Stomatal function and cam photosynthesis

1. Stomatal Function and CAM Photosynthesis Dr. Mark A. McGinley Associate Professor Honors College and Department of Biological Sciences Texas Tech University

2. Stomata

3. Stomata A stomate is a pore in the leaf that a leaf can open or shut When a stomate is open CO 2 can diffuse into the leaf H 2 O can diffuse out of the leaf Which do you think occurs faster? Why?

4. Rates of Diffusion Through the Stomata T he concentration gradient between the concentration of water vapor inside a leaf (essentially 100%) and outside of a leaf is much great than the concentration gradient between CO 2 inside and outside the leaf Water is a smaller molecule than carbon dioxide Thus rate diffusion of CO 2 out of the leaf is much faster than rate of diffusion of H 2 O into the leaf

5. When Do Plants Need CO 2 ? Carbon dioxide is needed in the Calvin Cycle which only occurs during the day? Thus, plants should be expected to open their stomata during the day

6. Stomata Open During Daytime Because water is lost faster from the leaf when temperatures are higher, plants risk losing a lot of water when they open their stomata during the daytime. In most plants, the benefits of opening their stomata in the daytime to allow CO 2 to be used in Calvin Cycle is greater than the costs of losing water.

7. How Do You Know it is Time to Water Your Plants?

8. Plants Rely on Turgor Pressure to Provide Support

9. Plants Respond to Water Stress (too little water ) by Closing Their Stomata When stomata are closed No water can diffuse out of the plant No CO 2 can enter the plant Therefore , Calvin Cycle stops and no photosynthesis occurs

10. Problem Facing Desert Plants Experience water stress most of the time If they kept their stomata closed all of the time then they would never be able to conduct photosynthesis!!!

11. Desert Plants Possible Solution In deserts the costs of water loss are much greater!! What happens if desert plants tried to open their stomata at night when it is cooler so they will not lose as much water.

12. Possible Solution Open Stomata at Night Open stomata at night CO2 diffuses in at night when rates of water loss are lower Close stomata in the daytime and conduct photosythesis as normal.

13. Problem With This Solution Because diffusion requires a concentration gradient and the concentration of CO 2 in the atmosphere is so low, a plant keeping its stomata open at night would quickly reach equilibrium so no more CO 2 would enter Thus, the plant would run out of CO 2 early the next day so photosynthesis would cease.

14. A Better Solution In order to keep CO 2 diffusing into a leaf all night the concentration gradient must be maintained. Can maintain the concentration gradient by removing the CO 2 from inside of the leaf Use the CO 2 in a reaction to form a new product and CO 2 can diffuse in all night

15. CAM Photosynthesis CAM stands for Crassulacean Acid Metabolism

16. CAM Photosynthesis CAM plants open their stomata at night When CO 2 enters the cell it enters a reaction that converts it to Malate (an acid) Malate build up in the plant cell over night Stored in a vacuole Plants close their stomata in the daytime Malate is broken down to release which enters the Calvin Cycle All other details of photosynthesis exactly the same in CAM plants as in regular C3 plants

17. CAM Plants Temporally separate the uptake of CO 2 from the use of CO 2 C3 plants CO 2 uptake in daytime, Calvin Cycle in daytime CAM plants CO 2 uptake at night, Calvin Cycle in daytime

18. CAM Photosynthesis

19. What a Great Solution! Why Don’t All Plants Do This?

20. Limitation of CAM Photosynthesis Because there is limited space inside the vacuole, there is a limit to how much malate can be stored at night Once that malate is used up the next day, there is no more CO2 so photosynthesis stops Thus, CAM plants have lower rates of photosynthesis during the day than C3 plants