1998 sll et al 2003 glob biogeochem cycl yellow

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Unformatted text preview: n sorghum sugarcane note: the vast majority of our calorie base comes from grasses (or from animals ea=ng grasses) We are primarily grass eaters, one way or another! photosynthesis CO2 + H 2O → CH 2O + O2 H2O CO2 O2 stomate gases inter and exit leaves via stomates The plight of plants: Challenge of acquiring CO2: CO2 is a trace gas (391 ppm; see CO2now.org) It must diffuse into leaves through small openings called stomates Water is lost through stomates O2 competes with CO2 (see discussion of RUBISCO) C3 photosynthesis 1.  CO2 diffuses into leaf 2.  Rubisco ‘grabs’ CO2, shuTles it into the Calvin- Benson cycle. This results in fixed carbon, CH2O (= photosynthesis = good) 3.  Alterna=vely, Rubisco ‘grabs’ O2 and shuTles it into the photorespiratory cycle. This does not result in fixed carbon (= photorespira=on = bad) CO2 concentra=on is less than or equal to ambient, ca. 390 ppm Photorespira=on increases with: increasing temperature decreasing CO2 concentra=on water limita=on C4 photosynthesis 1.  CO2 diffuses into leaf 2.  CO2 is grabbed by PEP- C. PEP- C has much stronger affinity for CO2 than Rubisco. 3.  The CO2 is transported into a protected environment (bundle sheath cell) where photosynthesis takes place. 4.  The concentra=on of CO2 in the bundle sheath cell is very high (ca. 10,000 ppm), virtually elimina=ng photorespira=on. Why don’t all plants use C4 photosynthesis? • There is an energe=c cost associated with PEP manufacture • An evolu=onarily young pathway (probably < 40 Million Years Old!) Despite the energe=c cost, restric=on to grasses, and evolu=onary youth, C4 photosynthesis accounts for ~25% of net primary produc=vity! Percent of grass species that Use C4 photosynthesis C3 and C4...
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