Final answer:
Celery becomes limp in a hypertonic solution because water moves out of the plant cells through osmosis, leading to a loss of turgor pressure. This causes the plant to lose its rigidity and become flaccid.
Step-by-step explanation:
Osmosis is the process by which water from plant cells escapes when a plant, such as celery, is submerged in a hypertonic solution, in which the concentration of solutes in the external solution is higher than in the plant cells.
Water moves from a region of higher water potential (within the cells) to an area of lower water potential (outside the cells) under hypertonic conditions because the water potential is lower outside the cells.
Turgor pressure decreases as a result of water leaving plant cells and the central vacuoles within the cells losing water. Maintaining cell stiffness and structural stability requires turgor pressure.
The plant cells, and subsequently the celery stalks, become floppy and flaccid when the turgor pressure drops. Turgor pressure loss is a physical symptom of this.