11.7: Amphipathic Lipids and Membranes
- Describe the structure of membrane lipids
- Describe membrane components and how they are arranged.
Cell Membrane
All living cells are surrounded by a cell membrane. Plant cells and animal cells contain a cell nucleus that is also surrounded by a membrane and holds the genetic information for the cell. Everything between the cell membrane and the nuclear membrane, including intracellular fluids and various subcellular components such as the mitochondria and ribosomes is called the cytoplasm.
Lipids found in Membranes
The lipids in cell membranes are amphipathic. They have dual characteristics: part of the lipid is ionic and therefore dissolves in water, whereas the rest has a hydrocarbon structure and therefore dissolves in nonpolar substances. The ionic part is referred to as hydrophilic, meaning “water loving,” and the nonpolar part as hydrophobic, meaning “water fearing” (repelled by water). The amphipathic lipids commonly found in membranes are phospholipids and glycolipids.
Phospholipids are lipids containing phosphorus. The phosphate ion (PO 4 3 - ) is one of the components used in their formation. It is a major component of cell membranes. A phospholipid consists of a hydrophilic (water-loving) head and hydrophobic (water-fearing) tail (see figure 11.7.1). The phospholipid is similar to a triglyceride in which a fatty acid has been replaced by a phosphate group and an amine alcohol.
Figure \(\PageIndex{1}\): A phospholipid consists of a head and a tail. The "head" of the molecule contains the phosphate group and is hydrophilic, meaning that it will dissolve in water. The "tail" of the molecule is made up of two fatty acids, which are hydrophobic and do not dissolve in water.
The phospholipid drawn above is a lecithin . Lecithins occur in all living organisms. Egg yolks are especially rich in lecithins. Commercial-grade lecithins isolated from soybeans are widely used in foods as emulsifying agents. An emulsifying agent is used to stabilize an emulsion—a dispersion of two liquids that do not normally mix, such as oil and water. Many foods are emulsions. Milk is an emulsion of butterfat in water. The emulsifying agent in milk is a protein called casein . Mayonnaise is an emulsion of oil in water, stabilized by lecithins present in egg yolk.
Glycolipids are sugar-containing lipids. These lipids contain sphingosine rather than a glycerol and hence may be addressed as sphingolipids. The structure of sphingosine is shown in figure 11.7.2 to the left.The glycolipids are found on the outer surface of the cell membrane, acting as distinguishing surface markers for the cell and thus serving in cellular recognition and cell-to-cell communication. Shown in figure 11.7.2 is a glycolipid composed of sphingosine, a fatty acid, and galactose. Sphingolipids are important constituents of the membranes of nerve and brain cells.
Figure \(\PageIndex{2}\): Sphingosine is an amino alcohol found in all sphingolipids, Glycolipids contain a sugar unit.
Sphingolipids may be phospholipids or glycolipids that contain the unsaturated amino alcohol sphingosine rather than glycerol. Shown below in figure 11.7.3 is the structure of a phospholipid that is also a sphingolipid.
Figure \(\PageIndex{3}\): A sphingolipid may also be a phospholipid, as evidenced by the phosphoric acid unit in its structure.
Lipid Bilayers
The membranes of all cells have a fundamentally similar structure. Bilayers shown in figure 11.7.4 are double layers of lipids arranged so that the hydrophobic tails are sandwiched between an inner surface and an outer surface consisting of hydrophilic heads. The hydrophilic heads are in contact with water on either side of the bilayer, whereas the tails, are inside the bilayer, are prevented from having contact with the water.
Figure \(\PageIndex{4}\): In a water solution, phospholipids form a bilayer where the hydrophobic tails point towards each other on the interior and only the hydrophilic heads are exposed to the water.
In the bilayer interior, the hydrophobic tails (that is, the fatty acid portions of lipid molecules) interact by means of London's forces. The interactions are weakened by the presence of unsaturated fatty acids. As a result, the membrane components are free to move about to some extent, and the membrane is described as fluid. Cholesterol deposits in the bilayers adds rigidity to the membrane structure. The fused ring portion of cholesterol is rigid and inflexible. Bilayers like this make up every cell membrane.
Very few ions or polar molecules could pass through their hydrophobic “sandwich filling” to enter or leave any cell. Ions and polar species do cross the membrane, aided by proteins that are present in the lipid bilayer. Some proteins span the hydrophobic interior of the bilayer. Small ions and molecules soluble in water enter and leave the cell by way of channels through these proteins. Other proteins are more loosely associated with the surface of the lipid bilayer like tiles in a mosaic. This is known as the fluid mosaic model of the membrane structure.
Figure \(\PageIndex{5}\): The phospholipid bilayer of a cell membrane contains embedded protein molecules which allow for selective passage of ions and molecules through the membrane.
Summary
Lipids are important components of biological membranes. The membrane lipids are amphipathic: part of the molecule is hydrophilic, and part of the molecule is hydrophobic. Membrane lipids may be classified as phospholipids, glycolipids, and/or sphingolipids. Proteins are another important component of biological membranes. Deposits of cholesterol adds rigidity to the membrane.