What is the main characteristic of a lipid bilayer?

The main characteristic of a lipid bilayer is that it is a flexible and dynamic structure. This flexibility allows for the movement of lipids and proteins within the bilayer, facilitating essential cellular functions such as cell signaling, transport, and membrane fusion. The lipid bilayer is composed of phospholipids arranged with their hydrophilic heads facing outward towards the aqueous environments, while the hydrophobic tails face inward, away from water. This arrangement creates a semi-permeable membrane that can adapt and change shape in response to various stimuli.

In addition to flexibility, the dynamic nature of the lipid bilayer is attributed to the fluid mosaic model, which describes how components such as proteins can move laterally within the membrane. This property is crucial for processes like endocytosis and exocytosis, where the membrane must change shape to encase materials for transport into and out of the cell.

Other characteristics that are excluded in the options reflect rigidity or inflexibility, which do not accurately represent the behavior of lipid bilayers in biological membranes. A lipid bilayer does not maintain a static shape, nor does it exhibit high permeability to all molecules, as its selective permeability is key to maintaining homeostasis in the cell.