What property of steroid hormones allows them to cross the phospholipid bilayer?

• A. They are charged molecules
• B. They are large proteins
• C. They are lipophilic
• D. They are hydrophilic

Answer: (C)

Verified step by step guidance

1. Understand the structure of the phospholipid bilayer: The cell membrane is primarily composed of phospholipids, which have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This structure creates a barrier to most water-soluble substances.
2. Recognize the nature of steroid hormones: Steroid hormones are derived from cholesterol, which is a lipid. This means they are lipophilic, or fat-loving, and can easily interact with the hydrophobic tails of the phospholipid bilayer.
3. Identify the property that allows passage through the bilayer: Because steroid hormones are lipophilic, they can dissolve in the lipid bilayer and pass through it without the need for transport proteins or channels.
4. Contrast with other molecules: Charged molecules and large proteins typically cannot pass through the lipid bilayer directly because they are either hydrophilic or too large to fit through the membrane without assistance.
5. Conclude why lipophilicity is key: The lipophilic nature of steroid hormones is crucial for their ability to cross the phospholipid bilayer, allowing them to enter cells and exert their effects.