What the Golgi does. From the perspective of the cell, the Golgi can be thought of as a black box with material entering from ER or endosomes, and then leaving with various consequences. (a) Sorting. Newly made secreted and membrane proteins arrive at the cis-Golgi from the ER in COPII-coated vesicles and are sorted from the trans-Golgi to the other organelles of the cell. COPI vesicles retrieve escaped ER residents, and are widely, but not universally, thought to also recycle Golgi enzymes from later to earlier compartments as the cisternae mature. (b) Modification of proteins. Newly made proteins from the ER receive a range of post-translational modifications as they move through the Golgi stack, as illustrated here by the processing of N-linked glycans. (c) Modification of the lipid bilayer. The membrane of the ER is primarily composed of phospholipids. Sphingolipids, such as sphingomyelin in mammals, and glycolipids are made in the Golgi using ceramide delivered from the ER, and cholesterol levels also increase toward the trans side. Thus, the membrane leaving from the trans-Golgi is very different in composition to that which arrived from the ER.