Cell
The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is sometimes called the building block of life. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an ostrich egg. In 1837 before the final cell theory was developed, a Czech Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting 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 of cells.
All cells have several different abilities:
-Reproduction by cell division: (binary fission/mitosis or meiosis).
-Use of enzymes and other proteins coded for by DNA genes and made via messenger RNA intermediates and ribosomes.
-Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is released and then used in metabolic pathways.
-Response to external and internal stimuli such as changes in temperature, pH or levels of nutrients.
-Cell contents are contained within a cell surface membrane that is made from a lipid bilayer with proteins embedded in it.
-Reproduction by cell division: (binary fission/mitosis or meiosis).
-Use of enzymes and other proteins coded for by DNA genes and made via messenger RNA intermediates and ribosomes.
-Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is released and then used in metabolic pathways.
-Response to external and internal stimuli such as changes in temperature, pH or levels of nutrients.
-Cell contents are contained within a cell surface membrane that is made from a lipid bilayer with proteins embedded in it.
There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.
