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Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to catalyze passage. They are often anchored in place within the membrane by tethers to proteins outside the cell, cytoskeletal elements inside the cell, or both. In living cells, however, many proteins are not free to move.

Therefore, the collection of lipids and proteins that make up a cellular membrane relies on natural biophysical properties to form and function. Scientists who model membrane structure and dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move laterally in the lipid bilayer. The portions of these proteins that are nested amid the hydrocarbon tails have hydrophobic surface characteristics, and the parts that stick out are hydrophilic (Figure 2).Īt physiological temperatures, cell membranes are fluid at cooler temperatures, they become gel-like.

Many of these proteins are embedded into the membrane and stick out on both sides these are called transmembrane proteins. In fact, proteins account for roughly half the mass of most cellular membranes. In addition to lipids, membranes are loaded with proteins. © 2010 Nature Education All rights reserved. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition. However, cholesterol is not present in bacterial membranes or mitochondrial membranes.

Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids in animal cell plasma membranes. Thus, the hydrophilic heads of the glycerophospholipids in a cell's plasma membrane face both the water-based cytoplasm and the exterior of the cell.Īltogether, lipids account for about half the mass of cell membranes. In water, these molecules spontaneously align - with their heads facing outward and their tails lining up in the bilayer's interior.

This is because they are two-faced molecules, with hydrophilic (water-loving) phosphate heads and hydrophobic (water-fearing) hydrocarbon tails of fatty acids. Like all lipids, they are insoluble in water, but their unique geometry causes them to aggregate into bilayers without any energy input. Glycerophospholipids are by far the most abundant lipids in cell membranes.

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