In which type of passive transport-simple diffusion or facilitated diffusion—a carrier protein is required for transport across the membrane?

Explanation

**Step-by-Step Solution:**

1. **Understanding Passive Transport**: Passive transport is a method by which substances move across cell membranes without the need for energy input. This process relies on the concentration gradient, meaning substances move from areas of higher concentration to areas of lower concentration.

2. **Types of Passive Transport**: There are two main types of passive transport:
- **Simple Diffusion**: This is the movement of small, nonpolar molecules directly through the lipid bilayer of the cell membrane. It does not require any assistance from proteins.
- **Facilitated Diffusion**: This process involves the movement of larger or polar molecules across the membrane with the help of specific proteins, known as carrier proteins or channel proteins.

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