Fundamental building blocks of all cell membranes are phospholipids, which consist of two hydrophobic fatty acid chains linked to a phosphate-containing hydrophilic head group. explain how the structure of the bilayer membrane acts as a stable barrier between two aqueous (water based) areas.

Phospholipids spontaneously form stable bilayers. In phospholipid layer in cell membrane, there are hydrophilic heads (polar part of the layer) and hydrophobic tails (nonpolar part of the layer).

Polar head groups are exposed to water and their hydrophobic tails buried in the interior of the membrane.

1) Small, non-polar molecules diffuse easily through this layer, no energy is required, this is passive transport.

2) Polar molecules cannot diffuse through phospholipid layer using passive transport, because hydrophobic tails does not allow that (polar molecules are reppeled), energy is required for that, this is active transport.

Also, the lighter (smaller) particles move more quickly than heavier (larger) ones at the same temperature.