C4 plants utilize a distinct method of photosynthesis compared to C3 plants, employing a two-step process to effectively concentrate carbon dioxide ($CO_2$) in specialized cells.

Both C3 and C4 plants perform photosynthesis, but their mechanisms differ due to adaptations to their respective environments. C3 plants, which represent the majority of plant species, rely on the Calvin cycle to convert carbon dioxide ($CO_2$) into glucose. While this method is efficient under ideal conditions, it presents a challenge: during hot and dry weather, C3 plants experience significant water loss through their stomata, the tiny pores in leaves that allow $CO_2$ to enter and water to escape.

In contrast, C4 plants have evolved an innovative strategy to minimize water loss. They engage in a two-step process to concentrate $CO_2$ in specialized cells located deeper within their leaves, thus shielding them from direct exposure to air and the drying effects of sunlight. This process is referred to as C4 photosynthesis.

The first step involves capturing $CO_2$ in the mesophyll cells, which are situated in the middle layer of the leaf. In this stage, $CO_2$ combines with a three-carbon compound to form a four-carbon compound (hence the designation ‘C4’). This four-carbon compound is then transported to specialized cells known as bundle sheath cells, which are located deeper within the leaf, away from the external atmosphere.

In the second step, the four-carbon compound releases its $CO_2$ within the bundle sheath cells. Because these cells are situated deep in the leaf, they are insulated from the air, thereby reducing water loss. The released $CO_2$ is subsequently utilized in the Calvin cycle to synthesize glucose, similar to the process in C3 plants.

This two-step mechanism enables C4 plants to keep their stomata closed for longer periods, thus minimizing water loss. Additionally, it enhances their ability to capture $CO_2$ efficiently, which is particularly advantageous in hot and arid environments where $CO_2$ may be scarce. Consequently, many C4 plants, such as maize and sugarcane, thrive in conditions that would pose significant challenges for C3 plants.