Which Compounds Are Included in Lipids?

January 10, 2026 •

4 min read

While the term 'lipids' is often used interchangeably with 'fats,' fats are actually just one subgroup of a much broader category of compounds. All lipids are organic compounds that are insoluble in water but soluble in non-polar solvents. This hydrophobic quality stems from their hydrocarbon-rich composition and is the key to their function in storing energy and forming cell structures.

Quick Summary

This article explores the diverse chemical compounds comprising lipids, including fatty acids, glycerolipids, phospholipids, sphingolipids, sterols, and waxes. It details the structural differences and biological roles of these essential molecules, from energy storage and cell membranes to hormone production.

Key Points

Fatty Acids: The fundamental building blocks of many lipids, consisting of a hydrocarbon chain and a carboxylic acid group, classified as either saturated or unsaturated based on double bonds.
Glycerolipids: Primarily triglycerides, formed from glycerol and three fatty acids, serving as the body's main energy storage form.
Phospholipids: Amphipathic molecules with a hydrophilic head and hydrophobic tails that form the lipid bilayer of all cell membranes.
Sterols: A class of lipids featuring a four-ring carbon structure, most notably cholesterol, which is vital for membrane structure and as a precursor for hormones.
Waxes: Protective, water-repellent coatings composed of long-chain fatty acids and alcohols.
Isoprenoids: A large and varied group of compounds derived from isoprene units, including fragrances, pigments like carotenoids, and natural rubber.
Amphipathic Nature: Some lipids, like phospholipids, have both water-loving and water-fearing parts, enabling them to form stable cellular structures.

The Core Components of Lipids: Fatty Acids

At the heart of many lipid structures are fatty acids, which are long hydrocarbon chains with a carboxylic acid group ($−COOH$) at one end. These simple yet crucial components determine many of a lipid's characteristics, including its saturation and melting point.

Saturated vs. Unsaturated Fatty Acids

Fatty acids are categorized based on their saturation, which refers to the number of double bonds in the hydrocarbon chain.

Saturated fatty acids have no double bonds, meaning their carbon chains are fully 'saturated' with hydrogen atoms. This results in straight chains that can pack tightly together, making them solid at room temperature, such as butter or animal fat. Examples include palmitic acid and stearic acid.
Unsaturated fatty acids contain one or more double bonds, which cause kinks in the hydrocarbon chain. These kinks prevent the molecules from packing tightly, causing them to be liquid at room temperature, like olive oil. They are further divided into monounsaturated (one double bond) and polyunsaturated (multiple double bonds) fatty acids.

Complex Lipids: Building on the Fatty Acid Foundation

Fatty acids often combine with other molecules to form more complex lipid compounds, each with specialized biological roles.

Glycerolipids: Energy Storage Molecules

Triglycerides, the most common glycerolipid, are formed when a glycerol molecule bonds with three fatty acid chains. These are the primary form of fat storage in both humans and plants, acting as a dense source of energy. When the body needs energy, it breaks down these triglycerides into fatty acids and glycerol.

Phospholipids: The Basis of Cell Membranes

Phospholipids are structurally similar to triglycerides, but with a crucial difference: one of the fatty acid chains is replaced by a phosphate-containing head group. This gives the molecule a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. This amphipathic nature causes phospholipids to spontaneously form a lipid bilayer in water, which is the fundamental structure of all cell membranes. They control the passage of substances into and out of the cell and serve as signaling molecules.

Sphingolipids: Specialized Membrane Components

Sphingolipids are a complex family of lipids with a sphingoid base backbone instead of glycerol. They are particularly abundant in the nervous system and play important roles in cell signaling and recognition. The most prominent examples are sphingomyelin, a key component of myelin sheaths, and glycosphingolipids, which are crucial for the immune response.

Other Major Lipid Categories

Beyond the fatty acid-derived lipids, other compounds are classified as lipids due to their hydrophobic properties.

Steroids and Sterols

Steroids are characterized by a fused four-ring carbon structure. The most well-known sterol is cholesterol, which adds rigidity to animal cell membranes and is the precursor for other important steroid hormones like testosterone and estrogen. Despite its hydrophobic nature, cholesterol has a small hydroxyl group that positions it within the cell membrane.

Waxes

Waxes are simple lipids formed by the esterification of a long-chain fatty acid with a long-chain alcohol. Their water-repellent properties make them ideal protective coatings for plants, feathers, and skin. Common examples include beeswax and carnauba wax.

Isoprenoids and Terpenes

Isoprenoids, also known as terpenes and terpenoids, are synthesized from five-carbon isoprene units. This diverse class of compounds includes pigments like carotenoids, fragrances like menthol, and even natural rubber. The fat-soluble vitamins A, D, E, and K are also isoprenoid-based lipids.

Comparison of Major Lipid Categories


Feature	Triglycerides	Phospholipids	Sterols	Waxes
Core Structure	Glycerol backbone with three fatty acids.	Glycerol backbone with two fatty acids and a phosphate head group.	Fused four-ring carbon skeleton.	Long-chain fatty acid esterified to a long-chain alcohol.
Hydrophobicity	Fully hydrophobic.	Amphipathic (hydrophilic head, hydrophobic tails).	Primarily hydrophobic.	Highly hydrophobic.
Function	Primary energy storage.	Major component of cell membranes; cell signaling.	Membrane fluidity; precursor for hormones, bile acids, and vitamins.	Protective coatings for waterproofing.
Solubility	Insoluble in water.	Forms bilayers in aqueous environments.	Insoluble in water.	Insoluble in water.
Example	Animal fats, vegetable oils.	Phosphatidylcholine, phosphatidylethanolamine.	Cholesterol, estrogen.	Beeswax, carnauba wax.

Conclusion: A Diverse and Vital Class of Compounds

In summary, the term 'lipids' encompasses a vast and structurally diverse range of compounds that are essential for life. From the simple energy-rich triglycerides found in our fat cells to the complex phospholipids forming our cell membranes and the signaling steroid hormones regulating our body, these hydrophobic molecules perform a wide array of vital functions. Their shared characteristic of water insolubility is what unites them as a class, while their individual structures dictate their specialized roles in biochemistry and physiology. Understanding the different compounds included in lipids is fundamental to appreciating their crucial biological importance.

Frequently Asked Questions

Lipids serve several vital functions, including long-term energy storage, forming the structural components of cell membranes (phospholipids and cholesterol), acting as signaling molecules, and providing insulation and protection for internal organs.

Fats, specifically triglycerides, are a type of lipid. The term 'lipids' is a broader category that also includes waxes, steroids, and phospholipids. All lipids are hydrophobic, but they have different structures and functions.

Fatty acids are the building blocks for more complex lipids like triglycerides and phospholipids. Their structure, including chain length and saturation, determines the properties of these larger lipid molecules.

Cholesterol is a type of lipid known as a sterol. It is an essential component of animal cell membranes, regulating membrane fluidity. It also serves as a precursor for synthesizing important steroid hormones and bile acids.

Phospholipids are amphipathic lipids with a hydrophilic head and hydrophobic tails. This structure allows them to arrange into a stable lipid bilayer in water, forming the fundamental barrier that defines a cell's boundary.

Isoprenoid lipids include a wide range of compounds derived from isoprene units. Notable examples are the fat-soluble vitamins A, D, E, and K, as well as carotenoids (pigments) and fragrances found in plants.

Saturated fats have straight fatty acid chains with no double bonds, allowing them to pack tightly and be solid at room temperature. Unsaturated fats have one or more double bonds, which cause kinks in the chain, making them liquid at room temperature.