Rocket holds up two celery stalks. One stands stiff. The other, left out of water overnight, droops like a wet noodle.
"Same stalk, same walls," he says. "Why is this one floppy?"
Raven presses the limp one. "What do you notice? It lost water. The walls are still there, but nothing is pushing on them."
Nova hovers over the seedling, whose leaf is now a deeper green. "Think about the big vacuole in every plant cell," she says.
"Full of water, it pushes out on the wall and the cell is firm," Rocket says. "Empty, the wall has no support."
"Which is why a thirsty plant wilts," Raven says. "And why it stands back up after watering." Nova hums. "Review time."
Each plant cell has a large central vacuole that takes up most of the cell. It regulates the water in the cell.
The liquid inside the vacuole pushes outward on the cell wall. That outward push is what keeps a stem firm and upright.
When a plant goes without water, water leaves the vacuoles. The vacuoles shrink and leave the cell walls unsupported, so the plant wilts.
Water the plant, and water refills the vacuoles. The cells push against their walls again, and the plant stands back up.
Celery in this course stays a lab material. Compost it or throw it away after the lab.
| What you see | What is happening in the cells |
|---|---|
| A stiff celery stalk standing upright | Full vacuoles push on rigid cell walls |
| A limp stalk left out of water | Vacuoles have shrunk; walls are unsupported |
| A wilted houseplant perks up after watering | Water refills the vacuoles and firms the cells |
| Green leaves on the seedling | Chlorophyll in chloroplasts reflects green light |
| A pale root with no green | No chlorophyll there is reflecting green light |
| Week review | True or false? |
|---|---|
| Plant cells have a cell wall outside the membrane. | ? |
| Chlorophyll absorbs green light and reflects red and blue. | ? |
| Xylem carries water up the stem. | ? |
| The central vacuole helps keep a plant cell firm. | ? |
| Animal cells have chloroplasts. | ? |
Strong week, biologist. Tomorrow is Nature Day: a family leaf hunt for vein patterns and the color green.