Leaves and CO2 Uptake 

There are three cell types in a leaf: epidermis (outer cells), mesophyll (middle cells), and veins (connect the leaf to the rest of the plant).

%%Mesophyll%% cells are surrounded by space which allows them to obtain CO2. Air enters these spaces via pores in the epidermis.

The %%epidermis%% cells secrete wax out of the leaf creating a cuticle which limits water loss. This cuticle does prevent CO2 diffusion into the leaf.

%%Stomata%% (pores) which allow the entry of CO2, H2O and nutrients are found on the epidermis and make up 1-2% of a plants surface area. The guard cells of the stomata regulate water loss and CO2 intake. They are two long cellulose molecules oriented radially. %%Guard cells%% swell/open when there is sufficient H2O and shrink/close when there is water loss. High concentrations of potassium ions and chloride ions cause guard cells to swell. Light often stimulates the stomata to open and high CO2 levels causes the stomata to close.

%%Abscisic acid (ABA)%% is a hormone produced during droughts which causes the stomata to close.

%%Crassulacean Acid Metabolism (CAM)%% is a temporal mechanism used to limit transpiration. At night, the stomata open and CO2 is stored as a 4-carbon molecule synthesized from PEP carboxylase which combines a bicarbonate ion (HCO3-) with phosphoenolpyruvate (PEP) and is stored in the vacuole. During the day, the stomata are closed and the stored CO2 is incorporated into the Calvin cycle. The CO2:H2O exchange rate is 1:50 which is 10 times higher than none CAM plants. The downsides of CAM is that is produced carbohydrates slowly since the storage of CO2 requires ATP. CAM plants exist often in desserts or as epiphytes (plants that do not touch the soil). 5-10% of angiosperms use CAM.

%%C4 plants%% reduce photorespiration by having a high concentration of CO2 near rubisco so it does not have to compete with oxygen. Rubisco is found deep in the leaf in a %%bundle sheath cell%% which creates a spatial mechanism that limits substrate competition. Rubisco reacts more readily with oxygen at elevated temperatures and oxygen is more abundant in the atmosphere compared to CO2. They use PEP carboxylase to create the 4-carbon molecule that provides the Calvin cycle with CO2. C4 plants lose less water and have increased efficiency of the Calvin cycle in bundle sheath cells. Examples of C4 plants are corn, and weeds. A downside to C4 is that is uses more ATP to turn CO2 into a 4-carbon molecule which is less efficient than a C3 plant.

%%C3 plants%% are plants that do not use a 4-carbon molecule to give the Calvin cycle CO2.