Cell membrane structure

The plasma membrane, or cell membrane, is composed of an amphipathic lipid bilayer. Amphipathic lipids have a hydrophilic (water-loving) head and hydrophobic (water-hating) tail. In an aqueous environment, these lipids naturally arrange into a spherical lipid bilayer. The water-loving, hydrophilic heads are facing towards the water so that the water-hating, hydrophobic tails can be buried inside the lipid bilayer, away from the water.

Lipid bilayers are highly fluid and elastic structures. The most abundant lipids in eukaryotic cell membranes are phospholipids. However membranes also contain other lipids such as sphingolipids, cholesterol, and glycolipids. These lipids have specific properties to adjust the nature of the membrane. Eg. for example a high proportion of cholesterol makes the membrane more rigid and increases its resistance to stiffening at low temperatures. In addition to lipids, the plasma membrane also contains proteins. Proteins are key components that move laterally through the membrane.

While the lipids form a barrier that encloses the cell, proteins fulfil a wider variety of functions. Importantly, proteins are responsible for transporting most types of molecules across the cell membrane. This makes the cell membrane a selective barrier. Since cells are highly specialized, the composition of the membrane is dependent on the cell’s function, and so can vary from organism to organism or cell type to cell type.

The multiple components of the plasma membrane which can move laterally through the membrane, are why the structure and function of the cell membrane is also known as the fluid mosaic model (Figure 1).

A three dimensional model of the bilayer cell membrane. Lipids with hydrophilic heads and hydrophobic tails form the main structure of the membrane. The lipids align to form two layers, with the heads facing outwards and the tails sandwiched in the middle. The lipid heads appear in different colors to represent the different types of lipids that make up the membrane. Proteins are embedded in the membrane, with some proteins represented as tubes that cross the membrane to allow molecules to travel in and out of the cell. Other proteins appear as tube like structures with protrusions on either side of the membrane, for molecules to fit into.

Figure 1. Model of the plasma membrane with phospholipids, sphingolipids, cholesterol, glycolipids (small spheres with tails in different colors), and proteins (larger red, yellow and blue objects).

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