Which one of the following substances does not pass across membranes by simple diffusion?

Options

A. `O_2`
B. `CO_2`
C. `H^+`
D. `H_2O`

Explanation

To determine which substance does not pass across membranes by simple diffusion, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Simple Diffusion**:
- Simple diffusion is the process where substances move across a semi-permeable membrane without the assistance of transport proteins. This typically occurs for small, non-polar molecules.

2. **Analyze the Options**:
- We have four options to evaluate: O2 (oxygen), CO2 (carbon dioxide), H+ (hydrogen ion), and H2O (water).

3. **Evaluate Each Substance**:
- **O2 (Oxygen)**:
- Oxygen is a small, non-polar molecule. It can easily diffuse through the lipid bilayer of cell membranes.
- Conclusion: O2 can pass through by simple diffusion.

- **CO2 (Carbon Dioxide)**:
- Carbon dioxide is also a small molecule and non-polar. It readily diffuses out of cells as a by-product of cellular respiration.
- Conclusion: CO2 can pass through by simple diffusion.

- **H+ (Hydrogen Ion)**:
- Hydrogen ions are charged particles and cannot easily pass through the hydrophobic lipid bilayer of the membrane. They typically require transport proteins to facilitate their movement across the membrane.
- Conclusion: H+ cannot pass through by simple diffusion.

- **H2O (Water)**:
- Water is a polar molecule, but it can still pass through membranes via osmosis, which is a form of simple diffusion specifically for water.
- Conclusion: H2O can pass through by simple diffusion.

4. **Final Conclusion**:
- Among the options provided, the substance that does not pass across membranes by simple diffusion is **H+ (Hydrogen ion)**.

### Final Answer:
The substance that does not pass across membranes by simple diffusion is **H+ (Hydrogen ion)**.