In 1838, a German botanist, Matthias Schleiden came to the conclusion that all plant tissues contain massive number of cells. In the following year, zoologist Theodor Schwann observed that all animal tissues are also made of cells and he proposed a cellular basis for all life. In 1858, Rudolf Virchow finally came to the conclusion that cells can arise only from preexisting cells. 

The three main component of cells are:  cell membrance, nucleus, and cytoplasm.  Cell memberance is a lipid bilayer structure that separates inside the cell from its surroundings, and let some molecules go in and out while prevent the others.  Cell membrance has proteins, and chlostrols embaded inside of it.  Chlostols is to give the cell membrane its strength, while proteins performs different functions mainly transporting some molecules into and out of the cells--transport proteins.  Cells can transport molecules via passive (no energy is required) or active (energy is required) transport.  Nucleus is the most important part of a cell.  It contains chromosomes, DNA, RNA, and functions as the headquarter of a cell.  Nucleus plays an imortant role in the cell division.  Cytoplasm is the space inside the cell.  It contains different organels such as mitochandrias (produces energy), Endoplasmic Reticulum (transports proteins), Golgi bodies (packages proteins), and ribosomes (makes proteins).