1. Evolution connection: Photosynthesis I Learning goals: Students will understand that 1) when photosynthesis evolved, there was little oxygen in Earth’s atmosphere, 2) photosynthesis is responsible for the increase in oxygen in the atmosphere, 3) the non-specific nature of the enzyme RUBISCO is an evolutionary relic of the low-oxygen atmosphere of early Earth, and 4) C 4 and CAM photosynthesis are adaptations that allow plants to sidestep the constraints imposed by these relics. For the instructor: This short slide set explains the existence of photorespiration using evolutionary theory. To integrate it best, use these slides immediately after you’ve discussed basic photosynthesis and photorespiration, as an introduction to C 4 and CAM carbon fixation. Each of the following slides comes with a sample script for the instructor. To review this script, download the PowerPoint file and view the Notes associated with each slide. Evolution Connection slideshows are provided by Understanding Evolution (understandingevolution.org) and are copyright 2011 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California. Feel free to use and modify this presentation for educational purposes.

2. Why do plants photorespirate? Soybean field photo by Scott Bauer/USDA Evolution connection: Photosynthesis I

3. … because Earth’s environment was very different when photosynthesis first evolved. Evolution connection: Photosynthesis I divergence of photosynthetic bacterial lineages Graph adapted from Xiong, J., and Bauer, C. E. (2002). Complex evolution of photosynthesis. Annual Review of Plant Biology. 53: 503-521.

4. When good enough is not good enough... Evolution connection: Photosynthesis I This image is by Richard Zmasek has a Creative Commons Attribution-NonCommercial- ShareAlike 3.0 (CC BY-NC-SA 3.0) license.

5. Trade-offs in hot, dry climates Evolution connection: Photosynthesis I Stomata open:Stomata closed: CO 2 H 2 O/O 2 water loss lack of carbon dioxide build up of oxygen photorespiration! Stomata photo from Micrographia.

6. Adaptations for two variants on photosynthesis: C4 and CAM Evolution connection: Photosynthesis I Normal photosynthesisC 4 photosynthesisCAM photosynthesis High temperatures lead to lots of photo-respiration. CO 2 concentrated near bundle-sheath cells. CO 2 concentrated by closing stomata during day. Redwood photo by Jo-Ann Ordano © California Academy of Sciences Sugar cane photo by George G. Hawxhurst © California Academy of Sciences. Barrel cactus photo by Gerald and Buff Corsi © California Academy of Sciences. Bundle-sheath cell photo by Paul Schulte High temperatures but low photo-respiration. High temperatures but low photo-respiration.

7. Evolution connection: Photosynthesis I Christin, P., Salamin, N., Muasya, A. M., Roalson, E. H., Russier, F., and Besnard, G. (2008). Evolutionary switch and genetic convergence on rbcL following the evolution of C 4 photosynthesis. Molecular Biology and Evolution. 25: 2361-2368. Xiong, J., and Bauer, C. E. (2002). Complex evolution of photosynthesis. Annual Review of Plant Biology. 53: 503-521.