Which type of molecule constitutes the inner bilayer of biological membranes?

The inner bilayer of biological membranes is primarily composed of phospholipids. These molecules have a unique structure that is essential for membrane formation. Each phospholipid consists of a hydrophilic "head" that is attracted to water and two hydrophobic "tails" composed of fatty acid chains. This amphipathic nature allows phospholipids to arrange themselves into a bilayer, with the hydrophilic heads facing the aqueous environment and the hydrophobic tails oriented inward, away from the water.

This arrangement creates a semi-permeable membrane that serves as a barrier to most water-soluble substances, while still allowing certain molecules to pass through. The flexibility and fluidity provided by the phospholipid bilayer are critical for the functionality of cells, including processes such as cell signaling and transport.

In contrast, triglycerides are primarily used for energy storage, fatty acids can be components of phospholipids but do not form the bilayer structure themselves, and steroids serve various roles in biological systems, including signaling and membrane fluidity, but do not constitute the main structural component of the membrane bilayer. Thus, phospholipids are the fundamental molecules that form the inner bilayer of biological membranes.