What structures are necessary in plants for photosynthesis to occur?

Answer 1

Photosynthesis happens in the leaves of most plants.
Cactus use needles instead of moisture losing leaves and they have their photosynthetic cells in the wall of the plant itself.

Plants need stalks and support structures to hold them up and they need roots to draw in water, nitrogen and other resources. They have to have cells to hole the photosynthetic organelles as well.

According to Wikipedia: http://en.wikipedia.org/wiki/Photosynthesis
"Photosynthesis is the conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water, the energy source is sunlight, and the end-products include glucose and oxygen. It is arguably the most important biochemical pathway."

Answer 2

Plants need water, light, and carbon dioxide to carry out photosynthesis. So they need roots to get in the soil to get water and nutrients. They need chloroplasts to receive light and carbon dioxide. photosynthesis is mainly carried out in the leaves of plants. So that is a necessary structure.