by Catherine Haug, November 25, 2010
This article discusses the following topics:
Introduction
- Essential elements for life (C, H, and O)
- What makes carbon so special?
- Carbon bonds (single, double, triple)
- Linear representation of hydrocarbon chains (diagrams)
What are fats?
- Glycerides (mono-, di- and tri-glycerides)
- Fatty acids
- Degree of saturation (saturated, mono-unsaturated, & poly-unsaturated)
- Omega number (omega-3, omega-6, and omega-9)
- Fatty acid composition of fats
Introduction
The essential elements for life
You are probably familiar with the Periodic Table, or Chart of the Elements; you can view one version on PTable.com. There are currently 118 recognized elements, but only a few of these are essential for life.
The most basic elements for life are carbon (C), hydrogen (H), oxygen (O), Nitrogen (N) and Phosphorous (P). The first four of these elements form the molecules that provide functionality in biological systems: carbohydrates, fats, and proteins, along with vitamins and cofactors. Phosphorous is most notably found in cell membranes.
In addition to these, there are many minerals that have biological function, but life would not be possible without these basic elements.
What makes carbon special?
Carbon has a unique ability to form long molecular chains, that can be pure carbon (as in graphite or diamond), or hydrocarbons, which include hydrogen along the carbon backbone. Along this backbone, carbon can also bond to other elements, but it is its bonds with oxygen (O) on the end of the molecule that make it a fat.
[NOTE: silicon, carbon’s relative in the periodic table, also has this ability to form chains.]
Carbon Bonds
The carbons in the hydrocarbon chain are connected to each other by bonds, or pairings of electrons, one from each carbon. Each carbon can make 4 bonds to other elements, and these bonds can be of three types:
- Single bonds, which are comprised of 2 electrons and represented as C-C;
- Double bonds, which are comprised of 4 electrons and represented as C=C; and
- Triple bonds, which are comprised of 6 electrons, but are not typically found in fats.
Single bonds between carbons are called “saturated;” when there are double bonds in the chain, it is called “un-saturated.”
Linear representation of hydrocarbon chains
In the 2D (two-dimensional) diagrams below (and elsewhere in this series), the hydrocarbon chain is shown as a zig-zag stick figure. The points of each zig/zag is a carbon, and the lines joining these points are the bonds. The hydrogens bonded to each carbon are not shown for simplicity, but all the other atoms bonded to carbon (O, N, P) are indicated.
The stick figures attempt to demonstrate the geometrical shape of these molecules, and the shape is determined by the arc (measured in degrees) made by the bonds on each side of a carbon point. Single bonds are separated by 109.5 degrees; double bonds by 120 degrees (to form a ‘Y’ shape), and triple bonds by 180 degrees (to form a linear shape). These angles are the degree of zig/zag along each part of the chain.
NOTE: 3D representations of hydrocarbon chains are also used. See my post on Biochemistry of Fats (link under construction).
What are fats?
Fats, for the most part, are part of the lipid family of organic substances, meaning that they are water-phobic.
Technically, the term ‘fat‘ applies to the storage form in fatty tissue, which is also the form in which dietary fats are found (for example, cream, lard, duck fat, corn oil, sesame oil, etc.). These fats are formed by a linking of individual fatty acids to a glycerol backbone, to make glycerides (see below).
A fatty acid is a hydrocarbon with an acidic group at the end. The acidic group, represented as ‘-COOH’, contributes a hydrogen ion when reacting with other substances (such as with glycerol to form a glyceride), thus making it acidic.
Short chain fatty acids (such as acetic acid or vinegar) are more acidic than the longer chain versions (such as stearic acid). Short chain fatty acids tend to be water soluble, whereas longer chain versions are hydrophobic and fall into the category of lipids (non-water soluble organic substances)
In addition to binding to glyccerol to form glycerides, fatty acids can also be linked to phosphate to form phospholipids, the form of fat that comprises most of the cell wall (membrane). Or they can be bound to plasma protein albumen for transport in the blood.
However, many people, including me, also refer to fatty acids as ‘fat,’ whether or not they are linked to glycerol.
NOTE: cholesterol is a lipid but is not a fat. It is a sterol, a special type of alcohol made up of interconnected rings of carbons.
Glycerides
As mentioned above, this is the storage form of fat in fatty tissues. [See Wikipedia: Fatty Acids for a table of fats from different sources (plant and animal), showing relative amounts of saturated, MUFA and PUFA fatty acids, and vitamin E.]
Glycerides are comprised of glycerol, a 3-carbon alcohol molecule, and one to three fatty acids. I like to describe this as a coat rack with 3 hooks and up to 3 coats hanging on the hooks. If only 1 fatty acid is attached, it is a monoglyceride; 2 fatty acids is a diglyceride; and 3 fatty acids is a triglyceride.
In the following sketch:
- The glycerol is the three C’s and their corresponding O’s arranged vertically on the left; the three fatty acids extend to the right from each of the O’s. In this example, each of the fatty acids has 18 carbons with distinctly different structure. (diagram from source 7)
- What makes fats unique are the different fatty acids that are attached to the glycerol backbone. These fatty acids are what give the fats their general characteristics, and also their bio-functionality. (See Fatty acid composition of fats, below, for more on this)
Fatty acids
NOTE: most fatty acids are not appreciably acidic in that they cause little change in the pH; they are acidic because they react like acids. That is, they will readily give up a hydrogen ion (H+) to another substance. The very short chain fatty acids (formic and acetic) are appreciably acidic and have a lower pH in water.
The simplest of fatty acids include a hydrocarbon chain (carbon & hydrogen) of at least one carbon, with a COOH group attached at one end.
- A 1-carbon fatty acid (C1) is HCOOH, also known as formic acid (such as from an ant sting).
- The 2-carbon fatty acid (C2), is CH3COOH, also known as acetic acid, the main component of vinegar.
- The 3-carbon fatty acid (C3), CH3CH2COOH, is called propionic acid; and
- The 4-carbon fatty acid (C4), CH3CH2CH2COOH [or CH3(CH2)2COOH] is called butyric acid, which gives butter its name.
Thus fatty acids can be categorized by the number of carbons in the chain, such as C18 for stearic acid, which has 18 carbons. They can also be categorized as:
- short-chain (SCFA), which have up to 6 carbons in the chain;
- medium-chain (MCFA), which have up to 12 carbons in the chain; or
- long-chain (LCFA) fatty acids, which have 14 or more carbons in the chain.
Fatty acids can be linear, as in all the examples above, or they can be rings, such as glucuronic acid, an essential acid for liver detoxification that is derived from glucose, a sugar. (6)
Glucuronic acid
Degree of Saturation
Fatty acids are also categorized by the number of double bonds in the chain; that is, the degree of saturation (diagrams from Wikipedia, (Sources 1 – 4))
- Saturated fats have no double bonds in the hydrocarbon chain (not counting the double bond to the Oxygen in the COOH group). For example, butyric acid (C4), lauric (C12) acid, myristic acid (C14), palmitic acid (C16) and stearic acid (C18).
Stearic acid, a saturated fat
- Mono-unsaturated fats (MUFA) have only one double bond in the chain; for example, oleic acid (C18), with the lone double bond in the 9th position from the end, making it an omega-9 fat:
Oleic acid, a mono-unsaturated fat (Omega-9)
- Poly-unsaturated fats (PUFA) have more than one double bond in the chain. For example, linoleic acid (C18) with two double bonds in the chain (6th and 9th positions from the end):
LInoleic acid, a poly-unsaturated Omega-6 fat
- and linolenic acid (C18), with three double bonds in the chain (3rd, 6th and 9th positions from the end):
Alpha-linolenic acid, a poly-unsaturated Omega-3 fat
3D representations
The 2-dimensional diagrams above are useful for understanding the chemical structure of these fatty acids, but to have a true understanding of the molecules, 3-dimensional diagrams are more useful, in that they depict the bends that occur at double bonds. Here’s a sampling, from Wikipedia (5):
3D fats
In the 3D representations above, the red ball indicates the oxygens in the COOH at one end; the black balls are carbon, and the grey balls are hydrogens.
- Saturated fatty acids: Arachidic (C20), stearic (C18) and palmitic (C16) acids are straight.
- Mono-unsaturated fatty acids (MUFA): Erucic (C22) and oleic acids (C18) have a kink.
- Poly-unsaturated (PUFA): Arachidonic acid (C20), an omega-6 with 4 double bonds forming a U shape; linoleic acid (C18), an omega-6 with 2 double bonds forming an L shape; and linolenic acid (C18), and omega-3 with 3 double bonds forming a J shape.
And here’s a 3-D respresentation of a triglyceride: trimyristin (each of the 3 fatty acids is myristoleic acid, C14 mono-unsaturate).(5)
Trimyristin (Triglyceride of Myristoleic Acid)
See a great, interactive 3-D diagram of a triglyceride at 3D Chem (5). ==>NOTE: this link takes a while to load. Do not try this link if your internet connection is dial-up.
For more on the geometry of fats and fatty acids, see Basic Biochemistry of Fats.
Omega Number
Unsaturated fats in biological systems are divided into two groups, named by the position of the last double bond:
- Omega-3 is a poly-unsaturated fat with the last double bond in the 3rd position from the end; for example, linolenic acid with double bonds in the 3rd. 6th and 9th positions (see diagram above).
- Omega-6 is a poly-unsaturated fat with the last double bond in the 6th position; for example, linoleic acid with double bonds in the 6th and 9th positions (see diagram above).
- Omega-9 is a mono-unsaturated fat with the only double bond in the 9th position from the end; for example, oleic acid (see diagram above).
Saturated fats don’t have an omega number because they have no double bonds.
Fatty Acid Composition of Fats
Remember that each fatty acid of a triglyceride can be different. For example, one can be a saturated fatty acid, another can be mono-unsaturated, and the third can bepoly-unsaturated omega-3 (or omega-6) as in the example triglyceride above.
Each triglyceride in a vat of fat (or bottle of oil) can have different combinations of fatty acids. Generally, the overall mix of fatty acids is known and represented by percentages. For example, the major fatty acids in olive oil (2):
- Oleic Acid): 55 – 83% (a monounsaturated C18 omega-9 fatty acid)
- Linoleic Acid: 3.5 – 21% (a polyunsaturated C18 omega-6 fatty acid)
- Palmitic Acid: 7.5 – 20% (a saturated C16 fatty acid)
- Stearic Acid: 0.5 – 5% (a saturated C18 fatty acid)
- Linolenic Acid: 0 – 1.5% (specifically alpha-Linolenic Acid, a C18 polyunsaturated omega-3 fatty acid)
Or described another way, by degree of saturation (2):
- 16% saturated (stearic and palmitic acids)
- 75% mono-unsaturated (oleic acid)
- 8% poly-unsaturated omega-6 (linoleic acid)
- 1% poly-unsaturated omega-3 (linolenic acid)
See Curezone (8), Wikipedia (9), or Summer Bee Meadow: Properties of Soapmaking Oils (10) for more on fat composition of fats and oils.
Sources
- Stearic acid diagram: en.wikipedia.org/wiki/Stearic_acid
- Oleic acid diagram: en.wikipedia.org/wiki/File:Oleic-acid-skeletal.svg
- Linoleic acid diagram: en.wikipedia.org/wiki/Linoleic_acid
- Linolenic acid diagram: en.wikipedia.org/wiki/Alpha-Linolenic_acid
- 3D diagrams: en.wikipedia.org/wiki/Fatty_acid; www.3dchem.com/3dmolecule.asp?ID=320; and en.wikipedia.org/wiki/Fat
- Glucuronic acid diagram : en.wikipedia.org/wiki/Glucuronic_acid
- Triglyceride diagram: en.wikipedia.org/wiki/Triglyceride
- Fat content of olive oil: www.oliveoilsource.com/page/chemical-characteristics#Fatty and curezone.com/foods/fatspercent.asp
- Fat content of cooking oils/fats: en.wikipedia.org/wiki/Lard
- Summer Bee Meadow on properties of soapmaking oils: www.summerbeemeadow.com/content/properties-soapmaking-oils
