Photosynthesis Final 29 PDF

Photosynthesis 🡪 🡪  Photosynthesis Photosynthesis is carried out by a number of different organisms All contain chlorophyll o Absorbs light energy and begins process of photosynthesis o Chlorophyll a (blue red) o Chlorophyll b (yellow green) o They absorb differen...

Photosynthesis 🡪 🡪  Photosynthesis Photosynthesis is carried out by a number of different organisms All contain chlorophyll o Absorbs light energy and begins process of photosynthesis o Chlorophyll a (blue red) o Chlorophyll b (yellow green) o They absorb different wavelengths of light  Prokaryotic Autotrophs: Cyanobacteria Make up largest group of photosynthesizing prokaryotes First organisms to harness the sun’s energy Unicellular, but may grow in colonies Live in many different environments Grow rapidly in nutrient rich water (known to cause algal blooms) o can be toxic to fish, birds, humans, and other mammals Eukaryotic Autotrophs: Algae, Photosynthetic Protists, and Plants Unlike cyanobacteria, algae, some protests, and plant cells contain chlorophyll within the photosynthetic membranes of discrete organelles called chloroplasts o gives leaves, stems and un-ripened fruit their characteristic green colour!  Plant Parts...Review! Colour Code! Leaves o The Factory! Transpiration o Cooling and Transport Stomata o The Gate Keepers  Chloroplasts Have two limiting membranes (outer and inner) enclosing an interior space filled with a liquid stroma Within the stroma, a system of membrane bound sacs called thylakoids stack on top of one another to form characteristic columns called grana o typical chloroplast has 60 grana each consisting of 30-50 thylakoids Inside the thylakoids is a water filled space called the Lumen Chloroplasts have their own DNA and ribosomes, reproduce via fission Stroma contains enzymes that catalyze the conversion of CO2 and water into carbohydrates 7 Chloroplasts http://www.youtube.com/watch?v=Oi2 _n2wbB9o&feature=related  Photosynthesis Converts light energy into the chemical bonds of glucose (plants are Autotrophs). It is an endergonic rxn that happens in two stages: The Light Dependant Rxn and the Light-Independent Rxn. The overall rxn is: 6CO2 + 6H2O + Light Energy 🡪 C6H12O6 + 6O2 Glucose  The Light Dependent Reaction Occurs in the Thylakoid Membrane The chlorophyll captures light energy and uses it to break down water molecules and create ATP and NADPH. The oxygen is released as a product. The ATP and NADPH are carried over to the Light Independent Rxn.  The Light Independent Reaction Occurs in the stroma CO2 from the air is added to the H+ ions (carried by NADPH) the glucose is formed  A General Overview http://www.youtube.com/watch?v=cX3Iev-JDgM  Tracking Atoms! The sugars that result from photosynthesis are produced by adding the hydrogen ions and electrons from water to carbon dioxide, NOT by splitting CO2 and adding water! oxidized H+ and e- 6CO2 + 12H2O 🡪 C6H12O6 + 6O2 + 6H2O reduced Light Dep. Rxn - In Depth Involves the splitting of water (photolysis) 12 H2O + Energy 🡪 6 O2 + 24 H+ + 24e- Requires light for the energy to split the water molecule Also forms a highly energetic compound, adenosine triphosphate (ATP) 3 Parts: o 1. PHOTOEXCITATION o 2. ELECTRON TRANSPORT o 3. CHEMIOSMOSIS  A Virtual Tour http://vcell.ndsu.nodak.edu/animations/photosynthesi s/movie-flash.htm  Pigments Chlorophyll A is the most important photosynthetic pigment. Other pigments called antenna or accessory pigments are also present in the leaf. o Chlorophyll B o Carotenoids (orange / red) o Xanthophylls (yellow / brown) These pigments are embedded in the membranes of the chloroplast in groups called photosystems. Each pigment absorbs a particular wavelength of light in the visible spectrum Pigment Absorption What wavelengths of light do you think plants use the least in photosynthesis? Photosystems use some wavelengths of light but reflect others…  Photoexcitation Photon = Packets of electromagnetic radiation Occurs in Photosystems (clusters of photosynthetic pigments embedded in the thylakoid membranes Photosystems absorb photons of particular wavelengths Consist of an antenna complex and rxn centre Two types: 700, 680 refers to wavelength absorbed (nm) o PS I (P700) o PS II (P680) Photosystems  Photoexcitation Antenna complex absorbs a photon of light Transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule in the centre of the rxn centre The electron of this chlorophyll moves from ground state to a higher potential energy level (excitation)  Non-Cyclic Electron Flow ETC! 1. Photon strikes PS II, excites electron o Usually, excited electron is captured by the primary electron acceptor (REDOX rxn) o P680 missing electron o P680 + pull s electrons from water Z protein associated with PS II splits water into oxygen, protons, electrons o Z protein holds two water molecules in place as an enzyme strips four electrons o Electrons are passed on to the electron carrier o Oxygen leaves chloroplast as a by-product o Protons remain in Thylakoid space, contributing to to gradient that drives chemosmosis 25  Non-Cyclic Electron Flow Electrons move from the electron acceptor by electron carrying molecules As electrons move they release energy and b6-f complex pumps H+ from the stroma, across the thylakoid membrane into the thylakoid space This creates a electrochemical gradient Light energy is absorbed by photosystem 1 Energy is transferred to the reaction centre P700 molecule Electrons pass onto another electron acceptor Lost electrons are replaced by electrons from photosystem II 27 Electrons from photosystem I are used by enzyme NADP reductase to reduce NADP+ to form NADPH 28  Non-Cyclic Electron Flow Chemiosmosis – Protons that have accumulated in the Thylakoid space creating an electrochemical gradient that drives phosphorylation of ADP to ATP o Called photophosphorylation, since light is required  Light Independent Rxn Calvin Cycle Also called C3 photosynthesis (since first compound formed is a 3C molecule Occurs in stroma of chloroplasts Cyclic series of reactions Three phases: carbon fixation, REDOX reactions, RuBP (ribulose 1,5-bisphosphate) regeneration  Calvin Cycle Phase 1: Carbon Fixation o CO2 are added to RuBP, forming unstable 6C intermediate molecules o Each of these instantly splits into two 3C molecules called PGA (3 phosphoglycerate) o The enzyme that catalyses these reactions is called Rubisco, a very large enzyme that works very slowly, most abundant protein on earth Phase 2: REDOX o Each PGA is phosphorylated, is activated by ATP and then reduced by NADPH o 6 G3P (glyceraldehyde 3 – phosphate) are made o One molecules of G3P exits as a final product 33  Calvin Cycle Phase 3: RuBP regeneration o The 5 G3P are rearranged to regenerate 3 RuBP o 3 ATP are used! The Bottom Line o For the net synthesis of one G3P, 9 ATP and 6 NADPH are used!  Overview In order for one G3P to exit the cycle (and go towards glucose synthesis), three CO2 molecules must enter the cycle, providing three new atoms of fixed carbon. When three CO2 molecules enter the cycle, six G3P molecules are made. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate three molecules of the RuBP acceptor. 35  Two Cycles of Calvin Carbon. 6 combine with 6 RuBP acceptors, making 12 molecules of glyceraldehyde-3-phosphate (G3P). 2 G3P molecules exit the cycle and goes towards making glucose. 10 G3P molecules are recycled, regenerating 6 RuBP acceptor molecules. 36 ATP- 18 ATP are converted to 18 ADP (12 during the fixation step, 6 during the regeneration step). NADPH. 12 NADPH are converted to 12 NADP+ (during the reduction step). A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. I 37  G3P Is the primary end product of photosynthesis It may be converted into glucose and polymerized into starch w/in the stroma Or it may be transported to the cytoplasm and used to produce glucose and sucrose. Sucrose is the main carb transported from mesophyll cells of the leaf to other cells of the plant Alternative Methods of Carbon Fixation Rubisco – the double agent! Normally rubisco adds CO2 to RuBP but when O2 is very plentiful it adds it to RuBP This is called Photorespiration – it occurs in light! It removes PGA molecules from the Calvin cycle Produces phosphoglycolate Decrease CO2 fixation and less sugar formed Rubisco is an evolutionary remnant (from a time in earth’s history when O2 was not a prevalent Some plants have adapted strategies to work around this! Rubisco - The Double Agent! HELPFUL! ANNOYING!! 41  C4 Plants Hot, tropical grasslands and savannahs C4 photosynthesis – a pathway of carbon fixation that reduces the amount of photorespiration that takes place by continually pumping CO2 molecules from mesophyll cells into bundle sheath cells, where rubisco brings them into the C3 Calvin cycle C4 refers to the four carbon oxaloacetate that is formed in the mesophyll CO2 level in leaf is increased, no photoresp. 42 CO2 and PEP make Oxaloacetate Oxaloacetate -> Malate (decarboxylated) -> pyruvate and CO2 43 C4 Plants 44  CAM Plants Crassulacean Acid Metabolism Plants in dry, desert environments Stomata open at night to take in CO2 and incorporate it into organic acids, and close during the day to allow the acids to release CO2 into the Calvin Cycle. 45 46