Lipids: Definition and Functions

Study Notes

Lipids are a diverse group of biomolecules that share the common characteristic of being hydrophobic (water-insoluble) and soluble in organic solvents.
They are named after the Greek word "lipos," meaning "fat."
Lipids consist of fatty acids or a steroid nucleus.
They play crucial roles in the body:
- Providing energy (triglycerides are the storage form)
- Forming structural components of cell membranes
- Serving as hormones (e.g., steroid hormones like cortisol, aldosterone, and sex hormones)
- Acting as fat-soluble vitamins with regulatory functions

Lipids are a significant source of energy in our diet and within the body.
They are stored as triglycerides in adipose tissue, which also provides insulation and organ protection.
Lipids are essential components of nervous tissue, contributing to its function in sensation and response.
They are transported in the blood as lipoproteins, which are crucial for delivering lipids and vitamins to cells.
Cholesterol plays a vital role in the synthesis of hormones, vitamin D3, bile salts, and sterols.
Prostaglandins, derived from lipids, influence blood pressure and are linked to arteriosclerosis.
Lipids serve as a valuable source of essential fatty acids, such as linoleic acid, linolenic acid, and arachidonic acid, which the body cannot synthesize on its own.
They can act as activators for certain enzymes and participate in electron transport within mitochondria.

Lipids are grouped into three main categories:
- Simple lipids: Esters of fatty acids with alcohols (e.g., triglycerides, also known as fats or oils)
- Conjugated (complex) lipids: Esters of fatty acids with alcohols and additional groups (e.g., phospholipids, glycolipids, lipoproteins, waxes)
- Derived lipids: Products obtained by hydrolysis of simple or complex lipids (e.g., fatty acids, cholesterol, ketone bodies)

Fatty acids are produced by hydrolyzing neutral fats (triglycerides).
They are organic monocarboxylic acids with an aliphatic chain ranging from 4 to 24 carbon atoms.
Insoluble in water and soluble in organic solvents.
The aliphatic chain can be saturated (no double bonds) or unsaturated (containing one or more double bonds).
Serve as fuel, building blocks for triglycerides, phospholipids, and sphingolipids.
Precursors for prostaglandins.

Fatty acids are typically written using condensed structural formulas or skeletal formulas.
Naming follows a systematic approach:
- The base name is derived from the corresponding hydrocarbon with the same number of carbon atoms, replacing the final "-e" with "-oic acid".
- Saturated fatty acids end with "-anoic acid" (e.g., octanoic acid).
- Unsaturated fatty acids end with "-enoic acid" (e.g., octadecenoic acid).
- Carbon atoms are numbered from the -COOH group.
- The carbon atom adjacent to the -COOH group is called the α-carbon atom.
- For example, octadecenoic acid (oleic acid) is written as 18:1, 9, indicating an 18-carbon chain with one double bond between carbon atoms 9 and 10.