Membrane phospholipids have hydrophilic heads that face outward; during membrane assembly, enzymes from the Golgi apparatus may leave the cell through vesicles that fuse with the membrane. The enzymes alter the positions of phospholipids, resulting in different arrays of phospholipids on the outer and inner sides of the membrane.
An individual phospholipid is a lipid that usually consists of two fatty acid tails covalently linked to a common phosphate group. A glycerol molecule is a colorless, odorless molecule that works with carbohydrate and lipid metabolism. A phosphate group is one phosphorus atom bound to four oxygen atoms, and it is one of several molecules that make up nucleic acids. Fatty acid chains may be either saturated or unsaturated and are long hydrocarbon molecules. Saturated fatty acid chains are organic compounds that contain only carbon-carbon single bonds. Unsaturated fatty acid chains contain single-carbon, double-carbon, and triple-carbon bonds and link carbon to hydrogen.
Overall, phospholipids are amphipathic, meaning that they have both hydrophilic and hydrophobic parts. The glycerol molecule and phosphate group make up the head that is hydrophilic, which is having a strong affinity toward water, and are polar. The two fatty acid chains that make up the tails are hydrophobic, which means having a weak or no affinity to water, and they do not dissolve in water and are nonpolar.
The hydrophilic head has an affinity for water and is, therefore, positioned so that it is oriented outward in the bilayer, toward the intracellular and extracellular fluids. The hydrophobic tails do not have an affinity for water and are therefore oriented inward, where there is no water. Similarly, when individual phospholipids are in an aqueous solution, they clump together to form a micelle. A micelle is a sphere of phospholipids that forms in response to the amphipathic nature of fatty acids, with the hydrophilic heads positioned outward and the hydrophobic tails positioned inward.
Most phospholipids are derived from glycerol and constructed within the smooth endoplasmic reticulum (ER), which is a network of membranes that helps process molecules in a cell and transports cell materials, of the cell. Primarily within the cytosol and just outside the ER, there is a variety of enzymes that are responsible for the synthesis of many different molecules. Fatty acid synthase, glycerol phosphate acyltransferase (GPAT), lysophosphatidic acid acyl transferase (LPAAT), phosphatase, and choline phosphotransferase are some the enzymes involved in the synthesis of phospholipids. Each enzyme plays a specific role in catalyzing, or assisting, a specific biochemical reaction. Fatty acid synthase is responsible for the synthesis of fatty acids, while the other enzymes are responsible for phospholipid synthesis. Because phospholipids contain fatty acids, the fatty acids must be made prior to the phospholipid. Once made, the lipids are transported via vesicles, which bud from the membrane of the endoplasmic reticulum. The vesicles then fuse with the membrane of the Golgi apparatus, which is an organelle that attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules to their correct places inside or outside a cell.
Cell membranes are typically disproportionate, in that the layer that faces the inside of the cell is often different in content and size from the layer that faces the extracellular fluid. The phospholipid bilayer forms a wall between the outside of the cell, where there is extracellular fluid, and the inside of the cell, where there is cytoplasm. The bilayer is fluid in that it adjusts to allow transport of molecules through the membrane. The two sides of the phospholipid bilayer contain very different phospholipids and other molecules, such as glycolipids, which are found in eukaryotic cell membranes, and cholesterol, which is found in animal cells. Glycolipids assist in maintaining cell membrane stability and expedite cell recognition, while cholesterol helps to regulate membrane proteins. The varieties of glycolipids and cholesterol products partially account for the variations in the makeup of the phospholipid bilayer. Enzymes in the Golgi apparatus and beyond modify these molecules by adding or snipping pieces and then prepare them for export to other parts of the cell.
In addition to the lipid bilayer and embedded proteins, the outer layer of the cell membrane is covered with a layer of carbohydrates, called a glycocalyx. A carbohydrate is an organic molecule that contains carbon, hydrogen, and oxygen and provides energy to cells, and the glycocalyx is an extracellular layer of polysaccharides and glycoprotein that coats the cell membrane. This can be a capsule or slime layer. It can create a fuzzy coat on the outside of the cell membrane and can facilitate cell attachment, moisture retention within the cell, and cell protection. A glycocalyx coating on a bacterium helps protect the bacterium from a body's immune system. One example of glycocalyx is the presence of dental plaque produced by a specific type of streptococcus bacteria, which is a gram-positive bacteria.
Phospholipids vary in the degree of saturated and unsaturated fatty acid chains. Unsaturated fatty acid chains have a double bond found in the carbon backbone, while saturated fatty acid chains do not have double bonds while having a maximum number of hydrogen atoms bonded to carbon atoms. The greater the proportion of unsaturated fatty acid chains to saturated fatty acid chains, the more fluid the cell membrane will be. Phospholipids can also vary in the length of the tails. Shorter tails allow more fluidity of the membrane. The Golgi apparatus sorts the various phospholipids, identifying and "tagging" them for distribution to their proper location, and vesicles bud from the Golgi apparatus and transport the phospholipids to their final destination.