The net movements of molecules of gasses and particles from a region of higher concentration to the region of lower concentration based on diffusion gradients without a separating medium is called Diffusion.
Osmosis is the movement of water molecules from the region from a region of higher water potential to that of lower water potential across a separating medium of semipermeable membrane.
Because of exposure of the lettuce to sunlight, there is rapid diffusion of water molecules from the cells of the lettuce on the sunny day than into the cells from the environment. Because the turgor pressure has dropped to zero, so water molecules diffuse from the region of higher water potential (the cells of the lettuce) to region of lower (sounding air). This results to shrinking of the cells, called witting.
However, as she waters the lettuce, water molecules diffuses into the cell and move up the cells through osmosis, the water molecules makes the lettuce cells turgid. The turgor pressure has returned to normal . This explains the firmness.
Water diffuses into the vacuole which has capacity to store water. The presence of water increases vacuole's volume which expands through osmosis, and therefore changes it shape. The changes in shape leads to firmness.
Ms. Milly can keep the stalk of the lettuce in flower pot filled with water. The continues to keep the cells of the lettuce turgid, by replacing the lost water through diffusion into the sunny environment to prevent wilting,
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