**Cell theory** was brought by two scientists named Theodor Schwann and Matthias Schleiden.

The theory states that:
1. All living organisms are made up of cells.
2. Cells are the basic unit of structure and function in all organisms.
3. New cells are made from existing cells.

Eukaryotic cells have DNA which is located inside the nucleus. On the other hand, prokaryotic cells lack a nucleus so their genetic material is found in the cytoplasm.

In transmission electron microscopy (TEM), a specimen of cells or tissues must be cut first into ultrathin slices. These slices are then examined under the electron microscope as electrons pass through them, creating a very detailed image of cell structures and proteins.

In scanning electron microscopy (SEM), the specimen does not have to be cut into ultrathin slices. Instead, the specimen is visualized by a pencil-like beam of electrons that scan over the surface of the specimen, creating a three-dimensional image.

A common procedure for both TEM and SEM is that the specimens to be examined must be placed in a vacuum. This is due to the fact that electrons are easily scattered by molecules in the air, which would affect the accurate visualization of the examined specimen.

Additionally, both SEM and TEM can only visualize specimens that are chemically preserved and dehydrated. This means that electron microscopy, in general, can only be used to visualize nonliving cells and tissues rather than living or free-floating specimens.

The cell structures that are common to all cells include the DNA or the genetic material; the cytoplasm, which is the fluid where the organelles are suspended in it; plasma membrane, also referred to as the cell membrane that serve as the cell’s barrier to the external environment; and the ribosomes, which are the organelles in charge of producing proteins.

The invention of microscope led the discovery of the cell. To explore the cell even more, scientists both use microscopes and other powerful techniques such as fluorescent labels to follow the molecules moving inside the cells. High-resolution videos made it easier for scientists to study how cells grow, divide, and develop.This major breakthrough allowed the scientists to have a deeper understanding about the cell theory and to develop the scientific ideas even further.