# Membranes --- **Membranes** are sheetlike [[biology|biological]] structures that either form solid barriers between things, or otherwise allow selective passage of particular substances. They can also provide structural support (probably...I'm pretty sure). There are two main kinds of membranes: [[plasma membrane|plasma membranes]] which are made up of many molecules associated with one another (usually phospholipids), and [[tissue membrane|tissue membranes]], which are made up of many cells that are all associated with each other. ___