2.2.8—Phospholipids

A phospholipid has a glycerol backbone, two fatty-acid tails and a phosphate-containing group. The phosphate-containing head is hydrophilic, while the fatty-acid tails are hydrophobic, so the molecule is amphipathic.

• Hydrophilic head: the phosphate-containing group interacts favourably with water.
• Hydrophobic tails: the two fatty-acid chains avoid water. Their saturation can affect how closely neighbouring tails pack.
• Amphipathic arrangement: in water, heads face the watery environments and tails point inward away from water.
• Bilayer consequence: two opposing layers create a hydrophobic core, forming the basic barrier of a cell-surface membrane; many water-soluble substances do not cross that core freely.

Because each phospholipid has a water-compatible end and water-avoiding ends, large numbers self-arrange into a bilayer rather than dissolving uniformly. The hydrophobic interior makes the membrane selectively restrictive, while membrane proteins can provide specific routes for substances that cannot cross the core unaided.

Do not describe a phospholipid as glycerol plus three fatty acids: two tails and a phosphate-containing head distinguish it from a triglyceride. The bilayer is a selective barrier, not an impermeable wall, and the phosphate head is only one part of the structure that creates its function.