Question

Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are stearic acid, ${69.6}^{\circ }\mathrm{C}$; oleic acid, ${13.4}^{\circ }\mathrm{C}$; linoleic acid, $-5{}^{\circ }\mathrm{C}$; and linolenic acid, $-11{}^{\circ }\mathrm{C}$. a. What structural aspect of these 18-carbon fatty acids can be correlated with the melting point? b. Draw all the possible triacylglycerols that can be constructed from glycerol, palmitic acid, and oleic acid. Rank them in order of increasing melting point. c. Branched-chain fatty acids are found in some bacterial membrane lipids. Would their presence increase or decrease the fluidity of the membrane (that is, give the lipids a lower or a higher melting point)? Why?

Short answer

a) Saturation level affects melting point. b) Rank: PPP > PPO/POP > OPP > OOO. c) Branched chains increase fluidity, lowering melting point.

Step-by-step solution

Identify the Structural Aspect for Melting Points

The melting points of fatty acids are largely influenced by their saturation. Saturated fatty acids (with no double bonds) usually have higher melting points due to stronger van der Waals forces, while unsaturated fatty acids (one or more double bonds) have lower melting points. Hence, stearic acid, being fully saturated, has a higher melting point than oleic, linoleic, and linolenic acids, which are unsaturated.

Understand Triacylglycerol Structure

Triacylglycerols consist of glycerol attached to three fatty acid chains. In this case, we consider triacylglycerols that include glycerol, palmitic acid (a saturated fatty acid with 16 carbons), and oleic acid (an 18-carbon unsaturated fatty acid with one double bond).

Draw Possible Triacylglycerols

Construct possible triacylglycerols by attaching palmitic and oleic acids to the three hydroxyl groups of glycerol. Possible combinations are: all three chains as palmitic acid (PPP), two palmitic acids and one oleic acid (PPO and POP), one palmitic and two oleic acids (OOO and OPO), and all three as oleic acid (OOO).

Rank Triacylglycerols by Melting Point

Rank the triacylglycerols by their saturation: fully saturated (PPP) will have the highest melting point, followed by those with a mix favoring palmitic acid chains over oleic acid chains (e.g., PPO, POP), and finally those high in oleic chains (OOO) will have the lowest melting point.

Analyze Effect of Branched-Chain Fatty Acids on Fluidity

Branched-chain fatty acids disrupt the regular packing of lipid molecules in a membrane, thereby decreasing the melting point and increasing fluidity. Therefore, their presence in a membrane will result in more fluid lipids, lowering the melting point the same way that unsaturation does.

Key concepts

Saturated Fatty Acids

Saturated fatty acids are a type of fatty acid that contains no double bonds between the carbon atoms of the hydrocarbon chain. This means that each carbon atom is "saturated" with hydrogen atoms. These types of fatty acids are usually solid at room temperature, owing to their straight chains which enable tight packing of the molecules. The regular structure leads to stronger van der Waals forces, which in turn increases their melting point.

Here’s a quick look at their characteristics:

• No double bonds
• Solid at room temperature
• Higher melting points compared to unsaturated fats
  due to stronger intermolecular forces
• Commonly found in animal fats such as butter

Saturated fatty acids play important roles in both structural and energy storage functions in organisms.

Unsaturated Fatty Acids

Unlike saturated fatty acids, unsaturated fatty acids have one or more double bonds within the carbon chain. These double bonds create kinks, preventing the molecules from packing tightly. As a result, they are often liquid at room temperature and have lower melting points.

Understanding their characteristics is key:

• Contain one (monounsaturated) or more (polyunsaturated) double bonds
• Liquid at room temperature
• Lower melting points due to less efficient packing of molecules
• Commonly found in plant oils such as olive oil and fish oil

The presence of cis-double bonds in natural unsaturated fats causes the chain to bend, contributing significantly to the fluidity of cellular membranes and other important biological functions.

Triacylglycerol Structure

Triacylglycerols, also known as triglycerides, are lipid molecules composed of one glycerol backbone esterified with three fatty acid chains. This structure serves as a significant energy storage form in organisms, providing more than twice the energy per gram than carbohydrates or proteins.

Key aspects of triacylglycerol include:

• Glycerol as the central backbone
• Three fatty acids can be of varying lengths and saturation
• Energy-rich, stored in adipose tissue
• Hydrophobic nature allowing them to store energy effectively
  without attracting water

The melting point of triacylglycerols depends on the fatty acids attached. Saturated acids increase melting point, whereas the inclusion of unsaturated acids results in lower melting points, demonstrating the interplay between fatty acid composition and lipid function.

Membrane Fluidity

Membrane fluidity is crucial for the function of biological membranes and is influenced strongly by the types of lipids that make up the membrane. The presence of saturated fatty acids tends to decrease fluidity, making the membrane more rigid.

Meanwhile, the integration of unsaturated fatty acids, with their kinked structures, prevents tight lipid packing and enhances fluidity.

Factors affecting membrane fluidity include:

• Fatty acid composition (saturated versus unsaturated)
• Chain length (shorter chains increase fluidity)
• Presence of cholesterol which can modulate fluidity

Branched-chain fatty acids, like those found in some bacterial cell membranes, also disrupt tight packing and increase fluidity. This fluid nature is vital for various cellular processes such as transport, signaling, and membrane fusion.