Organelles are structures within a cell that have specific functions; membrane-bound organelles are organelles protected by a single or double plasma membrane. Mitochondria, lysosomes, the endoplasmic reticulum and the Golgi apparatus are examples of membrane-bound organelles.
Membrane-bound organelles are one of the defining characteristics of eukaryotic cells. Prokaryotic cells such as bacteria do not possess these organelles. Certain species of bacteria possess primitive protein pockets that fulfill some organelle functions but are not defined structures protected by a membrane.
Mitochondria are unique organelles that contain their own DNA. They produce the cell's energy through respiration. Because of this, they are also known as the powerhouse of the cell. Mitochondria are covered by two membranes: a smooth outer membrane and a folded inner membrane. Mitochondrial DNA is passed down through the mother and is a useful tool in genetic tests.
The endoplasmic reticulum produces lipids and proteins. The rough outer membrane of the endoplasmic reticulum is covered with ribosomes and is responsible for protein synthesis; the smooth endoplasmic reticulum lacks ribosomes and synthesizes lipids. Ribosomes and the Golgi apparatus assist the rough endoplasmic reticulum in protein synthesis. Lysosomes are the digestive system of the cell. They use enzymes to digest food, waste, toxins and dead cellular material.