What defines a solution as hypotonic compared to another?

A solution is defined as hypotonic when it has a lower solute concentration compared to another solution, especially in the context of cellular environments. This means that the osmolarity, which is a measure of the total concentration of solute particles in a solution, is lower in the hypotonic solution.

When a cell is placed in a hypotonic solution, water moves into the cell due to osmosis, aiming to equalize the solute concentrations on both sides of the cell membrane. This influx of water can lead to cell swelling or even bursting, as there is a greater concentration of water molecules in the hypotonic solution compared to the inside of the cell.

In summary, the defining characteristic of a hypotonic solution is its lower osmolarity relative to another solution, which significantly impacts cellular function and behavior through osmotic pressure differences.