Lecture 8 1 Summarize the importance of compartmentation in plant metabolism and give examples of processes specific to different organelles compartments Biochemical reactions that sustain life are extremely diverse Secondary metabolites are not directly required for life ie systems that make rubber and other products we use Compartmentation helps facilitate these diverse metabolisms which are dependent upon highly regulated enzymes and communication pathways coarse control changes slowly hours days and controls abundance of proteins in the cell fine control changes quickly seconds minutes and controls the extent to which each protein is active Flux is the general dynamic and interactive regulation down an enzyme pathway Process examples light driven ATP and NADPH synthesis occur in the Thylakoid Photorespiratory reactions occur in Peroxisome sugar accumulation in Vacuole etc 2 Describe the basic reactions of oxygenic photosynthesis and describe how it is different from non oxygenic photosynthesis 6CO2 12H2O light C6H12O6 6O2 6H2O Water is the electron donor Plants algae and cyanobacteria exhibit this type of photosynthesis Non oxygenic photosynthesis uses sulfur instead of oxygen and occurs in Purple Bacteria early mechanism without O2 Photosynthesis takes place in the Chloroplast Light reactions and the Calvin Cycle are important steps in the process of oxygenic photosynthesis 3 Explain how photosynthesis in multicellular plants encompasses the capture of light energy synthesis of reducing agents and ATP and assimilation of carbon and how these coordinated processed are regulated Calvin Cycle 3CO2 3 ribulose 1 5 biphosphate 9ATP 6NADPH 6 triose phosphates o Common to all photosynthetic organisms Main reaction is the CO2 CH2O Overall this cycle generates 3C sugars Main enzyme is called Rubisco which is the most abundant protein on earth Calvin cycle needs a lot of ATP to function so it s very energetically expensive It is a redox reaction lose e oxidized gain e reduced This cycle must be maintained Overall products NADPH NADP and triose phosphate o Rubisco appears in the carboxylation reaction the first stage This uses CO2 and then releases two 3 phosphoglycerate for the next step which is reduction o The light reactions sustain the Calvin Cycle by regenerating ATP and NADPH Physical light is turned into chemical energy in the ATP and NADPH molecules An electrochemical gradient is present with H and produces the ATP The light harvesting complex contains multiple proteins and pigments also called the energetic funnel These pigments absorb light Chlorophyll B absorbs light at a HIGHER energy than Chlorophyll A Chlorophyll B is in the outer part of the antenna and A is in the inner part Antenna also called Carotenoids or accessory pigments Chl B can transfer light energy to A but NOT vice versa o Longer wavelength of light means lower energy in visible light Other pigments can expand the range of light able to be absorbed and used ie accessory pigments o Chl A absorbs light around 425 and 650nm while Chl B absorbs around 450 and 675nm around 100nm and up o Respiration occurs at lower light intensity while photosynthesis begins to occur Z Scheme aka Non Cyclic E Flow Photosystem I occurs second and uses P700 Photosystem II occurs first and uses P680 These two work together to produce the ATP and NADPH needed in the Calvin cycle as well as release O2 into the atmosphere o PSI P700 Chl A becomes excited by light Chl A e receives electrons from electron transport chain o PSII P680 oxygen evolving step receives electrons from the splitting of water molecules to reduce the chlorophyll donates electron to electron transport chain and helps create the electron H gradient o Purple bacteria only have PSII Ferredoxin is present in the reaction of NADP NADPH o Photoinhibition is when the ETC gets stuck and not enough NADPH is coming back around synthesis of starch makes the calvin cycle keep moving and store it until it needs to make sucrose 4 Explain what photorespiration is and under what conditions it is likely to occur Occurs in a hot dry atmosphere when stomata partially close and produce less sugar because of declining CO2 levels which starve the calvin cycle Rubisco binds O2 instead of CO2 and further reduced CO2 End product splits and releases a 2 carbonn compound from the chloroplast Peroxisomes and mitochondria then rearrange and split this compound finally releasing CO2 5 Describe the advantages and disadvantages of photorespiration and its evolutionary significance This process occurs in the light and consumes O2 while producing CO2 It generates no ATP but instead uses it Also produces no sugar and decreases photosynthetic output Likely evolved from a time when the earth had less O2 and more CO2 than today s levels Plants that cannot carry out this process are susceptible to excessive light damage We view this as a waste of CO2 since we depend on carbon fixation in our food 6 Describe how C4 photosynthesis is different from C3 photosynthesis and under what conditions it was thought to evolve C3 photosynthetic plants are called so because their first organic product is a three Carbon compound 3 phosphoglycerate These plants experience the above described photorespiration C4 plants are called so because they preface the calvin cycle with an alternate mode of carbon fixation that forms a 4 carbon compound as its first product o C4 plants have distinct photosynthetic cells bundle sheath cells and mesophyll cells Calvin cycle is confined to the chloroplasts of the bundle sheath cells but incorporation of CO2 in mesophyll cells occurs before this step Cyclic electron flow is their only photosynthetic method of generating ATP Basically Mesophyll cells pump CO2 into the bundle sheath keeping CO2 concentration in those cells high enough for rubisco to bind to CO2 rather than O2 Involves PEP Caboxylase Sugar production is enhanced and photorespiration is minimized Lecture 9 about photosynthesis 1 Describe absorption and action spectra and explain what Engelmann s experiments demonstrated Used a pigment spectrophotometer to measure light absorption in certain parts of plants Showed absorbance levels of light at certain wavelengths in Chl A at 475 and 700 Chl B at 500 and 600 The combined absorption efficiency across entire visible spectrum went above a total of about 90 Found that rubisco has a much lower affinity for O2 and a higher one for CO2 Carbon fixation was decreased rubisco acted
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