The cell membrane of a cell is said to be semi-permeable because

The cell membrane is called semi-permeable because it selectively allows certain substances to pass through it while preventing others from entering or leaving the cell. The membrane is made up of a phospholipid bilayer, which acts as a barrier to large or charged molecules, like proteins and ions, while allowing smaller molecules, like oxygen and water, to pass through by diffusion or active transport. This process of selective permeability helps to maintain the internal environment of the cell and allows it to function properly. The membrane is also involved in various cellular processes, but it is not directly involved in energy production. The fatty acids in the membrane contribute to its structure, but they are not the only building blocks of the membrane.