Lipids are macromolecules, large organic molecules, that carry out many different functions needed for life. Lipids function to store energy, compose the membrane of cells, and act as chemical signalers. In terms of the elements found in lipids, all lipids contain oxygen, hydrogen, and carbon. Some lipids also contain phosphorus and nitrogen. The elements that a lipid is made out of depends upon the type of lipid and the role it carries out. Let’s examine some of the different types of lipids. The four main types of lipids are sterols, phospholipids, waxes, and fats.
Properties Of Lipids
Lipids are somewhat different from other biological molecules because they are hydrophobic. Unlike hydrophilic (water-loving) molecules that dissolve in water, lipids don’t dissolve when in the presence of water. You may have seen this property in action if you have looked at salad dressing. You’ll notice that there is a vinegar layer, which is mostly water, and an oil layer. The water and oil form separate layers due to the hydrophobic properties of the lipids in the salad dressing.
Lipids are made out of a form of alcohol known as glycerol, fatty acids, and carbon. Carbon is the basic building block for all known organic molecules. Carbon atoms are found in all living things, and carbon is able to create such a diverse amount of life because it can form many different types of molecules. Lipids, like other organic molecules, are made out a skeleton or base created from carbon atoms and other molecules that attach themselves to the skeleton. When fatty acids and glycerol bond with the carbon-based skeleton, lipids are created. Lipids are broken down by a process known as hydrolysis, and certain enzymes like lipase (a subclass of esterases) are responsible for starting this process.
Function Of Lipids
Lipids perform many different functions in the body. Lipids are compounds capable of storing chemicals and triglycerides function as energy storage/reserves of energy for the body. Lipids are also important compounds that comprise the cell membranes of cells in eukaryotic organisms. Lipids don’t just function as components of the cell membrane, they also regulate how permeable the membrane is. Lipids are even stores of fat soluble vitamins like vitamin A, D, E, and K.
Some lipids, such as steroid hormones and prostaglandins, function as metabolic regulators in the cell. Cholesterol itself is found within the blood and cell membranes of most organisms. Cholesterol is what helps cell membranes maintain their fluidity, through the interaction between cholesterol and lipid complexes. Cholesterol is necessary for the creation of steroids, vitamin D, and bile acids. Polyunsaturated phospholipids give the membrane of cells flexibility and fluidity.
Lipoproteins are composed of proteins and lipids, and they are what let lipids be transported through aqueous environments and around the rest of the body. Another form of lipids, fatty acids (such as linolenic acids and linoleic acids) are necessary for the creation of a type of molecule called ecosanoids. Ecosanoids like thromboxanes and prostaglandins are involved in body defense/repair and immune system processes like blood clotting, inflammation, fever and pain.
Lipids function as electrical insulators for nerve fibers, with the lipids found in the myelin sheaths of the fibers. Lipids also function as signaling molecules, transmitting signals from cell to cell and system to system.
The Role Of Lipids In The Metabolism
Lipids play a critical role in the function of the metabolism, with sterols, phospholipids and triglycerides being the primary types of lipids involved in the metabolic system. The lipid metabolism degrades certain lipids and synthesizes others, creating the functional lipids used in individual tissues.
Excess carbohydrates are broken down and converted into triglycerides, and this is accomplished through the utilization of acetyl-CoA and fatty acids. Acetyl-CoA is broken down and used to create fatty acids, and the fatty acids then undergo a process known as esterification which creates triglycerides.
The fatty acids themselves are created through a synthesis process that reduces acetyl-CoA molecules. Unsaturated fatty acids can be synthesized through a process known as desaturation. During desaturation, double bonds are plugged into the fatty acid chain. Linoleic acid is an example of a double unsaturated fatty acid, and mammalian cells cannot synthesize it, meaning that mammals must consume organisms which have created the acid to obtain it for themselves.
Acetyl-CoA is generated through the breakdown of fatty acids by either peroxisomes or mitochondria. This breakdown is referred to as beta-oxidation. Two carbon fragments are broken off of the carboxyl end of the fatty acid, and the carbon fragments are used to generate acetyl-CoA. The acetyl-CoA will eventually be used to make ATP through the electron transport chain of the citric acid cycle.
Types Of Lipids
Fats And Oils
Saturated fats, unsaturated fats, and oils are structures are referred to as triglycerides, and they are created by the reaction of glycerol with one of several different fatty acids. These fats can either be liquid or solid at room temperature, depending upon how the triglyceride is structured.
Though you may have heard that saturated fats are healthier for you than unsaturated fats, the reason for this difference isn’t apparent until you know how the fats are different in their chemical makeup. The atoms of carbon found in a molecule of glycolipids can only bond a maximum of four times with other atoms. However, saturated fats operate a bit differently. Saturated fats have carbon atoms that form a single bond with atoms like hydrogen in the molecule. This means that they create a long straight chain or tail of fatty acid that allowed many molecules to be packed into a small amount of space. The effect of these long, tightly packed chains of saturated fats is that products which contain many saturated fats like butter or lard are frequently solid at room temperature. In contrast, unsaturated fats have carbon atoms that form double bonds with other atoms in the molecule. The double bonds end up bending the tail of the fatty acid, and as a result of the molecules in the fatty acid can’t pack as tightly together as they can in saturated fats. Because of this, products like olive oil which have high unsaturated fat content are usually liquid at room temperature.
Waxes are organic compounds, much like fats. Waxes are usually comprised of long chains of hydrocarbons. Many of the natural waxes have something called esters within them. Esters are chemical compounds made out of at least one alkoxy (O-alkyl) group, and they are usually made out of an alcohol and a carboxylic acid. Waxes often have long chain alcohols and long chain carboxylic acids.
Much like fats and oils, waxes are usually insoluble in water. If you’ve ever gone to a sandwich store and received a sandwich wrapped in wax paper, perhaps you’ve spilled water or something on the paper and seen how the wax repels the water away from it, how it forms beads on the surface of the wax paper and rolls off of it. Wax doesn’t dissolve in water, so this means that it’s a useful compound for the creation of protective layers, something that many plants and other organisms take advantage of.
Phospholipids are a subtype of lipids that have a phosphate group within them. Phospholipids are major elements in the construction of cell membranes. Most phospholipids have a simple organic molecule like choline, a phosphate group, and a diglyceride. Because of this, phospholipids contain nitrogen and phosphorous.
Phospholipids have a hydrophobic, or water-avoiding tail, and a hydrophilic or water-loving head. This forms what is known as the phospholipid bilayer. This phospholipid bilayer is what makes up the cell membrane and protects the organelles and other materials inside our cells from the outside world.
Though it might be surprising to find out that steroids are lipids, if you know something about their properties it will come as less of a surprise. Much like other lipids, steroids are insoluble in water. Steroids include hormones like estrogen and testosterone as well as cholesterol. Hormones are chemicals which transmit signals through the body and act to regulate the development and functions of the body. Steroids are made out of molecules of cholesterol and they are formed from carbon/hydrogen rings. Cholesterol is important to maintaining the function of cell membranes, as it plays a role in the permeability and structure of the membranes.