The outer layer of a human red blood cell is called the cell membrane or plasma membrane. It is a thin, flexible structure that surrounds the entire cell and acts as a barrier between the cell's internal environment and the external environment.

The cell membrane is composed of a phospholipid bilayer, which is a double layer of phospholipids arranged with their hydrophilic (water-loving) heads facing outward and their hydrophobic (water-hating) tails facing inward. This creates a lipid barrier that is impermeable to most molecules.

Embedded within the phospholipid bilayer are various proteins, glycolipids, and glycoproteins. These molecules perform a variety of functions, including:

* Transport of molecules into and out of the cell

* Attachment of the cell to other cells and molecules

* Recognition of specific molecules, such as hormones and antigens

* Generation of electrical signals and maintenance of cell shape

The cell membrane is essential for the survival of the red blood cell. It provides the cell with its shape, protects it from damage, and allows it to function properly in its role of transporting oxygen throughout the body.