C4 photosynthesis: To prevent the wasteful effects of photorespiration some plant like corn and sugarcane that grow in hot dry climates have evolved a different system for fixing CO2. The anatomy of these plants leaves is different from normal leaves. They are said to exhibit Kranz anatomy. The xylem and phloem of these leaves are surrounded by thick walled parenchyma cells called bundle sheath cells where most of the cells photosynthesis takes place.
What happens is this: The plant opens its stomata in the early morning to let CO2 in. The CO2 diffuses into the mesophyll cells where it is combined with a 3C compound called phosphoenolpyruvic acid or PEP for short.
This produces the 4C compound oxaloacetic acid which is then converted to malic or aspartic acid.
Note the conversion of PEP to OAA
The malic or aspartic acid is then moved through plasmodesmata (at the expense of ATP) into the bundle sheath cells .
In the bundle sheath cells the 4C compounds is broken into CO2 and PEP.
Because the CO2 collected in the many mesophyll cells is being concentrated into a few bundle sheath cells , the plants can keep a higher concentration of CO2 in the bundle sheath cells (where the dark reactions and photosythesis are occurring) than it can elsewhere in the leaf. This higher concentration of CO2 prevents photorespiration and allows the plant to close its stomata during the hot hours of the day.
The C4 pathway is more expensive energetically than normal photosynthesis, but not as expensive as photorespiration ( where on a bad day plants may lose 30% of fixed carbon. For this reason C4 plants are thought to have evolved in places where photorespiration was a serious problem for plants.
From:Koning, Ross E. "Photosynthesis: the Synthesis part". Plant Physiology Website. 1994. http://koning.ecsu.ctstateu.edu/plant_biology/synthpart.html (3/27/99) the following diagram....