Final answer:
Biological membranes are comprised of bilayers made from phospholipids, with phosphatidylcholine being a significant constituent.
Step-by-step explanation:
Biological membranes contain bilayers primarily made from a class of lipids known as phospholipids. Phospholipids are unique in their structure, as they have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. This arrangement allows them to form a bilayer where the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward, towards the aqueous environment. Phosphatidylcholine is an important phospholipid present in these bilayers. Alongside lipids, proteins play crucial roles in the function of biological membranes, with integral proteins spanning the bilayer and peripheral proteins attached to its surface.