When the RNA of a eukaryote cell has finished assembling new protein or lipid or sugar molecules in the endoplasmic reticulum, the molecules gradually pinch off a little piece of the endoplasmic reticulum around themselves. They seal themselves into a little bubble. This bubble floats into the Golgi bodies, a little further away from the cell’s nucleus.
The Golgi bodies, like vacuoles and the endoplasmic reticulum, are another set of tunnels whose walls are made out of lipid membranes. They were probably originally part of the cell membrane, like vacuoles. Cells probably evolved the first Golgi bodies a little after developing the endoplasmic reticulum, around two billion years ago.
Inside the Golgi bodies, if the atoms on the new molecule are in a certain order, then a few more hydrogen or carbon atoms stick on to that molecule when they come across it. These atoms make the molecule match up to other parts of the cell where they are needed.
So if the cell wall needs repairing, the RNA adds some atoms to a lipid molecule so that that molecule will fit into the cell wall, and not anywhere else in the cell. It’s like a puzzle piece that will only fit in one place in the puzzle. All the molecules leaving the Golgi body go either to the cell membrane, or through the cell membrane out of the cell, or to the lysosomes.
After the new atoms label the molecules, the molecules pinch off part of the Golgi body to make another little bubble around themselves. This bubble floats into the cytoplasm of the cell. In the cytoplasm, the bubbles float around until they find the right place for themselves, whether that’s repairing the cell membrane, leaving the cell to do something outside the cell, or going over to a lysosome.