Final answer:
In the plasma membrane of a typical human cell, proteins constitute about 50 percent of the membrane's composition by mass.
Step-by-step explanation:
The composition of a plasma membrane varies with cell type; however, proteins make up a significant portion of the membrane's mass. For a typical human cell, proteins account for about 50 percent of the composition by mass, lipids for about 40 percent, and carbohydrates for the remaining 10 percent. It's essential to note that proteins within the plasma membrane are crucial for its function, including facilitating selective permeability, interactions with other cells, cell adhesion, and hormone receptor activities, which highlights why a considerable mass of the membrane is made up of protein content.