Cells need membranes to define and compartmentalise space, facilitate and regulate the flow of materials, detect external signals and mediate interactions between cells. Our current understanding of membrane structure represents the culmination of more than a century of studies, beginning with the recognition that lipids are an important membrane component and moving progressively from a lipid monolayer to a phospholipid bilayer.

Once proteins were recognised as important components, Hugh Davson and James F Danielli proposed their “sandwich” model, with the lipid bilayer surrounded on both sides by layers of proteins. This model stimulated much research and was presumed to apply to the variety of membranes revealed by electron microscopy. As membranes and membrane proteins were examined in more detail, however, the “sandwich” model was called increasingly into question and was eventually discredited.

In its place, SJ Singer and GL Nicholson&’s fluid mosaic model emerged and is now the universally accepted description of membrane structure. According to this model, proteins with varying affinities for the hydrophobic membrane interior float in and on a fluid lipid bilayer. Prominent lipids in most membranes include phospholipids (both phosphoglycerides and phosphorsphingolipids) and glycolipids such as cerebrosides and gangliosides.

In eukaryotic cells, sterols are also important membrane components; these include cholesterol in animal cells and several related phytosterols in plant cells. Sterols are not found in the membranes of prokaryotes, but at least some bacterial cells contain somewhat similar compounds called hopanoids.

In terms of their association with the lipid bilayer, membrane proteins are classified as integral, peripheral or lipid-anchored. Integral membrane proteins have one or more short segments of predominantly hydrophobic amino acids that anchor the protein to the membrane. In many cases, these are transmembrane segments, such that portions of the protein are exposed on both sides. Most of these transmembrane segments are a-helical sequences of about 20-30 predominantly hydrophobic amino acids. Peripheral membrane proteins are hydrophilic and remain on the membrane surface. Lipid-anchored proteins are also hydrophilic in nature but are covalently linked to the membrane by any of several lipid anchors that are embedded in the lipid bilayer.

Most membrane phospholipids and proteins are free to move within the plane of the membrane unless they are specifically anchored to structures on the inner or outer membrane surface. Transverse diffusion, or “flip-flop”, between monolayers is not generally possible, except for phospholipids when catalysed by enzymes called phospholipid translocators or flippases. As a result, most membranes are characterised by an asymmetric distribution of lipids between the two monolayers and an asymmetric orientation of proteins within the membranes, so that the two sides of the membrane are structurally and functionally dissimilar.

Membrane proteins function as enzymes, electron carriers, transport molecules and receptor sites for chemical signals such as neurotransmitters and hormone. Membrane proteins also stabilise and shape the membrane and mediate intercellular communication and cell-cell adhesion. Many proteins in the plasma membrane are glycoproteins, with carbohydrate side chains that protrude from the membrane on the external side, where they play important roles as recognition markers on the cell surface.

The writer is Associate Professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also fellow, Botanical Society of Bengal, and can be contacted at tapanmaitra59@yahoo.co.in