Phospholipids are amphipathic molecules whose hydrophilic phosphate heads and hydrophobic fatty-acid tails self-arrange into stable membrane bilayers in water-rich environments.
Coverage 2021–2021 · Updated 15 Jul 2026
Phospholipids are amphipathic: a polar phosphate-containing head interacts with water while non-polar tails avoid it, so they self-assemble into bilayers.
In water, heads face the aqueous environments and tails pack away from water in the interior. This arrangement creates a flexible hydrophobic barrier that separates compartments while allowing selected molecules to cross.
A bilayer requires:
In a cell membrane, water contacts both outer and inner head surfaces, while the tail core makes it difficult for many ions and polar molecules to pass without transport proteins.
The bilayer is not formed with tails facing water. Reversing the orientation would expose the least water-compatible part and would not create a stable membrane barrier.
This objective is assessed through structured response, commonly using Outline / Annotate.
Outline / Annotate
Build the answer around this relationship: Phospholipid heads are hydrophilic and phosphate-containing.
Reversing the hydrophilic phosphate head and hydrophobic fatty-acid tails.
Representative question
Annotate the diagram to illustrate the amphipathic nature of phospholipids.
B1.1 becomes easy when every answer follows structure -> property -> function. Carbon skeletons and functional groups create molecular diversity. Condensation builds larger molecules and hydrolysis breaks them. Alpha-glucose stores energy as starch and glycogen; beta-glucose forms strong cellulose. Surface carbohydrates enable recognition. Lipids are hydrophobic, triglycerides store energy, phospholipids self-assemble into bilayers, and steroids cross membranes because they are mostly non-polar.
