1. 7.11-7.14 Photorespiration: Stopping Photosynthesis Other types of Photosynthesis (besides C3) Pgs. 117-120 Objective: I can describe alternative mechanisms of carbon fixation (i.e. different types of photosynthesis) that have evolved in hot and dry climates.

2. Review & Context of New Material  “Normal, regular” photosynthesis = C 3 Photosynthesis…why C 3 ?  Calvin Cycle involves mostly 3-carbon compounds (PGA, G3P, etc.)  Most plants are C 3 plants…and most plants do not survive well in hot temps…  …plants in hot temps evolved different ways to do light-independent reactions  Light-dependent reactions & Calvin cycle happen normally!  Added steps to prepare for Calvin C.

3. Review of Stoma (plural = stomata)  “Hole” on suface of leaf…for what?  Allows CO 2 in (came from?) and O 2 out (came from?)  Can also let H 2 O out (why bad if hot?) ■ Guard cells will help close stoma to save H 2 O ■ Inside the leaf: O 2 increases CO 2 decreases ■ Calvin Cycle stops BEFORE run out of CO 2

4. Photorespiration  Process where RuBisCO catalyzes O 2 instead of CO 2 (due to conc. levels)  Will ultimately produce CO 2 (try return conc. levels)  TRYING to do Calvin Cycle…  But costs too much energy (not ideal) RuBisCO CO 2 O2O2 Calvin Cycle Glucose O2O2 O2O2 O2O2 O2O2 O2O2 O2O2 CO 2 Must find a way to keep CO 2 levels high & O 2 levels low (prevent photorespiration) in hot temperatures… ATPNADPH No ATP left!

5. C 3 Photosynthesis Mesophyll Cell Bundle Sheath Cell (impermeable to gas) Selectively Permeable Membrane (not allow gas) RuBisCO CO 2 O2O2 O2O2 Calvin Cycle Glucose O2O2 O2O2 O2O2 O2O2 O2O2 Too hot!!! Will do photorespiration, which we don’t want!!! This is why plants die in extreme heat, even if water regularly (“Normal “Plants) Light Rxns

6. C 4 Photosynthesis  A different enzyme (NOT RuBisCO) bonds CO 2 with 3-C molecule to make a 4-carbon molecule  Thus, C 4 Photosyn.  Molecule is moved to a Bundle sheath cell: impermeable to gas  O 2 can’t enter; ■ 4-C loses CO 2  Calvin Cycle can run! CO 2 can’t leave: conc. for plants normally living in hot environments…

7. C 4 Photosynthesis (different plant) Mesophyll Cell Bundle Sheath Cell (impermeable to gas) Selectively Permeable Membrane (not allow gas) O2O2 O2O2 O2O2 O2O2 O2O2 O2O2 O2O2 O2O2 CO 2 Enzyme CO 2 RuBisCO Calvin Cycle C 4 plants include sugar cane, corn, and many grasses in hot, dry environments Few stomata open Light Rxns

8. CAM: Crassulacean Acid Metabolism  Plant family Crassulaceae: desert plant  Stomata open at night (min. H 2 O loss)  Performs “half” of C 4 pathway  “Stores” CO 2 inside of 4-carbon molec.  Drives MORE CO 2 to diffuse in  During day, stomata close  Finish rest of C 4 pathway – makes CO 2 from the 4-carbon molecules  High (super saturated) concen. of CO 2 inside cell can go to Calvin Cycle

9. CAM Mesophyll Cell CO 2 Enzyme CO 2 RuBisCO Calvin Cycle CO 2 Stoma Open Stoma Closed NightDay O2O2 O2O2 CO 2 Light Rxns To summarize (next slide = comparison)…

10. Comparing Photosynthesis Types  C 3 photosynthesis  Carbon fixation directly happens in first step of Calvin Cycle in one cell @ 1 time  C 4 photosynthesis (spatial separation)  Carbon fixation happens forming a 4-C molecule in a different SPACE from rest of Calvin cycle (two cells physically separated)  CAM (Crassulacean Acid Metabolism)  Carbon fixation happens at a different TIME from the rest of Calvin Cycle (temporal separation)  Photorespiration = bad (not even carbon fixation, but oxygen reacting with Rubisco)

11. Photosynthesis & Pollution  Global Warming vs. Climate Change  Greenhouse effect =  Greenhouse gases include  CO 2, H 2 O, and CFCs…  Ozone Layer – Protects from UV  CFCs also destroy Ozone (O 3 ) http://www.mhhe.com/biosci/bio_animations/ 02_MH_Photosynthesis_Webhttp://www.mhhe.com/biosci/bio_animations/ 02_MH_Photosynthesis_Web/