LipidsEdit

Lipids are a broad and diverse class of biomolecules defined largely by their hydrophobic or amphipathic character. They are insoluble in water but soluble in nonpolar solvents, a property that underpins their roles in energy storage, membrane structure, signaling, and insulation. The lipid family ranges from simple fatty acids to complex sterols and sphingolipids, with many derivatives serving as precursors to hormones, vitamins, and signaling molecules. Because of their variety, lipids are often categorized by key structural features, such as the presence of ester bonds, ring systems, or amphipathic head groups. Fatty acids, Triglycerides, Phospholipids, Sphingolipids, and Cholesterol exemplify the major subgroups that underpin cellular organization and metabolism. The lipid story intersects with nutrition, physiology, and medicine, reflecting how organisms store energy, build membranes, and respond to internal and external cues. Adipose tissue stores much of the body's energy as triglycerides, while cellular membranes rely on phospholipids and cholesterol to create boundaries that regulate traffic and communication. Eicosanoid signaling molecules derived from fatty acids illustrate how lipids can translate metabolic state into physiological responses.

Classification and structure

Lipids are best understood through their principal subclasses and the typical chemical features they share.

Fatty acids: carboxylic acids with long hydrocarbon chains that may be saturated or unsaturated. They are the building blocks for many other lipids and are stored in cells as part of larger esters. Fatty acids can vary in chain length and degree of unsaturation, which influences their physical properties and biological roles.
Triglycerides: esters formed from glycerol with three fatty acid chains. They are the primary form of long-term energy storage in animals and plants and are concentrated in Adipose tissue as energy reserves. Triglycerides are hydrophobic and can be mobilized when energy is needed.
Glycerolipids and phospholipids: glycerol backbones with fatty acids and various head groups. The most abundant of these in membranes are Phospholipids, which typically have two fatty acids attached to glycerol and a phosphate-containing head group. Their amphipathic nature drives the formation of the lipid bilayer, a fundamental component of the Cell membrane.
Sterols and cholesterol: steroid-like ring structures that provide rigidity to membranes and serve as precursors for steroid hormones and bile acids. Cholesterol is the most well-known sterol in animals and is distributed in membranes and lipoproteins.
Sphingolipids: lipids based on a sphingoid backbone rather than glycerol. They contribute to membrane structure and signaling, and include species such as sphingomyelin found in specialized membrane domains. Sphingolipids participate in cell recognition and signaling pathways.
Prenol lipids and isoprenoids: a diverse set of lipids derived from isoprene units. They include vitamins (such as Vitamin A), pigments, and electron carriers (like Coenzyme Q). These lipids play roles in photosynthesis, vision, and energy metabolism.
Other lipid categories: lipids also encompass certain fat-soluble vitamins, waxes, and various bioactive molecules that do not fit neatly into a single subclass but contribute to physiology in important ways.

Functions

Lipids fulfill a wide array of functions that are essential for life.

Energy storage and density: lipids, particularly Triglycerides, store large amounts of energy per unit mass and can be mobilized to fuel cellular processes during fasting or increased activity. This energy reserve is centralized in tissues such as Adipose tissue.
Membrane structure and dynamics: Phospholipids and Cholesterol assemble into the Lipid bilayer, creating a selective barrier that organizes cellular compartments and supports membrane protein function. The composition and organization of membranes influence fluidity, permeability, and signaling.
Signaling and regulation: lipids serve as precursors to signaling molecules, including Eicosanoids derived from arachidonic acid and other fatty acids, as well as steroid hormones synthesized from Cholesterol. These molecules coordinate inflammation, vascular function, and metabolic responses.
Insulation and protection: lipids such as fats provide insulation and cushioning for organs, and myelin sheaths in nerves rely on lipid-rich membranes for rapid electrical conduction.
Digestion and nutrient transport: dietary lipids enable the absorption of fat-soluble vitamins and other nutrients. Bile acids, derived from cholesterol, emulsify fats to aid digestion and absorption.
Cellular organization and energy metabolism: various lipids participate in intracellular signaling, vesicle formation, and energy production through pathways like beta-oxidation, where fatty acids are broken down in mitochondria to generate ATP.

Metabolism

Lipid metabolism encompasses digestion, transport, storage, and catabolic and anabolic pathways.

Digestion and emulsification: dietary lipids are emulsified by bile salts in the digestive tract, increasing the surface area for enzymatic hydrolysis by lipases that release free fatty acids and monoglycerides.
Absorption and transport: fatty acids and monoacylglycerols are taken up by intestinal cells and reassembled into triglycerides, which are packed into Chylomicrons and enter the circulation via the lymphatic system. In the bloodstream, lipoproteins such as Very-low-density lipoprotein, Low-density lipoprotein, and High-density lipoprotein participate in lipid transport to tissues.
Storage and mobilization: excess fatty acids are stored in Adipose tissue as triglycerides, and mobilized when energy is needed by hydrolysis to free fatty acids and glycerol.
Beta-oxidation and energy production: fatty acids are transported into mitochondria and catabolized via beta-oxidation to generate acetyl-CoA, which feeds the citric acid cycle and oxidative phosphorylation.
Lipogenesis and ketogenesis: lipogenesis builds fatty acids and triglycerides from acetyl-CoA when energy and carbon skeletons are abundant; in prolonged fasting or carbohydrate restriction, acetyl-CoA can be diverted to ketone body production in the liver, providing an alternative fuel for tissues such as the brain. Lipogenesis and Ketone bodies illustrate these adaptive pathways.

Nutrition and health

Dietary lipids influence health outcomes through multiple mechanisms.

Types of dietary fat: unsaturated fats (monounsaturated and polyunsaturated) are generally associated with favorable cardiovascular risk profiles when they replace saturated fats, while trans fats are widely discouraged due to adverse effects on cholesterol and inflammation. The balance of fatty acids, including essential Omega-3 and Omega-6 fatty acids, supports membrane function and signaling.
Essential fatty acids: humans require certain fatty acids that cannot be synthesized de novo, such as linoleic acid and alpha-linolenic acid, which must be obtained from the diet. These are foundational for normal development and physiology.
Lipids and disease: lipid profiles, including levels of LDL and HDL, relate to cardiovascular risk in many populations, though the interpretation of these markers is nuanced and depends on overall diet, genetics, and lifestyle. Medical approaches often target lipid metabolism through interventions such as Statin therapy to modulate cholesterol synthesis and lipoprotein distribution.
Controversies and evolving guidelines: research on dietary fats, cholesterol, and heart disease has produced decades of debate and evolving guidelines. While consensus supports prioritizing whole-food patterns and balancing fatty acid intake, proponents of various dietary traditions argue for different emphases on saturated fats, carbohydrate quality, and protein sources. The body of evidence continues to evolve as new large-scale trials and mechanistic studies refine our understanding of how lipids influence health.

Evolutionary and comparative perspectives

Lipids are universal across life, with notable variation reflecting environmental and physiological needs. Membrane lipids exhibit diversity that supports adaptations to temperature, nutrient availability, and metabolic strategies across bacteria, archaea, plants, fungi, and animals. In vertebrates, complex lipid systems underlie nervous system function, with specialized lipids such as myelin being essential for rapid nerve signaling. Comparative studies highlight how lipid composition correlates with lifestyle and habitat, shedding light on the interplay between nutrition, metabolism, and physiology.