Final answer:
Chloroplasts are predominantly found in the mesophyll cells of plant leaves where they conduct photosynthesis. Cyanobacteria perform photosynthesis too but do not have chloroplasts; instead, they have similar structures within the cellular membrane.
Step-by-step explanation:
Chloroplasts are located predominantly in the mesophyll cells in the leaves of plants. These organelles are responsible for photosynthesis, a process that converts light energy into chemical energy stored in glucose. The structure of chloroplasts includes a double membrane and internal stacked, disc-shaped structures called thylakoids, which contain the pigment chlorophyll, crucial for capturing light energy. While cyanobacteria do perform photosynthesis and contain chlorophyll, it is not within a chloroplast but rather in folds of the cellular membrane called thylakoids. These photosynthetic bacteria are significant as they are thought to be the ancestors of chloroplasts via an endosymbiotic event with a eukaryotic host cell.