Cell membranes are one of the most important cell structures. They protect cells from their environment and help regulate what goes in and out of cells. Cell membranes are made up of a lipid bilayer, with proteins embedded in it. The lipid bilayer is made up of two layers of lipids (fats), with the hydrophobic tails pointing towards each other and the hydrophilic heads pointing outwards. Proteins are found in the spaces between the lipid molecules, and they can be either integral or peripheral. Integral proteins span the entire width of the membrane, while peripheral proteins only attach to one side.
The cell membrane is semi-permeable, meaning that it allows some substances to pass through it while keeping others out. This is accomplished by special proteins called transmembrane proteins, which act as channels or pores that allow specific molecules to cross the cell membrane. The size, charge, and shape of molecules all affect how easily they can pass through the cell membrane. Smaller molecules like oxygen and carbon dioxide can diffuse across the cell membrane, while larger molecules like glucose need help from transport proteins to get across.
Cell membranes also play an important role in communication between cells. Specialized protein receptors on the surface of cell membranes bind to signaling molecules sent from other cells. This binding triggers a change in the receptor protein that can cause changes in other proteins inside the cell, leading to a cellular response. Cell membranes also have sugar groups on their surface that serve as identification tags for cells. These sugar tags help immune cells recognize foreign cells and viruses so they can destroy them before they cause infections