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How are fats, phospholipids, and steroids alike?
Multiple Choice
- They each contain a polar phosphate group.
- They do not dissolve in water.
- They each contain only one fatty acid.
- They are all solid at room temperature.
- They all contain at least one carbon ring.
The Correct Answer and Explanation is:
Correct Answer: They do not dissolve in water.
Explanation:
Fats, phospholipids, and steroids are all classified as lipids, which are a diverse group of hydrophobic (water-fearing) molecules. Despite their structural differences, one unifying feature of these three types of lipids is that they do not dissolve in water due to their nonpolar nature.
1. Hydrophobic Nature
Water is a polar solvent, which means it can dissolve other polar substances or ions. Lipids, on the other hand, are largely made up of nonpolar hydrocarbons—long chains or rings of carbon and hydrogen atoms. This nonpolar structure makes them insoluble in water. Fats and oils form droplets in water rather than mixing, and steroids (like cholesterol) do not dissolve either, though they can integrate into cell membranes.
2. Fats
Fats are composed of glycerol and three fatty acids. They are known as triglycerides. Because of the long hydrocarbon tails of the fatty acids, fats are nonpolar and hydrophobic. They serve as long-term energy storage in animals.
3. Phospholipids
Phospholipids have a glycerol backbone, two fatty acid tails (nonpolar), and a phosphate-containing head (polar). While the head is hydrophilic (water-attracting), the tails are hydrophobic, making the whole molecule amphipathic. This property allows them to form bilayers in cell membranes, but as a whole, they are still considered largely insoluble in water unless in a structured form like a membrane.
4. Steroids
Steroids have a core structure of four fused carbon rings. Examples include cholesterol, testosterone, and estrogen. They are nonpolar or slightly amphipathic but overall do not dissolve in water.
Why the Other Options Are Incorrect:
Thus, the best unifying trait is that they do not dissolve in water.