Re: why are plants stiff andimmobile due to their cellular structure?

Good question!

First, let's figure out what is different between plants and animals. Animals have lots of different organs. We have muscles, bones, skin, veins, nerves, and so on. Each organ has a very specific task and is specialized to do that job. The skin, for instance does not have to hold the body upright. The bones are there to do that. Plants also have specialized cells, but they are not centralized like those found in animals. For instance, most animals have lungs to exchange air. Plants have hundreds of stomata on each leaf that control the movement of air and moisture.

Now let's look at an organ plants do not have - bones. The giant redwoods lack a skeleton but still manage to stand up somehow. So let’s look at the cells themselves. The answer is what makes plant cells generally look the same. All of them have a cell wall. This wall is a stiff layer that holds the cell in that familiar rectangular shape. And when you pack together millions of these cells, the cell walls work together to make the tissue stiff, unlike animal cells which, which a couple of exceptions will be softer and much more flexible.

Here are a couple of pages that highlight the differences between plants and animals.
http://www.sciencenet.org.uk/database/bio/cells/b00292a/b00309c.html
http://www.sparknotes.com/biology/cellstructure/celldifferences/se ction1.html