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Photosynthesis What is photosynthesis? What is it for? Where does it occur? (Always?) What are the two major processes? How much detail will you need? How can we know if photosynthesis is taking place? Variations on a theme…. Refer to chapters 6 and 7 in text. What is photosynthesis? “Photosynthesis involves the conversion of light energy into chemical energy.” The primary storage of this chemical energy will be in 6 carbon sugars. ATP will be used as an intermediate in this solar-to-carb energy transfer. What is it for? ATP will ultimately be re-charged when energy is released from the carbs in cellular respiration when the cell has business to do. far red↑ - Visible light is one part of the electromagnetic spectrum. - The strongest absorption is on blue-violet (400-500nm) and near red (650-700). - The middle of the spectrum – green – is NOT absorbed: It is reflected. Hence, plants appear green. http://www.uic.edu/classes/bios/bios100/lecturesf04am/absorption-spectrum.jpg The absorption spectrum of selected plant pigments: http://www.mrothery.co.uk/images/Imag53.gif The action spectrum of an unspecified plant: - This shows the rate of photosynthesis relative to light wavelength. - Does the pattern look familiar? If so (it better!), why? Why is it not just like… that other spectrum? Where does photosynthesis occur? - The chloroplast is the site of photosynthesis in eukaryotes. - Two chief parts: “photo-” light-dependent reactions in thylakoids, “-synthesis” light-independent reactions (aka Calvin cycle) in stroma. - note ATP/ADP and NADPH/NADP+ bridging the two - note two photosystems (II comes before I) for light energy capture. - note electron transport chain… (See! Aren’t you glad you made the effort in respiration!) http://www.55a.net/firas/ar_photo/11/photosynthesis-overview.gif What are those two major processes, again? 1. Light-dependent reactions -An e- is energized, and handed off to the electron transport chain. - Water split, providing e- to replace them, with O2 off as waste. This is photolysis. - plastoquinone in ETC pumps protons into the thylakoid lumen… continued next slide http://cache.eb.com/eb/image?id=72207&rendTypeId=35 -Light absorbed by chlorophyll a in photosystem II: -Light absorbed by chlorophyll a in photosystem I: ← NB -2 e- are energized, and handed off to NADP+, which is reduced to NADPH. - The proton motive force powers ATP synthase, in photophosphorylation, using chemiosmosis. -ATP and NADPH head off to the Calvin cycle. http://cache.eb.com/eb/image?id=72207&rendTypeId=35 1. Light-dependent reactions (cont.) 1. Another view of the light-dependent reactions - This adds the free energy of the reactants. - Different wavelengths stimulate PSII (680) and PSI (700). - e- -accepting intermediates after PSI - ALSO, the “cyclic pathway”: http://www.agen.ufl.edu/~chyn/age2062/lect/lect_04/7_11.gif Cyclic photophosphorylation: Instead of passing energized e- to NADP+, some are sent back to the ETC: This pumps out more H+ (therefore more ATP) without making NADPH, because more ATP than NADPH is needed in the Calvin cycle! (That default path is called non-cyclic photophosphorylation.) Deep breath. Recap. What’s the other process, again? 2. Light-independent reactions - Carbon fixation: - CO2 is attached to RuBP by Rubisco* (enzyme). (H comes from light-dependent reactions.) - The intermediate falls into two 3C units (3PG), which are reduced by ATP and NADPH. - every third time around a G3P, aka TP (a triose phosphate) is spun off, which then may become glucose or other organic molecules. - Another ATP is invested to replace the original RuBP. * “probably the most abundant protein on Earth” Not to panic: Here is the “Magical, Magical, POOF” The regeneration stage can be broken down into steps. 1. Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule. 2. Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6phosphate (6C). A phosphate ion is lost into solution. 3.Then fixation of another CO2 generates two more G3P. 4. F6P has two carbons removed by transketolase, giving erythrose-4-phosphate. The two carbons on transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P). 5. E4P and a DHAP (formed from one of the G3P from the second CO2 fixation) are converted into sedoheptulose-1,7-bisphosphate(7C) by aldolase enzyme. 6. Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle that are unique to plants) cleavessedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution. 7. Fixation of a third CO2 generates two more G3P. The ketose S7P has two carbons removed by transketolase, giving ribose-5-phosphate (R5P), and the two carbons remaining on transketolase are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 CO2, with generation of three pentoses that can be converted to Ru5P. 8. R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase. Xu5P is converted into RuP byphosphopentose epimerase. 9. Finally, phosphoribulokinase (another plant-unique enzyme of the pathway) phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP. Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons), with only one G3P available for subsequent conversion to hexose. This requires nine ATP molecules and six NADPH molecules per three CO2 molecules. (courtesy Wikipedia) OR, LOOK AT THIS → http://www.youtube.com/watch?v=JWxCfWOqIDc&safety_mode=true&persist_safety_mode=1&safe=active http://bancroft.berkeley.edu/Exhibits/Biotech/calvin.html 'There is no such thing as pure science. By this I mean that physics impinges on astronomy on the one hand, and chemistry and biology on the other. The synthesis of a really new concept requires some sort of union in one mind of the pertinent aspects of several disciplines....It's no trick to get the right answer when you have all the data. The real creative trick is to get the right answer when you have only half of the data in hand and half of it is wrong and you don't know which half is wrong. When you get the right answer under these circumstances, you are doing something creative. From Calvin’s autobiography Following the Trail of Light: A Scientific Odyssey (1992): What is used in the light-dependent reactions? What is produced? What is used in the light-independent reactions? What is produced? Make sure you write down the answers! intermembrane space outer membrane stroma inner membrane lumen granum thylakoid lamella “space” in Mader http://upload.wikimedia.org/wikipedia/commons/1/11/Chloroplast-new.jpg Draw and annotate a diagram showing the structure of a chloroplast as seen in an electron micrograph. Why are thylakoids so thin? Why are there so many of them? Why the ‘empty’ (fluidfilled) space around the grana? Where are chloroplasts found? Why are they green? Where is the light actually absorbed? botit.botany.wisc.edu/.../Chloroplast_EN.html How can we know if photosynthesis is taking place? 6 CO2 + 6 H2O → C6H12O6 + 6 O2 ------------------------------------------------------------------- Directly: 1. Measure the CO2 consumed. 2. Measure the O2 generated. Indirectly 3. Determine the net increase in biomass. 4. Measure the reduction of NADP+ by providing a colorimetric substitute for NADP+ (This is what you do in the AP lab.) What do you think would influence the rate of photosynthesis? What would be the effect…, and why? light? (intensity? duration?) temperature? pH? CO2 levels? .... www.climateaudit.org/?p=884 http://www.marietta.edu/~spilatrs/biol103/photolab/rspCrve.gif generalhorticulture.tamu.edu/.../light.html Variations on a theme…. Oxygen actually binds to Rubisco more readily than does CO2, so in low CO2 (like stomata closed to conserve water) O2 is taken into the Calvin cycle, and CO2 is released: Called photorespiration, but generates no ATP. Ouch. Dry environments cause problems for photosynthesis: Two specialized alternative photosynthetic adaptations that you will want to know… C4 pathway: In arid climates is helps to be able to just crack the windows: In order to increase the carbon locally available to the Rubisco carbon is first fixed, more aggressively, to PEP by PEP carboxylase, resulting in a 4-C molecule. This is carried to bundle sheath cells around the leaf vein, (which is where the Calvin cycle occurs in these plants) and handed off into the Calvin cycle. About 7600 species of plants use C4 carbon fixation, which represents about 3% of all terrestrial species of plants. All these 7600 species are angiosperms. C4 carbon fixation is less common in dicots than in monocots, with only 4.5% of dicots using the C4 pathway, compared to 40% of monocots. ... Fortysix percent of grasses are C4 and together account for 61% of C4 species. These include the food crops maize, sugar cane, millet, and sorghum. (Wikipedia) CAM pathway: www.steve.gb.com/science/photorespiration.html Another route is to simply close the windows during the day: By attaching the CO2 to some other organic molecule at night, and then releasing it to Rubisco in the presence of light, while the stomata are closed: In both C4 and CAM carbon is first fixed by some other molecule: - is C4 this is separated from Calvin by space (structural). - in CAM they are separated by time. The majority of plants possessing CAM are either epiphytes (e.g., orchids, bromeliads) or succulent xerophytes (e.g., cacti…), but CAM is also found in …[a scattering of others]. Plants which are able to switch between different methods of carbon fixation include … Dwarf Jade Plant, which normally uses C3 fixation but can use CAM if it is drought-stressed,[and … Purslane, which normally uses C4 fixation but is also able to switch to CAM when drought-stressed. CAM has evolved convergently many times. It occurs in 16,000 species (about 7% of plants), belonging to over 300 genera and around 40 families, but this is thought to be a considerable underestimate. It is found in quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the great majority of plants using CAM are angiosperms (flowering plants). (Wikipedia) Welwitschia ↑: Google this one! Rafflesia arnoldii, producing the world’s largest flower, doesn’t photosynthesize. ↑http://scienceray.com/biology/earths-largest-flower/ Wikipedia → The flower of Rafflesia arnoldii grows to a diameter of around one meter (3 ft) and weighing up to 11 kilograms (24 lb).[citation needed] It lives as a parasite on the Tetrastigma vine, which grows only in primary (undisturbed) rainforests.Rafflesia lacks any observable leaves, stems or even roots, yet is still considered a vascular plant. Similar to fungi, individuals grow as thread-like strands of tissue completely embedded within and in intimate contact with surrounding host cells from which nutrients andwater are obtained. This plant produces no leaves, stems or roots and does not have chlorophyll. It can only be seen when it is ready to reproduce. Perhaps the only part of Rafflesiathat is identifiable as distinctly plant-like are the flowers; although, even these are unusual since they attain massive proportions, have a reddish-brown coloration and stink of rotting flesh, which is why it was nicknamed the "carrion flower". This scent attracts insects such asflies which then pollinate the rare plant. How do reactant molecules reach the chloroplasts in leaves? To make one molecule of glucose takes how much ATP and NADPH? Compare C4 and CAM pathways to each other, and to the C3 (conventional) pathway. In the light reactions, what is the electron donor? Where do the electrons end up? How might you measure photosynthesis? Describe how the two stages of photosynthesis are dependent on each other Relate chloroplast form to function. photosynthesis chemiosmosis absorption spectrum cyclic photophosphorylation pigment non-cyclic photophosphorylation action spectrum RuBP chloroplast rubisco light-dependent reactions 3PG thylakoid G3P light-independent reactions TP Calvin cycle outer membrane stroma inner membrane photosystems lamella electron transport chain intermembrane space chlorophyll a granum photosystem II: 6 CO2 + 6 H2O → C6H12O6 + 6 O2 photolysis photorespiration thylakoid lumen PEP photosystem I PEP carboxylase ATP synthase bundle sheath cells photophosphorylation, CAM pathway C4 pathway