Water channels are made up of — different types of aquaporins.

Options

A. Two
B. Four
C. Eight
D. Six

Answer

**Step-by-Step Solution:**

1. **Understanding Aquaporins**: Aquaporins are specialized proteins that form water channels in cell membranes. The term "aqua" refers to water, and "porins" relates to pores, indicating that these proteins create pores specifically for water transport.

2. **Function of Aquaporins**: These proteins are crucial for the movement of water between cells. They allow water molecules to pass through the cell membrane while being selective, meaning that only water can pass through these channels and not other molecules or ions.

3. **Location of Aquaporins**: Aquaporins are found in various organelles, including the outer membranes of mitochondria and plastids, and they can also be present in some bacteria.

4. **Mechanism of Action**: The transport of water through aquaporins is a passive process, meaning it does not require energy. Water moves through these channels following the osmotic pressure gradient, which is the difference in concentration of water across the membrane.

5. **Types of Aquaporins**: In total, there are 8 different types of aquaporins that facilitate the transport of water in plants and other organisms.

6. **Final Answer**: Therefore, the correct completion of the statement "Water channels are made up of — different types of aquaporins" is "8".

---