How are fats, phospholipids, and steroids alike?
A) They are all solid at room temperature.
B) They all contain at least one carbon ring.
C) They each contain only one fatty acid.
D) They each contain a polar phosphate group.
E) They do not dissolve in water.
The Correct Answer and Explanation is:
The correct answer is:
E) They do not dissolve in water.
Explanation:
Fats, phospholipids, and steroids are all types of lipids, a broad class of biological molecules that share some important chemical characteristics. The key property that unites these three is their hydrophobic nature, meaning they do not dissolve in water.
Let’s break down why this is true and why the other options are incorrect:
1. Why do fats, phospholipids, and steroids not dissolve in water?
Water is a polar molecule, meaning it has regions of partial positive and negative charge, which makes it very good at dissolving other polar substances or ionic compounds. Lipids, on the other hand, are mostly composed of nonpolar hydrocarbon chains or rings. Nonpolar molecules do not have charged regions and cannot form hydrogen bonds with water molecules. Because water and nonpolar molecules do not mix well (like oil and water), lipids are generally hydrophobic and insoluble in water.
- Fats (triglycerides) consist mainly of three fatty acid chains linked to a glycerol molecule. The long hydrocarbon tails are nonpolar and repel water.
- Phospholipids have two fatty acid tails (nonpolar) and a phosphate group head (polar). While the head is hydrophilic (water-attracting), the tails are hydrophobic, so phospholipids behave uniquely by forming bilayers in water rather than dissolving.
- Steroids have a structure composed of four fused carbon rings. These rings are largely hydrophobic, making steroids insoluble in water.
2. Why the other options are incorrect:
- A) They are all solid at room temperature.
This is not true. Fats (especially saturated fats) may be solid, but many fats are liquid (like oils). Phospholipids and steroids are generally not solids in their natural biological states. - B) They all contain at least one carbon ring.
Only steroids contain carbon rings. Fats and phospholipids are primarily linear or branched hydrocarbon chains. - C) They each contain only one fatty acid.
Fats contain three fatty acids, phospholipids have two fatty acids, and steroids have none. - D) They each contain a polar phosphate group.
Only phospholipids contain a phosphate group. Fats and steroids do not.
Summary:
Fats, phospholipids, and steroids all share the characteristic of being nonpolar and hydrophobic, which means they do not dissolve in water. This property is fundamental to their roles in biological membranes, energy storage, and signaling. This hydrophobic nature distinguishes lipids from other biological macromolecules like carbohydrates and proteins, which are more soluble in water.