Two major structural adaptations have been linked with the adaptation of the membrane to extreme pH and temperature environments: the synthesis of membrane-spanning, bipolar lipids and the binding of the glycerol moiety and the hydrocarbon chains by an ether bound. Bipolar lipids can form lipid monolayers, in which each polar headgroup points out on one side of the membrane. Monolayers are more rigid, less permeable and thermally more resistant than lipid bilayers.