Chloroplasts are found only in plant cells and have a double membrane. Within the stroma are the membrane structures, the thylakoids, which appear in stacks known as grana. The chloroplasts contain the plant cell’s chlorophyll and are responsible for the green color of plants. Under an electron microscope, the thylakoids sacs can be viewed as stacked on top of each other to form grana, which are connected by thin tubules between individual thylakoids.