Final answer:
The passage of water-soluble substances through the plasma membrane is facilitated by transport proteins like channel and carrier proteins, specifically aquaporins for water.
Step-by-step explanation:
The passage of water-soluble substances through the plasma membrane is made possible by transport proteins, such as channel proteins and carrier proteins. These proteins are crucial because they allow polar compounds, which would otherwise be blocked by the nonpolar central layer of the plasma membrane, to move through the membrane. This process, known as facilitated diffusion, is a type of passive transport that does not require energy. An example of a channel protein is an aquaporin, which specifically facilitates the rapid transport of water through the membrane.