Gasoline is a liquid fuel, a complex mixture of refined petroleum products that powers internal combustion engines. Fats, or triglycerides, are biological molecules used by living organisms for long-term energy storage and insulation. Despite their vastly different origins and uses, their fundamental chemical structure is surprisingly similar. This similarity dictates their shared physical properties and high energy potential.
The Shared Molecular Backbone: Hydrocarbons
The core chemical similarity between gasoline and fat is that both are composed predominantly of hydrocarbons. A hydrocarbon is an organic compound made up solely of carbon (C) and hydrogen (H) atoms. Gasoline is a complex blend of many different hydrocarbons, typically containing molecules with four to twelve carbon atoms, such as octane (C8H18).
Fats, specifically triglycerides, are built from a small glycerol molecule chemically bonded to three long fatty acid tails. These fatty acid tails are long chains of carbon atoms that are almost completely saturated with hydrogen atoms. The length of these chains is generally much longer than those in gasoline, often ranging from 12 to 24 carbons.
In both substances, carbon atoms form the backbone, with hydrogen atoms covalently bonded along the chain. This structure is highly stable and stores significant energy, serving as the basis for their utility as concentrated fuel sources. Although fats include a small oxygen-containing group to link the fatty acids to glycerol, the vast majority of the molecule’s mass comes from the hydrocarbon tails.
The Critical Property of Nonpolarity
The shared hydrocarbon structure results in both gasoline and fat possessing the property of nonpolarity. Polarity is the result of uneven electron sharing, as seen in water. In contrast, carbon-hydrogen bonds are characterized by an even sharing of electrons, which creates a large, electrically neutral, nonpolar molecule.
This nonpolar nature explains why oil and water do not mix, a phenomenon known as hydrophobicity, or “water-fearing.” Water molecules are strongly polar and preferentially bond with each other, effectively excluding nonpolar substances like gasoline and cooking oil.
The principle of “like dissolves like” therefore applies: nonpolar gasoline can dissolve other nonpolar substances, such as certain greases and waxes. Similarly, fats are dissolved and transported in the body by specialized nonpolar carriers.
Energy Density and Release
The most profound shared consequence of the hydrocarbon structure is their function as highly concentrated energy stores. The numerous, stable carbon-hydrogen bonds hold a massive amount of potential energy. When these bonds are broken and the atoms are rearranged into new, more stable molecules, this trapped energy is released.
By mass, the energy content of fat is remarkably similar to that of gasoline. Fat contains approximately 39 kilojoules per gram, or 9 Calories per gram, while gasoline has about 43.9 kilojoules per gram. This near-equal energy density highlights the efficiency of the hydrocarbon structure for storing energy, regardless of whether it is derived from petroleum or a biological process.
The process that releases this energy is chemically identical in both cases: oxidation. For gasoline, this happens as rapid oxidation, or combustion, in an engine’s cylinder, which quickly breaks the hydrocarbon molecules. This reaction requires oxygen and produces the final products of carbon dioxide and water, releasing heat and mechanical work.
For fat, the process is controlled, slow oxidation, called metabolism or cellular respiration. The body utilizes oxygen to gradually break down the fatty acid chains, releasing energy in small, manageable packets that the cells capture to produce adenosine triphosphate (ATP). Despite the difference in speed and control, both processes fundamentally involve breaking the same type of high-energy chemical bonds in the presence of oxygen to yield carbon dioxide and water.