It contains proteins that provide a number of critical functions.

Because optimal cell activity is dependent on the transport of nutrients and useful materials into the cell as well as the removal of waste products from the cell, the cell membrane contains proteins and other molecules that execute a number of these functions. Some proteins are attached to these phospholipid mats to help move nutrients (such as oxygen and water) and wastes (such as carbon dioxide); others help the cell connect with and attach to the right kinds of materials (as well as other cells); and still others keep the cell from connecting with toxic materials and the wrong kinds of cells, foreign or otherwise.

Enzymes are specialized proteins that help break down bigger nutrients or mix multiple nutrients into more usable forms. Protein molecules may be linked to the surface of one of the cell membrane's layers, or they may be totally entrenched within the layer dwelling alongside the phospholipids, depending on their design and function. Some proteins that funnel nutrients into and out of the gap between the inner and outer layers of the cell membrane traverse just one of the phospholipid layers. Others are large enough to span both and are designed to transfer nutrients into the cell or funnel wastes away from the cell. Proteins also assist the cell in maintaining its form.

Furthermore, cell membranes participate in a variety of cellular processes such as cell adhesion, ion conductivity, and cell signaling, as well as serving as the attachment surface for several extracellular structures such as the cell wall and the glycocalyx, as well as the intracellular network of protein fibers known as the cytoskeleton. Cell membranes can be artificially rebuilt in the field of synthetic biology.