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Learning paths

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Learning path

Full curriculum

Full curriculum
  1. Amphipathic molecules and self-assembly in water
  2. Fatty acids, saturation and triacylglycerols
  3. Phospholipids and membrane-forming amphipathic structure
  4. Cells as membrane-bounded chemical systems
  5. Prokaryotic and eukaryotic cell organization
  6. Osmolarity, tonicity and cell-volume responses
  7. Modes of cell-cell signaling and target-cell specificity
  8. Second messengers as intracellular signaling intermediates
  9. Fluid-mosaic organization of biological membranes
  10. Phosphorus cycling through rocks, soils, water and biomass

Unit content

Phospholipids and membrane-forming amphipathic structure

A phospholipid is an amphipathic lipid with a strongly polar or charged head region attached to hydrophobic hydrocarbon chains. This combination makes phospholipids especially well suited to form bilayers in water.

A common class, the glycerophospholipids, contains:

  • a glycerol backbone;
  • two fatty-acid-derived hydrocarbon chains attached by ester linkages;
  • a phosphate-containing head group attached to the third glycerol position.

The phosphate group is strongly polar and often carries negative charge. Additional groups attached to phosphate can modify the net charge, size and hydrogen-bonding behavior of the head.

The two hydrocarbon chains provide a large nonpolar region. The complete molecule is therefore amphipathic:

polar head
    |
  glycerol
   /    \
 tail  tail

Why phospholipids favor bilayers

In water, the polar heads can remain hydrated while the hydrocarbon tails reduce their exposed nonpolar surface by packing together. Because many phospholipids have two tails and an approximately cylindrical overall shape, they commonly assemble into bilayers rather than small spherical micelles.

The bilayer has a hydrophobic interior and hydrated surfaces. This structure creates a barrier between aqueous compartments without requiring neighboring phospholipids to be covalently bonded to one another.

Chain composition changes bilayer properties

Fatty-acid chain length and unsaturation affect packing. Longer, straighter chains generally make stronger van der Waals contacts. Cis double bonds create kinks that frustrate close packing and tend to increase fluidity at a given temperature.

Cells can therefore alter membrane physical properties by changing lipid composition.

Phospholipids are not the only biological membrane lipids. Sphingolipids, glycolipids and sterols use different chemical architectures. What they share with phospholipids is the broader principle that amphipathic structure lets polar surfaces coexist with a hydrophobic membrane interior.

This unit explains the molecular material from which bilayers form. Membrane proteins, selective permeability, transport and electrochemical gradients are separate cellular properties built on top of this lipid foundation.