Evolution connection: Photosynthesis IEvolution 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) C4 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 C4 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.

Why do plants photorespirate?

Soybean field photo by Scott Bauer/USDA

Evolution connection: Photosynthesis IEvolution connection: Photosynthesis I

… because Earth’s environment was very different when photosynthesis first evolved.

Evolution connection: Photosynthesis IEvolution connection: Photosynthesis I

divergence ofphotosynthetic

bacterial lineages

Graph adapted from Xiong, J., and Bauer, C. E. (2002). Complex evolution of photosynthesis. Annual Review of Plant Biology. 53: 503-521.

When good enough is not good enough . . .

Evolution connection: Photosynthesis IEvolution 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.

Trade-offs in hot, dry climates

Evolution connection: Photosynthesis IEvolution connection: Photosynthesis I

Stomata open: Stomata closed:

CO2

H2O/O2

• water loss • lack of carbon dioxide• build up of oxygen• photorespiration!

Stomata photo from Micrographia.

Adaptations for two variants on photosynthesis: C4 and CAM

Evolution connection: Photosynthesis IEvolution connection: Photosynthesis I

Normal photosynthesis C4 photosynthesis CAM photosynthesis

High temperatures lead tolots of photo-respiration.

CO2 concentrated nearbundle-sheath cells.

CO2 concentrated by closingstomata 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 lowphoto-respiration.

High temperatures but lowphoto-respiration.

Evolution connection: Photosynthesis IEvolution 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 C4 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.