Why do guard cells contain chloroplasts?

Guard cells contain chloroplasts primarily so they can manufacture food through photosynthesis. This is an essential function for these cells because, although their primary role is to regulate gas exchange by opening and closing the stomata, having chloroplasts allows them to produce glucose and other organic compounds needed for energy and growth. The energy produced through photosynthesis supports the metabolic needs of the guard cells, enabling them to effectively control the stomatal movements.

When light is available, guard cells convert light energy into chemical energy, which helps in the transport of potassium ions into the cells. The influx of potassium ions causes water to enter the guard cells through osmosis, leading to their swelling and the opening of the stomata. This process not only facilitates gas exchange but also plays a critical role in photosynthesis itself, as it allows carbon dioxide to enter the plant while managing water loss through transpiration.