Question

On a per-carbon basis, where does the largest amount of biologically available energy in triacylglycerols reside: in the fatty acid portions or in the glycerol portion? Indicate how knowledge of the chemical structure of triacylglycerols provides the answer.

Short answer

The largest amount of energy per carbon is in the fatty acid portions due to their numerous carbon-hydrogen bonds.

Step-by-step solution

Understand the Structure of Triacylglycerols

Triacylglycerols are composed of a glycerol backbone attached to three fatty acids. Each fatty acid is a long chain of carbon and hydrogen atoms, ending with a carboxyl group. The glycerol portion consists of three carbon atoms, each bonded to a hydroxyl group.

Analyze the Energy Content in Fatty Acids

Fatty acids in triacylglycerols consist mainly of long hydrocarbon chains. These chains are energy-rich because they contain numerous carbon-hydrogen bonds, which release a significant amount of energy when oxidized.

Compare with Energy Content in Glycerol

Glycerol has a simpler structure with fewer carbon atoms and is less energy-dense. It has three hydroxyl groups, which means it has more oxygen and fewer high-energy carbon-hydrogen bonds compared to fatty acids.

Draw a Conclusion Based on Structure and Energy Content

The largest amount of biologically available energy, on a per-carbon basis, resides in the fatty acid portions of triacylglycerols. This is because fatty acids have more carbon-hydrogen bonds and are less oxidized, allowing for more energy to be released during metabolic processes than glycerol.

Key concepts

Fatty Acid Energy Content

Fatty acids are crucial components of triacylglycerols, which primarily consist of long chains of carbon and hydrogen atoms. These chains are typically saturated or unsaturated, dictating how many hydrogen atoms attach to the carbon backbone. The energy content of fatty acids is largely due to the abundance of carbon-hydrogen (\( C-H \) bonds) bonds. When our bodies metabolize fatty acids, these bonds are broken, releasing substantial amounts of energy.

• Compared to other macronutrients, fatty acids release more energy per gram.
• This is because they oxidize more extensively than carbohydrates or proteins during metabolism.

This makes them an excellent source of energy storage in organisms, particularly during periods of extended physical activity or fasting.

Glycerol Structure

Glycerol is a simple polyol compound forming the backbone of triacylglycerols. Its structure is characterized by three carbon atoms, each bonded to a hydroxyl (\( OH \) bond) group. This configuration results in glycerol having a more oxidized structure. Therefore, it possesses fewer high-energy carbon-hydrogen bonds compared to fatty acids.

• The presence of hydroxyl groups increases its solubility in water but not energy content.
• Glycerol can be converted into glucose in the liver through gluconeogenesis during energy shortages.

Hence, while it plays a role in energy metabolism, its energy potential is significantly less compared to the hydrocarbon-rich fatty acids.

Biological Energy Storage

Triacylglycerols serve as a primary form of energy storage in biological systems. Their structure makes them highly efficient for storing energy due to several reasons:

• Being hydrophobic, they do not require water for storage, making them compact and lightweight.
• The fatty acid chains contain large numbers of energy-rich carbon-hydrogen bonds.

During periods when the body requires additional energy, such as fasting or intense exercise, triacylglycerols are broken down into free fatty acids and glycerol: - Free fatty acids are metabolized to produce ATP, the energy currency of cells. - Glycerol can be converted into glucose for further use in energy pathways. Thus, triacylglycerols are especially vital for long-term energy storage and energy supply during prolonged activities.

Oxidation of Carbon-Hydrogen Bonds

The energy derived from triacylglycerols primarily hinges on the oxidation of carbon-hydrogen bonds. This process occurs when the bonds within fatty acid molecules break and react with oxygen during metabolism to produce energy:

• Fatty acids undergo beta-oxidation, followed by entry into the citric acid cycle.
• Both processes release stored energy by oxidizing carbon-hydrogen bonds into carbon dioxide and water.

The substantial energy provided from these reactions is utilized by cells to perform various functions. Because the carbon-hydrogen bonds are highly reduced, converting them releases a significant energy amount, making them valuable sources for metabolic activities as opposed to the already oxidized bonds in glycerol.