Vesicles are small, membrane-bound SACS that can contain a variety of substances. The term 'vesicles' can refer to both SACS and CYSTS, which are larger and more rigid than vesicles. SACS, which have four letters, are the smaller of the two and are filled with liquid or gas. CYSTS, which have five letters, are larger and contain a thicker, more solid material. Both SACS and CYSTS are found in many different organisms, including plants, animals, and fungi.