Question

The plasma membrane is composed of ______ % of phospholipids?

Answer 1

Phospholipids fall under the category of complex lipids. Complex lipids are basically esters of fatty acids with alcohols containing additional groups such as phosphate, nitrogenous base, carbohydrate, proteins, etc. The phospholipids contain phosphoric acid and frequently a nitrogenous base as the ‘additional groups’.
The plasma membrane is the outermost covering of the cell. It has many physiological roles and provides shape and rigidity to the cell. Usually, it is made up of proteins, lipids and some carbohydrates.

Complete answer:
The chemical constituents of the cell membrane are arranged in a ‘sandwich pattern’. It is a kind of a lipid layer sandwiched between two protein layers and garnished from both sides with some carbohydrates.
The proportions of these proteins and lipids varies from cell to cell. Some cells may have high contents of lipid in their cell membrane while others may have protein in greater quantities. For example, in human beings, the cell membrane of the erythrocytes (RBC) has 52% proteins and 40% lipids.
However, by weight, one can say that, the cell membrane is composed of roughly 50% proteins and 50% lipids.

Note:
The chemical structural organization of cell membrane was best described by Singer and Nicholson in their fluid mosaic model. It shows that fluidity of the cell membrane depends on lipids since the lipids (phospholipids) are unsaturated fatty acids. They form the main part of the cell membrane and the protein molecules just ‘float’ over the lipids and make a coating on both sides. The carbohydrates (oligosaccharides) remain attached to the outer side with the amino groups of proteins.