Let's analyze the options step by step to determine which type of cell transport requires energy because it moves substances against their concentration gradient. **1. Understanding the Types of Cell Transport:** - **Passive Transport:** - Does **not** require energy. - Moves substances **down** their concentration gradient (from high to low concentration). - Examples include diffusion, osmosis, and facilitated diffusion. - **Facilitated Diffusion:** - A type of passive transport. - Uses transport proteins to help move molecules across the cell membrane. - Still moves molecules **down** their concentration gradient, so no energy is needed. - **Active Transport:** - **Requires** energy (usually in the form of ATP). - Moves substances **against** their concentration gradient (from low to high concentration). - Examples include the sodium-potassium pump. - **Active Diffusion:** - This term is not standard in cell biology. - Sometimes, people refer loosely to active processes involving diffusion, but the correct term for energy-consuming transport against gradients is *active transport*. **2. Analyzing the options:** - **Active diffusion:** - Since the term is not standard, it seems to refer to active transport processes that involve diffusion-like movement against the gradient. - **Energy is required.** - **Facilitated diffusion:** - Does **not** require energy. - Moves molecules along their gradient. - **Passive:** - Also does **not** require energy. - Moves molecules down the gradient. - **All of them require energy:** - This is incorrect because passive and facilitated diffusion do not require energy. **3. Conclusion:** The process that **requires energy because it moves substances against their concentration gradient** is **active transport**. **4. Final answer:** **Active diffusion** (or more correctly, *active transport*) is the process that requires energy because it moves molecules against their concentration gradient. --- **Correct choice:** **Active diffusion**