Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail Figure Many steroids also have the —OH functional group, which puts them in the alcohol classification sterols.
Figure Steroids such as cholesterol and cortisol are composed of four fused hydrocarbon rings. Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver and is the precursor to many steroid hormones such as testosterone and estradiol, which are secreted by the gonads and endocrine glands.
It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help in the emulsification of fats and their subsequent absorption by cells. Although cholesterol is often spoken of in negative terms by lay people, it is necessary for proper functioning of the body.
It is a component of the plasma membrane of animal cells and is found within the phospholipid bilayer. Being the outermost structure in animal cells, the plasma membrane is responsible for the transport of materials and cellular recognition and it is involved in cell-to-cell communication.
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. Figure 3. Stearic acid is a common saturated fatty acid. Figure 4. Biology Molecular Biology Basics Lipids. Omar T. Oct 20, Explanation: Non-polar substances such as fats and oils cannot be dissolved in water because they have made up of only carbon-carbon and hydrogen-carbon bonds.
Related questions What are lipids? Examples of a saturated and unsaturated fatty acid are shown below. As you can see from the structural schematics above, the presence of a double bond in the palmitoleic acid dramatically alters the structure of the fatty acid, which, in turn, dramatically alters its melting point.
Because of the linear nature of the saturated fatty acid, these molecules can pack more tightly in the solid phase, leading to an increased number of dispersion interactions and hence the requirement of more energy to break them. Alternatively, the kink resulting from the double bond in unsaturated fatty acids prevents efficient packing, decreasing the overall strength of the dispersion forces holding the fat molecules together.
The general rule of thumb is that saturated fatty acids have a higher melting point than unsaturated fatty acids. Triglycerides containing unsaturated fatty acids are liquid at room temperature whereas triglycerides containing saturated fatty acids are solid at room temperature. This is the difference between an oil and a fat. Also affecting the melting point is the length of the hydrocarbon chain — longer chains have a higher melting point than shorter chains.
This is again related to the strength of the dispersion forces that longer chains afford. Naturally occurring oils and fats are usually made up of a mixture of triglycerides, meaning that each glycerol backbone can contain up to 3 different fatty acid molecules. However, there are trends. For instance, triglycerides from animal fats typically have a higher percentage of saturated fatty acids compared to triglycerides extracted from plants oils. As such, they also have different impacts on human health.
When speaking of the double bonds in naturally occurring unsaturated fatty acids, we are usually referring to a cis double bond formation. Cis bonds are formed in nature as opposed to trans bonds because the enzymes responsible for desaturation reactions — reactions that transform a saturated carbon-carbon bond into an unsaturated carbon-carbon bond — operate in a way that only results in cis bond formation.
If enough autooxidation occurs, the oil will go rancid — this is why food oils tend to have a much shorter shelf life than fats saturated fatty acids are more stable. To address this issue, and prevent food waste, food scientists implemented a method to partially hydrogenate unsaturated oil mixtures until a desired texture was obtained.
By reducing the double bonds in an unsaturated fatty acid mixture, the melting point can also be altered. This change in the fatty acid physical property favors some cooking applications.
For instance, Crisco, which was widely used for baking, is actually made up of partially-hydrogenated cottonseed oil. Before hydrogenation, cottonseed oil is liquid at room temperature; after hydrogenation, it is a solid at room temperature, but it melts upon cooking.
Margarine is made in a similar fashion. Lipids are formed from long chains of hydrocarbons. Hydrocarbon compounds are notable for the long sequence of carbon to carbon bonds with hydrogen atoms bonded to the carbon atoms. Carbon atoms may bond with up to four additional atoms.
A single pair of electrons shared between two atoms is termed a single bond. Saturated lipids have single bonds between the carbons on the chain carbons always form single bonds with hydrogens. In unsaturated lipids, one of the carbon to carbon bonds is double bonded four electrons are shared between the atoms.
This double bond reduces the number of hydrogen atoms on the molecule and creates a bend in the chain. Simply put, the saturated lipids have as many hydrogen atoms as possible surrounding the chain of carbons, where unsaturated lipids have less than the maximum number of possible hydrogen atoms surrounding the carbon chain as a result of the double bond between two or more of the carbon atoms.
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