Lipids
Lipids
Lipids act as a long-term energy resource and tend to be utilized at a slower rate. They are a diverse group of organic compounds including fats, oils, hormones, and certain components of membranes that are grouped together because they do not interact appreciably with water. Membranes function to separate individual cells from their environments and to compartmentalize the cell interior into structures that carry out special functions. This compartmentalizing function is very important. Membranes and the lipids that make them up are essential to life.
Almost all the molecular components of living cells in organisms are soluble in water. Molecules such as proteins, nucleic acids, and carbohydrates have an affinity for water and are called hydrophilic (“water-loving”). Lipids, however, are not hydrophilic but hydrophobic (“water-fearing”) because of an abundance of carbon and hydrogen.
There are three major groups of lipids:
Almost all the molecular components of living cells in organisms are soluble in water. Molecules such as proteins, nucleic acids, and carbohydrates have an affinity for water and are called hydrophilic (“water-loving”). Lipids, however, are not hydrophilic but hydrophobic (“water-fearing”) because of an abundance of carbon and hydrogen.
There are three major groups of lipids:
- triglycerides
- steroids
- phospholipids
Triglycerides
Triglycerides are better known as fats. As their name suggests, the triglycerides are composed of one molecule of glycerol bonded with three molecules of fatty acids, each linked by an ester bond to one of the three OH groups of a glycerol molecule.
Fatty acids rarely occur as free molecules in nature. Instead, they are usually found as components of many complex lipid molecules such as fats (energy-storage compounds) and phospholipids (the primary lipid components of cellular membranes).
The main source of fatty acids in the diet is triglycerides. In humans, fat constitutes an important part of the diet, and in the more-developed countries it can contribute as much as 45 percent of energy intake. Most of what humans eat is in the form of triglycerides. The primary cellular function of fatty acids is as long term energy storage. The body will store a small amount of excess nutrients as glycogen, but after a certain amount of time, these nutrients are converted to triglycerides for storage. Triglycerides are efficient energy-storing molecules because more energy can be stored in a pound of fat than in a pound of glycogen.
Fatty acids rarely occur as free molecules in nature. Instead, they are usually found as components of many complex lipid molecules such as fats (energy-storage compounds) and phospholipids (the primary lipid components of cellular membranes).
The main source of fatty acids in the diet is triglycerides. In humans, fat constitutes an important part of the diet, and in the more-developed countries it can contribute as much as 45 percent of energy intake. Most of what humans eat is in the form of triglycerides. The primary cellular function of fatty acids is as long term energy storage. The body will store a small amount of excess nutrients as glycogen, but after a certain amount of time, these nutrients are converted to triglycerides for storage. Triglycerides are efficient energy-storing molecules because more energy can be stored in a pound of fat than in a pound of glycogen.
Cholesterols
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Cholesterols have almost nothing in common with triglycerides other than the fact that they too are hydrophobic. All cholesterols are derivatives of a common structure made up of four fused carbon rings instead of chains, unlike the other lipids. Each cholesterol contains different chemical groups attached to these rings. Cholesterols play a significant part of the structure of cell membranes and as hormones.
Cholesterol is found in the cell membrane of cells where it aids in maintaining the correct fluidity of the membrane. It also serves as the precursor molecule for the synthesis of other steroids including bile salts (aids in the digestion of fats) and steroid hormones (such as testosterone, estrogen, progesterone, and cortisol).
Cholesterol is found in the cell membrane of cells where it aids in maintaining the correct fluidity of the membrane. It also serves as the precursor molecule for the synthesis of other steroids including bile salts (aids in the digestion of fats) and steroid hormones (such as testosterone, estrogen, progesterone, and cortisol).
Phospholipids
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Phospholipids are derivatives of triglycerides. They are composed of a glycerol molecule with two fatty acids, called a diglyceride. The third carbon contains a phosphate group and usually some added polar molecule. Much of the diglyceride is hydrophobic just as in the triglycerides. However, phosphate is hydrophilic. Because part of phospholids want to dissolve in water and part of them does not, they serve as the basis for the structure of all the membranes of the cell.