A small uncharged molecule moves across a membrane down its concentration gradient. What transport process is most likely involved in the movement of this molecule?

Question

BlackTom AI · Accepted Answer

Small uncharged molecules (for example O2 or CO2) cross the phospholipid bilayer directly by simple diffusion because they are lipid-soluble enough to move through the membrane without assistance.

1. Relevant knowledge points and concepts
- Passive versus active transport: Passive transport moves substances down their concentration gradients without cellular energy (ATP); active transport moves substances against their gradients and requires energy.  
- Diffusion (simple diffusion): Movement of molecules from high to low concentration across a membrane or space; no transport proteins required for small, nonpolar or uncharged molecules. Fick’s law (qualitative form) states flux ∝ concentration difference (J = −D dC/dx).  
- Osmosis: Special case of diffusion referring specifically to water movement across a semipermeable membrane.  
- Facilitated diffusion: Also passive (down gradient) but requires membrane proteins (channels or carriers), typically for charged, polar, or larger molecules that cannot cross the hydrophobic core.

2. Step-by-step reasoning
- The molecule is described as small and uncharged: such molecules are generally able to dissolve in the hydrophobic interior of the lipid bilayer.  
- The movement is down its concentration gradient: that identifies the process as passive (no energy input).  
- Evaluate the choices: Osmosis applies only to water, so it cannot be the general pathway for a small uncharged solute. Facilitated diffusion requires specific membrane proteins and is usually used for ions or polar molecules that cannot cross the bilayer by themselves. Active transport is excluded because it moves substances against gradients and requires ATP.

3. Intermediate conceptual “calculation”
- Using Fick’s principle qualitatively: greater concentration difference → greater net flux J. For a small uncharged molecule with permeability through lipid bilayer, J will be nonzero and directed from high to low concentration without carrier proteins.

4. Why the correct answer is correct
- Simple diffusion fits all given facts: small, uncharged, moving down its concentration gradient and therefore crossing by diffusion through the lipid bilayer without energy or transport proteins.

5. Why other options are incorrect
- Osmosis: only for water.  
- Facilitated diffusion: requires channels/carriers and typically for polar/charged species.  
- Active transport: requires energy and moves against concentration gradients.

Conclusion: The correct transport process is Diffusion, which matches the provided answer.

A small uncharged molecule moves across a membrane down its concentration gradient. What transport process is most likely involved in the movement of this molecule?单项选择题

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