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) 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.

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:
CO2
H2O/O2
• 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 photosynthesis C4 photosynthesis CAM photosynthesis
High temperatures lead to
lots of photo-respiration.
CO2 concentrated near
bundle-sheath cells.
CO2 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 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.