Cell
The cell is the basic structural and functional unit of all known livingorganisms. It is the smallest unit of life that is classified as a living thing (except virus, which consists only from DNA/RNA covered byprotein and lipids), and is often called the building block of life.Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Humans contain about 100 trillion (1014) cells . Most plant and animal cells are between 1 and 100 µm and therefore are visible only under the microscope.
The cell was discovered by Robert Hooke in 1665. 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, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain thehereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
The word cell comes from the Latin cella, meaning "small room".The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.
anatomy
There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells can either exist as a single celled organism or be found in multicellular organisms.
The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the otherorganelles of eukaryotes. There are two kinds of prokaryotes:bacteria and archaea; these share a similar structure.
The nuclear material of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. Here, the undefined nuclear region in the cytoplasm is called the nucleoid.
A prokaryotic cell has three architectural regions:
The nuclear material of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. Here, the undefined nuclear region in the cytoplasm is called the nucleoid.
A prokaryotic cell has three architectural regions:
- On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.
- Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membranethough some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea). The cell wall consists ofpeptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cells and fungal cells) also have a cell wall.
- Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Aprokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease).[8] Though not forming a nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance.