Question

Arrange these following four molecules in increasing order of their biological energy content (per mole): (a) Sucrose (b) Myristic acid, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{12} \mathrm{COOH}\) (c) Glucose (d) Capric acid, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{COOH}\)

Short answer

Glucose < Sucrose < Capric Acid < Myristic Acid.

Step-by-step solution

Understand the Molecules

The four molecules need to be arranged based on their biological energy content. Sucrose and glucose are carbohydrates, whereas myristic acid and capric acid are fatty acids. Carbohydrates and fats store energy differently due to their structure; fats generally have higher energy content per mole compared to carbohydrates because of their higher number of reducible bonds.

Evaluate Chemical Structures

Examine the structure of each molecule: Sucrose is a disaccharide made of glucose and fructose molecules; glucose is a monosaccharide; myristic acid is a long-chain saturated fatty acid with a 14-carbon chain; and capric acid is a saturated fatty acid with a 10-carbon chain. Longer fatty acids tend to hold more energy than shorter ones.

Compare Energy Content of Carbohydrates

Among sucrose and glucose, sucrose is a larger molecule being a disaccharide compared to glucose which is a monosaccharide. Thus, sucrose contains more energy per mole than glucose.

Compare Energy Content of Fatty Acids

Between myristic acid and capric acid, myristic acid has a longer carbon chain (14 carbons) compared to capric acid (10 carbons). Longer carbon chains in fatty acids mean more reducible bonds, thus more energy content. Hence, myristic acid has more energy per mole than capric acid.

Arrange All Molecules by Energy Content

Now, considering all the molecules and their energy contributing features, the order from lowest to highest energy content is: Glucose < Sucrose < Capric Acid < Myristic Acid.

Key concepts

Carbohydrates

Carbohydrates are one of the main types of nutrients and a significant source of energy for the body. They are primarily composed of carbon, hydrogen, and oxygen atoms in a 1:2:1 ratio, forming a simple structure known as sugars. There are two main types of carbohydrates: simple carbohydrates (sugars) and complex carbohydrates (starches and fibers).

• Simple carbohydrates are quick energy sources but do not usually supply any other nutrients or fiber.
• Complex carbohydrates, on the other hand, have more complex chemical structures, which means they provide longer-lasting energy and contain additional nutrients and fiber.

When discussing biological energy content, glucose and sucrose are prevalent examples.
Glucose is a monosaccharide, meaning it is a single sugar molecule. It is a primary energy currency in biological systems, rapidly metabolized to release energy.
Sucrose, however, is a disaccharide composed of glucose and fructose. It has a larger structure, leading to slightly more energy per mole when compared to glucose.

Fatty Acids

Fatty acids are critical components of lipids in your body. They are long hydrocarbon chains terminated with a carboxyl group (COOH). The main types are saturated and unsaturated fatty acids.

• Saturated fatty acids have no double bonds between carbon atoms, allowing them to pack closely together. This structure enables them to store energy densely.
• Unsaturated fatty acids contain one or more double bonds, introducing kinks in the chains and resulting in less dense packing.

Among the fatty acids, energy content generally increases with the length of their carbon chains.
Myristic acid and capric acid are examples of saturated fatty acids. Myristic acid has a 14-carbon chain, while capric acid has 10 carbons. Given that longer chains present more carbon-carbon and carbon-hydrogen bonds, myristic acid stores more energy than capric acid.

Chemical Structures

Chemical structures of organic molecules determine their properties and functions, especially in terms of energy storage. Carbohydrates like glucose and sucrose have ring structures with several hydroxyl (OH) groups, making them soluble in water and fast to metabolize.
Fatty acids, in contrast, consist of long, unbranched hydrocarbon chains. Their chemical structure makes them low in solubility in water, as they are predominantly hydrophobic. This allows them to pack tightly in cells, storing substantial amounts of energy in the form of triglycerides.
The length and saturation level of the fatty acid carbon chains directly affect the energy they store. The more bonds present, especially in longer and fully saturated chains, the more potential energy is held. In essence, the chemical structure with more bonds facilitates higher energy content, thanks to extended reducibility and fewer polar interruptions.