In biology, a membrane refers to a thin surface that serves as a boundary, partition, or lining that separates two areas. Membranes often serve to protect areas.
An example of a biological membrane is the cell membrane. Most cell membranes are made of two layers of phospholipids. For this reason, cell membranes are sometimes referred to as lipid bilayers. The phosphate heads of the phospholipids are hydrophilic and, therefore, face outwards. The fatty acid tails of the phospholipids are hydrophobic and face inwards.
Cell membranes are said to be selectively permeable. This means that cell membranes allow some things into the cell, but keep other things out. In this way, the cell membrane serves as a protective barrier to a cell’s internal contents.
Embedded within the cell membrane are proteins. These proteins serve as channels through which only specific substances can pass. This is one way in which the cell membrane is able to be selectively permeable.