Consider the anatomy of the leaf of a C4 plant. In which type of cell – mesophyll or bundle-sheath – would you expect the oxygen-releasing reaction of photosynthesis to occur? Why? Does this localization of oxygen release increase or decrease photorespiration? Why?

The leaves of a C4 plant exhibit a special anatomy. In C4 plants, there are 2 distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells, which surround the vascular tissue of the leaf, forming two separate rings. The inner ring of the leaf is the Bundle-Sheath cells, which contain starch-rich chloroplasts. The outer ring (between the Bundle-Sheath cells and the surface) consists of the mesophyll cells. This peculiar anatomy of the C4 plant is called the Kranz Anatomy. This type of anatomy is advantageous to certain plants as it provides a site in which carbon dioxide can be concentrated around Rubisco (the first enzyme in the Calvin Cycle), thus reducing the photorespiration.

Of the two cells, the bundle-sheath cells are more likely to exhibit the oxygen-releasing reaction of photosynthesis to occur (Mostly due to the occurrence of the Calvin Cycle in the bundle-sheath cells). To better understand this, however, we must follow how photosynthesis works in a C4 plant. The first step in the pathway occurs in the outer ring (mesophyll cells). Here, the pyruvate converts to PEP by the enzyme pyruvate-phosphate dikinase; this reaction requires inorganic phosphate and ATP plus pyruvate, giving phosphoenolpyruvate, AMP, and PPi (inorganic pyrophosphate) as products. The pathway then continues in the outer ring by the incorporation of CO2 into organic compounds. In order to do this, however, the mesophyll cells use the enzyme PEP carboxylase to add CO2 to phosphoenolpyruvate (PEP). This forms the four-carbon product oxaloacetate. [Here PEP plays an important role as it has a much higher affinity for CO2 than rubisco and has no affinity for O2. As a result, under certain conditions (when it is hot and dry and stomata are partially closed), PEP carboxylase can fix carbon efficiently when rubisco cannot, causing CO2 concentration in leaf to fall and O2 concentration to rise.] After the carbon-fixation, the mesophyll cells transport the four-carbon products through plasmodesmata to the bundle-sheath cells located in the inner ring. Here within the bundle-sheath cells, the four-carbon product releases CO2. This CO2 is later reassimilated into organic material by rubisco and the Calvin Cycle, which is confined solely to the bundle-sheath cells. From here on, the bundle-sheath cells (which contain the thylakoids) utilize the conventional C3 pathway (of Calvin Cycle) to generate carbohydrates and release O2. (The decarboxylation also generates pyruvate, which is transported back to the mesophyll cell for conversion into PEP).

The primary function of the Kranz Anatomy is to provide a site in which carbon dioxide can be concentrated around RuBisCO, thus reducing photorespiration. In order to facilitate the maintenance of a significantly higher carbon dioxide concentration in the bundle sheath compared to the mesophyll, the boundary layer of the Kranz has a low conductance to carbon dioxide, a property which may be enhanced by the presence of suberin. In other terms, the mesophyll cells of a C4 plant pump CO2 (powered by ATP) into the bundle sheath, keeping the CO2 concentration in the bundle-sheath cells high enough for rubisco to bind carbon dioxide rather than oxygen. As a result, the C4 photosynthesis minimizes photorespiration and enhances sugar production. This adaptation is especially advantageous in hot regions with intense sunlight, where stomata partially close during the day.