Basics of C4 Plants
Rubisco is an inefficient enzyme because oxygen can substitute for CO2 in the active site. This happens when excess oxygen become available in the chloroplast. This results in reducing the production of glucose. All plants have a mechanism to detect water stress and close the stomata's accordingly....a little closure when slightly dry and complete closure when really dry. .....to reduce or prevent transpiration. It does not take a "rocket scientist" to understand how oxygen concentration can increase in a leaf if the stomata's are partially closed ....or closed....if it's cell are actively carrying on photosynthesis - it is being produced in the light dependent reactions. So on hot windy dry days C3 plants often have their stomata's partially closed and rubisco uses oxygen instead of carbon dioxide for the carboxylation reaction in the Calvin Cycle. This means less glucose, less growth, etc during hot dry times.
So "mother nature" found a way to solve this problem, a modification that allows plants to efficiently operate the Calvin cycle when slightly...sometime more than slightly...water dry. What evolved was not a more efficient Rubisco, but rather a way to keep the CO2 concentration around the enzyme higher than the oxygen.....preventing rubisco from utilizing oxygen in the active site. Rubisco is a large quaternary "structured" enzyme and it must have been difficult for evolution to make the correct mutation(s) to solve this problem. And evolution often cleverly made slight modifications on things that were already available to solve such problems. This is exactly what happened. Some reactions that were already being used in respiration were modified to concentrate CO2 around rubisco. Carbon dioxide was added to phosphoenol pyruvate [PEP]..... a three carbon molecule involved in glycolysis....by phosphoenol pyruvate carboxylase. This enzyme is very efficient and oxygen does not react with it so four carbon molecules readily made.....made by reacting CO2 to phosphoenol pyruvate.....when CO2 concentrations are low. Evolution went one step farther to streamline the process. It modified many of the photosynthetic cells just to do this reaction.....to make four carbon molecules.....called malatee....from phosphoenol pyruvate and CO2. After malate is made in these specially modified cells, it is shuttled into the non-modified cells where the Calvin cycle occurs....the bundle sheath cells that are making glucose by the Calvin cycle reactions. A reaction releases CO2 from oxaloacetate again making PEP. Thus shuttle and reaction keeps the CO2 so concentrated around rubisco that it only utilizes CO2.... never oxygen.... and the photosynthetic rate remains at the normal pace. The phosphoenol pyruvate is shuttled back out to the modified cells to again be carboxylated making oxaloacetate which is shuttled back to the bundle sheath cells, etc
In summary, C4 plants continue to grow even when they are water stressed if there is a slight opening in the stomate. This is due to the CO2 being shuttled into the bundle sheath cells making rubisco "act efficiently". Photosynthetic rate is reduce in C3 plants that have this same small stomate opening because they can't concentrate CO2 around rubisco ......that then acts inefficiently.
An Excellent Description and Diagram of C4 Photosynthesis.
Summarized differently: Unlike CAM plants, in C4 plants photosynthesis is separated in space rather than in time. These plants fix CO2 in their mesophyll cells using PEP carboxylase to form a four-carbon molecule (hence C4). This product is then transferred to seperate bundle sheath cells where its CO2 is removed and added to RuBP by RUBISCO as in C3 plants.
As you now understand, C4 Photosynthesis is a version of photosynthesis that demands much less water and carbon dioxide than conventional (C3) photosynthesis. As a result, C4 plants are common in warm or dry areas, and they are also capable of using elevated carbon dioxide levels (such as those produced by today's industrial society) to increase their growth rates.