ch369_ch 13-photosynthesis2

ch369_ch 13-photosynthesis2 - Photosynthesis uses the...

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Unformatted text preview: Photosynthesis uses the energy of sunlight to convert CO 2 into carbohydrates and other organic molecules that are useful to plants. Plant cells with chloroplasts. Chloroplasts- where photosynthesis occurs. chloroplast Photosynthesis - Chapter 13 Photosynthesis includes “light reactions” & “dark reactions”. Light reactions: Using the energy from sunlight to produce ATP and NADPH. Photosynthesis wiki Light reactions. Summary of "light reactions" Photosystem II and cytochrome b6f work together, using energy from light to generate a pH gradient across the membrane separating stroma and lumen. The protons that are moved across the membrane are pulled from water, generating O 2 as a waste product. ATP synthase uses energy from proton Fow to make ATP. Photosystem I reduces NADP + to NADPH (which is useful in dark reactions). The “cyclic reaction” pumps protons to contribute to the proton gradient. Next, the “ dark reactions ”. Dark reactions = Using the energy stored in ATP and NADPH to convert CO 2 into useful stuff like sugars and other organic molecules needed for growing plants. An enzyme called “ ribulose bisphosphate carboxylase/oxygenase ” (or Rubisco ) catalyzes CO 2 fxation into carbohydrates. An enzyme called “ ribulose bisphosphate carboxylase/oxygenase ” (or Rubisco ) catalyzes CO 2 fxation into carbohydrates. Rubisco adds CO 2 to a 5-carbon sugar and cleaves the product to two 3-carbon units. This step is part oF the “ Calvin cycle ” (next page). Calvin cycle consumes lots of ATP and NADH, but that’s OK, its free !! (from solar power) The process just described, where CO 2 is directly combined with ribulose-1,5-bisphosphate, is called “ C3 metabolism ” or “ C3 photosynthesis ”. Regulation of carbon Fxation by Rubisco Regulation of the Calvin cycle is linked to the availability of light. (the Calvin cycle is less active in the dark). High pH in stroma, low pH in lumen (caused by H + pumping) causes carboxylation of rubisco, forming an essential Mg 2+ binding site. Relatively low pH in lumen indicates that light reactions are working and pumping H + , providing energy for dark reactions. Summary: “The dark reactions are most active when there is light available.” inactive rubisco Carboxylated lysine, active rubisco This next part surprised me ! Rubisco’s substrate binding site has some difFculty distinguishing between O 2 and CO 2 . Rubisco sometimes Fxes O 2 rather than CO 2 !!! This is called the “ oxygenase ” reaction of Rubisco, or “ photorespiration ”. When Rubisco uses O 2 instead of the CO 2 it is a big waste of energy and material for plants. Although the 2-phosphoglycolate that is produced in photorespiration can be salvaged, the salvage pathway consumes ATP, and also causes valuable nitrogen to be lost as ammonia in one of the salvage steps....
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This note was uploaded on 08/26/2010 for the course CH 369 taught by Professor Kbrowning during the Spring '07 term at University of Texas.

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ch369_ch 13-photosynthesis2 - Photosynthesis uses the...

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