The cell is the basic structure, function and biologic unit of all organisms. Cells are the smallest unit of life that can replicate independently, Due to cell theory cells are referred to as the “building blocks of life". Cells consist of cytoplasm enclosed within the membrane, which contains many molecules like proteins and nucleic acids in the cell. Organisms can be classified as unicellular or multicellular. While the number of cells in plants and animals varies from species to species, humans contain almost or about 100 trillion cells. Most plant and animal cells are visible under a microscope, with dimensions between 1 and 100 micrometers.
The cell was discovered by Robert Hooke in 1665. The cell theory, first developed in 1839 by Theodor Schwann, which states that all organisms are composed of one or more cells, that all cells come from preexisting cells (cell division), that vital functions of an organism occur within cells (building blocks of life). Every cell contains hereditary information necessary for all cell functions and for transmitting information to the next span of cells. Cells are said to have emerged on Earth 3.5 billion years ago. It was coined by Robert Hooke in which he compared the cork cells he saw through a microscope to small rooms monks lived in.
There are two different types of cells, eukaryotes, which contain a nucleus, and prokaryotes, which do not. Prokaryotic cells are usually single-celled, while eukaryotic cells can be either or.
Prokaryotic cells are simpler and smaller than eukaryotic cells, and don’t have membrane-bound organelles such as the nucleus. The DNA of a prokaryotic cell consists of a single chromosome that is in constant contact with cytoplasm.
Enclosing the cell is the cell envelope which generally consists of a cell wall covering the plasma membrane. The envelope separates the interior of the cell from its environment, serving as a protective filter for the cell. The cell wall which consists of peptidoglycan and also acts as a barrier against exterior forces. It also prevents the cell from expanding and bursting from pressure due to its environment. Some eukaryotic cells also have a cell wall.
Eukaryotic cells are plants, animals, fungi, protozoa, and algae. The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present. The eukaryotic DNA is organized in one or more molecules, called chromosomes, which are associated with proteins. All DNA is stored in the nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also can contain some DNA.
All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, and regulates what moves in and out. Cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, contain information necessary to build proteins and enzymes.