What is the minimum number of cell membranes that a water molecule must move through in getting from soil into a xylem vessel -

Options

A. 0
B. 1
C. 2
D. 6

Answer

To determine the minimum number of cell membranes that a water molecule must move through to get from the soil into a xylem vessel, we can analyze the pathways that water takes as it travels through plant tissues.

### Step-by-Step Solution:

1. **Understanding the Pathways**: Water can move through plants via two main pathways: the apoplastic pathway and the symplastic pathway. The apoplastic pathway allows water to move through the cell walls and intercellular spaces without crossing any cell membranes, while the symplastic pathway involves water moving through the cytoplasm of cells, crossing cell membranes.

2. **Starting Point - Soil to Root**: Initially, water is absorbed from the soil by the root hairs, which are part of the epidermis. Here, water moves through the apoplastic pathway (cell wall to cell wall) without crossing any membranes.

3. **Crossing the Endodermis**: As water moves toward the center of the root, it encounters the endodermis. The endodermis has a Casparian strip, which is a band of cell wall material that is impregnated with suberin, making it impermeable to water. Therefore, water must cross the endodermal cell membrane to continue its journey.

4. **Transition to Symplastic Pathway**: After crossing the endodermal cell membrane, water enters the symplastic pathway. This means it is now moving through the cytoplasm of the endodermal cells.

5. **Crossing the Pericycle**: The next membrane that water must cross is the membrane of the pericycle cells, which are located just inside the endodermis. This is the second membrane that water must cross.

6. **Entering the Xylem**: After crossing the pericycle membrane, water can then enter the xylem vessels. At this point, the water is in the xylem and can move upward through the plant.

### Conclusion:
The minimum number of cell membranes that a water molecule must move through to get from the soil into a xylem vessel is **2** (the endodermal membrane and the pericycle membrane).

### Final Answer:
**2**