B1.1.10 Difference between saturated, monounsaturated and polyunsaturated fatty acids Notes

The Structure of Fatty Acids: A Quick Overview

1. All fatty acids share a common structure: a long hydrocarbon chain (a chain of carbon atoms bonded to hydrogen) with a carboxyl group (-COOH) at one end. However, these chains can vary in two key ways:
   1. Length: The number of carbon atoms in the chain (typically between 14 and 20 in biological systems).
   2. Saturation: The number and position of double bonds between carbon atoms.

1. Saturated fatty acids: No double bonds. Every carbon atom is fully "saturated" with hydrogen atoms.
   1. Unsaturated fatty acids: One or more double bonds between carbon atoms. These are further divided into:
   2. Monounsaturated fatty acids (MUFA): Contain one double bond.
   3. Polyunsaturated fatty acids (PUFA): Contain two or more double bonds.

Saturated Fatty Acids: The Straight Chains

Characteristics

1. No double bonds: A rigid, linear structure.
2. High melting point: Tightly packed molecules require more energy (heat) to separate, making saturated fats solid at room temperature.
3. Sources: Found in animal fats (e.g., butter, lard) and some plant oils like coconut oil.

Biological Role

1. Saturated fats are ideal for long-term energy storage and thermal insulation in animals.
2. For example, marine mammals like whales rely on blubber, rich in saturated fatty acids, to retain heat in cold waters.

Monounsaturated Fatty Acids: The First Bend

Characteristics

1. One double bond: The kink reduces the molecule’s ability to stack.
2. Moderate melting point: MUFAs are typically liquid at room temperature but may solidify when cooled.
3. Sources: Found in plant-based oils like olive oil, avocado oil, and nuts.

Biological Role

1. MUFAs are common in plant oils, which are liquid at room temperature.
2. These oils provide energy storage in seeds and are easier to mobilize during germination.

Polyunsaturated Fatty Acids: The Curvy Chains

Characteristics

1. Two or more double bonds: The chain is highly curved and cannot pack tightly.
2. Low melting point: These fats remain liquid even at low temperatures.
3. Sources: Found in fish oils (e.g., salmon, mackerel) and plant oils like sunflower and flaxseed oil.

Biological Role

1. Polyunsaturated fats are common in plants and cold-water fish.
2. Plants store them in seeds for energy, while fish benefit from their fluidity, which keeps cell membranes functional in cold environments.

Melting Points and Energy Storage: Why It Matters

1. The melting point of a fatty acid depends on how tightly the molecules can pack:
   1. Saturated fats: Straight chains pack tightly, forming solid fats (e.g., butter).
   2. Monounsaturated fats: A single kink reduces packing, making oils semi-liquid (e.g., olive oil).
   3. Polyunsaturated fats: Multiple kinks prevent packing, keeping oils liquid (e.g., sunflower oil).

How Double Bonds Affect Melting Point

1. Double bonds in fatty acids introduce kinks in the hydrocarbon chain, disrupting their ability to pack tightly together.
   1. This reduces the strength of van der Waals interactions between molecules.
   2. As a result, unsaturated fatty acids with one or more double bonds require less energy to separate, leading to lower melting points.
   3. The more double bonds present, the greater the disruption, making polyunsaturated fatty acids liquid at lower temperatures compared to monounsaturated and saturated fatty acids.

The Bigger Picture: Health and Function

1. The type of fatty acid in your diet affects health:
   1. Saturated fats: Excessive intake raises LDL cholesterol, increasing the risk of heart disease.
   2. Monounsaturated fats: Improve heart health by lowering LDL cholesterol while maintaining HDL cholesterol.
   3. Polyunsaturated fats: Essential for brain function but prone to oxidation, leading to rancidity.