Final answer:
Water diffusion across the plasma membrane is regulated by osmosis and facilitated by aquaporins, specialized proteins that control water passage based on the cell's water balance requirements.
Step-by-step explanation:
The rate and direction of water diffusion across the plasma membrane is primarily regulated by osmosis and facilitated by aquaporins, which are specialized channel proteins. The gradient of solute concentration across the membrane strongly influences osmosis, wherein water moves from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).
The plasma membrane is selectively permeable, and its structure plays a significant role in regulating the diffusion process. Aquaporins can transport water solely or together with other molecules like glucose, urea, and ions, and are allosterically regulated to maintain the cell's water balance. Discovery of aquaporins was so crucial that it led to the awarding of the Nobel Prize in Chemistry in 2003.