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Lipids

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Biology

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.

The molecular structure of lipids

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

In lipids, fatty acids do not bond to one another but to glycerol only!

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.

It is important to remember that lipids are macromolecules composed of fatty acids and glycerol, but lipids are not polymers, and fatty acids and glycerol are not monomers of lipids! This is because fatty acids with glycerol do not form repetitive chains, like all other monomers. Instead, fatty acids attach to glycerol and lipid are formed; no fatty acids attach to one another. Therefore, lipids are not polymers because they contain chains of non-similar units.

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.

You can read more about the structure and the function of these key molecules in the article Triglycerides.

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.

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

PhospholipidsTriglycerides
Phospholipids and triglycerides have fatty acids and glycerol.
Both phospholipids and triglycerides contain ester bonds (between glycerol and fatty acid).
Both phospholipids and triglycerides may have saturated or unsaturated fatty acids.
Both phospholipids and triglycerides are insoluble in water.
Contain C, H, O, as well as P.Contain C, H, and O.
Consist of two fatty acids and a phosphate group.Consist of three fatty acids.
Consist of a hydrophobic 'tail' and a hydrophilic 'head'.Completely hydrophobic.
Form a bilayer in cell membranes.Do not form bilayers.

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 of the test sample into one of the test tubes.

  2. Add 5 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.

ResultMeaning
No emulsion is formed, and there is no colour change. A lipid is not present. This is a negative result.
An emulsion that is white/milky in colour has formed.A lipid is present. This is a positive result.

Lipids - Key takeaways

  • Lipids are biological macromolecules and one of the four most important in living organisms. They are composed of glycerol and fatty acids.
  • The covalent bond that forms between glycerol and fatty acids during condensation is called the ester bond.
  • Lipids are not polymers, and fatty acids and glycerol are not monomers of lipids. This is because fatty acids with glycerol do not form repetitive chains, like all other monomers. Therefore, lipids are not polymers since they contain chains of non-similar units.
  • The two most significant types of lipids are triglycerides and phospholipids.
  • Triglycerides have three fatty acids attached to glycerol. They are entirely insoluble in water (hydrophobic).
  • Phospholipids have two fatty acids and one phosphate group attached to glycerol. The phosphate group is hydrophilic, or 'water-loving', making the head of a phospholipid. Two fatty acids are hydrophobic, or 'water-hating', making the tail of a phospholipid.
  • The emulsion test is used to test for the presence of lipids.

Lipids

No. Fatty acids are parts of lipids. Fatty acids and glycerol together make up lipids.

A lipid is an organic biological macromolecule composed of fatty acids and glycerol. Lipids have many functions including energy storage, the structural components of cell membranes, cell recognition, insulation, and protection.

Two significant lipids in the human body are triglycerides and phospholipids. Triglycerides store energy, while phospholipids form bilayers of cell membranes.

The four types of lipids are phospholipids, triglycerides, steroids, and waxes.

Lipids are broken down into molecules of fatty acids and glycerol.

Final Lipids Quiz

Question

What is the definition of lipids?

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Answer

Lipids are biological macromolecules, and one of the four most important in living organisms.

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Question

What four groups do lipids include?

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Answer

Lipids include fats, oils, steroids and waxes.

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Question

Are lipids soluble in water?

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Answer

No, lipids are insoluble in water. However, they are soluble in organic solvents including alcohols and acetone.

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Question

What is the chemical structure of lipids?

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Answer

Lipids are organic molecules, which means they contain carbon and hydrogen. Lipids contain oxygen too, and sometimes other elements such as phosphorus, nitrogen, and sulphur.

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Question

What is the molecular structure of lipids?

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Answer

Lipids are composed of glycerol and fatty acids.

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Question

During which reaction are fatty acids joined with glycerol?

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Answer

Fatty acids are joined with glycerol during condensation.

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Question

What is the covalent bond between fatty acids and glycerol called?

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Answer

A glycosidic bond.

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Question

What are the different functions of lipids?

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Answer

Lipids are energy storage molecules; they build cell membranes, facilitate cell recognition, provide insulation and protection.

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What is the role of lipids in cell membranes?

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Answer

Lipids in cell membranes help membranes stay flexible and do not allow lipid-soluble molecules to pass through these membranes.

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Question

How do the lipids insulate the body?

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Answer

Lipids are stored beneath the body surface to keep our bodies warm. They offer a layer of insulation.

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Question

What are triglycerides built of?

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Answer

Glycerol

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Question

True or false: Triglycerides are soluble in water.  

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Answer

True.

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Question

What are phospholipids?

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Answer

Phospholipids are lipids that consist of one glycerol, two fatty acids and one phosphate group.

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Question

What is the 'head' and what is the 'tail' in a phospholipid molecule?

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Answer

The 'head' is the phosphate group with glycerol, while the 'tail' is the two fatty acids.

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Question

Are phospholipids soluble in water?

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Answer

One part of a phospholipid is soluble, and that is the 'head' - phosphate group and glycerol. The other part, the 'tail' (two fatty acids), is insoluble in water.

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Question

How are phospholipids organised in the bilayer?

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Answer

The 'head' is orientated toward the inside or the outside of the cell, and the 'tail' is orientated towards the inside of the bilayer, towards other 'tails'.

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Question

What is the test for lipids called?

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Answer

The emulsion test. 

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Question

How is the emulsion test carried out?

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Answer

  1. 2  of the test sample is added into a test tube. 
  2.  5  of ethanol is added.
  3. The end of the test tube is covered and shaken well.
  4. The liquid from the test tube is poured into a new test tube that was previously filled with water. Another option is to add water to the existing tube. 
  5. The change is observed and recorded. A positive result: An emulsion that is white/milky in colour has formed. 

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