Final answer:
Lipid bilayers are considered two-dimensional fluids due to the movement of their components, facilitated by unsaturated fatty acids. In contrast, spherical liposomes are less fluid because their shape restricts the movement of phospholipids.
Step-by-step explanation:
The fluidity of the lipid bilayer is primarily attributed to the interactions of hydrophobic tails within its structure. Phospholipids, having a hydrophilic head and two hydrophobic tails, form this bilayer which serves as a barrier that separates water and other materials on each side. These membranes can be considered two-dimensional fluids as the components are free to move about, particularly influenced by the presence of unsaturated fatty acids which weaken the interactions between the tails, thereby increasing fluidity.
However, when phospholipids are heated in an aqueous solution, they tend to form spherical structures called micelles or liposomes. These liposomes have their hydrophilic heads on the outside and hydrophobic tails on the inside, exhibiting less fluidity compared to the planar bilayer sheets. The structural rigidity of spherical liposomes arises from their shape, which limits the movement of individual phospholipids within the structure, making them less fluid than the bilayer sheets.