Table of Contents
Overview of Lipids
Lipids are a broad group of biologically important molecules that are characterized mainly by their poor solubility in water and good solubility in non-polar (fatty) solvents. They are not defined by a single common structure, but by shared physical and chemical properties.
Major roles of lipids in organisms include:
- Energy storage
- Structural components of membranes
- Protection and insulation
- Signaling (hormones, second messengers)
- Vitamins and pigments
A helpful way to approach lipids is to distinguish between:
- Saponifiable lipids (contain ester bonds and can be split with strong base: fats, oils, phospholipids, glycolipids)
- Non-saponifiable lipids (no ester bonds: steroids, some vitamins, terpenoids)
Below, we focus on the main classes and their biological significance.
Fatty Acids – Basic Building Blocks of Many Lipids
Fatty acids are often the fundamental units from which many lipids are built.
General Structure
A typical fatty acid has:
- A long, unbranched hydrocarbon chain (usually 4–24 carbon atoms)
- A terminal carboxyl group $-COOH$
In shorthand notation, a fatty acid like palmitic acid can be written as:
C16:0— 16 carbons, 0 double bonds
A fatty acid with one double bond, such as oleic acid, isC18:1.
Saturated vs. Unsaturated Fatty Acids
- Saturated fatty acids
- Contain only single bonds between carbon atoms in the chain
- Have straight chains that can pack closely
- Typically solid or semi-solid at room temperature (e.g., many animal fats, some tropical plant fats)
- Unsaturated fatty acids
- Contain one or more double bonds in the chain
- Monounsaturated: one double bond (e.g., oleic acid,
C18:1) - Polyunsaturated: two or more double bonds (e.g., linoleic acid,
C18:2; linolenic acid,C18:3) - Most natural double bonds in fatty acids are in the cis configuration, causing a “kink” in the chain and preventing tight packing
- Usually liquid at room temperature (typical of many plant oils and fish oils)
The number and position of double bonds strongly influence:
- Melting point
- Fluidity of membranes in which they are incorporated
- Metabolic and signaling properties
Essential Fatty Acids
Humans (and many animals) cannot introduce double bonds beyond certain positions in the fatty acid chain. Therefore, some polyunsaturated fatty acids must be obtained from the diet:
- Linoleic acid (an omega-6 fatty acid)
- α-Linolenic acid (an omega-3 fatty acid)
These are essential fatty acids, required for:
- Normal growth and development
- Synthesis of signaling molecules (e.g., eicosanoids)
- Proper function of cell membranes, especially in the nervous system and retina
Triacylglycerols (Fats and Oils)
Structure and Formation
Triacylglycerols (often simply called triglycerides, fats, or oils) are the main storage lipids in many organisms.
They consist of:
- A glycerol molecule (a 3-carbon alcohol)
- Three fatty acid molecules, each esterified to one of the hydroxyl groups of glycerol
Each ester bond forms through a condensation reaction between a hydroxyl group of glycerol and the carboxyl group of a fatty acid, releasing water.
Fats vs. Oils
- Fats
- Triacylglycerols rich in saturated fatty acids
- Typically solid at room temperature
- Common in animals (e.g., butter, lard), and some plants (e.g., coconut fat, cocoa butter)
- Oils
- Triacylglycerols rich in unsaturated fatty acids
- Liquid at room temperature
- Typical of many plant seeds (e.g., sunflower, olive, rapeseed oils) and fish
In nature, a given triacylglycerol often contains a mixture of different fatty acids, so there is a large variety of possible structures.
Biological Functions of Triacylglycerols
- Long-term energy storage
- Higher energy content per gram than carbohydrates or proteins because:
- Fatty acids are more reduced (contain more hydrogen and less oxygen)
- Triacylglycerols are stored in nearly anhydrous form (without water)
- Insulation and protection
- Subcutaneous fat helps maintain body temperature
- Fat deposits around organs cushion against mechanical shock
- Buoyancy
- In aquatic animals, fat can contribute to buoyancy
Phospholipids – Main Components of Biological Membranes
General Structure
Phospholipids are the dominant structural lipids in cell membranes. A typical membrane phospholipid contains:
- A glycerol backbone
- Two fatty acid chains (usually one saturated, one unsaturated)
- A phosphate group attached to the third carbon of glycerol
- Often an additional polar head group attached to the phosphate, e.g.:
- Choline (phosphatidylcholine)
- Ethanolamine (phosphatidylethanolamine)
- Serine (phosphatidylserine)
- Inositol (phosphatidylinositol)
This produces an amphipathic molecule, with:
- A hydrophobic (water-repelling) “tail” (fatty acids)
- A hydrophilic (water-attracting) “head” (phosphate + polar group)
Amphipathic Nature and Bilayer Formation
In aqueous environments, phospholipids spontaneously arrange themselves so that hydrophobic tails avoid water and hydrophilic heads interact with it. This self-assembly results in structures such as:
- Micelles (spherical aggregates)
- Lipid bilayers, where two leaflets of phospholipids align tail-to-tail
Biological membranes are based on such phospholipid bilayers, which:
- Provide a semi-permeable barrier between cell interior and exterior
- Serve as the matrix into which membrane proteins and other lipids are embedded
- Can form curved structures (vesicles, organelles) important for compartmentalization
Influence of Fatty Acid Composition
The degree of saturation and chain length of the fatty acids in membrane phospholipids influences:
- Membrane fluidity (more unsaturated → more fluid)
- Permeability to different molecules
- Function of membrane proteins embedded in the bilayer
Organisms can adjust their membrane lipid composition in response to temperature changes to maintain appropriate membrane fluidity.
Glycolipids – Lipids with Attached Sugars
Glycolipids resemble phospholipids but carry carbohydrate groups instead of (or in addition to) phosphate-based head groups.
Structure
Common glycolipids in animals:
- Have a glycerol or sphingosine backbone
- One or two fatty acids
- One or more sugar residues (e.g., glucose, galactose, or more complex oligosaccharides) as the hydrophilic head
Examples include:
- Cerebrosides (one sugar)
- Gangliosides (complex oligosaccharides including sialic acids)
Biological Roles
- Membrane components, especially in the outer leaflet of the plasma membrane
- Contribute to the glycocalyx, a carbohydrate-rich coat on the cell surface
- Important in:
- Cell recognition and communication
- Cell adhesion
- Determining blood group antigens (e.g., ABO system)
- Acting as receptors for toxins or pathogens in some cases
In plants, specific glycolipids are abundant in chloroplast membranes and are important for photosynthetic function.
Steroids and Sterols
General Structure
Steroids are lipids characterized by a four-ring carbon skeleton (steroid nucleus). A major subgroup are sterols, which contain:
- The steroid ring system
- A hydroxyl group $-OH$ (making them weakly amphipathic)
- A hydrophobic hydrocarbon side chain
The most prominent sterol in animal cells is cholesterol.
Cholesterol
Cholesterol is a key component of animal cell membranes and has several roles:
- Modulates membrane fluidity and stability
- At higher temperatures: reduces fluidity (rigid ring structure)
- At lower temperatures: prevents tight packing of phospholipid fatty acids, thus counteracting membrane solidification
- Serves as a precursor for:
- Steroid hormones (e.g., cortisol, aldosterone, sex hormones such as estrogen and testosterone)
- Bile acids (important for fat digestion)
- Vitamin D
Plants and fungi have related sterols (e.g., sitosterol in plants, ergosterol in fungi) with similar roles.
Steroid Hormones
Derived from cholesterol, these hormones are:
- Lipid-soluble and can cross cell membranes
- Often bind to intracellular receptors (in cytoplasm or nucleus)
- Regulate gene expression and thus affect long-term processes such as:
- Metabolism
- Salt and water balance
- Development of sexual characteristics
- Stress response
Steroid hormones highlight the importance of lipids as signaling molecules besides their structural and storage functions.
Waxes
Waxes are esters of long-chain fatty acids with long-chain alcohols (not glycerol).
Properties
- Highly hydrophobic
- Usually solid and water-repellent
- Resistant to degradation and often chemically relatively inert
Biological Functions
- Protective coatings:
- On leaves, stems, and fruits of plants, reducing water loss and providing some defense against pathogens
- On insect cuticles, limiting evaporation and serving as a barrier
- In bird feathers and mammalian fur, contributing to water repellency (e.g., secretion from the uropygial gland of birds)
- Sometimes involved in communication (e.g., insect waxes with signaling functions)
Other Biologically Important Lipids
A variety of other molecules are classified as lipids because of their hydrophobic or amphipathic nature. Some important examples:
Eicosanoids
- Derived from 20-carbon polyunsaturated fatty acids, especially arachidonic acid
- Include prostaglandins, thromboxanes, leukotrienes
- Act as local hormones with very short lifetimes
- Important in:
- Inflammation and immune responses
- Regulation of blood clotting
- Constriction or dilation of blood vessels
- Smooth muscle contraction (e.g., in airways, uterus)
Isoprenoids (Terpenoids) and Fat-Soluble Vitamins
Many isoprenoid compounds (derived from isoprene units) are lipids, including:
- Fat-soluble vitamins (A, D, E, K)
- Stored and transported in association with lipids
- Essential in:
- Vision (vitamin A)
- Calcium and phosphate metabolism (vitamin D)
- Antioxidant protection (vitamin E)
- Blood clotting (vitamin K)
- Plant pigments and fragrances, and other biologically active terpenoids
These link lipid chemistry to diverse physiological and ecological roles.
Lipids in Nutrition and Health (Conceptual Overview)
Lipids are essential in the diet but must be present in appropriate types and amounts. Key aspects include:
- Energy supply and essential fatty acids
- Absorption of fat-soluble vitamins
- Influence on blood lipid profiles (e.g., LDL and HDL cholesterol)
- Role of trans fatty acids (formed by industrial hydrogenation or high-temperature processing) in increasing cardiovascular risk
The balance between saturated, unsaturated, and trans fats, as well as the overall quantity of dietary lipids, affects human health over the long term.
Summary
- Lipids are a diverse group of water-insoluble or poorly soluble molecules.
- Fatty acids are basic building blocks; their saturation level strongly influences physical properties and biological roles.
- Triacylglycerols are major storage forms of energy and provide insulation and protection.
- Phospholipids and glycolipids are key structural components of biological membranes and participate in cell recognition and signaling.
- Sterols and steroids like cholesterol and steroid hormones modify membrane properties and serve as crucial signaling and regulatory molecules.
- Waxes form protective, water-repellent layers in plants and animals.
- Various specialized lipids (eicosanoids, fat-soluble vitamins, terpenoids) have important regulatory, protective, and ecological functions.
Together, these lipid classes illustrate how variations on a basic hydrophobic theme can support storage, structure, protection, signaling, and many other vital processes in living organisms.