Cell Membrane

The cell membrane (plasma membrane) is a thin, flexible barrier that surrounds the cell-s cytoplasm, separating the cell interior from the external environment. It is composed of a semipermeable phospholipid bilayer with embedded proteins, which controls the transport of materials into and out of the cell.

• All cells have a cell membrane; in plants, fungi, and bacteria, a rigid cell wall exists outside the cell membrane, but animal cells have only the membrane.
• Semipermeable barrier: small nonpolar molecules (e.g. O-, CO-) diffuse through the lipid bilayer easily, while ions and polar molecules require protein channels or transporters to cross.
• Fluid mosaic model: the membrane-s phospholipids and proteins can move laterally, giving the membrane flexibility and allowing cell shape changes (e.g., red blood cells squeezing through capillaries).

• Classic trap - Don-t confuse the cell membrane with the cell wall. If a question asks what encloses an animal cell, the answer is the plasma membrane (animals have no cell wall).
• Function clues - Descriptions like -selectively permeable barrier- or -phospholipid bilayer with proteins- are referring to the cell (plasma) membrane.
• Structure ID - On diagrams, the thin line surrounding the cell (just inside any cell wall) is the plasma membrane.