What part of the cactus photosynthesizes?
In wiki there is a good answer for your question http://en.wikipedia.org/wiki/Cactus#Leaves

In plants photosynthesis takes place in chloroplasts. Chloroplasts may be in the cells of fruits, stems, but most of all in leaves. In some succulents (such as cacti), the main photosynthetic activity is associated with a stem.