Lipids

A lipid is a long-chain hydrocarbon that is soluble in nonpolar solvents and is primarily composed of carbon and hydrogen atoms. Unlike the other major groups of biological molecules (carbohydrates, proteins, and nucleic acids), lipids do not form polymers (chains) through dehydration reactions, or chemical reactions that combine two molecules with the elimination of a water molecule. Many lipids include fatty acid chains. A fatty acid is a molecule consisting of a hydrophilic head made up of a carboxyl group (${-}{\rm{COOH}}$) and a hydrophobic tail made up of a hydrocarbon chain. Lipids are not soluble in water, giving them important roles in cells. Phospholipids form the cell membrane, the protective barrier separating a cell's interior from its external environment. Lipids also serve as long-term energy storage molecules. In addition, some lipids play important roles in cell signaling, growth, and development.

The main role of lipids is to form a barrier between a cell's watery interior and the outside environment. The cell membrane is composed of a double layer of phospholipids, called a lipid bilayer. A phospholipid is a lipid molecule composed of glycerol (a carbon compound) bonded to two fatty acids and a phosphate group. It forms the basis of cell membranes. The phosphate group, along with a glycerol, forms a hydrophilic (water-soluble) head. The hydrophilic head and hydrophobic tail work together to orient the bilayer, such that the heads face out (toward the aqueous environments inside and outside the cell) and the tails face in (toward each other). The dual nature of phospholipids provides the cell membrane with selective permeability, or the allowance of certain molecules to pass through the cell membrane, because few molecules can cross the hydrophilic and hydrophobic barrier without assistance. Small, uncharged, nonpolar molecules, such as oxygen gas, can cross unassisted because of their low molecular weight. However, large, charged, and polar molecules need assistance crossing the membrane. By employing transmembrane proteins to form channels and actively move substances through the lipid bilayer, a cell has some control over which molecules enter and exit the cell. Weak bonds between lipids in the bilayer mean that the surface is fluid, moving as needed to accommodate molecules, such as transport proteins, embedded in the membrane. The chemical and physical properties that cause phospholipids to form the large bilayer of cell membranes in water also act on a smaller scale. A micelle is an aggregate of phospholipid molecules in water whose hydrophobic tails interact at the center. The hydrophobic tails of the phospholipids join together to avoid contact with water. Micelles are used to transport hydrophobic substances through water. Slightly larger phospholipid structures, called vesicles, can be used to transport hydrophilic substances. In addition to transport, these double-layered phospholipid structures can be used for cellular storage, as chemical reaction chambers, and for cellular digestive purposes. For example, a substance such as medicine can be enclosed in a liposome, a small, fluid-filled sac surrounded by a phospholipid bilayer. Surrounded by a miniature cell membrane, liposomes allow for drug delivery to tissues that would otherwise be unreachable.

Lipids also serve as long-term energy storage molecules. While lipids and carbohydrates are both organic molecules consisting largely of carbon and hydrogen, lipids tend to be smaller than carbohydrates, and they store more energy. Gram for gram, lipids produce over twice as much energy as carbohydrates. Cells store lipids in droplets made of triacylglycerol molecules. A triacylglycerol (also called triglyceride) is a lipid composed of three fatty acid chains bonded to a glycerol head. These molecules are commonly referred to as fats and oils.

A saturated fatty acid has only single bonds in the fatty acid tail, whose carbons are bonded with the maximum number of hydrogens. Because the fatty acid has no double bonds, it is a straight molecule. The straight shape of saturated fatty acids allows them to stack densely, giving them thicker textures at room temperature, such as with cheese, butter, and coconut oil. A fatty acid that has one or more double bonds between carbons—and thus fewer than the maximum number of hydrogens possible—is an unsaturated fatty acid. The double bonds form kinks in the hydrocarbon chain. An unsaturated fatty acid containing only a single double bond is monounsaturated, while an unsaturated fatty acid containing more than one double bond is polyunsaturated. The presence of double bonds forms kinks in unsaturated fatty acids, which prevent the molecules from stacking too densely. The double bonds maintain the looser texture of olive and soybean oils, liquids at room temperature. Doctors have discouraged the use of saturated fats because of possible links between their consumption and heart disease. In response, an altered form of unsaturated fats called trans fats became popular. Trans fats are partially hydrogenated unsaturated oils that take on the texture of thick and creamy saturated fats. The double bonds of unsaturated fats are modified by this process, which allows the kinked molecules to stack densely. Studies have found that consuming trans fats is likely more dangerous than consuming saturated fats, and their use has been banned in some countries. Another group of lipids, the steroids, plays key roles in the growth and maintenance of the body. A steroid is a lipid, such as cholesterol, containing three six-carbon rings and one five-carbon ring. Some vitamins, such as vitamin D, and hormones, such as testosterone and estrogen, are also made from steroids. An important steroid is cholesterol, which helps with the production of steroid hormones, bile salts, and vitamin D and controls cell membrane fluidity. Cholesterol is the precursor to vitamin D, the sex hormones, and bile compounds, which are important for digestion. Cholesterol is also a vital component of the cell membrane, preventing it from becoming too sticky at low temperatures or too loose at high temperatures.

However, cholesterol can have negative health implications. Cholesterol is transported in the cardiovascular system by lipoproteins: lipids bound to proteins. Lipoproteins come in several varieties, including high density and low density. A high-density lipoprotein (HDL) is a structure containing a high proportion of proteins and some lipids that transports hydrophobic molecules in water. A low-density lipoprotein (LDL) is a structure containing a high proportion of lipids and some proteins that transports hydrophobic molecules in water. HDL is commonly called "good" cholesterol, and LDL is commonly called "bad" cholesterol, although cholesterol is only one component of these structures. They are so named because high levels of LDL are associated with an increased risk of heart disease and stroke. Low-density lipoproteins deliver molecules that can build up in blood vessels, leading to blockages and disease. Like LDL cholesterol, very low-density lipoprotein, or VLDL, is also considered a “bad” cholesterol, leading to blood vessel blockage and heart disease. Very low-density lipoprotein is different from low-density lipoprotein in that it contains a higher percentage of triglycerides. While cholesterol is in some edible animal products, such as eggs and meats, diet alone is not responsible for a person's cholesterol levels. Genetics and body chemistry play a major part. Human bodies synthesize most if not all of the cholesterol they need, so people do not need to consume it at all. Cholesterol levels can be managed through a combination of diet, exercise, and medicine as needed.