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With your neighbor: Identify 3 things that you already know about photosynthesis.

With your neighbor: Identify 3 things that you already know about photosynthesis. Identify a question that you have about photosynthesis. Autotrophs make organic compounds from CO 2. Ex. Plants, cyanobacteria. Ex. some bacteria. Ex. Archaea. Ex. Animals.

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With your neighbor: Identify 3 things that you already know about photosynthesis.

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  1. With your neighbor: • Identify 3 things that you already know about photosynthesis. • Identify a question that you have about photosynthesis.

  2. Autotrophs make organic compounds from CO2 Ex. Plants, cyanobacteria Ex. some bacteria Ex. Archaea Ex. Animals

  3. Photosynthesis: Making organic compounds using light energy In plants, photosynthesis occurs in chloroplasts. Stroma—dense fluid inside chloroplasts Grana—Stacks of thylakoid membranes which surround thylakoidspace containing chlorophyll

  4. Stomata—pores on the leaf through which CO2 enters and O2 exits Mesophyll—tissue inside the leaf that contains many chloroplasts

  5. Overall photosynthesis reaction: 6CO2 + 6H2O + light  C6H12O6 + 6O2 Since cell respiration is exergonic, photosynthesis is endergonic and requires light as the source of energy

  6. Photosynthesis is divided into 2 parts • Light-dependent reactions occur in the grana • Light-independent reactions (Calvin Cycle) occur in the stroma

  7. The light reactions convert solar energy to chemical energy (ATP & NADPH). Light has characteristics of waves AND of particles (photons). Shorter wavelengths of light have more energy/photon

  8. Light is absorbed by pigments. Different pigments absorb different wavelengths. • Chlorophyll a and b absorb red and blue light best (reflect green)

  9. Carotenoids are accessory pigments that are usually masked by chlorophyll. Carotenoids absorb blue light, so they look yellow and orange. We can only see them in the fall when chlorophyll is no longer produced.

  10. When chlorophyll absorbs light, e-s in the chlorophyll jump to a higher energy level (they now have more POTENTIAL energy) • When the e-s fall back down, 1 of 3 things can happen • Energy is lost as heat • Energy is retransmitted = fluorescence • Energy may trigger another reaction

  11. Chlorophyll is organized with proteins and other organic molecules into photosystems= light harvesters. Primary electron acceptor prevents high energy electrons from dropping back to ground state. Photosystem II absorbs light at a wavelength of 680 nm Photosystem I absorbs light at a wavelength of 700 nm

  12. The Light-dependent Reactions

  13. Photosystem II absorbs light & excited e-s are captured by primary e-acceptor • 2. H20 donates e-s to chlorophyll  • 3. Electrons fall down an electron transport chain. • 4. The exergonic “fall” to lower energy levels is harnessed by thylakoid membrane to make ATP through a proton gradient and ATP synthase = • PHOTOPHOSPHORYLATION H20 2H+ + ½O2 + 2 e- released into air

  14. 5. The bottom energy level is the reaction center of Photosystem I. 6. Primary e- acceptor passes excited e-s to another electron transport chain where NADP+ is reduced to NADPH which carries e-s to the Calvin Cycle

  15. This is called noncyclic electron flow

  16. Cyclic electron flow—only Photosystem I Makes ATP but not NADPH!

  17. Both the mitochondrial inner membrane and the thylakoid membrane of the chloroplast contain an electron transport chain.

  18. The Calvin Cycle does not directly require light, but it uses products from the light reactions Rubisco catalyzes the addition of CO2 to ribulosebisphosphate (RuBP)

  19. The product of the Calvin Cycle is glyceraldehyde3-phosphate (G3P), a 3 carbon sugar. From one round of the Calvin Cycle (3 CO2s entering), one G3P is produced using 9 ATP and 6 NADPH which came from the light reactions Two rounds of the Calvin Cycle yield one glucose. Most plants are called C3 plants because the product of the Calvin Cycle is a 3 carbon sugar.

  20. Problem arising from photosynthesis in C3 plants: As CO2 enters leaves, H2O is lost through stomata CO2 So when it’s hot and dry, stomata close, lowering [CO2] in the leaf. This is bad because rubisco binds to O2 just as well as to CO2. H2O Since there’s more O2 than CO2 around, rubisco will bind to O2, causing photorespiration (sugars being oxidized BACK to CO2). This reduces the efficiency of the plant.

  21. SOLUTION #1—C4 PLANTS Before the Calvin Cycle, these plants sequester the CO2 by converting it into a 4C compound using the enzyme PEP carboxylase in the mesophyll cells. The 4 C compound is transported into the bundle sheath cells, where the CO2 is released so that it can bind to rubisco and enter the CalvinCycle.

  22. SOLUTION #2—CAM PLANTS CAM plants open their stomata during the night and close them during the day. When CO2 enters at night, it’s converted to organic acids. During the day, when ATP and NADPH are made, CO2 is released from the organic acids to enter the Calvin Cycle.

  23. C4 plants separate the light reactions from the Calvin Cycle in space (diff. cells) CAM plants separate the light reactions from the Calvin Cycle in time (night/day)

  24. Identify 3 differences between cellular respiration and photosynthesis. Identify 3 similarities between cellular respiration and photosynthesis.

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