HO - 02.3 Lipids

Lipids

Definition: Lipids are organic molecules that are mostly hydrophobic (non-polar) and do not form polymers.
Solubility: They are soluble in non-polar solvents like chloroform and ether.
Types of Lipids:
- Cholesterol
- Free fatty acids
- Triglycerides
- Phospholipids

Structure: Fatty acids consist of a carboxylic acid group and an aliphatic chain, which can be highly variable in length (from less than 5 to more than 20 carbon atoms). They serve as the main components of fats and oils.
Examples of Fatty Acids:
- Palmitic acid (16:0)
- Palmitoleic acid (16:1 n-7), which is unsaturated due to a double bond.
Notation: The notation 16:0 indicates the number of carbon atoms and the number of double bonds.

Definition: Glycerol is a 3-carbon polyol (sugar alcohol) that serves as the backbone to which fatty acids attach.
Structure:
- Contains three hydroxyl (-OH) groups.
- Often referred to as glycerin.
Function: Glycerol combines with fatty acids through esterification, which is a dehydration reaction, creating fats and oils.

Formation: Triglycerides are formed by attaching three fatty acids to a glycerol molecule.
Variety: The fatty acids attached can differ in their lengths and saturation levels.
Energy Storage: Fats and oils provide high energy density for storage and serve as energy reserves in most organisms. Although commonly called triglycerides, a more accurate term is triacylglycerides.

Fats:
- Solid at room temperature.
- Composed of saturated fatty acids, giving them a higher melting point (e.g., butter derived from milk).
Oils:
- Liquid at room temperature.
- Contain at least one unsaturated fatty acid, resulting in a lower melting point.

Cis vs. Trans Fatty Acids:
- Cis Fatty Acids: Have hydrogen atoms on the same side of a double bond, causing a bend in the structure (e.g., oleic acid).
- Trans Fatty Acids: Have hydrogen atoms on opposite sides, resulting in a straight structure (e.g., elaidic acid).
Melting Points: Oleic acid has a lower melting point (14 ºC) compared to elaidic acid (45 ºC) due to the structural differences.

Structure: Phospholipids consist of a glycerol backbone, two fatty acid tails (which may include both saturated and unsaturated bonds), and a phosphate group.
Amphipathic Nature: This unique structure makes phospholipids amphipathic (having both hydrophilic and hydrophobic properties).
Role in Cell Membranes: Phospholipids spontaneously form bilayers in aqueous environments, forming the foundational structure of cell membranes.

Types of Lipids: Cholesterol is a crucial lipid that serves as a precursor for steroids in animal signaling, with examples including estradiol, testosterone, and cortisol.
Function: Cholesterol plays a role in various physiological processes, including hormonal signaling.

Waxes: Another class of lipids that consist of long aliphatic chains (e.g., carnauba wax).
Glycolipids: These molecules include sugars and are essential in many cellular functions, including immune responses and structural roles in bacterial cell walls (e.g., Escherichia coli lipopolysaccharide).

Fats are primarily used for long-term energy storage in humans and animals, while plants primarily use fats and oils along with starch for their energy needs.
Caloric Density: Lipids provide 9 calories per gram, while carbohydrates offer only 4 calories per gram, highlighting the energy efficiency of lipids despite slower metabolism.

Carbohydrates: Composed of carbon chains with numerous hydroxyl groups, they serve as energy storage and structural components. Monosaccharides can form polymers through dehydration.
Lipids: Comprised of carbon chains terminating in a carboxyl group. They are hydrophobic and do not form polymers but can be amphipathic when linked to polar molecules.