In plant cells, chloroplasts perform photosynthesis, a process that converts light energy from the sun into chemical energy in the form of glucose. Plants can later use this stored chemical energy to carry out activities integral to life, such as growth and reproduction.
Chloroplasts are primarily found in the cells that make up a plant's leaves, organs that are specialized to capture light. The process of photosynthesis performed within the chloroplasts uses water, light and carbon dioxide, and it gives off glucose and oxygen. Photosynthesis is divided into light and dark reactions. The light reactions use water and light to synthesize ATP and NADPH; the dark reactions use the energy stored in that ATP along with carbon dioxide and NADPH to produce glucose, ADP, and NADP+. ADP and NADP+ are recycled for use in the light reactions, and the process continually repeats itself.
Chloroplasts, despite the integral nature of their function, are likely unrelated to the plant cells in which they reside. Chloroplasts, like mitochondria, have their own DNA, and they divide independently of the plant cell cycle. Also, a cell is unable to replenish its chloroplasts when they are removed. Evidence like this has led scientists to develop endosymbiotic theory. Endosymbiotic theory posits that chloroplasts and other key organelles originated separately from their host cells, and at some point in history, a single-celled organism engulfed a chloroplast, forming an enduring symbiotic relationship that persists pervasively today.