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Lipids

A lipid is a type of organic molecule. Fats, oils and waxes are different examples of lipids.

Lipids are composed of carbon, hydrogen and oxygen atoms.

Triglycerides are an example of a lipid. Triglycerides are composed of one glycerol and three fatty acids.

A triglyceride is composed of one glycerol and three fatty acids.
A triglyceride is composed of one glycerol and three fatty acids.
  • The main function of lipids is as an energy store. Lipids can be used as a source of energy when needed.
  • When a person eats too much, excess food is converted into fat for energy storage.

  • Lipids are used as an insulating (heat conserving) material. The skin contains lipids that keep the skin waterproof.
  • Lipids are an important component of biological membranes.
A space-filling diagram of arachidonic acid. The grey spheres are carbon atoms, the white spheres are hydrogen atoms and the red spheres are oxygen atoms. This fatty acid is found in poultry.
A space-filling diagram of arachidonic acid. The grey spheres are carbon atoms, the white spheres are hydrogen atoms and the red spheres are oxygen atoms. This fatty acid is found in poultry.

A fatty acid is a lipid. Fatty acids are carboxylic acids, that is they have the carboxyl group $$\ce{R-COOH}$$.

The R group refers to a chain of carbon and hydrogen atoms (collectively known as hydrocarbon).

The length of the hydrocarbon chain varies among lipids. Lipids containing long-chained fatty acids have higher melting points due to greater intermolecular forces between lipid molecules.

Fatty acids are the components of larger lipids such as triglycerides and phospholipids. The properties of the larger lipid depend on the fatty acids they contain.

There are two main types of fatty acids that lipids can contain:

  • Saturated fatty acids form straight chains.
  • This is because their hydrocarbon chains have no double bonds.

    Saturated fatty acids have a higher melting point. They are normally solids at room temperature.

    Meat, eggs and cheese are all high in saturated fats!

  • Unsaturated fatty acids form bent chains.
  • This is because their hydrocarbon chains contain at least one double bond between carbon atoms.

    Unsaturated fatty acids have a lower melting point. They are normally liquids at room temperature.

    Sunflower seeds and nuts contain a lot of unsaturated fats!

Saturated fats can line up next to each other to form stable structures.
Saturated fats can line up next to each other to form stable structures.

Triglycerides are a type of lipid. They are often called fats.

Triglycerides are composed of one glycerol molecule (the backbone of the triglyceride) and three fatty acids.

The fatty acid tails are shown in red while the glycerol backbone is in black. The R-groups vary depending on the fatty acid type.
The fatty acid tails are shown in red while the glycerol backbone is in black. The R-groups vary depending on the fatty acid type.

Each of the fatty acids is linked to the glycerol via an ester bond. Triglycerides form their ester bond through a condensation reaction.

In animals, triglycerides are the main long-term energy store. However, converting triglycerides to glucose is slow. For quick release, energy is stored in the carbohydrate glycogen.

Triglycerides can also be found in some parts of plants such as nuts and oilseeds.

Triglycerides contain around twice as much energy per unit mass as carbohydrates. This is ideal for animals and seeds as they have a high energy requirement and a less reliable energy source than plants.

Phospholipids are a major component of cell membranes.

Phospholipids have a glycerol backbone like triglycerides. However, they differ from triglycerides in the groups attached to the glycerol backbone.

While triglycerides have three fatty acid groups, phospholipids have two fatty acids and a phosphate group attached. Like triglycerides, the fatty acids are attached by ester bonds.

Phospholipids are amphipathic, meaning that they both attract and repel water. The phosphate head attracts water (hydrophilic), and the fatty acid tail repels water (hydrophobic). This differs from most other lipids, which are entirely non-polar.

Cell membranes are made from a phospholipid bilayer.

The amphipathic nature of phospholipids allows for them to form a membrane. When exposed to an aqueous environment, the hydrophobic tails will face away from the aqueous environment while the hydrophilic heads will face the water.

This feature is key to the spontaneous assembly of the lipid bilayer of membranes. Phospholipids placed in an aqueous solution automatically form a membrane.

Phospholipids can aggregate in different manners in an aqueous environment.
Phospholipids can aggregate in different manners in an aqueous environment.

As shown in the image, phospholipids can also form liposomes and micelles in an aqueous environment.

Liposomes are employed in drug delivery. They are artificial vacuoles created by pharmaceutical industries.

An emulsion test allows you to check whether a substance might be a lipid. The test uses the fact that lipids can dissolve in ethanol but not water:

  • Grind the sample and place in a clean, dry test tube.
  • Pour 95% ethanol into the tube. Shake vigorously.
  • Lipids can dissolve in ethanol but not water.

  • Pour this solution into another test tube of distilled water.

If the sample contains lipids, the lipids will form tiny lipid globules in the mixture called an emulsion. This appears as a cloudy white layer in the mixture.

If the mixture doesn't contain lipids, the mixture will stay colourless.

The cloudy white emulsion has a milky appearance.
The cloudy white emulsion has a milky appearance.