Final answer:
The outermost boundary of the living part of a plant cell is the cell wall, a rigid structure made primarily of cellulose that provides support and shape to the cell.
Step-by-step explanation:
The outermost boundary of the living part of a plant cell is the cell wall. The cell wall is a rigid structure that encases the plasma membrane and provides the cell with structural support, protection, and shape. Unlike animal cells that only have a cell membrane, plant cells have this additional layer. The primary component of the plant cell wall is cellulose, which is a polysaccharide made up of glucose units. This is why when you bite into a vegetable such as celery, you experience a crunch, as you are breaking through these rigid cell walls.