Lipids are biological macromolecules. They are essential in living organisms, along with carbohydrates, proteins, and nucleic acids.

Lipids include fats, oils, steroids and waxes. They are hydrophobic, meaning they are insoluble in water. However, they are soluble in organic solvents such as alcohols and acetone.

The chemical structure of lipids

Lipids are organic biological molecules, just like carbohydrates, proteins, and nucleic acids. This means they consist of carbon and hydrogen. Lipids contain another element along with C and H: oxygen. They may contain phosphorus, nitrogen, sulphur or other elements.

Figure 1 shows the structure of a triglyceride, a lipid. Notice how the hydrogen and oxygen atoms are bonded to carbon atoms in the backbone of the structure.

Fig. 1 - Structure of a triglyceride

The molecular structure of lipids

Lipids are composed of glycerol and fatty acid. The two are bonded with covalent bonds during condensation. The covalent bond that forms between glycerol and fatty acids is called the ester bond.

Glycerol is an alcohol and an organic compound as well. Fatty acids belong to the carboxylic acid group, meaning they consist of a carboxyl group ⎼COOH (carbon-oxygen-hydrogen).

Triglycerides are lipids with one glycerol and three fatty acids, while phospholipids have one glycerol, a phosphate group, and two fatty acids instead of three.

The function of lipids

Lipids have numerous functions that are significant for all living organisms:

Energy storage

Lipids serve as a source of energy. When lipids are broken down, they release energy and water, both valuable for cellular processes.

Structural components of cells

Lipids are found in both cell-surface membranes (also known as plasma membranes) and the membranes surrounding organelles. They help membranes stay flexible and allow lipid-soluble molecules to pass through these membranes.

Cell recognition

Lipids that have a carbohydrate attached are called glycolipids. Their role is to facilitate cellular recognition, which is crucial when cells form tissues and organs.

Insulation

Lipids that are stored beneath the body surface insulate humans from the environment, keeping our bodies warm. This happens in animals as well - aquatic animals are kept warm and dry due to a thick layer of fat underneath their skin.

Protection

Lipids serve as a protective shield around vital organs. Lipids also protect our biggest organ - the skin. The epidermal lipids, or lipids that form our skin cells, prevent the loss of water and electrolytes, prevent sun damage, and serve as a barrier against various microorganisms.

Types of lipids

The two most significant types of lipids are triglycerides and phospholipids.

Triglycerides

Triglycerides are lipids that include fats and oils. Fats and oils are the most common types of lipids found in living organisms. The term triglyceride comes from the fact that they have three (tri-) fatty acids attached to glycerol (glyceride). Triglycerides are entirely insoluble in water (hydrophobic).

The building blocks of triglycerides are fatty acids and glycerol. Fatty acids that build triglycerides can be saturated or unsaturated. Triglycerides composed of saturated fatty acids are fats, while those consisting of unsaturated fatty acids are oils.

The primary function of triglycerides is energy storage.

Phospholipids

Like triglycerides, phospholipids are lipids built of fatty acids and glycerol. However, phospholipids are composed of two, not three, fatty acids. Like in triglycerides, these fatty acids can be saturated and unsaturated. One of the three fatty acids that attach to glycerol is replaced with a phosphate-containing group.

The phosphate in the group is hydrophilic, meaning it interacts with water. This gives phospholipids one property that triglycerides don't have: one part of a phospholipid molecule is soluble in water.

Phospholipids are often described as having a 'head' and a 'tail'. The head is the phosphate group (including glycerol) that attracts water (hydrophilic). At the same time, the tail is the two hydrophobic fatty acids, meaning they 'fear' water (you can say that they orientate themselves away from water). Have a look at the figure below. Notice the 'head' and the 'tail' of a phospholipid.

Fig. 2 - Phospholipid structure

Because of having both a hydrophilic and a hydrophobic side, phospholipids form a bilayer ('bi' stands for 'two') which makes up the cell membranes. In the bilayer, the 'heads' of phospholipids face the outside environment and the inside cells, interacting with water present inside and outside cells, while the 'tails' face inside, away from the water. Figure 3 shows the orientation of phospholipids inside the bilayer.

This property also allows for the creation of glycolipids. They form on the surface of the outer cell membrane, where carbohydrates attach to the hydrophilic heads of phospholipids. This gives phospholipids another vital role in living organisms: cell recognition.

Similarities and differences between phospholipids and triglycerides

How to test for the presence of lipids?

The emulsion test is used to test for the presence of lipids.

Emulsion test

To perform the test, you need:

• test sample. Liquid or solid.

• test tubes. All test tubes should be completely clean and dry.

• ethanol

• water

Steps:

1. Place 2 ${\mathrm{cm}}^3$ of the test sample into one of the test tubes.

2. Add 5${\mathrm{cm}}^3$ of ethanol.

3. Cover the end of the test tube and shake well.

4. Pour the liquid from the test tube into a new test tube that you previously filled with water. Another option: You can add water to the existing test tube after step 3 instead of using a separate tube.

5. Observe the change and record.