If you've ever forgotten to water your house plants, then you've probably noticed that they begin to wilt. Most of us naturally know that wilting plants need water, but exactly why is it that dehydrated (thirsty) plants wilt?
The answer is that plant cells contain many organelles (compartments), one of which is a very large vacuole (storage compartment) for water. When filled with water, this vacuole pushes out against the cell wall (a rigid layer which wraps around the plant cell to support it). This resulting outward pressure is called turgor pressure.
When it rains, or when you water a house plant, some of the water absorbed by the plant's roots is used to carry out cellular processes, some is used to transport nutrients (the plant's equivalent of an animal's blood circulation), and the leftover water is stored in vacuoles in the cells.
So, when a plant is well-hydrated, its vacuoles swell with water. Thus, the turgor pressure inside each cell is high. This supports the wall of each cell and makes the plant cells stiff. This stiffness is what allows plant stems to stand up straight (plants rely on turgor pressure since they do not have bones to support their "limbs" against gravity).
In contrast, when a plant gets dehydrated, it must use its vacuoles as a source of water since water is so important for every cell to function. So, some of the stored water must exit the vacuoles so that it can be used. This is similar to a town's water tower: when the town is well-supplied with water, the tower stays full, but when there is a water shortage, the stored water in the tower is drained out and used to support the townspeople.
As you can imagine, when the vacuoles are drained, they shrink and thus do not push outward on the cell wall anymore. This lack of turgor pressure causes the plant to wilt.
Little Lion Experiment:
Cut a grape in half and peel the skin off of it. If you don't have grapes, then cut a thin (1/4") slice of an apple. Notice how the fruit is rather stiff.
Next, to cause dehydration, cover the piece of fruit with salt for 10 minutes. The salt will draw some of the water out of the fruit. For the best results, scrape the wet salt off of the fruit and replace it with a new sprinkling of salt every 2 minutes.
Now, you have dehydrated the fruit cells so that their water vacuoles are depleted (i.e. they contain less water than they used to). This is similar to what happens when you forget to water your house plants. Feel the fruit to see how dehydration affected the stiffness of the plant. Can you explain your results with regard to turgor pressure?
Food for thought: if you left the grape in salt for a very long time, you'd end up with something similar to a raisin. A raisin, after all, is just a dehydrated grape! It still has the same amount of skin around it, but that skin is wrinkled because the volume of the raisin is much less than the volume of the original grape. This difference in volume shows how much water was lost. So, since grapes are so much larger than raisins, you can see that the main component of grape cells (and in fact all living cells) is water!