Final answer:
The fluidity of the lipid bilayer refers to its ability to move and change shape, which is essential for cell membrane flexibility, semi-permeability, and transport of materials.
Step-by-step explanation:
The fluidity of the lipid bilayer refers to the ability of the phospholipids and proteins to move around within the plane of the cell membrane, allowing the membrane to change shape and to have flexibility. In other words, fluidity describes how molecules within the lipid bilayer can move past one another, and is closely related to the membrane's flexibility and semi-permeable nature. Unsaturated fatty acids with C-C double bonds in the tails of the lipids increase this fluidity; the more unsaturated the lipids are, the more fluid the membrane is. Temperature also affects membrane fluidity, as colder temperatures tend to decrease fluidity while warmer temperatures increase it. Membrane fluidity is crucial for various cellular processes, including the transport of materials into and out of the cell, and for the cell's ability to adapt to changing conditions.