BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.9. Cell Membranes

5.9.3. Phospholipids

The Structure of Phospholipids is described in detail in Chapter 3 (Fig. 3.18). A phospholipid molecule consists of a polar¹ HEAD (a phosphate group) and two non-polar hydrocarbon tails (fatty acid residues). Polarity means that electrical charges are unevenly distributed within the molecule, which makes it soluble in Water. A characteristic feature of phospholipids is that their head is hydrophilic, whereas their tails are hydrophobic.

A small amount of phospholipid, when spread over a water surface, forms a monolayer, as shown in Fig. 5.14. The non-polar hydrocarbon tails project out of the water, while the polar hydrophilic heads lie on its surface.

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Fig. 5.14. A monolayer of phospholipid molecules on a water surface.

If there is more phospholipid than is needed to simply cover the water surface, or if a phospholipid is shaken with water, particles known as micelles are formed. The hydrophobic tails of the phospholipid molecules are tucked away inside them, thereby shielding them from contact with water (Fig. 5.15, A). Figs. 5.15, B and 5.15, C illustrate structures that consist of a double layer of phospholipid molecules—the so-called bilayer. It is now well established that the phospholipid bilayer is the foundational Structure of Cell membranes.

Fig. 5.15. A spherical micelle (A) and a double layer (bilayer) of phospholipid molecules (B), shown in cross-section. C. A three-dimensional model of the bilayer.

¹ Recall that polar groups or molecules have an affinity for water (they are hydrophilic), whereas non-polar groups do not mix with water (they are hydrophobic) (see Section 3.1.2).