Final answer:
Small water-soluble substances use facilitated transport to cross the cell membrane with the help of specific transport proteins, while osmosis is the passive movement of water through aquaporins.
Step-by-step explanation:
Small water-soluble substances typically move across the cell membrane through a process known as facilitated transport or facilitated diffusion. This is a type of passive transport that involves the movement of molecules down their concentration gradient, from an area of higher concentration to an area of lower concentration, with the aid of membrane proteins. These specific proteins span the entire membrane and include channel proteins, gated channel proteins, and carrier proteins, which form channels or bond with the substances to allow them to pass through the membrane without expending energy.
Osmosis is another form of passive transport that is specifically the movement of water molecules through a semipermeable membrane. It's driven by the water concentration gradient across the membrane, which also does not require cellular energy. Aquaporins are special channel proteins in the membrane that facilitate the movement of water and are critical for maintaining the cell's water balance.