Final answer:
Lettuce and spinach become crisp in water because their cells absorb water and become turgid due to the creation of turgor pressure, which stiffens the plant tissues. This is a result of osmosis and is balanced by the cell wall's ability to prevent cell bursting, creating a firm texture in the vegetables.
Step-by-step explanation:
The reason lettuce and spinach become crisp in water is due to the absorption of water causing cells to swell and become turgid. This is a result of water moving into the cells through osmosis, entering the cytosol and then the vacuoles (tonoplasts), creating higher osmotic pressure inside the plant cells. This in turn results in turgor pressure, which stiffens the cells and makes the tissue crisp. When the osmotic and turgor pressures reach equilibrium, intake of water ceases, thereby maintaining the structure without causing cell bursting thanks to the cell wall.
In contrast, in a hypertonic solution, plant cells lose water and become plasmolyzed. This process involves the plasma membrane detaching from the cell wall while remaining attached at certain points, which leads to leaves and stems becoming flaccid or floppy due to loss of turgor pressure. Thus, when we place limp vegetables in water, they regain turgidity and crispness thanks to osmoregulation mechanisms that plants use to control osmotic effects.