Water channels are

Options

A. Made up of eight similar types of aquaporins
B. Involved in active transport
C. Involved in facilitated diffusion
D. More than one option is correct

Answer

**Step-by-Step Text Solution:**

1. **Definition of Water Channels:**
Water channels are specialized proteins that facilitate the movement of water across cell membranes. They are primarily composed of proteins known as aquaporins.

2. **Structure of Aquaporins:**
Aquaporins are integral membrane proteins that form channels through which water molecules can pass. They typically consist of multiple subunits, with each aquaporin channel being formed by the assembly of several similar protein subunits.

3. **Function of Water Channels:**
The primary function of water channels, specifically aquaporins, is to allow the rapid transport of water in and out of cells. This process occurs along the concentration gradient, meaning water moves from areas of higher concentration to areas of lower concentration.

4. **Transport Mechanism:**
The transport of water through aquaporins is a passive process known as facilitated diffusion. This means that it does not require energy input from the cell, as it relies on the natural movement of water molecules.

5. **Nature of Water Channels:**
Water channels are semi-permeable, allowing only specific molecules (like water) to pass while blocking others. This selectivity is crucial for maintaining cellular homeostasis.

6. **Clarification of Misconceptions:**
It is important to note that water channels are not involved in active transport. Active transport requires energy to move substances against their concentration gradient, which is not the case for water movement through aquaporins.