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PHOTOSYNTHESIS

PHOTOSYNTHESIS. Photosynthetic Carbon Fixation removes tons of CO 2 from the atmosphere every year. The greenhouse effect , contributed to by CO 2 , has some beneficial effects, the problem lies in the recent rate of increase in CO 2 emissions from fossil fuel consumption.

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PHOTOSYNTHESIS

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  1. PHOTOSYNTHESIS

  2. Photosynthetic Carbon Fixation removes tons of CO2 from the atmosphere every year.

  3. The greenhouse effect, contributed to by CO2, has some beneficial effects, the problem lies in the recent rate of increase in CO2 emissions from fossil fuel consumption.

  4. Leaves: The green machine.

  5. The spectrum of Photosynthetic organisms

  6. The formula for photosynthesis is, in a sense, the opposite of that for aerobic respiration.

  7. A Brief overview of the structures involved in Photosynthesis : where the chloroplasts live.

  8. An actual dicot leaf cross section micrograph. The upper and lower-most epidermal layers are essentially composed of transparent “window” cells with several layers of green photosynthetic cells in between.Cells in the lower layers are loosely connected with the air spaces connected through stomatal openings (bottom center) to the outside.The circular groups of cells in the center are cross sections of small vascular bundles (leaf “veins”) that contain xylem (deliver water) and phloem (take sugar away) cells.

  9. Stomatal opening is controlled by two guard cells that swell when illuminated and fully hydrated (left) causing the stomate to open. Darkness or low water content cause the guard cells to relax (right) closing the opening and limiting CO2 - O2 gas exchange and water flow (transpiration) powered by evaporation.

  10. Each vascular bundle (see detail) has an outer grouping of phloem cells that carry sugar away from the leaves and an inner grouping of xylem cells that carry water up to the leaves. The branching“veins” of a leaf are the continuation of the vasculature in the leaf. phloem The plant’s vascular system conducts water from the roots to the leaves. xylem

  11. Schematic overview of photosynthesis

  12. We’ll consider the light reaction first

  13. Schematic overview of the light reaction

  14. In the photosystems ( II & I) energy is funneled to a primary reaction center where a high energy electron is created and captured in a high energy state before it can “decay” by releasing its energy as longer wavelength (red) light.

  15. Slight structural differences distinguish chlorophyll a & b which are slightly different shades of green. The large porphyrin ring structure holds a magnesium atom (green) in the center complexed to four nitrogen atoms (blue). Pigment molecules. Carotenoids, such as the b-carotene shown above, are orange to yellow pigments that capture light in the blue -to- near uv end of the spectrum.

  16. Chlorophyll released from the other components of a photosystem can generate high energy electons when illuminated with white light, but they quickly decay re-emitting red light similar to the yellow chemical fluorescence emitted by a glow stick.

  17. The visible spectrum

  18. An elegant experiment showing that oxygen is evolved from water only by light absorption in the red and blue ends of the visible spectrum. An algal filament illuminated by white light separated by a prism attracts oxygen-seeking bacteria only to those cells receiving red or blue light.

  19. And that’s why leaves are green!

  20. Jack Frost at work. As the days shorten, leaves are “programmed” to slowly recycle their contents back to the tree and then drop off. As the chlorophyll fades the remaining yellow-orange carotenoid pigments show through and may be joined by new red anthocyanin pigments synthesized in response to cold.

  21. Another look at the steps in the light reaction

  22. The guys in yellow represent light energy input. The “water wheel” shows how electron energy is used to pump hydrogen ions across the thylakoid membrane, creating a H+ gradient that then is used to make ATP.Finally the electron is re-energized and used to load hydrogen onto an NADP “pickup truck” for delivery to the Calvin cycle. A light reaction analogy.

  23. A slightly more realistic representation of the Light Reaction with all of the components (compare to the previous slide).

  24. On to the sugar factory

  25. 3-PGA is 3-phosphoglyceric acid; it becomes G3P, glyceraldehyde-3-phosphate, when H is added from NADPH. One of these G3Ps (or PGAL) is then removed and the remaining 5 G3Ps (= 15 carbons) are re-arranged back into the three 5C pick-up molecules, RuBP, ribulose-bis-phosphate (a 5C sugar). The re-arrangement steps (step 4 on the left) are amazingly complicated and are (thankfully) not shown in detail here. The RuBP + CO2pick-up step is catalyzed by the enzyme RUBISCO = ribulose-bis-phosphate carboxylase. Melvin (no lie, that’s his name) Calvin won the Nobel Prize years back for figuring this out, although his post-doc Benson is rumored to have done much of the work (such is life).

  26. Summary Schematic

  27. One last point: remember how stomata control both transpiration and gas exchange? So, in hot dry weather, many plants have a problem

  28. Low water content causes the guard cells to relax (right) closing the opening and limiting CO2 - O2 gas exchange and water evaporation. Unfortunately the Rubisco enzyme has difficulty distinguishing between CO2 and O2 when the former is low and the later is high, so it begins to join O2 onto RubP, leading to photorespiration and a loss of ATP. As a result the plant must go dormant, or die.

  29. The standard photosynthesis process is termed C3 photosynthesis because 3-PGA, the first stable Calvin cycle intermediate, is a 3C compound. The C3 process RUBISCO pick-up enzyme, however, gets “confused” when O2 inside the leaf is high and CO2 is low causing a wasteful process called photorespiration to occur instead of the standard Calvin cycle events. This happens when leaf stomates are closed under hot dry conditions. The result is that C3 plants go dormant (think the grass in your lawn) during a summer drought. C4 and CAM plants have a different enzyme (not RUBISCO) that loads CO2 onto a 3C molecule to make a 4C product. These enzymes work well even when O2 is high in the leaf and CO2 is low. C4 plants, like crabgrass and corn, can grow well even when it’s hot and dry, as the C4 pick up enzyme creates a 4C acid in some cells that is then transported to other cells where C is transferred to the normal Calvin cycle. CAM plants, like cacti, keep their stomates open only at night and pick up CO2 to create 4C acids (using ATP made the previous day) that are stored until the next day when they are then fed into the Calvin cycle, even though the cactus stomates are now closed.

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