Final answer:
The mistake in the definition is that it includes small molecules in the process of osmosis, which is actually the movement of water only through a semipermeable membrane from higher to lower water concentration to reach equilibrium.
Step-by-step explanation:
The mistake in the provided definition of osmosis is that it states osmosis as the passage of water and small molecules across a semipermeable membrane. However, osmosis specifically refers to the movement of water only, not other small molecules, through a semipermeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration). This process is driven by the water concentration gradient and not directly by the solute concentration gradient, although they are inversely related.
Osmosis is a special case of diffusion, where the net movement of water is through a membrane that prevents solute molecules from passing, leading to an equilibrium where the water concentration on both sides of the membrane is balanced. This balance is essential for maintaining cell structure and function, which is why health workers do not infuse plain water directly into the bloodstream, as it could disrupt the osmotic balance and harm cells.