1.2 Many common features underlie the diversity of biological membranes. 1.2.1 All membranes share a“ three-layer” sheet-like appearance(of 6 to 10 nm thick under electron microscopic examination, with two electron-dense layers separated by a less dense central region. 1.2.2 All membranes are closed entities1.2 Many common features underlie the diversity of biological membranes. 1.2.1 All membranes share a “three-layer”, sheet-like appearance (of 6 to 10 nm thick) under electron microscopic examination, with two electron-dense layers separated by a less dense central region. 1.2.2 All membranes are closed entities