Importance of Lipids
The molecules used by the body for nutrition and cellular processes are proteins, carbohydrates, water, vitamins, minerals and lipids. Of these, only protein, carbohydrates and lipids provide energy in the form of calories. Lipids are commonly known as fats, and they are able to supply more than twice the amount of energy that the same amount of protein and carbohydrates can. In addition, they perform critical functions inside the body and are required for life to exist.
Types
As with all classifications of molecules, lipids have unique properties, but the one that sets them apart from all other molecules is their lipophilicity, or solubility in fats and oils. This property allows them to pass through the plasma membrane of cells and modify cellular activity. Lipids take on several forms and can be found in the body as fatty acids, triglycerides, cholesterol, phospholipids and steroids. In order to fully understand the importance of lipids, you must first understand the critical jobs they perform.
Fatty Acids
Fatty acids have two parts: a hydrophilic carboxylic acid head and a hydrocarbon alkane chain. Fatty acids can be classified in several ways. Unsaturated fatty acids are those that have a double bond in their hydrocarbon chain, while saturated fatty acids will have no double bonds in the hydrocarbon chain. Fatty acids are used as building blocks to make triglycerides. A triglyceride is three fatty acids grouped together by a glycerol backbone. These molecules are used for energy storage in the body.
Cholesterol
The structure of cholesterol is very different from fatty acids. It has three parts: a hydrophilic hydroxyl group that is water soluble and four hydrophobic hydrocarbon rings in the middle and a hydrophobic hydrocarbon tail, both of which are lipid soluble. The combination of these three elements gives cholesterol hydrophobic characteristics overall, but it is considered an amphipathic molecule (having hydrophilic and hydrophobic properties). The ability to be amphipathic allows the cholesterol molecule to span the plasma membrane and provide a structure that keeps it rigid. Cholesterol is also the main building block of all steroid hormones.
Phospholipids
Phospholipids are what make up cellular membranes. They consist of a hydrophilic phosphate head and a hydrophobic fatty acid tail. This structure enables them to align themselves up in an organized fashion, hydrophilic heads on the outside of the membrane and hydrophobic tails on the inside. This alignment will create a water-tight barrier that separates the inside of the cell from the outside. This is called the phospholipid bilayer. In order to pass through the cell membrane easily, molecules must have lipid-soluble characteristics.
Steroids
The cholesterol hydrocarbon ring is the main element that defines the structure of all steroid hormones. Through the use of site-specific enzymes, the cholesterol backbone is modified, one step at a time, into the functional molecule. Steroid hormones are chemical messengers in the body and best known for their regulation of the reproductive system. Most people associate steroid hormones with testosterone, but estrogen, progesterone and prostaglandin are also made from cholesterol. These hormones are best known for regulating the female cycle and sperm production.
Signal Transduction
In addition to all the functions of lipids already described, they also help regulate signal transduction in the body. Signal transduction is the process by which hormones and proteins in the body communicate with each other. Through a series of steps, initiated by the interaction of a hormone or protein with a special domain on the outside of the cellular membrane called a lipid raft, changes can be made in cellular processes.
Energy Supply and Storage
The best known function of lipids is probably energy supply and storage. One gram of fat contains about nine calories of energy, while the same amount of protein and carbohydrate contains only about four. This makes fat a valuable energy source. In addition, fat can be stored in tissue depots and later mobilized to provide energy under starvation or stressful conditions.
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