There are two types of photosynthesis- C3 and C4. Both process involve the conversion of light energy into organic matter, such as glucose. The difference between the two is in how they capture carbon dioxide gas from the atmosphere.
In C3 plants, the first step of photosynthesis takes place in the chloroplasts of mesophyll cells. These cells have many small vacuoles that help to keep the cell turgid (swollen). When CO2 enters these cells, it diffuses into the stroma, where it reacts with RuBP to form 2 molecules of 3-PGA.
In contrast, C4 plants have a special type of leaf anatomy called Kranz Anatomy. In these leaves, there are two types of photosynthetic cells- bundle sheath cells and mesophyll cells. The bundle sheath cells surround the vascular bundles (the xylem and phloem) and have few or no chloroplasts. The mesophyll cells are located outside of the vascular bundles and have many chloroplasts.
The initial steps of photosynthesis in C4 plants take place in mesophyll cells, just like in C3 plants. However, instead of diffusing directly into the stroma, CO2 first binds to a four-carbon compound called PEP (Phosphoenolpyruvate). This reaction creates a molecule called oxaloacetate which quickly breaks down to release 2 molecules of 3-PGA.
The 3-PGA then diffuses into bundle sheath cells where it is converted into G6P (glucose 6 phosphate). This process uses up ATP and NADPH from the light reactions stage. Finally, G6P is transformed into hexoses (such as sucrose) which can be transported out of the cell for use in other parts of the plant