Explanation:

Photosynthesis in Cactus:
Photosynthesis is a process in which plants use sunlight, carbon dioxide, and water to produce glucose and oxygen. It is a vital process that occurs in the chloroplasts of plant cells. In cactus, photosynthesis primarily takes place in the stem, although other parts of the plant also contribute to the process.

Stem:
The stem of a cactus is green and contains specialized structures called chloroplasts. These chloroplasts contain chlorophyll, a pigment that is responsible for capturing sunlight. The stem of a cactus is thick and succulent, which allows it to store water and perform photosynthesis even in arid environments. The stem also has stomata, which are tiny openings that allow the exchange of gases, including carbon dioxide and oxygen.

Leaves:
Unlike most other plants, cacti have modified leaves called spines. These spines serve multiple purposes, such as reducing water loss and protecting the plant from herbivores. However, spines do not perform photosynthesis. Cacti have reduced or absent leaves, as these would lead to excessive water loss in their native desert environments.

Roots:
The roots of a cactus primarily function to anchor the plant in the ground and absorb water. While the roots play a crucial role in water uptake, they do not directly participate in photosynthesis.

Conclusion:
In conclusion, photosynthesis in cactus primarily occurs in the stem. The stem contains chloroplasts with chlorophyll, allowing the plant to capture sunlight and convert it into energy through photosynthesis. The reduced or absent leaves of the cactus help to minimize water loss in desert environments. The spines, which are modified leaves, do not participate in photosynthesis. While the roots of the cactus are important for water absorption, they do not directly contribute to the process of photosynthesis.