Answer: A fluid phospholipid bilayer with embedded proteins.
Explanation: Biological membranes are composed of phospholipids that interact with each other to form a bilayer in which proteins are embedded. Phospholipids have the fatty acyl chains (hydrophobic tails) which face each other to form the interior of the bilayer, while their polar head groups face the outward environment interacting with water molecules. The fluid mosaic model for structure of biological membranes is due to the interactions between the hydrophobic tails of the lipids. The fatty acyl chains of the phospholipids (nonpolar/ hydrophobic tails) in the interior of the membrane form a fluid hydrophobic region while the integral proteins float in the sea of these lipids. The proteins and lipids move freely and laterally in the plane of the bilayer, but movement there is restriction in the movement from one face of the bilayer to the other. The proteins are embedded at regular intervals and are held by hydrophobic interactions between the membrane lipids and the hydrophobic regions of the proteins.
The membrane mosaic is fluid because most of the interactions among its components are noncovalent, thereby leaving the lipid and protein molecules free to move laterally in the plane of the membrane.
