Photosynthesis - Session 4 PDF

THE BASICS OF PHOTOSYNTHESIS Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter Chapter 4: through photosynthesis (form inorganic matter:CO2)....

THE BASICS OF PHOTOSYNTHESIS Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter Chapter 4: through photosynthesis (form inorganic matter:CO2). – Sunlight is transformed to energy stored in the form Photosynthesis of chemical bonds (c) Euglena (d) Cyanobacteria (b) Kelp (a) Mosses, ferns, and flowering plants Photosynthesis Why is Photosynthesis important? Makes organic molecules (glucose) out of inorganic materials (carbon dioxide and water). It begins all food chains/webs.Thus all life is supported by this process. It also makes oxygen gas. Without photosynthesis there would be little to no oxygen on the planet. Photosynthesis-starts to ecological food webs! WHY ARE PLANTS GREEN? Different wavelengths of visible light are seen by the human eye as different colors. Gamma Micro- Radio X-rays UV Infrared rays waves waves Visible light Wavelength (nm) WHY ARE PLANTS GREEN? WHY ARE PLANTS GREEN? Plants are green because of Plant Cells the electromagnetic have Green spectrum, energy and Chloroplasts “special pairs” of chlorophyll molecules in each plant cell. The thylakoid membrane of the As such, plants look green chloroplast is impregnated with because they absorb red photosynthetic light most efficiently and pigments (i.e., chlorophylls, the green light is reflected. carotenoids).  THE COLOR OF LIGHT SEEN IS THE COLOR PHOTOSYNTHESIS NOT ABSORBED Absorbing Light Energy to make Chloroplasts chemical energy: glucose. absorb light Reflected – Pigments: Absorb different colors of energy and Light light light convert it to – Main pigment: Chlorophyll a chemical energy Accessory pigments: Chlorophyll b and Carotenoids These pigments absorb all wavelengths Absorbed light (light) BUT not green! Transmitted Chloroplast light Photosynthesis means "putting together Chloroplasts: Sites of Photosynthesis with light Plants use sunlight to turn water Photosynthesis and carbon dioxide into – Occurs in chloroplasts, organelles in certain glucose. plants. – All green plant parts have chloroplasts and carry Plants use glucose as food for out photosynthesis energy and as a building block The leaves have the most chloroplasts for growing. The green color comes from chlorophyll in the chloroplasts Autotrophs make glucose and The pigments absorb light energy. heterotrophs are consumers of it.  Photosynthesis occurs in chloroplasts Chloroplast Pigments In most plants, photosynthesis occurs Chloroplasts contain several pigments – Chlorophyll a primarily in the leaves, in the chloroplasts – Chlorophyll b – Carotenoids A chloroplast contains: – stroma, a fluid – Different pigments absorb light differently – grana, stacks of thylakoids The thylakoids contain chlorophyll – Chlorophyll is the green pigment that captures light for photosynthesis The location and structure of chloroplasts Chlorophyll a & b Chloroplast Chl a has a methyl MESOPHYLL CELL LEAF LEAF CROSS SECTION group Mesophyll Chl b has a carbonyl group Porphyrin ring delocalized e- CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Thylakoid Stroma Thylakoid compartment Phytol tail  AN OVERVIEW OF PHOTOSYNTHESIS: Two- Fall Colors Step reaction The light reactions During the fall, the green chlorophyll pigments are convert solar Light Chloroplast greatly reduced revealing the other pigments. energy to chemical energy NADP+ – Produce ATP & NADPH ADP Carotenoids are pigments that are either +P The Calvin cycle makes Light Calvin cycle red or yellow. sugar from carbon reactions dioxide Plants must continually replace their chlorophyll. – ATP generated by the light In response to shorter days in the fall, plants reactions provides the energy for sugar synthesis produce less chlorophyll. But existing chlorophyll – The NADPH produced by the molecules break down at a steady rate. Therefore, light reactions provides the in the fall, the amount of chlorophyll in plants electrons for the reduction of carbon dioxide to decreases and the green color fades. glucose AN OVERVIEW OF PHOTOSYNTHESIS Redox Reaction Photosynthesis is the process by which The transfer of one or more electrons from one autotrophic organisms use light energy reactant to another. to make sugar and oxygen gas from carbon dioxide and water. Two types: 1. Oxidation 2. Reduction Carbon Water Glucose Oxygen dioxide gas PHOTOSYNTHESIS  PHOTOSYNTHESIS Oxidation Reaction 2 Phases The loss of electrons from a – Light-dependent reaction substance. – Light-independent reaction Or the gain of oxygen. Light-dependent: converts light energy Oxidation into chemical energy; produces ATP and NADPH molecules to be used to fuel 6CO2 + 6H2O → C6H12O6 + 6O2 light- independent reaction glucose Light-independent: uses ATP produced to make simple sugars/ glucose Reduction Reaction Steps of Photosynthesis The gain of electrons to a substance. “THE LIGHT REACTION” Or the loss of oxygen. Light hits reaction centers of chlorophyll, found in chloroplasts. Chlorophyll vibrates and causes water Reduction to break apart. 6CO2 + 6H2O → C6H12O6 + 6O2 Oxygen is released into air glucose Hydrogen remains in chloroplast attached to NADPH  Steps of Photosynthesis 1. Light Reaction (Electron Flow) The DARK Reactions= Calvin Cycle Two types of photosystems CO2 from atmosphere is joined to H from cooperate in water molecules (NADPH) to form the light glucose reactions ATP Glucose can be converted into other mill molecules. Water-splitting NADPH-producing photosystem photosystem PHOTOSYNTHESIS 1. Light Reaction Light-dependent reaction (LIGHT (Electron Flow) Reaction) Occurs in the thylakoid membrane – Requires light – Occurs in chloroplast (in thylakoids) Uses PS II and PS I – Chlorophyll (thylakoid) traps energy from P680 rxn center (PSII) - chlorophyll a light – Light excites electron (e-) P700 rxn center (PS I) - chlorophyll a Kicks e- out of chlorophyll to an electron transport Uses Electron Transport Chain (ETC) chain Electron transport chain: series of proteins in Generates O2, ATP and NADPH thylakoid membrane – Total byproducts: ATP, NADPH, O2  1. Light Reaction Plants produce O2 gas by splitting H2O (Electron Flow) Primary The O2 liberated by photosynthesis is made Electron Acceptor 2e- from the oxygen in water (H+ and e-) Enzyme Primary Reaction 2e- Electron Acceptor 2e- ETC SUN 2e- 2e- NADPH P700 Photon ATP P680 Photon H2O Photosystem I 1/2O2 + 2H+ Photosystem II Photosystem II (or water- plastoquinone oxidoreductase) 1. Light Reaction (Electron Flow) is the first protein complex in the light-dependent reactions Photosystem II regains electrons by splitting of oxygenic photosynthesis. It water, leaving O2 gas as a by-product is located in the thylakoid membrane of plants. It Primary captures photons and uses the electron acceptor energy to extract electrons Primary from water molecules. As these electron acceptor electrons flow down the chain, they are used to pump hydrogen ions across the membrane, providing even Photons more power for ATP synthesis.. Plastoquinone is one of the electron acceptors associated with Photosystem Photosystem I is one of two II in photosynthesis. It accepts two photosystems in the photosynthetic electrons and is reduced to Energy for light reactions of plants. synthesis of Plastoquinol and as such acts as an Photosystem I is an integral electron and energy carrier in the PHOTOSYSTEM I membrane protein complex that electron transport process. uses light energy to catalyze the Plastocyanin is a copper-containing transfer of electrons across the protein that mediates electron-transfer. PHOTOSYSTEM II by chemiosmosis thylakoid membrane.  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 Light absorption in PSII. When light is absorbed by one of the many harnessed by ATP synthase to make ATP pigments in photosystem II, energy is passed inward from pigment to pigment until it reaches the reaction center. There, energy is – In the stroma, the H+ ions combine with NADP+ transferred to P680 (PSII primary donor), boosting an electron to a to form NADPH high energy level. The high-energy electron is passed to an acceptor molecule and replaced with an electron from water. This splitting of water releases the oxygen we breathe. The production of ATP by chemiosmosis in ATP synthesis. The high-energy electron travels down an electron transport chain, losing energy as it goes. Some of the released energy drives pumping photosynthesis of H+ ions from the stroma into the thylakoid interior, building a gradient. H+ ions flow down their gradient and into the stroma, they pass through ATP synthase, driving ATP production in a process known as chemiosmosis. Thylakoid compartment (high H+) Light Light Light absorption in PSI. The electron arrives at photosystem I and joins the P700 (PSI primary donor) special pair of chlorophylls in the reaction center. When light energy is absorbed by pigments and passed inward to the Thylakoid reaction center, the electron in P700 is boosted to a very high energy membrane level and transferred to an acceptor molecule. The special pair's missing electron is replaced by a new electron from PSII (arriving via the electron Antenna transport chain). molecules NADPH formation. The high-energy electron travels down a short second leg of the electron transport chain. Stroma At the end of the chain, the electron is passed to NADP (low H+) ELECTRON TRANSPORT CHAIN + to make NADPH. PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE  PHOTOSYNTHESIS Chloroplast Light-independent reaction (Dark Reaction) – Does not require light Stroma – Calvin Cycle Outer Membrane Thylakoid Granum Inner Membrane Occurs in stroma of chloroplast Requires CO2 Uses ATP and NADPH as fuel to run Makes glucose sugar from CO2 and Hydrogen Calvin Cycle Calvin cycle: Light-independent reaction Carbon Fixation (light independent rxn). C3 plants (80% of plants on earth). Occurs in the stroma. Uses ATP and NADPH from light rxn. Uses CO2. To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH. The majority of plants and crop plants are C3 plants, referring to the fact that the first carbon compound produced during photosynthesis contains three carbon atoms  Light Independent Reactions 2 molecules Carbon from CO2 is converted to glucose (ATP and NADPH drive the reduction of CO2 to C6H12O6.) Light Independent Reactions Light Independent Reactions The Calvin cycle reactions can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO2, two other chemicals are present to Calvin Cycle initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end. CO2 is added to the 5-C sugar RuBP by the RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound enzyme rubisco. that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is “fixed” from its inorganic form into organic molecules. This unstable 6-C compound splits to two ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into molecules of PGA or 3-phosphoglyceric acid. another three-carbon compound called G3P. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized. PGA is converted to Glyceraldehyde 3-phosphate One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the (G3P). carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation G3P is the 3-C sugar formed by three turns of the step. ATP is also used in the regeneration of RuBP. cycle. Summary—Light Independent Reactions PHOTOSYNTHESIS What affects photosynthesis? – Temperature: Temperature Low = Rate of photosynthesis low a. Overall input CO2, ATP, NADPH. Temperature Increases = Rate of photosynthesis b. Overall output glucose. increases If temperature too hot, rate drops PHOTOSYNTHESIS Photorespiration What affects photosynthesis? – Carbon Dioxide: As CO2 Occurs on hot, dry, bright days. increases, rate of photosynthesis increases Stomata close. – Light intensity: as light Fixation of O2 instead of CO2. increases, rate of photosynthesis increases Produces 2-C molecules instead of 3-C sugar molecules. Produces no sugar molecules or no ATP.  Photorespiration Competing Reactions Because of photorespiration: Plants have Rubisco grabs CO2, “fixing” it into a special adaptations to limit the effect of carbohydrate in the light independent photorespiration. reactions. O2 can also react with rubisco, inhibiting its 1. C4 plants active site – not good for glucose output 2. CAM plants – wastes time and energy (occupies Rubisco) C4 Plants Photorespiration (C2 photosynthesis) When Rubisco reacts with O2 instead of Hot, moist environments. CO2 15% of plants (grasses, corn, sugarcane). Occurs under the following conditions: – Intense Light (high O2 concentrations) Divides photosynthesis spatially. – High heat Light rxn - mesophyll cells. Photorespiration is estimated to reduce Calvin cycle - bundle sheath cells. photosynthetic efficiency by 25% A four-carbon compound is produced (malate) Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. When does photorespiration occur? Leaf Anatomy In C3 plants (those that do C3 photosynthesis), all When it is hot, plants close their processes occur in the mesophyll cells. stomata to conserve water They continue to do photosynthesis  Mesophyll cells use up CO2 and produce O2  creates high O2 concentrations inside the plant Bundle sheath  photorespiration occurs cells Bundle sheath cells form a sheath around the entire vascular tissue in plants leaves and constitute a distinct leaf cell type, as defined by their elongated morphology, their position adjacent to the vein and by differences in their chloroplast development compared to mesophyll cells. C4 Pathway C4 Photosynthesis Certain plants have developed ways to limit the amount of photorespiration In C4 plants, photosynthesis occurs – C4 Pathway* in both the mesophyll – CAM Pathway* and the bundle sheath * Both convert CO2 into a 4 carbon cells. intermediate  C4 Photosynthesis  C4 Pathway How does the C4 Pathway limit photorespiration? CO 2is fixed into a 4- Bundle sheath cells are far from the carbon intermediate surface– less O2 access Has an extra PEP Carboxylase doesn’t have an enzyme– PEP affinity for O2  allows plant to collect a Carboxylase that lot of CO2 and concentrate it in the initially traps CO2 bundle sheath cells. instead of Rubisco– makes a 4 carbon intermediate PEP: Phosphoenolpyruvate carboxylase C4 Pathway CAM Pathway Fix CO at night and 2 The 4 carbon intermediate store as a 4 carbon is “smuggled” into the bundle sheath cell molecule The bundle sheath cell is Keep stomates not very permeable to closed during day to CO2 prevent water loss CO2 is released from the 4C malate goes through Same general the Calvin Cycle.  C3 Pathway process as C4 Pathway  Summary of C4 Photosynthesis C4 Pathway – Separates by space (different locations) CAM Pathway – Separates reactions by time (night versus day)