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What is the cellular theory?

Cellular theory is a theory that asserts that the cell is the constituent unit of the living beings.

Before the discovery of the cell, it was not recognized that the living beings were made of building blocks like cells.

The cellular theory is one of the basic theories of Biology.

Are there living beings without cell?

The virus are considered the only alive beings that do not have cells. Virus are constituted by genetic material (DNA or RNA) enwrapped by a protein capsule. They do not have membrane and cell organelles neither self-metabolism.

In 1665 Robert Hooke, an English scientist, published his book Micrographia, in which he described that pieces of cork viewed under the microscope present small cavities similar to pores and filled with air. Based on later knowledge of what were the walls of those cavities constituted? What is the historical importance of that observation?

The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. The observation leaded to the the discovery of the cells, a fact only possible after the invention of the microscope. In that work, Hooke established the term “cell”, now widely used in Biology, to designate those cavities seen under the microscope.