The hydrophilic head groups of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer. Unlike the phospholipids and fats discussed earlier, steroids have a ring structure. Although they do not resemble other lipids, they are grouped with them because they are also hydrophobic. All steroids have four, linked carbon rings and several of them, like cholesterol, have a short tail.
Cholesterol is a steroid. Cholesterol is mainly synthesized in the liver and is the precursor of many steroid hormones, such as testosterone and estradiol. It is also the precursor of vitamins E and K.
Cholesterol is the precursor of bile salts, which help in the breakdown of fats and their subsequent absorption by cells. Although cholesterol is often spoken of in negative terms, it is necessary for the proper functioning of the body. It is a key component of the plasma membranes of animal cells. Waxes are made up of a hydrocarbon chain with an alcohol —OH group and a fatty acid.
Examples of animal waxes include beeswax and lanolin. Plants also have waxes, such as the coating on their leaves, that helps prevent them from drying out. Figure 5. Steroids such as cholesterol and cortisol are composed of four fused hydrocarbon rings. Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids, and steroids.
Fats are a stored form of energy and are also known as triacylglycerols or triglycerides. Fats are made up of fatty acids and either glycerol or sphingosine. Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain. If only single bonds are present, they are known as saturated fatty acids. Unsaturated fatty acids may have one or more double bonds in the hydrocarbon chain. Phospholipids make up the matrix of membranes.
They have a glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the fluid nature of the membrane.
It is also the precursor of steroid hormones such as testosterone. Improve this page Learn More. Skip to main content. Module 3: Important Biological Macromolecules. Search for:. Lipids Learning Outcomes Distinguish between the different kinds of lipids Identify several major functions of lipids.
For an additional perspective on lipids, explore this interactive animation. Similarly, animals employ waxes for protective purposes; for instance, earwax in humans prevents foreign material from entering and possibly injuring the ear canal area.
Phospholipids are similar to fats except they have two fatty acid chains bonded to a glycerol plus they contain the element phosphorus. Phospholipids are unique because they have a hydrophobic and a hydrophilic water-soluble end. Phospholipids are biologically important because they are the main structural components of cell membranes.
The cell membrane is called a phospholipid bilayer because it consists of two phospholipid layers oriented so that the hydrophyllic? Therefore, water and other cellular fluids are contained. The hydrophobic ends for both molecules face each other on the inside and allow for passage of acceptable, and some objectionable, materials through the cell membrane.
Steroids are structurally different from the other lipids. The carbon skeleton of steroids is bent to form four fused rings that do not contain fatty acids. The most common steroid, cholesterol, is needed to make both the male testosterone and female estrogen sex hormones, and it is a component of cell membranes and is needed for the proper function of nerve cells.
Excessive amounts of cholesterol, however, have been linked to heart disease. Another popular steroid group is the anabolic steroids that are man-made and mimic the effect of the male hormone, testosterone.
Originally intended as a treatment for anemia and certain diseases that destroy muscle, athletes have recently been using them to increase muscle mass, stamina, and strength? However, the performance-enhancement drugs come with a price. The anabolic steroids are linked to increased cholesterol levels, mood swings, reduced sex drive, possible infertility, and possible connections between liver damage and the resulting liver cancer. Certain beneficial fat-soluble hormones, such as cortisol, are also familiar steroids.
Fatty acids are chain-like molecules that are important components of several types of lipids. The illustrations below show two different fatty acid molecules. Each has a characteristic carboxyl group the -COOH attached to a chain of carbons with hydrogen atoms attached to the carbon chain. Two things are noteworthy. First, the hydrocarbon chain is very non-polar and therefore doesn't dissolve in water very well. However, hydrocarbon chains do associate with each other readily.
Second, note that the unsaturated fatty acid has two hydrogens removed, and this allows formation of a double bond, i. Note also that the double bond tends to produce a bend or a kink in the fatty acid. The illustration to the right shows two other common fatty acids: stearic acid, which is a straight 18 carbon chain with no double bonds, and oleic acid, which is an 18 carbon chain with a single double bond, which cause a bend in the carbon chain.
A fat molecule is a type of lipid that consists of three fatty acid molecules connected to a 3 carbon glycerol backbone, as shown on the right. The three fatty acids can be different from one another. Since the hydrocarbon chains are very non-polar, fats do no dissolve in water; instead, fat molecules tend to coalesce with one another. Since a fat molecule has 3 fatty acids connected to a glycerol molecule, they are also called trigylcerides. Phospholipids constitute another important class of lipids.
These are similar to similar to trigylcerides in that they have a glycerol backbone, but there are only two fatty acids connected to glycerol. The third carbon of the glycerol backbone is attached to a phosphate group an atom of phosphorus bonded to four atoms of oxygen , and the phosphate group is attached to a base molecule of choline, serine, or ethanolamine.
The part of the phospholipid with phosphate and the base is actually very polar, and it tends to rotate away from the two fatty acids. This makes phospholipid molecules have a hairpin shape. The head of the hairpin is very polar and therefore likes to associate with water it is hydrophilic , while the two fatty acid chains the "tails" are very non-polar and tend to avoid water hydrophobic and associate with other hydrocarbon chains.
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