Lipids

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

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      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.

      Trygliceride, lipids biological molecules Study SmarterFig. 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.

      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 "true" 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 lipids 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.

      Phospholipid, lipids, biological molecules Study SmarterFig. 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

      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 cm3 of the test sample into one of the test tubes.

      2. Add 5cm3 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.
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      Frequently Asked Questions about Lipids

      Are fatty acids lipids?

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

      What is a lipid, and what is its function?

      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.

      What are lipids in the human body?

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

      What are the four types of lipids?

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

      What are lipids broken down into?

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

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