1. Cell Membrane:
- The phospholipid bilayer itself is selectively permeable, meaning it allows some substances to pass through while blocking others. Water can pass through the membrane directly, but the rate is limited.
- Aquaporins: These are protein channels specifically designed for water transport. They facilitate the passage of water molecules across the membrane much faster than simple diffusion.
2. Osmosis:
- This is the passive movement of water across a semi-permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration).
- The concentration gradient between the inside and outside of the cell is crucial. If the cell's internal environment has a higher solute concentration, water will tend to move *out* of the cell.
3. Cell Wall (in plant cells):
- While the cell wall doesn't directly prevent water entry, it provides structural support and limits the cell's ability to expand indefinitely. This helps regulate the amount of water that can enter the cell.
4. Active Transport:
- In some cases, cells can use energy to actively pump water *out* of the cell, even against the concentration gradient. This is less common but can be important in specific situations.
In summary, it's not about preventing water from entering, but about regulating the flow and maintaining a balance. Cells have mechanisms to both allow and control water movement based on their needs and the surrounding environment.
