Lipids or commonly called fats are organic molecules mostly composed of a carboxyl end and a long hydrocarbon tail. The names and structures of these Lipids are:
Short chain fatty acids are made of less than six carbon (C) atoms, medium chain fatty acids are about six to twelve C atoms long, whilst fatty acids more than 14 C atoms long are referred to as long chain fatty acids.
When fatty acids combine with a long chain alcohol, waxes are formed. These waxes can be found in fruits where they act as a natural protective layer. They can also be used in the pharmaceutical industry as a base, an example of which is the wool wax Lanolin, a main ingredient used in Olay due to its easy absorption into the skin
Functions and Properties of Lipids
Lipids have many functions and uses in the body which can be seen in the following points
- Provides energy as per gram of fat, 9kcal is produced.
- Acts as an energy reserve as excess/ unused energy from carbohydrates, proteins and lipids will be stored in adipose tissues in the form of Triacylglycerol.
- Acts as an insulator to keep the body warm protecting it from very cold temperatures.
- Provides mechanical insulation acting as a cushion around vital organs protecting it from any mechanical shock.
- Supplies the body with essential fatty acids called Linolenic Acid and Linoleic Acid.
- Aids in the formation of cell membranes in which phospholipids are responsible i.e. the phospholipid bilayer.
- Steroids are a class of lipids, from which cholesterol can be derived. Cholesterol can be found within cell membranes and is used to make steroid hormones.
- Through the presence of fat, vitamins A, D, E and K can be retained in the body hence why they are called fat soluble vitamins.
- Increases the palatability of foods
- When digestion of fats takes place in the body this gives longer feeling of fullness known as satiety as fats digest longer within the body.
In the second point made about the storage of excess energy in adipose tissue in the form of Triacylglycerol (TAG), this molecule plays an important role in animals and energy production. In the case of Polar bears that undergo long periods of hibernation, under the skin of these bears a lot of TAG will be created as it provides a great deal of insulation and energy to maintain survival during hibernation period. As Triacylglycerol is a more reduced form carbon, more energy is produced per gram compared to the carbohydrates glycogen and starch.
Saturated Fats vs. Unsaturated Fats
Lipids can be found in two physical states, solid and liquid. The reasoning behind these two different states is due to the bonding that occurs within the molecule. As fats such as margarine and butter remain solid at room temperature this is because within their structure they are made up of straight chain hydrocarbon chains that have a single bond between the C atoms. Therefore they are saturated and can be referred to as saturated fats. However, fats such as oils remain liquid at room temperature as double bonds are present in the bonding taking place between the C atoms that make up the hydrocarbon chain. As such, they are unsaturated and are therefore called unsaturated fats.
|No double bonds present
|Double bonds present
|At room temperature: solid
||At room temperature: liquid
|Found in most animal fats
||Found in plant and fish fats
|Does not form kinks, so molecules are packed closely together
||Forms kinks within structure, so molecules are not packed closely together
Nomenclature of Fatty Acids
In the naming of Fatty Acids:
- Fatty Acids have both an ethyl end and a carboxyl end
Methyl end Carboxyl end
- To count the number of carbons present in the fatty acid, one must start counting from the carboxyl end.
- The number of double bonds if present should also be counted where the first carbon atom that is encountered when counting from the carboxyl end is the one which will be present in its naming.
- When naming the fatty acid it should be written in the following form. As this structure is Oleic acid it would be written as:
no. of C no. of double the no C atom at which
atoms present bonds present the double bond is present
Trends in the Physical properties of Lipids
The two properties that can be examined in fatty acids are the melting point (oC) and solubility (mg/100 mL in H2O).
Melting point (oC)
– Melting point increases as the number of C atoms present in saturated fatty acids increases.
– Melting point is lower in unsaturated fatty acids as compared to saturated fatty acids which have a higher melting point. Hence when more double bonds are present, the melting point decreases.
Solubility (mg/100 mL in H2O)
– As the number of C atoms present in fatty acids increases, solubility decreases.
Essential Fatty Acids
When a substance is considered essential, this means that the body can synthesize it and subsequently only through the diet can this substance be obtained. This is exactly what is meant by an essential fatty acid. As our bodies cannot synthesize fatty acids composed of double bonds located before the ninth carbon from the methyl end, they have to be obtained through the dies. These fatty acids are Omega-3 and Omega-6 also called Alpha-Linolenic Acid and Linoleic Acid respectively.
Omega infers the position of the first double bond in relation to the methyl end.
Therefore in Omega-3 and Omega-6, the first double bonds are found at C atom 3 and at C atom 6 respectively, both of which are found when counting from the methyl end.
Alpha-Linolenic Acid, Omega-3
Sources: flaxseed, soybean oil, dark green leafy vegetables, salmon, tuna
Linoleic Acid, Omega-6
Sources: seeds, nuts, sunflower oil, cottonseed oil
Trans Fatty Acids
When unsaturated fatty acids are hydrogenated, hydrogen is applied converting the double bonds present to single bonds where the new bonds formed have added hydrogen bonds to them. The process of hydrogenation also creates Trans fatty acids which can be found in fried foods. These Trans fats are very unhealthy as they increase the formation of LDL cholesterol which leads to a high risk of heart disease. However fatty acids are naturally found in abundance in the cis configuration, therefore mostly cis fatty acids are found in the diet except for in fried foods as stated before.
Formation of Triacylglycerol (TAG)
Triacylglycerol also commonly known as triglycerides are formed when the three hydroxyl groups on glycerol and the carboxyl group of three fatty acids (the fatty acids could be the same or different) undergo a condensation reaction. In a condensation reaction water is removed from between the molecules as the OH group from the fatty acids and the hydrogen from the glycerol are removed forming water. As these groups have been removed and are no longer present, the glycerol and fatty acids form ester bonds thereby creating a Triacylglycerol or a Triglyceride.
When fats are hydrolysed, the opposite occurs where water is added to the Triglyceride/Triacylglycerol, splitting into glycerol and three fatty acids. This reaction is catalyzed in the body by the enzyme lipase. Strong bases can be used in the hydrolysis of fats such as sodium hydroxide NaOH or potassium hydroxide KOH. However instead of fatty acids being produced, instead the sodium or potassium salt of a fatty acid is produced with glycerol.
Membrane Lipids – Formation of Phosphotidate
In the formation of Phosphotidate, this incorporates the combination of glycerol-3-phosphate with two fatty acids. In glycerol-3-phosphate, the third carbon atom is bonded to a phosphate group. On C atoms 1 and 2 an ester linkage is formed between two fatty acids where one fatty acid is saturated and the other unsaturated. The third C atom contains the phosphate group at which X in the diagram denotes the bonding of a group. When hydrogen is present in the position that X is located the structure is therefore a Phosphotidate. Therefore it can be stated that in the general structure of Phosphotidate, it entails a glycerol backbone bonded to a phosphate group at the third carbon and a saturated and unsaturated fatty acid bonded to carbon atoms one and two. As the second carbon atom is unsaturated, this causes the fatty acid to have a kink within its structure which is beneficial to the membrane making it less compact and more fluid. All naturally occurring fatty acids would have the cis configuration meaning that the hydrogen atoms bonded remain on one side of the double bond. However, the hydrogen atoms in the trans configuration are located on the opposite sides of the double bond. As stated before the trans configuration / trans fats are unhealthy.
Non polar tail, hydrophobic end
Polar head, hydrophilic end
Due to the structure of Phosphotidate it can be stated that it is amphipathic. There are two parts to the molecule, the hydrophilic (water loving) and the hydrophobic (water hating) parts which are the phosphate and fatty acid groups respectively. The hydrophilic part is referred to as the polar head and the hydrophobic part the nonpolar tail.
Membrane Lipids – Formation of Sphingosine
Sphingosine comprises of an unbranched 18 carbon alcohol. It has a trans configuration between C-4 and C-5, with hydroxyl groups at C-1and C-3 and at C-2 an amino group.
With sphingosine, the lipid that can be formed is ceramide where sphingosine is the backbone and at the amino group on C-2, a fatty acid forms and amide bond.
Membrane Lipids – Formation of Cholesterol
In cholesterol, there is a sterol nucleus at which there are four fused rings, three rings have six carbons and one has five carbons. An alkyl side chain at carbon 17 is present along with a double bond between C-5 and C-6 and a hydroxyl group at C-3 and. In all cholesterol comprises of a total of 27 carbon atoms.
Cholesterol is predominantly present in cell membranes of animals through orientation in the phospholipid membrane. How this occurs is that hydroxyl group of the cholesterol molecule aligns itself with the polar head of phospholipid molecule and a hydrogen bond is formed between the oxygen (O) of the ester linkage that is formed within the phospholipid molecule, i.e. the glycerol and fatty acids. The cholesterol’s sterol nucleus and alkyl side chain also act together with the fatty acid hydrocarbon chains of the phospholipids.
The fluidity of the cell membrane is regulated by cholesterol. As cholesterol is situated between the phospholipids it helps to maintain membrane fluidity through the various temperature changes that occur within the body and in turn affect the membrane. When there is an increase in temperature, cholesterol decreases the fluidity of the membrane by limiting the movement of phospholipids, while at lower temperatures, cholesterol inhibits the membrane from becoming more compact by maintaining fluidity.
- Garrett, Reginald H. and Charles M. Grisham. 2010. Biochemistry. Fourth edition. Boston: Brooks/Cole, Cengage Learning.
- Whitney, Ellie and Sharon R. Rolfes. 2012. Understanding Nutrition. California: Wadsworth
- Cengage Learning