Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
10.1 Storage Lipids
This chapter explains the structure, chemistry, and biological roles of storage lipids, which are major energy reserves in many organisms. It introduces fatty acids as fundamental building blocks, describes triacylglycerols as primary storage lipids, and compares fats and oils with carbohydrates in energy storage efficiency and metabolic function.
Fatty Acids Are Hydrocarbon Derivatives
• Fatty acids are carboxylic acids with hydrocarbon chains containing 4 to 36 carbons, typically unbranched and in even numbers in biological systems.
• They can be classified as saturated (no double bonds) or unsaturated (one or more double bonds). Monounsaturated fatty acids have one double bond, and polyunsaturated fatty acids have multiple double bonds.
• The hydrocarbon chain is nonpolar and hydrophobic, while the carboxyl group is polar and ionized at physiological pH, contributing to amphipathic behavior.
• Fatty acids rarely occur in the free (unesterified) form inside cells; instead, they are commonly incorporated into complex lipids such as triacylglycerols, phospholipids, and glycolipids.
• Double bonds in natural unsaturated fatty acids are usually in the cis configuration, introducing kinks in the hydrocarbon chain and affecting packing and melting behavior.
• Trans fatty acids are produced during industrial hydrogenation. They behave more like saturated fatty acids in biological membranes, allowing tighter packing and increasing melting points.
Nomenclature of Fatty Acids
• Fatty acids are named by both common and systematic nomenclature. Systematic names are based on the number of carbons and double bonds, such as octadecanoic acid (stearic acid) or cis-9-octadecenoic acid (oleic acid).
• A shorthand notation is also used: for example, 18:1(Δ9) indicates 18 carbons, 1 double bond at carbon 9 from the carboxyl end.
• The ω (omega) system counts from the methyl end. Omega−3 and omega−6 fatty acids are essential polyunsaturated fatty acids that humans must obtain from the diet.
• Common fatty acids include palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2), and arachidonic acid (20:4).
Physical Properties of Fatty Acids
• The melting point of fatty acids increases with chain length and decreases with the number of double bonds.
• Saturated fatty acids pack tightly in a nearly crystalline arrangement stabilized by hydrophobic interactions, resulting in higher melting points and solid fats at room temperature.
• Unsaturated fatty acids with cis double bonds contain kinks that prevent tight packing, resulting in lower melting points and liquid oils at room temperature.
• Trans fatty acids pack more like saturated fatty acids, raising melting points and contributing to plaque formation in arteries.
• The degree of unsaturation strongly affects biological membrane fluidity and metabolic properties.
Triacylglycerols Are the Primary Storage Form of Lipids
• Triacylglycerols (also called triglycerides) consist of glycerol esterified to three fatty acids. They are nonpolar, hydrophobic molecules stored in adipocytes and seeds.
• Triacylglycerols are neutral lipids because the carboxyl groups of fatty acids are bound in ester linkages and are no longer charged.
• Fats (solid at room temperature) are rich in saturated fatty acids; oils (liquid) have high unsaturated fatty acid content.
• Triacylglycerols are highly reduced and yield more energy per gram (approximately 38 kJ/g) than carbohydrates (about 17 kJ/g) during oxidation.
• They are stored in an anhydrous form, unlike glycogen, which binds water. As a result, triacylglycerols provide more than six times the energy storage efficiency of glycogen.
• Adipocytes store triacylglycerols as large lipid droplets, which can be mobilized by lipases when energy is needed.
Biological Advantages of Triacylglycerols as Energy Reserves
• Triacylglycerols are more reduced than carbohydrates, meaning their oxidation yields significantly more ATP.
• Unlike glycogen, triacylglycerols are stored without water, increasing their energy density and making them ideal for long-term energy storage in animals and plants.
• Adipose tissue not only stores triacylglycerols but also provides thermal insulation and protection for internal organs.
• Migratory birds and hibernating animals rely heavily on stored triacylglycerols to meet large metabolic demands during periods without food intake.
Waxes Are Storage Lipids and Water-Repellent Coatings
• Waxes are esters of long-chain saturated and unsaturated fatty acids with long-chain alcohols.
• They are highly hydrophobic and have high melting points, making them ideal for protective coatings.
• Plants secrete waxes to prevent water loss from leaves and fruits. Animal examples include beeswax, lanolin (from wool), and spermaceti (from whales).
• Waxes protect skin and hair, provide waterproofing for feathers, and serve as metabolic fuels for some species.
Comparison of Lipids with Carbohydrates for Energy Storage
• Carbohydrates are quick-access energy sources used for short-term metabolic needs, while lipids serve as long-term storage.
• Lipids yield more ATP per carbon atom because they are in a more reduced state than carbohydrates.
• Glycogen can be rapidly mobilized and metabolized anaerobically, whereas lipid metabolism requires oxygen and is slower.
• Mammals store glycogen in liver and muscle for rapid energy use but rely on adipose tissue triacylglycerols for sustained energy supply.
Lipid Droplets and Lipid Mobilization
• Lipid droplets consist of a core of triacylglycerols and sterol esters surrounded by a phospholipid monolayer and associated proteins.
• Specialized proteins such as perilipins regulate lipid droplet metabolism by controlling access of lipases.
• Lipases hydrolyze triacylglycerols into free fatty acids and glycerol, which are then released into the bloodstream.
• Fatty acids are transported bound to serum albumin, taken up by tissues, and oxidized in mitochondria for ATP production.
In a Nutshell
Storage lipids, primarily triacylglycerols and waxes, function as the main long-term energy reserves in living organisms. Fatty acids provide the structural foundation of these lipids, and their physical and chemical properties determine whether storage lipids are solid fats or liquid oils. Triacylglycerols are highly efficient energy stores due to their reduced state and anhydrous form, providing more than twice the energy of carbohydrates. Waxes serve structural and protective roles. Together, these lipids are essential for energy metabolism, thermal regulation, and cellular protection in both plants and animals.