The characteristic that all lipids have in common is that
a. they all contain nitrogen
b. they do not have a high energy content
c. none of them dissolves readily in water
d. they are all acidic when mixed with water
e. they are all made of fatty acids and glycerol
The Correct Answer and Explanation is:
The correct answer is:
c. none of them dissolves readily in water
Explanation:
Lipids are a diverse group of organic molecules, including fats, oils, waxes, phospholipids, and steroids. Despite their variety, all lipids share one key characteristic: they are hydrophobic (water-fearing) and do not dissolve readily in water.
Why is this? Water is a polar molecule, meaning it has a positive and a negative end. Molecules that dissolve well in water are generally polar or charged themselves, so they can interact favorably with water molecules. Lipids, on the other hand, are primarily composed of long chains of hydrocarbons (fatty acids) that are nonpolar — they lack charged regions. Because water and nonpolar molecules do not mix well, lipids tend to be insoluble or only sparingly soluble in water.
Now, let’s look at the other answer options to clarify why they are incorrect:
- a. they all contain nitrogen: This is false. Lipids primarily contain carbon, hydrogen, and oxygen. Nitrogen is present in some biomolecules like proteins and nucleic acids but not a general component of all lipids.
- b. they do not have a high energy content: This is incorrect. Lipids actually contain a high amount of energy per gram (about 9 kcal/g), more than carbohydrates or proteins, making them an efficient energy storage form.
- d. they are all acidic when mixed with water: This is incorrect. Lipids themselves do not dissociate to release hydrogen ions like acids do. While fatty acids are acidic, many lipids like triglycerides are neutral and not acidic.
- e. they are all made of fatty acids and glycerol: This is partially true for many lipids (such as triglycerides), but not for all. For example, steroids and phospholipids have different structures that do not always include fatty acids and glycerol.
In summary, the defining feature of lipids is their poor solubility in water due to their hydrophobic, nonpolar nature. This characteristic underlies many biological functions, such as forming cell membranes (phospholipid bilayers) and storing energy efficiently in a water-insoluble form.