2.7
Structure of Lipids
Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar ca…
Lipids are a group of hydrophobic molecules that include triglycerides, which are used to store energy, and phospholipids, which are the major structural components of cell membranes.
Triglycerides and most phospholipids consist of glycerol and fatty acid chains.
Fatty acids have methyl groups at one end of the chain and carboxyl groups on the opposite end. The carbon connected to the carboxyl group is called the alpha carbon, and the methyl group carbon is known as the omega carbon.
Fatty acids vary in length and the presence of double bonds in the hydrocarbon chain. Those with double bonds are unsaturated fatty acids and those with only single bonds are saturated fatty acids because they are saturated with the largest number of hydrogen atoms possible.
Fatty acids often have common names but can be systematically named according to the number of carbon atoms and the number and position of the double bonds in the carbon chain.
There are several common numbering methods. The carboxyl referencing system counts the position of all the double bonds from the carboxyl carbon, which is numbered as 1. The omega referencing system counts the position of the double bond closest to the omega carbon, with the omega carbon numbered as 1.
For example, linolenic acid, an omega 3-fatty acid, has 18 carbons and double bonds at positions 9,12, and 15, counting from the carboxyl end. This will be called 18:3 Δ9, 12, 15 according to the carboxyl referencing system and 18:3 (ω-3) according to the omega referencing system.
Unsaturated fatty acids can occur in two configurations: cis and trans. In the cis configuration, the hydrogens on the carbons involved in the double bond are on the same side of the bond, whereas in the trans configuration, the hydrogens are located on the opposite sides.
Structurally, cis fatty acids have bent chains, whereas trans fatty acids have straight chains.
Consumption of trans fats has been found to be responsible for various cardiovascular diseases, whereas consumption of cis fatty acids, such as omega-3 and omega-6, are known to be beneficial to health.
Triglycerides, commonly known as fats, are primarily used for the storage of energy. They are composed of three fatty acids linked to glycerol through ester linkages between the hydroxyl ends of glycerol and the carboxyl ends of the fatty acids resulting in the formation of a nonpolar molecule.
If all the three fatty acids are of the same type, they are known as simple triglycerides. If the three fatty acids differ, they are known as mixed triglycerides.
Another type of lipid, phospholipids, is an important structural feature of biological membranes.
They have hydrophilic heads consisting of phosphate groups modified with a hydrophilic alcohol group and hydrophobic fatty acid tails. The composition of these heads and tails can vary, resulting in different types of membrane lipids.
Glycerophospholipids are a common type of phospholipid that consists of two fatty acids and a highly polar group attached to each carbon of glycerol through ester and phosphodiester linkages, respectively.
View the full transcript and gain access to JoVE Core videos
Q1: What are the main structural components of lipids?
Lipids are primarily composed of carbon, hydrogen, and oxygen atoms arranged in hydrophobic hydrocarbon chains or rings. These structures contain fatty acids—long-chain carboxylic acids with nonpolar C-H bonds—that make lipids water-repellent. The hydrophobic nature of lipids allows them to form membranes and store energy efficiently in cells.
Q2: How do triglycerides differ from phospholipids in structure?
Triglycerides consist of three fatty acids bonded to a glycerol backbone through ester linkages, making them entirely hydrophobic. Phospholipids have two fatty acids and a phosphate-containing head group, creating a dual-nature molecule with hydrophobic tails and a hydrophilic head. This structural difference enables phospholipids to form bilayers in cell membranes.
Q3: What role do noncovalent attractions play in lipid organization?
Noncovalent attractions in biomolecules, such as hydrophobic interactions, drive lipid self-assembly in aqueous environments. Hydrophobic fatty acid tails cluster together to minimize water contact, while hydrophilic head groups orient toward water. These weak interactions allow lipids to form flexible structures like membranes and micelles essential for cellular function.
Q4: Why are lipids considered organic molecules?
Lipids are organic molecules because they contain carbon atoms bonded to hydrogen and oxygen in covalent arrangements. Like other water organic molecules inorganic ions found in cells, lipids are built from carbon skeletons with functional groups. Their carbon-based structure classifies them as organic compounds distinct from inorganic salts and minerals.
Q5: How do cholesterol and steroid lipids differ structurally from fatty acids?
Cholesterol and steroid lipids contain a four-ring carbon structure rather than long hydrocarbon chains found in fatty acids. Despite lacking extended chains, steroids remain hydrophobic due to their carbon-hydrogen composition. This rigid ring structure allows steroids to embed in membranes and function as signaling molecules, contrasting with the flexible chains of triglycerides.
Q6: What functional groups are present in lipid molecules?
Lipids contain functional groups including carboxyl groups in fatty acids, ester linkages in triglycerides, and phosphate groups in phospholipids. These functional groups and types of organic compounds determine lipid solubility and reactivity. The ester bonds linking fatty acids to glycerol are formed through condensation reactions, creating the characteristic structure of storage lipids.
Q7: How do lipids compare structurally to carbohydrates and proteins?
Unlike carbohydrates with their sugar units or proteins with amino acid chains, lipids lack repeating monomer units and are not true polymers. Carbohydrate chemistry mono and polysaccharides involves glycosidic bonds between sugars, while lipids use ester linkages. Proteins form through peptide bonds between amino acids, making lipids structurally distinct as nonpolar, energy-dense molecules.