvin Cycle • CO2 from atmosphere is joined to H from water molecules (NADPH) to form glucose • Glucose can be converted into other molecules with different flavors! Redox Reaction • The transfer of one or more electrons from one reactant to another. • Two types: 1. Oxidation 2. Reduction Oxidation Reaction • The loss of electrons from a substance. • Or the gain of oxygen. Oxidation 6CO2 + 6H2O C6H12O6 + 6O2 glucose Reduction Reaction • The gain of electrons to a substance. • Or the loss of oxygen. Reduction 6CO2 + 6H2O C6H12O6 + 6O2 glucose • Two types of photosystems cooperate in the light reactions ATP mill Water-splitting NADPH-producing photosystem photosystem 1. Light Reaction (Electron Flow) • Occurs in the Thylakoid membranes • During the light reaction, there are two possible routes for electron flow. A. Cyclic Electron Flow B. Noncyclic Electron Flow A. Cyclic Electron Flow • Occurs in the thylakoid membrane. • Uses Photosystem II only • P700 reaction center- chlorophyll a • Uses Electron Transport Chain (ETC) • Generates ATP only ADP + P ATP Cyclic Photophosphorylation (addition of phosphate to ADP to make ATP.) • Process for ATP generation associated with some Photosynthetic Bacteria • Reaction Center => 700 nm Plants produce O2 gas by splitting H2O • The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-) In the light reactions, electron transport chains generate ATP, NADPH, & O2 • Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons • The excited electrons are passed from the primary electron acceptor to electron transport chains – Their energy ends up in ATP and NADPH Chemiosmosis powers ATP synthesis in the light reactions • The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane – The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP – In the stroma, the H+ ions combine with NADP+ to form NADPH Chemiosmosis SUN H+ H+ (Proton Pumping) Thylakoid E PS II T PS I C high H+ H+ H+ concentration H+ H+ H+ H+ Thylakoid H+ ATP Synthase Space ADP + P low H+ ATP H+ concentration B. Noncyclic Electron Flow • Occurs in the thylakoid membrane • Uses PS II and PS I • P680 rxn center (PSII) - chlorophyll a • P700 rxn center (PS I) - chlorophyll a • Uses Electron Transport Chain (ETC) • Generates O2, ATP and NADPH Noncyclic Photophosphorylation • Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product Primary electron acceptor Primary electron acceptor Photons Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis B. Noncyclic Electron Flow • ADP + ATP P (Reduced • NADP + H ) + NADPH (Reduced) • Oxygen comes from the splitting of H2O, not CO2 H 2O 1/2 O2 + 2H+ (Oxidized) How the Light Reactions Generate ATP and NADPH Primary NADP electron acceptor Energy Primary to make 3 electron acceptor 2 Light Light Primary electron acceptor Reaction- 1 center NADPH-producing chlorophyll photosystem Water-splitting photosystem 2 H + 1/2 Summary—Light Dependent Reactions a. Overall input light energy, H2O. b. Overall output ATP, NADPH, O2. Light Independent Reactions aka Calvin Cycle Carbon from CO2 is converted to glucose (ATP and NADPH drive the reduction of CO2 to C6H12O6.) Light Independent Reactions aka Calvin Cycle CO2 is added to the 5-C sugar RuBP by the enzyme rubisco. This unstable 6-C compound splits to two molecules of PGA or 3-phosphoglyceric acid. PGA is converted to Glyceraldehyde 3-phosphate (G3P), two of which bond to form glucose. G3P is the 3-C sugar formed by three turns of the cycle. Summary—Light Independent Reactions a. Overall input CO2, ATP, NADPH. b. Overall output glucose. Review: Photosynthesis uses light energy to make food molecules • A summary of Chloroplast the chemicalLight processes of photosynthesis Photosystem II Electron transport CALVIN chains CYCLE Stroma Photosystem I Cellular respiration Cellulose Starch Other LIGHT REACTIONS CALVIN CYCLE organic compounds Photorespiration (Competing Reactions) • Occurs under the following conditions: – Intense Light (high O2 concentrations, hot, dry, bright days) – High heat (Stomatas close) • Rubisco grabs CO2, “fixing” it into a carbohydrate in the light independent reactions. • O2 can also react with rubisco, inhibiting its active site – not good for glucose output – wastes time and energy (occupies Rubisco) • So Fixation of O2 instead of CO2. • Produces no sugar molecules or no ATP. • Photorespiration is estimated to reduce photosynthetic efficiency by 25% Types of Photosynthesis C3 C4 CAM Rubisco: the world’s busiest enzyme! Types of Photosynthesis • Certain plants have developed ways to limit the amount of photorespiration – C3 Pathway – C4 Pathway* – CAM (Crassulacean Acid Metabolism) Pathway* * Both convert CO2 into a 4 carbon intermediate C4 Photosynthesis C3 Photosynthesis : C3 plants • Called C3 because the CO2 is first incorporated into a 3-carbon compound. • Stomata are open during the day. • RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2. • Photosynthesis takes place throughout the leaf. • Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy).. • Most plants are C3. C4 Photosynthesis : C4 plants • Called C4 because the CO2 is first incorporated into a 4-carbon compound. • Stomata are open during the day. • Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis. • Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy) C4 Photosynthesis : C4 plants • Adaptive Value: – Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration. – Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis. • C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants. CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism • Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis. • Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis CAM Photosynthesis : CAM plants. • Adaptive Value: – Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). – May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells). • CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads Leaf Anatomy • In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells. Mesophyll cells Bundle sheath cells C4 Pathway • In C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells. C4 Pathway • CO2 is fixed into a 4- carbon intermediate • Has an extra enzyme– PEP Carboxylase (Phosphoenolpyruvat e carboxylase) that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate C4 Pathway • The 4 carbon intermediate is “smuggled” into the bundle sheath cell • The bundle sheath cell is not very permeable to CO2 • CO2 is released from the 4C malate goes through the Calvin Cycle C3 Pathway How does the C4 Pathway limit photorespiration? • Bundle sheath cells are far from the surface– less O2 access • PEP Carboxylase doesn’t have an affinity for O2 allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is) CAM Pathway • Fix CO2 at night and store as a 4 carbon molecule • Keep stomates closed during day to prevent water loss • Same general process as C4 Pathway How does the CAM Pathway limit photorespiration? • Collects CO2 at night so that it can be more concentrated during the day • Plant can still do the calvin cycle during the day without losing water CAM Plants Night (Stomates Open) Day (Stomates Closed) Vacuole C-C-C-C C-C-C-C C-C-C-C CO2 Malate Malate Malate CO2 C3 C-C-C ATP C-C-C glucose PEP Pyruvic acid Summary of C4 Photosynthesis • C4 Pathway – Separates by space (different locations) • CAM Pathway – Separates reactions by time (night versus day) Bio cell uses photosynthesis to generate electricity • Bio cell inserted into a cactus and a graph showing the intensity of the electric current generated as a function of light that fell on the plant (in black, glucose, and red, O2).Scientists at the research institute CNRS, France, changed the chemical energy generated by photosynthesis of a plant into electrical energy. • The research demonstrates a new route for artificial photosynthesis , a promising area of research that aims to develop a strategy for conversion of sunlight into electricity even more efficient and more environmentally friendly than solar cells . Artificial photosynthesis • Artificial photosynthesis is a research field that attempts to replicate the natural process of photosynthesis, converting sunlight, water, andcarbon dioxide into carbohydrates and oxygen. Sometimes, splitting water into hydrogen and oxygen by using sunlight energy is also referred to as artificial photosynthesis. Photoelectrochemical cell • Research is being done into finding catalysts that can convert water, carbon dioxide, and sunlight to carbohydrates. For the first type of catalysts, nature usually uses the oxygen evolving complex. Having studied this complex, researchers have made catalysts such as blue dimer to mimic its function, but these catalysts were very inefficient. Another catalyst was engineered by Paul Kögerler, which uses four ruthenium atoms. • The carbohydrate-converting catalysts used in nature are the hydrogenases. Catalysts invented by engineers to mimic the hydrogenasesinclude a catalyst by Cédric Tard, the rhodium atom catalyst from MIT, and the cobalt catalyst from MIT. Dr. Nocera of MIT is receiving funding from the Air Force Office of Scientific Research to help conduct the necessary experiments to push forward in catalyst research. Dye-sensitized solar cell Possibly the most exciting technological development in nanotechnology is a photovoltaic cell that uses photosynthesis to generate electricity. The first solar photovoltaic chip was made using ground-up spinach tissue by scientist Shuguang Zhang at MIT. He was building on work by a group of researchers who had earlier figured out how to harness energy from a plant. That group was able to extract electrical current using a plant’s photosynthesis for a period of three weeks. Zhang’s chip converted approximately 12% of the light energy absorbed to electrical current. This compares to the 24% efficiency of silicon power cells. In the future, it is hoped that by adding layers of chips, efficiency will be increased to 20%. size of this photosynthetic solar chip is Ten to twenty nanometers or, small enough to fit about a hundred of them in the width of a human hair. Would result in lightweight computers and other electronic devices, not to mention more environmentally friendly. Electricity Generation by Photosynthetic Biomass Advantages • Dye-sensitized cells can be made at one-fifth of the price of silicon cells. • The solar energy can be immediately converted and stored, unlike in PV cells, for example, which need to convert the energy and then store it into a battery (both operations implying energy losses). Furthermore, hydrogen as well as carbon-based storage options are quite environmentally friendly. • Renewable, carbon-neutral source of energy, which can be used for transportation or homes. Also the CO2 emissions that have been distributed from fossil fuels will begin to diminish because of the photosynthetic properties of the reactions. Disadvantages • Artificial photosynthesis cells (currently) last no longer than a few years[ (unlike PV and passive solar panels, for example, which last twenty years or longer). • The cost for alteration right now is not advantageous enough to compete with fossil fuels and natural gas as a viable source of mainstream energy.
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