Eukaryotes are organisms which are composed of one or more eukaryotic cells. The word is from the Greek eu, true, and karyon, meaning nut or kernal. Eukaryotic cells are one of two types of biological cells found in nature, and are vastly more complex than prokaryotic cells. Eukaryotic cells have a distinct membrane bound nucleus that houses the majority of the genetic material as chromosomes. This is surrounded by an extensive network of internal membranes such as the endoplasmic reticulum and Golgi apparatus that function to compartmentalise various metabolic pathways. They also contain semi-autonomous membrane bound organelles, such as mitochondria, that have a limited amount of DNA that encodes for many of their functional proteins.