Membranes form the boundaries of cells. In eukaryotic cells, membranes also create internal organelles that are critical for compartmentalizing cell functions and making the system more robust (note, from textbox below, that 95–98% of cell membrane mass is internal). As physical boundaries, they define the cell and organelle volumes and must be moved in cell motility and membrane trafficking. As chemical boundaries, they keep critical components in the cytoplasm and other factors outside the cytoplasm and possibly the cell. The hydrophobic barrier provided by lipid bilayers blocks movement of charged and hydrophilic substances into and out of the cell. An important robust property of cells is that plasma membranes can often be lysed multiple times before a cell will die. They serve a gateway function in that they need to facilitate entry of some components and the exit of others. Also, chemical and physical elements of the cellular microenvironment, including its architecture, need to be communicated to the cytoplasm. Cell shape is typically defined by the cytoskeleton for most mammalian cells, but membrane area to cell volume ratio affects the shape as well as the mechanical properties of the membrane. Membrane mechanics play a critical role in many cytoplasmic as well as cellular phenomena because cell membranes have a significant bending stiffness and resistance to stretch as well as a fluid nature.

Membranes as Compartment Boundaries

The basic role of membranes is to compartmentalize cells by blocking the free diffusion of water-soluble components. Biological membranes are organized around lipid bilayers that have a hydrophobic core and two hydrophilic surfaces. Hydrophobic molecules and surfaces cause an increase in the order of water by not forming hydrogen bonds and creating a surface like an air–water interface. Because order causes a decrease in entropy, the hydrocarbon layer of the bilayer separates readily from water by hiding in the center of the bilayer, making the bilayers quite stable. Breaking a bilayer and exposing the hydrophobic edges to water is energetically very unfavorable. For this reason, holes in bilayers reseal rapidly to cover the exposed hydrocarbon tails. Many membrane proteins extend across the bilayer and they have a hydrophobic surface that fits with the hydrophobic domain of the bilayer. The specific transport of ions and certain molecules by transmembrane proteins occurs through hydrophilic channels in those proteins (Chapter 8).