Cell borders are the boundaries that separate individual cells from each other. They are composed of a phospholipid bilayer, which is a double layer of lipids (fats). The phospholipid bilayer is semi-permeable, meaning that it allows some molecules to pass through while blocking others.

The cell border also contains a variety of proteins, including:

* Integral proteins: These proteins are embedded in the phospholipid bilayer. They can have a variety of functions, such as transporting molecules into and out of the cell, or providing structural support.

* Peripheral proteins: These proteins are not embedded in the phospholipid bilayer. They are instead attached to the inner or outer surface of the membrane. Peripheral proteins can have a variety of functions, such as regulating the activity of integral proteins, or providing a link between the cell membrane and the cytoskeleton.

The cell border is a dynamic structure that is constantly changing in response to the cell's environment. For example, the cell membrane can change its composition to adapt to changes in temperature or pH. The cell membrane can also fuse with other membranes, such as the membranes of neighboring cells or the membranes of organelles within the cell.

The cell border is essential for the cell's survival. It provides a barrier that protects the cell from its surroundings, and it allows the cell to transport molecules into and out of the cell. Without the cell border, the cell would not be able to function properly.