Arrange these four molecules in increasing order of their...

Arrange these four molecules in increasing order of their biological energy content per mole:
(a) Mannose
(b) Stearic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH}$
(c) Fructose
(d) Palmitic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{14} \mathrm{COOH}$

Key Concepts

Biological Energy Content

Biological energy content refers to the amount of energy stored in molecules that can be released through metabolic processes, such as cellular respiration. It is usually measured per mole and is critical in determining the efficiency and capacity of a molecule to serve as an energy source for organisms.

Carbohydrates

Carbohydrates are organic molecules typically composed of carbon, hydrogen, and oxygen in roughly a 1:2:1 ratio. They serve as a primary energy source in many organisms. While not as energy-dense as lipids, carbohydrates are quickly metabolized to provide immediate energy, albeit with a lower energy yield per mole when fully oxidized.

Fatty Acids

Fatty acids are long-chain hydrocarbons with a terminal carboxyl group. They are highly reduced, meaning they have a greater number of carbon-hydrogen bonds relative to oxygen content, which results in a higher energy yield per mole upon oxidation. Fatty acids are stored as fats and can provide a dense, concentrated source of energy.

Degree of Reduction

The degree of reduction of a molecule refers to the extent to which it carries electrons that can be extracted during oxidation. Highly reduced molecules, such as fatty acids, contain more such electrons and thus provide more energy when oxidized compared to less reduced molecules like carbohydrates.

Carbon Chain Length and Energy Yield

The energy yield of fatty acids is not only a function of their high reduction state but also of their carbon chain length. Longer carbon chains offer more sites for reaction during oxidation, thereby releasing more energy per mole. Variations in chain length directly correlate with the overall biological energy content of these molecules.

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