The theory of endosymbiosis explains the origin of chloroplasts and mitochondria and their double membranes. This concept postulates that chloroplasts and mitochondria are the result of years of evolution initiated by the endocytosis of bacteria and blue green algae. According to this theory, blue green algae and bacteria were not digested; they became symbiotic instead. Endocytosis is when a substance gains entry into a cell without passing through its cell membrane.

Theory of endosymbiosis