Which of the following best explains why alcohol can pass directly into the bloodstream after consumption?

• A. Alcohol is broken down in the stomach before entering the bloodstream.
• B. Alcohol requires enzymatic modification before it can enter the blood.
• C. Alcohol is actively transported into the blood by carrier proteins.
• D. Alcohol is a small, nonpolar molecule that can diffuse across cell membranes.

Answer: (D)

Verified step by step guidance

1. Understand the concept of diffusion: Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, and it does not require energy. Small, nonpolar molecules can diffuse directly across the lipid bilayer of cell membranes.
2. Recognize the properties of alcohol: Alcohol is a small molecule and is nonpolar, which allows it to easily pass through the hydrophobic core of the phospholipid bilayer in cell membranes.
3. Identify the mechanism of transport: Alcohol does not require enzymatic modification, active transport, or carrier proteins to enter the bloodstream. Instead, it diffuses passively across cell membranes.
4. Relate the structure of cell membranes to alcohol's ability to diffuse: Cell membranes are composed of a phospholipid bilayer with hydrophobic tails. Small, nonpolar molecules like alcohol can move through this layer without assistance.
5. Conclude why alcohol enters the bloodstream directly: Alcohol's small size and nonpolar nature enable it to diffuse across cell membranes, bypassing the need for enzymatic modification or active transport mechanisms.