The organelle composed of the sugar cellulose and functions to give plant cells support is the cell wall.

Here's why:

* Cellulose: Cellulose is a complex carbohydrate made up of long chains of glucose molecules. It's a major component of the plant cell wall, providing structural support and rigidity.

* Cell Wall Function: The cell wall acts as a protective barrier, maintaining the cell's shape, and preventing it from bursting due to osmotic pressure. It also provides structural support for the entire plant, allowing it to grow upright and withstand environmental forces.