Question

Arrange these biological molecules (nucleic acid, cellulose, unsaturated fatty acid, saturated fatty acid) in order from highest to lowest energy density? Take note of which chemical bonds have the highest potential energy.  Can you explain which bonds have high or low potential energy and why? Also, covalent bonds between which of the atoms will have the highest potential energy?

Answer 1

The potential energy of a chemical bond formed between two atoms is inversely proportional to the strength of the bond. The weak bonds which are easily breakable have the highest potential energy and vice versa.

In terms of energy density, the energy stored in a unit mass of biomolecule depends on the strength of the chemical bonds between the atoms in the molecule. Fats have higher energy density due to the presence of long alkyl chain with less number of oxidized carbon atoms. Unsaturated fatty acids will have lower potential energy as compared to saturated fatty acids due to the presence of double bonds in unsaturated fatty acids. Lower the potential energy, the higher the energy density. A breakage of stronger bonds will release higher energy. Cellulose will have the next level of energy density due to the higher number of oxidized carbon atoms as compared to fatty acids. Nucleic acids will have the least energy density with higher potential energy due to the presence of multiple hydrogen bonds and phosphodiester bonds.

Covalent bonds that are formed by sharing of electrons between two atoms form the strongest bonding pair with the lowest resting potential energy. This is because, the formation or breakage of a covalent bond is associated is a high amount of heat energy, either release or absorption. Hence the reserve potential energy is low.

Ionic bonds have higher potential energy than covalent bonds, as they are formed by the attraction between oppositely charged atoms. They are easily formed and broken as compared to covalent bonds.

Weak forces that keep the atoms attracted together like hydrogen bonds have the highest potential energy. They are very easy to break and form as the atoms that are covalently bound to other atoms form hydrogen bonds.

Covalent bonds formed between equally charged atoms are more stable than the bond between unequally charged atoms. For example a bond between two carbon atom is stronger than the bond between hydrogen and oxygen. The former is an example of a pure covalent bond and the latter is the example of the polar covalent bond. The single covalent bond has the highest potential energy than the double and triple bonds.