Lipid bilayer

Lipids include a diverse group of water insoluble biomolecules that are mainly nonpolar in nature.

3 main types of lipids include:

  1. Triglycerides
  2. Phospholipids
  3. Steroids

Different types of lipids have a variety of biological roles.

The main function of triglycerides is similar to carbohydrates. They serve as fuel molecules that provide energy to the cellular metabolism.

Phospholipids are components of plasma membranes.

Different types of steroids perform their functions as hormones.

Chemical structure and function of triglycerides in living organisms

The main function of triglycerides is to store fat and to provide living organisms with the energy for metabolism.

Triglycerides are the most common form of fat in the human body and stored in fat cells.

When the body requires an energy the hormone glucagon signals the breakdown of the triglycerides to release free fatty acids and glycerol molecules.

Triglycerides are broken down to CO2 and water, releasing large amounts of energy, through beta oxidation of fatty acids and the citric acid cycle.

Beta-oxidation is the process by which fatty acids are broken down to generate acetyl-CoA for the citric acid cycle, and NADH and FADH2 for the use in the electron transport chain.

The glycerol component of triglycerides can be converted into glucose via gluconeogenesis to serve as the source of energy for the brain.

Thus, the chemical structure of triglyceride consists of two main components:

  1. glycerol,
  2. fatty acids.

Glycerol is a three-carbon alcohol that has a hydroxyl group attached to each carbon.

Fatty acids are long hydrocarbon chains containing a single carboxyl group at one end

The number of carbons in the fatty acid may range from 4 to 36; most common are those containing 12–18 carbons. 

In the triglyceride molecule, the fatty acids are attached to each of the three carbons of the glycerol molecule with an ester bond through an oxygen atom. 

Triglyceride structure - glycerol and fatty acidsDuring this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar.

Because of their chemical structure, triglycerides are also called triacylglycerols.

Some fatty acids have common names that specify their origin.

Fatty acids may be saturated or unsaturated. Unsaturated fatty acids have one or more double bonds.

Cis and trans in the name of triglycerides or fats indicate the configuration of the molecule around the double bond.

  • If hydrogens are present in the same plane, it is referred to as a cis fat;
  • If the hydrogen atoms are on two different planes, it is referred to as a trans fat.

cis and trans fatty acids

Omega 3 and omega 6 essential fatty acids

Essential fatty acids are the fatty acids required but not synthesized by the human body.

Consequently, they have to be supplemented through ingestion via the diet.

Omega-3 and omega-6 fatty acids fall into this category and are only two known for humans.

Essential Fatty Acid Pathways

Essential Fatty Acid (omega 3 and omega 6)

The farthest carbon away from the carboxyl group is numbered as the omega (ω) carbon, and if the double bond is between the third and fourth carbon from that end, it is known as an omega-3 fatty acid.

Omega-6 fatty acids have a carbon-carbon double bond in the n-6 position.

Omega-3 fatty acids are polyunsaturated and include: 

  • alpha - linoleic acid (ALA),
  • eicosapentaenoic acid (EPA),
  • docosahexaenoic acid (DHA).

Salmon, trout, and tuna are good sources of omega-3 fatty acids.

Omega-6 fatty acids include:

  • linoleic acid (LA),
  • gamma-linolenic acid (GLA),
  • arachidonic acid (AA).

Dietary sources of omega-6 fatty acids include food oils, poultry, eggs, nuts, hulled sesame seeds, cereals.

Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, reduce triglycerides in the blood, lower blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation, and may help reduce the risk of some cancers in animals.

General structure and function of phospholipids

Phospholipids are an one of the main types of lipids and major constituents of the plasma membrane.

General phospholipid molecule structure - phosphatidylcholine

Like triglycerides, phospholipid structure consist of fatty acid chains attached to a glycerol or sphingosine backbone.

Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol, and the third carbon of the glycerol backbone is occupied by a modified phosphate group. 

A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid; it is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids.

The phosphate group is modified by an alcohol.

Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes.

Function of steroids in the human body: cholesterol and steroid hormones

Cholesterol chemical structureThe other type of lipids is steroids.

In contrast to the phospholipids and triglycerides, steroids have a fused ring structure.

Though 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.

Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols).

Cholesterol functions

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.

An overview of different types of lipids