why is the cell known as the structure and fundamental unit of living beings?

Answer 1

Technically, a human being is nothing more than a vast network of communicating cells. Each cell is basically its own living structure. A human cell is not that different from simple bacteria. There are many differences, one primarily being that each cell in a human being cannot survive without the surrounding cells. Cells comprise your organs, your muscles, your skin, and a significant portion of your bones. Each of these cell types have different functions, but their internal structures are all very similar. Therefore these cells are your building blocks and are, fundamentally, you.

The best way to look at it is to think of embryonic development. Everyone begins as one cell, a fertilized egg. The cell then doubles. Then those cells each double. And so on until you have an entire human being, a massive cluster of cells.

Answer 2

All living things are made of cells, so cells are the structural parts of living things.

Fundamental unit can be interpreted as meaning basic unit. If you think of the levels of organization in living things (organism, systems, organs, tissues, cells) you can't go any farther down the levels than a cell and still have a unit that is alive. Just a part or piece of a cell, like a nucleus or ribosome, is not alive by itself. But a cell is alive. So the cell is the basic unit of organization for life.