Gases move across membranes by ...... For this membranes must be ........................

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks how gases move across membranes and what condition the membranes must meet for this movement to occur.

2. **Identifying the Process:**
Gases move across membranes primarily through a process known as **diffusion**. This is the movement of molecules from an area of higher concentration to an area of lower concentration.

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