Final answer:
Channel proteins are responsible for the passage of water-soluble substances through the plasma membrane.
Step-by-step explanation:
The passage of water-soluble substances through the plasma membrane is made possible by channel proteins in the membrane. Channel proteins have hydrophilic domains that provide a hydrated opening through the membrane layers, allowing polar compounds to avoid the nonpolar central layer of the plasma membrane. Aquaporins, a type of channel protein, specifically allow water to pass through the membrane at a very high rate.