Every living cell needs to obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules, minerals, etc.) from the environment, maintain water balance with its surroundings, and remove waste materials from the cell. The boundary of any cell in which all these substances must move through is the plasma membrane, composed of a phospholipid bilayer with a number of  protein molecules scattered throughout, along with the carbohydrates (glycoproteins), glycolipids and sterols. A phospholipids has both polar and non-polar regions. The tails of the two phospholipids layers are oriented towards each other. The hydrophilic heads which contain the phosphate portion, face out to the environment as well as into the cytoplasmof the cells interior, where they form hydrogen bonds with surrounding water molecules. Phospholipids are not bonded to each other. This gives the membrane its flexibility                                                                                            

Membrane  proteins

Some of these are embedded in the phospholipids layers, others move freely. Some are complexed to carbohydrate molecules, forming glycoproteins.

Cell adhesion (Intercellular joining) proteins, these are responsible for cell junctions, and some help maintain cell shape. Collagen is an important glycoprotein of the extracellular matrix, of which the adhesion can attach itself to.

Communication proteins at gap junctions permit cells to communicate with each other.

Receptor proteins serve as binding or attachment sites, especially for hormones or other molecular messengers. Some work in conjunction with carrier proteins, opening gated channels for ion movement. Some receptors are sensitive to nutrient levels, this can affect the metabolic rate or even cell division.

Transport proteins, these are carriers which have binding sites that attract specific molecules. When active transport takes place a molecule binds to the carrier it changes shape and moves the substance through the membrane. This requires energy, and this is where the transport protein is needed. Ions, amino acids, sugars and other small nutrient  molecules are moved through transport proteins.

Recognition proteins, Glycoprotein’s (carbohydrate-protein hybrids) and some glycolipids serve as surface receptors for cell recognition and identification. These play an important role for the immune system. They can distinguish between our own cells or foreign cells. These proteins are also used to guide cell attachment/adhesions in developmental processes.

Enzymatic proteins, Many of these are embedded in membranes which attract reacting molecules to the membrane surface. Enzymes needed for metabolic pathways can be aligned adjacent to each other to act like an assembly line for the reactions.