1- The building block of nature, or life really are cells.
2- No, because some unicellular organisms like bacteria only consist of 1 cell.
3- An example is bacteria, amoeba, etc.
4- Multicellular organism consist of different cells and mostly have different specialized cells. Unicellular organisms carry out their functions like multicellular, but all in 1 cell. They are known to be smaller than multicellular organisms.
5- No, because cells are the building blocks of life and you need cells to form together to form tissues, later into organs and organ systems. Tissues and organs function due to the amount of cells functioning. Tissues and organs are the formation of cells that serve a function to create more complex systems.