MCAT Biochemistry Review
Chapter 5: Lipid Structure and Function
5.3 Energy Storage
Triacylglycerols are a class of lipids specifically used for energy storage. From the body's point of view, lipids in general are a fantastic way to store energy. This is true for two major reasons. First, the carbon atoms of fatty acids are more reduced than those of sugars, which contain numerous alcohol groups. The result of this is that the oxidation of triacylglycerols yields twice the amount of energy per gram as carbohydrates, making this a far more energy-dense storage mechanism compared to polysaccharides like glycogen. Second, triacylglycerols are hydrophobic. They do not draw in water and do not require hydration for stability. This helps decrease their weight, especially in comparison to hydrophilic polysaccharides. One final perk for vertebrates surviving in colder temperatures (like penguins, polar bears, and arctic explorers) is that the layer of lipids serves a dual purpose of energy storage and insulation—it helps to retain body heat so that less energy is required to maintain a constant internal temperature.
Triacylglycerols, also called triglycerides, are composed of three fatty acids bound by ester linkages to glycerol, as shown in Figure 5.8. For most naturally occurring triacylglycerols, it is rare for all three fatty acids to be the same.
Figure 5.8. Triacylglycerol Structure The fatty acids used here are palmitic acid, oleic acid, and α-linolenic acid.
Overall, these compounds are nonpolar and hydrophobic. This contributes to their insolubility in water, as the polar hydroxyl groups of the glycerol component and the polar carboxylates of fatty acids are bound together, decreasing their polarity.
Triacylglycerol deposits can be observed in cells as oily droplets in the cytosol. These serve as depots of metabolic fuel that can be recruited when the cell needs additional energy to divide or survive when other fuel supplies are low. Special cells in animals, known as adipocytes, store large amounts of fat and are found primarily under the skin, around mammary glands, and in the abdominal cavity. In plants, triacylglycerol deposits are also found in seeds as oils. Triacylglycerols travel bidirectionally in the bloodstream between the liver and adipose tissue. The physical characteristics of triacylglycerols are primarily determined by the saturation (or unsaturation) of the fatty acid chains that make them up, much like phospholipids.
The two main methods of energy storage in the body are as triacylglycerols in adipose tissue or as carbohydrates in glycogen. Each method has its advantages and disadvantages. For example, glycogen offers access to metabolic energy in a faster water-soluble form; however, because of its low energy density, glycogen can only provide energy for a bit less than one day. In contrast, a moderately obese individual with 15 to 20 kg of stored triacylglycerols in adipocytes could draw upon fat stores for months, but it takes more time to mobilize this energy.
FREE FATTY ACIDS AND SAPONIFICATION
Free fatty acids are unesterified fatty acids with a free carboxylate group. In the body, these circulate in the blood bound noncovalently to serum albumin. Fatty acids also make up what we know as soap, which can be produced through a process called saponification.
Saponification is the ester hydrolysis of triacylglycerols using a strong base. Traditionally, the base that is used is lye, the common name for sodium or potassium hydroxide. The result is the basic cleavage of the fatty acid, leaving the sodium salt of the fatty acid and glycerol, as shown in Figure 5.9. The fatty acid salt is what we know as soap.
Figure 5.9. Saponification Ester hydrolysis of a triacylglycerol using lye (NaOH).
Soaps can act as surfactants. A surfactant lowers the surface tension at the surface of a liquid, serving as a detergent or emulsifier. This is important to how soap works. If we try to combine an aqueous solution and oil, as with vinegar and olive oil in salad dressing, these solutions will remain in separate phases. If we were to add a soap, however, the two phases would appear to combine into a single phase, forming a colloid. This occurs because of the formation of micelles: tiny aggregates of soap with the hydrophobic tails turned inward and the hydrophilic heads turned outward, thereby shielding the hydrophobic lipid tails and allowing for overall solvation, as shown in Figure 5.10. We saw these earlier when we discussed membrane lipids.
Figure 5.10. Cross-Section of a Micelle Micelles organize in aqueous solution by forcing hydrophobic tails to the interior, allowing the hydrophilic heads to interact with water in the environment.
Saponification also occurs naturally, although more slowly, in corpses and oil paintings, as the triacylglycerols are hydrolyzed by naturally occurring bases. In corpses, the result of this process is known as adipocere, or grave wax.
Nonpolar compounds can dissolve in the hydrophobic interior of the water-soluble micelle, meaning that our cleaning agents can dissolve both water-soluble and water-insoluble messes and then wash them all away together. Micelles are also important in the body for the absorption of fat-soluble vitamins (A, D, E, and K) and complicated lipids such as lecithins. Fatty acids and bile salts secreted by the gallbladder form micelles that can deliver the fatty acids, vitamins, and cholesterol to the cells of the small intestine.
MCAT Concept Check 5.3:
Before you move on, assess your understanding of the material with these questions.
1. How does the human body store spare energy? Why doesn't the human body store most energy as sugar?
2. Describe the structure and function of triacylglycerols.
3. What bonds are broken during saponification?
4. Why does soap appear to dissolve in water, and how is this fact important to cleaning?