Cells are the basic structural and functional and unit of all known living organisms and are referred to as "building blocks of life". Organisms can be classified as unicellular and multicellular on the basis of number of cells. Organisms having single cell are known as Unicellular and those with multiple number of cells are known as multicellular organisms. In 1665 Robert Hooke discovered the cell. Matthias Jakob Schleiden and Theodor Schwann, were the first to develop the cell theory in 1839 which states that all organisms are composed of one or more cells, and all cells emerge from pre existing cells, vital functions of an organism occur within cells and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation. Cells evolved on Earth around 3.5 billion years ago.
On the basis presence of nucleus cells are further divided into two types- eukaryotic and prokaryotic. Eukaryotic organisms contains nucleus isolated from the cytoplasm by nuclear envelope and prokaryotic organisms which do not contain nucleus instead they are nucleoid suspended in the cytoplasm. Prokaryotic cells are usually single-celled organisms while eukaryotic cells can be either single-celled or part of multicellular organisms. The cytoplasm contains many biomolecules such as proteins and nucleic acids. and the cell is separated from the environment by the cell membrane.
Last date updated on July, 2014