in prokaryotes, such a bacteria, algae and rickettsia, the cell itself is the organism. in some eukaryotes, like yeast and fungi, aggregation of the cells occurs to form tissues. in the higher forms of life, sch as plants and animals, the tissues are further organized to form the whole organ systems, the crucial difference between the eukaryotic and the prokaryotic cell is the presence in the former of internal membrance structures, particularly the nucleus. such strustures are not seen in prokaryotic cells