CELL

Cell (biology)

The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms  A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called 

Cell
Onion (Allium cepa) root cells in different phases of the cell cycle (drawn by E. B. Wilson, 1900)
A eukaryotic cell (left)

Structure of an animal cell

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants  and animals).^[ While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (10¹³) cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.

Cells were discovered by Robert Hooke  in 1665, who named them for their resemblance to cells inhabited by Christian monks  in a monastery. 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 cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells. Cells emerged on Earth at least 3.5 billion years ago.

Overview

Cells are of two types: eukaryotic , which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms , while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

Main article: Prokaryote

Structure of a typical prokaryotic cell

Prokaryotes include bacteria and archaea, two of the three  domains of life . Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes  including cell signaling . They are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter.

A prokaryotic cell has three architectural regions:

• Enclosing the cell is the cell envelope  – generally consisting of a plasma membranecovered by a cell wall  which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma(archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressuredue  to a hypotonic environment. Some eukaryotic cells (plant cells and fungalcells) also have a cell wall.
• Inside the cell is the cytoplasmic region   that contains the genome  (DNA), ribosomes and various sorts of inclusions.The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids , which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia  notably Borrelia burgdorferi  which causes Lyme disease.Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
• 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.

Structure of a typical animal cell

Structure of a typical plant cell

Eukaryotic cells

Main article: Eukaryote

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic . these cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. the main distinguishing feature of eukaryotes as compared to prokaryotes

   prokaryot is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus  an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:

• 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 linear molecules, called chromosomes , which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus , separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondriaalso contain some DNA.
• Many eukaryotic cells are ciliated  with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
• Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants  Eukaryotic flagella are less complex than those of prokaryotes.

Comparison of features of prokaryotic and eukaryotic cells

	Prokaryotes	Eukaryotes
Typical organisms	bacteria, archaea	protists, fungi, plants, animals
Typical size	~ 1–5 µm[16]	~ 10–100 µm[16]
Type of nucleus	nucleoid region; no true nucleus	true nucleus with double membrane
DNA	circular (usually)	linear molecules (chromosomes) with histoneproteins
RNA/proteinsynthesis	coupled in the cytoplasm	RNA synthesis in the nucleus protein synthesis in the cytoplasm
Ribosomes	50S and 30S	60S and 40S
Cytoplasmic structure	very few structures	highly structured by endomembranesand a cytoskeleton
Cell movement	flagellamade of flagellin	flagella and ciliacontaining microtubules; lamellipodia and filopodiacontaining actin
Mitochondria	none	one to several thousand
Chloroplasts	none	in algae and plants
Organization	usually single cells	single cells, colonies, higher multicellular organisms with specialized cells
Cell division	binary fission(simple division)	mitosis (fission or budding) meiosis
Chromosomes	single chromosome	more than one chromosome
Membranes	cell