Amphipathic describes a molecule that has both a hydrophilic region, which is attracted to water, and a hydrophobic region, which is not attracted to water. In IB Biology, phospholipids are the key example.
This concept is assessed at both SL and HL in B1.1 Carbohydrates and lipids, particularly when explaining the formation of phospholipid bilayers.
A phospholipid contains glycerol bonded to two fatty acids and a phosphate-containing group. Its two chemically different regions interact differently with water:
| Phospholipid region | Interaction with water | Result in a bilayer |
|---|---|---|
| Polar phosphate-containing head | Hydrophilic; attracted to polar water molecules | Faces the aqueous cytoplasm or extracellular fluid |
| Non-polar fatty acid tails | Hydrophobic; cluster away from water | Face inward, forming the bilayer’s hydrophobic core |
When phospholipids are placed in water, they naturally arrange themselves into a phospholipid bilayer. The hydrophilic heads remain in contact with water on both surfaces, while the hydrophobic tails point toward one another, away from water.
This arrangement forms a stable, continuous boundary around cells and organelles. The hydrophobic core has low permeability to ions, polar substances and many large molecules, helping the membrane control movement between aqueous environments.
Because the membrane’s interior is non-polar, small non-polar molecules such as oxygen and carbon dioxide can diffuse between phospholipids more readily than ions or polar molecules. Those hydrophilic substances generally require specific channel proteins, carrier proteins or pumps to cross the membrane, supporting selective permeability.
A common misconception is that “amphipathic” means a molecule dissolves equally well in water and lipids. It does not. It means that different regions of the same molecule have different affinities for water.
Exam technique: For an “explain” question, do not merely state that phospholipids are amphipathic. Identify the hydrophilic phosphate-containing heads and hydrophobic fatty acid tails, describe their orientation in water, and link this arrangement to spontaneous bilayer formation.
