Chloroplasts in a plant cell, in this case a spinach leaf. Inside the chloroplasts, the stacks of thylakoid membranes are where the action takes place. The dark spots are starch granules. In close proximity are the mitochondria, with curved membranes, and the endothelial reticulum. This system of membranes navigates between the energy producing organelles until it reaches the nucleus, where it also forms the nuclear membranes.