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
– CO2 can diffuse into the leaf
– H2O can diffuse out of the leaf
– Which do you think occurs faster? Why?

4. Rates of Diffusion Through the
Stomata
• The 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
CO2 inside and outside the leaf
• Water is a smaller molecule than carbon dioxide
• Thus rate diffusion of CO2 out of the leaf is much
faster than rate of diffusion of H2O into the leaf

5. When Do Plants Need CO2?
• 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 CO2 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 CO2 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 CO2 in the
atmosphere is so low, a plant keeping its
stomata open at night would quickly reach
equilibrium so no more CO2 would enter
• Thus, the plant would run out of CO2 early the
next day so photosynthesis would cease.

14. A Better Solution
• In order to keep CO2 diffusing into a leaf all
night the concentration gradient must be
maintained.
• Can maintain the concentration gradient by
removing the CO2 from inside of the leaf
– Use the CO2 in a reaction to form a new product
and CO2 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 CO2 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 CO2 from
the use of CO2
• C3 plants
– CO2 uptake in daytime, Calvin Cycle in daytime
• CAM plants
– CO2 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