When an animal cell is placed in an isotonic solution, nothing significant happens to the cell in terms of volume or shape. An isotonic solution has the same concentration of solutes as the cytoplasm of the cell. This means that the movement of water into the cell is equal to the movement of water out of the cell, leading to a state of equilibrium.
In this state, the cell maintains its normal shape, and cellular functions can occur normally without interruption. Since there is no net movement of water, the cell neither swells (as it would in a hypotonic solution) nor shrinks (as it would in a hypertonic solution). This balance is crucial for the health and function of the cell, allowing it to carry out metabolic processes effectively.