Final answer:
An isotonic environment is where the concentration of solutes is the same inside and outside the cells, leading to a balanced entry and exit of water across the cell membrane, maintaining cell size and function.
Step-by-step explanation:
An isotonic environment is balanced so water enters and leaves cells at the same rate. When the concentration of solutes in the external environment matches the concentration of solutes inside the cells, the solution is said to be isotonic. In such conditions, there is no net movement of water across the cell membrane because the water molecules that enter are in equilibrium with the ones that leave.
In contrast, in a hypertonic solution, the external solute concentration is higher than that inside the cell, leading to water exiting the cell. Conversely, in a hypotonic environment, the solute concentration is lower outside the cell than inside, causing water to flow into the cell. These imbalances lead to cellular swelling in hypotonic solutions or shrinking in hypertonic environments, which can be damaging to cell structure and function.