<P> For many centuries, the scientists cited disagreed with the significance of the structure they were seeing as the cell membrane . For almost two centuries, the membranes were seen but mostly disregarded this as an important structure with cellular function . It was not until the 20th century that the significance of the cell membrane as it was acknowledged . Finally, two scientists Gorter and Grendel (1925) made the discovery that the membrane is "lipid - based". From this, they furthered the idea that this structure would have to be in a formation that mimicked layers . Once studied further, it was found by comparing the sum of the cell surfaces and the surfaces of the lipids, a 2: 1 ratio was estimated; thus, providing the first basis of the bilayer structure known today . This discovery initiated many new studies that arose globally within various fields of scientific studies, confirming that the structure and functions of the cell membrane are widely accepted . </P> <P> The structure has been variously referred to by different writers as the ectoplast (de Vries, 1885), Plasmahaut (plasma skin, Pfeffer, 1877, 1891), Hautschicht (skin layer, Pfeffer, 1886; used with a different meaning by Hofmeister, 1867), plasmatic membrane (Pfeffer, 1900), plasma membrane, cytoplasmic membrane, cell envelope and cell membrane . Some authors who did not believe that there was a functional permeable boundary at the surface of the cell preferred to use the term plasmalemma (coined by Mast, 1924) for the external region of the cell . </P> <P> Cell membranes contain a variety of biological molecules, notably lipids and proteins . Composition is not set, but constantly changing for fluidity and changes in the environment, even fluctuating during different stages of cell development . Specifically, the amount of cholesterol in human primary neuron cell membrane changes, and this change in composition affects fluidity throughout development stages . </P> <P> Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms: </P>

What is the main composition of the cell membrane ​
find me the text answering this question