Final answer:
Water molecules require aquaporins to effectively cross cell membranes due to the hydrophobic nature of the membrane, enhanced by facilitated diffusion that allows for rapid and significant water transport regulated to meet cell needs and maintain water balance.
Step-by-step explanation:
Water molecules require aquaporins, which are transport proteins, to move rapidly and in large quantities across a membrane due to the hydrophobic nature of the lipid bilayer that composes the cell membrane.
Although small amounts of water can cross the membrane unassisted, most of the osmosis occurs through facilitated diffusion mediated by aquaporins, allowing for more efficient and faster transport. This is essential because the cell membrane is impermeable to polar molecules like water without these specialized channels.
Aquaporins not only facilitate the transport of water but can also assist in the movement of other substances such as glucose, glycerol, urea, ammonia, carbon dioxide, and even ions in certain cases.
These protein channels are allosterically regulated to allow cells to adjust water balance in response to changing conditions. Different types of aquaporins exist, and mutations in these can lead to genetic diseases.
The significance of aquaporins was highlighted by the Nobel Prize in Chemistry awarded to Peter Agre in 2003 for their discovery.