Which of the following moves rapidly across the membrane?
**Core Concept**
The movement of molecules across cell membranes is a fundamental concept in physiology and pharmacology. This process is governed by the principles of diffusion, facilitated diffusion, and active transport. In this context, the term "rapidly across the membrane" implies a process that does not require energy or a carrier molecule.
**Why the Correct Answer is Right**
The correct answer is likely to be a molecule that can diffuse quickly across the cell membrane due to its small size and lipophilicity. This process is facilitated by the fluid mosaic model of the cell membrane, where lipid-soluble molecules can pass through the phospholipid bilayer.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely to be a molecule that requires active transport, such as glucose, which is transported against its concentration gradient using a carrier molecule.
**Option B:** This option may be a molecule that is too large to diffuse quickly across the cell membrane, such as a protein or a large molecule.
**Option C:** This option may be a molecule that requires facilitated diffusion, such as glucose, which requires a carrier molecule to transport it across the cell membrane.
**Clinical Pearl / High-Yield Fact**
A key concept to remember is that small, lipid-soluble molecules can diffuse quickly across the cell membrane, whereas larger molecules require active transport or facilitated diffusion.
**Correct Answer: A. Oxygen.**