Algae are classified depending on their characteristics and structure; including photosynthetic pigmentation, storage products, flagella, the structures of the cell wall, nucleus, chloroplast, pyrenoid (part of the chloroplast used in producing starch) and the eyespot (organelle composed of tightly packed lipids). Algae can be either prokaryotic (lacking a cellular membrane; classified in the monera kingdom) or eukaryotic (the cellular organelles are bound by a nuclear membrane; classified in the protista or plantae kingdoms). There are about sixteen different types of alga known or defined and are grouped depending on their structure and location of growth.