Chemical nature and structure
Triacylglycerols belong to the class of lipids—low-molecular-weight bioorganic compounds found in all living organisms. Their molecules consist of a glycerol backbone with attached fatty acid residues. The acid residues within the molecule always contain four or more carbon atoms, with chains of 16 and 18 carbons being the most common.
Two approaches are used to depict molecules in chemistry:
- Vertical projection: optimal and convenient for writing chemical equations.
- Spatial configuration: less convenient when writing reactions, but much more accurately reflects the actual geometry and structure of the molecule.
Types of isomerism
Triacylglycerol molecules exhibit several types of isomerism simultaneously:
- Positional isomerism: associated with changes in the positions of acyl radicals on the glycerol backbone (relevant when two or three different acid residues are present).
- Enantiomerism (optical isomerism): occurs when the central carbon atom in the glycerol molecule is bonded to different substituents.
- Structural isomerism of radicals: caused by the different structures of the carbon skeletons of the fatty acids themselves while maintaining the same total number of carbon and hydrogen atoms.
Composition of natural fats
Any native fat extracted from biological material (by pressing or rendering) is a complex mixture of molecules. Finding a triacylglycerol with three completely identical acid residues is practically impossible. The composition strictly depends on the biological source:
- Animal fats (beef, mutton, pork). They are stored in adipose tissue (up to 25–30% of the composition) and provide thermal insulation for the body. They are dominated by saturated acids: palmitic acid (26–28%) and stearic acid (16–25%). Among unsaturated fatty acids, oleic acid dominates (38–41%). Under normal conditions, these are solid substances that melt upon heating.
- Plant fats (oils). They accumulate in plant seeds and remain liquid. The stearic acid content in them is extremely low (2–2.4%). However, they are rich in unsaturated acids: olive oil is dominated by oleic acid (67%), and sunflower oil by linoleic acid (59%).
- Milk fat. It is characterized by a significant presence of myristic acid (12.5%) and the presence of low-molecular-weight butyric acid (3.5%).
Physicochemical properties
Fats are insoluble in water (they exhibit hydrophobicity), but readily dissolve in many organic solvents. The best solvent for them is diethyl ether.
An interesting feature of fats is the absence of a sharp melting or freezing point. Phase transitions always occur over a specific temperature range. Precisely for this reason, solid animal fat is a plastic mass, as a liquid phase admixture is always present within it.
In the structure of unsaturated acids (oleic, linoleic, linolenic), double bonds have an exclusively cis-configuration (hydrogen atoms are located on the same side of the $C=C$ double bond). Residues with a trans-configuration (e.g., elaidic acid) are practically absent in natural lipids.
Energy value and phospholipids
Fats are incredibly energy-dense substances. This is due to the presence of long hydrocarbon chains and a low oxygen content compared to carbohydrates. The complete oxidation of 1 gram of fat releases about 38 kJ of energy, which is more than twice the energy yield of glucose (~15.5 kJ).
An important related group is phospholipids, a special variety of glycerol esters. Unlike fats, their main function is not energetic, but structural: together with proteins, they form cell membranes, walls, and internal cellular partitions.
Briefly: answers to topical questions
What are the main biological functions of triacylglycerols in the human body?
The main biological functions of triacylglycerols include supplying the body with energy, thermal insulation, and the production of metabolic water.
- Energy function — fats are energy-dense substances; their oxidation releases about 38 kJ of energy per 1 g of substance.
- Thermal insulation function — animal fats are deposited in subcutaneous adipose tissue, which protects the body against heat loss.
- Metabolic function — the oxidation of fats in tissues generates the highest amount of metabolic water (1.09 ml per 1 g of substance).
What chemical reactions are characteristic of triacylglycerols?
Triacylglycerols are characterized by hydrolysis reactions at ester bonds and addition reactions at double bonds.
- Hydrolysis with pure water — proceeds slowly due to the hydrophobicity of fats.
- Alkaline hydrolysis (saponification) — proceeds rapidly upon heating with alkalis or a soda solution, yielding glycerol and fatty acid salts (soaps).
- Addition reactions — an iodine molecule is capable of adding across each multiple (double) bond in unsaturated fatty acid residues.
How does the enzymatic hydrolysis of triacylglycerols occur in the gastrointestinal tract?
Enzymatic hydrolysis of triacylglycerols takes place in the lumen of the small intestine with the participation of pancreatic enzymes and bile acids.
- Emulsification — bile acid salts break down large fat droplets into smaller ones, increasing the contact surface area with the enzyme.
- Medium optimization — bicarbonates neutralize acidic chyme, establishing the required pH optimum (around 8.0).
- Hydrolysis — pancreatic lipase, with the participation of the colipase protein cofactor, breaks down emulsified fat.
The main hydrolysis products are $\beta$-monoacylglycerol (80%), free fatty acids, and diacylglycerol.
What do the iodine number and saponification number of fats indicate?
The iodine number indicates the number of multiple (double) bonds in a fat molecule.
- Iodine number — determined based on the principle of iodine molecule ($I_2$) addition across each double bond in unsaturated fatty acid residues. This indicator is used to determine fat structure and calculate the number of double bonds.
Information regarding what the saponification number of fats indicates is absent in the sources (only the alkaline hydrolysis process—saponification—is mentioned).
Class and exam questions
Why do fats yield more energy than carbohydrates?
This is due to their chemical structure. Triacylglycerols contain long hydrocarbon chains and very few oxygen atoms compared to carbohydrates, making them more energy-dense substrates for oxidation.
Why is solid fat plastic and soft?
Fats do not have a precise melting point; this process is spread over a temperature range. Consequently, even in solid fat, a liquid phase admixture is always retained, which imparts plasticity to the mass.
Do trans fats occur in nature?
They are practically absent in natural occurring fats. The radicals of all unsaturated fatty acids naturally feature a cis-configuration of double bonds.
What else to study on this topic
- Spatial configuration and vertical projection of triacylglycerol molecules
- Stereochemistry of unsaturated fatty acids and the arrangement of hydrogen atoms
- Mechanisms of optical isomerism at the central carbon atom of glycerol
- Chemical composition and functions of cell membranes (phospholipids and proteins)
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