Photosynthesis160-page18

Photosynthesis160-page18 - that produce oxygen occur in the...

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Photosynthesis - 18 C 4 Photosynthesis A few specialized plants of hot, dry environments have evolved mechanisms to minimize photorespiration. When CO 2 diffuses into a leaf mesophyll cell, it is combined with a 3-carbon compound, phosphoenolpyruvate (PEP), forming a 4- carbon acid, oxaloacetate which is converted to malate. This 4-carbon acid is then transferred to the bundle sheath cells of the leaf. This is a more efficient trap for carbon dioxide since the 4-carbon acid can accumulate during non-light periods, concentrating carbon dioxide when photosynthesis cannot occur, and can be used during periods of low moisture when stomata are closed to prevent water loss. This is called C 4 photosynthesis because of the 4-carbon acids formed before the Calvin cycle occurs. C-4 plants also perform the two stages of photosynthesis in separate cells to keep O 2 away from Rubisco, thereby preventing photorespiration. The light reactions
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Unformatted text preview: that produce oxygen occur in the leaf mesophyll cells that surround the bundle sheath cells of veins. The chloroplasts of the mesophyll cells have many thylakoids. No Calvin cycle occurs in the mesophyll cells in the absence of CO 2 . The Calvin cycle occurs in specialized bundle sheath cells that surround the veins of the leaf. The chloroplasts in the bundle sheath cells have very few thylakoids, but much stroma. The accumulated 4-carbon acids are shunted to the bundle sheath cells. In addition, many plasmodesmata occur between the mesophyll cells and the bundle sheath cells to transport ATP and NADPH produced in the light reactions to the bundle sheath cells for use in the Calvin cycle. ADP, P i and NADP + are returned to the mesophyll cells, as is the 3-carbon acid, pyruvate, that forms when the CO 2 is released for carbon fixation in the bundle sheath cell....
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This note was uploaded on 12/29/2011 for the course BIO 151 taught by Professor Edwards during the Spring '10 term at SUNY Stony Brook.

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