The extracellular matrix is composed primarily of glycoproteins (proteins covalently bonded with carbohydrate, usually short chains of sugars) secreted by cells. The most abundant glycoprotein in the ECM is usually collagen which forms strong fibers outside the cells (the striped purple structures at left). These collagen fibers are embedded in a web of proteoglycans--molecules especially rich in carbohydrates (up to 95%) and can form large complexes (the blue, willowy structures). Another glycoprotein, fibronectin, binds to receptor proteins called integrins that built into the plasma membrane (which is about 0.00001 millimeters thick). Integrins span the membrane and bind to microfilaments on their cytoplasmic side, and are able to transmit between the ECM and the cytoskeleton.

Membrane proteins serve many functions, including transport, enzymatic activity, signal transduction, intercellular joining, cell-cell recognition, and attachment. A protein that spans the membrane may provide a hydrophilic (water-loving) channel across the solute-selective membrane. Some transport proteins hydrolyze ATP to actively pump substances across the membrane. A protein built into the membrane may be an enzyme, active site exposed to substances outside of the cell. Sometimes, enzymes are present in a series, carrying out sequential steps in a metabolic pathway. Membrane proteins may have a binding site with a specific shape that fits a chemical messenger, like a hormone. The signal may change the conformation of the protein that relays the message to the inside of the cell. Membrane proteins of adjacent cells may be hooked together in various ways. Some glycoproteins work as identification, recognized by other cells. Also, fibers may be bonded to membrane proteins, which helps the cell maintain its shape and fixes the location of certain membrane proteins. Proteins adhered to ECM can coordinate extracellular and intracellular changes.

The animation at right illustrates the fluid mosaic model. Originally, it was theorized that the phospholipid bilayer was sandwiched by sheets of proteins, but now the working model is one where proteins move in and around the bilayer. The phospholipids themselves have hydrophilic heads and hydrophobic tails. The proteins also have hydrophilic and hydrophobic regions which correspond with the phospholipids.