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Phospholipids

Phospholipids are a special type of lipid. While a usual triglyceride is composed of a glycine molecule bonded to three fatty acid molecules, a phospholipid replaces one of the fatty acids with a phosphate group (Figure 1).

Picture of a molecular model of a phospholipid.
Figure 1: A phospholipid molecule containing 2 capric acid molecules and a phosphate molecule. The long carbon-carbon chains of the capric acid molecules are hydrophobic, while the phosphoric acid group (phosphate) is hydrophilic. The differing properties of these chemical groups contained in the same molecule make this an amphipathic molecule, which gives it the ability to interact with both hydrophilic (water based) and hydrophobic (non-water based) environments.

Unlike the fatty acids, which are hydrophobic, the phosphate group is strongly polar, and prefers being in contact with water. This makes the entire molecule one that contains both hydrophobic, and hydrophilic regions. This type of molecule is called ampithatic.

The presence of both of these regions in the same molecule means that the hydrophobic regions of the molecule must be buried away from water, while the hydrophilic regions must be exposed to it. This is difficult for a single molecule to achieve in a single solvent, as the molecule would by definition be either hydrophobic or hydrophilic.

However, many phospholipid molecules together are able to satisfy both their phosphate group's need to be exposed to water, and their fatty acid's need to be buried away from water. One conformation that achieves this is the phospholipid bilayer (Figure 2). A layer of phospholipid molecules has a hydrophilic surface formed by the phosphate groups and a hydrophobic surface formed by the fatty acid groups. This would allow for a membrane to form between a hydrophobic and hydrophilic environment, however, the two hydrophobic surfaces can themselves stick together, allowing a membrane between two hydrophilic environments.

Picture of a molecular model of a phospholipid bilayer.
Figure 2: Due to the ampithatic properties of the phospholipid molecule, an energetically favourable conformation for the molecules to adopt is the phospholipid bilayer. The hydrophobic fatty acid chains are buried in the bilayer and face each other, and are adjacent to other fatty acid chains, buried away from water molecules. At the same time the hydrophilic phosphate groups are on the membrane surfaces where they can be exposed to water molecules. The phospholipid bilayer is the basis behind cell membranes, being able to form selectively permeable membranes between compartments where different biological reactions can take place, including between cells and the extracellular space, and between organelles within cells.

This phospholipid bilayer is the basis behind cell membranes. This membrane is a way of separating water based solutions from one another using a simple molecular structure. This allows compartmentalization, meaning that certain compounds and reactions can occur in specific parts of an organism which are specialized in those reactions. The bilayer is a simple to form and flexible way of achieving this. Many membrane proteins can be present in the membranes, which allow communication and transport across the membrane.

All in all, membranes are fascinatingly complex structures built upon the relatively simple chemical properties of phospholipids.