Cell

A cell is, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. It is enclosed by a plasma membrane, a selective barrier that allows nutrients to enter and waste products to leave. Cells are the basic building blocks of all organisms, and the human body is composed of trillions of cells.

Cell theory is the fundamental scientific theory of biology holding that cells are the basic units of all living tissues. Its modern formulation rests on two tenets: biogenesis, the principle that all cells come only from other cells, and the recognition of cells as the fundamental units of organisms. The cell doctrine states that cells are generated from cells, and that the only way to make more cells is by division of those that already exist.

The theory was first proposed by Theodor Schwann and Matthias Jakob Schleiden in 1838. Schwann's microscopic investigations showed that animal tissues, like those of plants, are composed of cells, and that each cell has a nucleus. Schleiden, by contrast, believed that cells formed through crystallization rather than cell division. With the atlas texts of histopathology published in the 1840s, the cell theory introduced and publicized by Schwann in 1838 and 1839 became universally accepted as scientific fact.

In 1852 Robert Remak published convincing evidence that cells are derived from other cells as a result of cell division. Rudolf Virchow published an editorial essay entitled Cellular Pathology, which popularized the concept of cell theory using the Latin phrase omnis cellula a cellula, meaning that all cells arise from cells. Remak, a former colleague who worked in the same laboratory as Virchow at the University of Berlin, had published the same idea three years before Virchow's essay. Virchow was one of the first to determine the causes of various diseases by examining their effects on tissues and organs, and over the course of his career he published more than 2,000 papers.

• 1665
  Hooke describes cork and names the cell
  Robert Hooke described cork and other plant tissues in 1665 and introduced the term cell because the cellulose walls of dead cork cells reminded him of the blocks of cells occupied by monks.^{4Cell theory | Definition, History, Importance, Scientists, First Proposed, & Facts · Encyclopaedia Britannica ↗}
• 1831
  Brown first describes nuclei
  The Scottish botanist Robert Brown was the first to describe observations of nuclei, which he observed in plant cells.^{7Microbiology, 3.2 Foundations of Modern Cell Theory · OpenStax, Rice University ↗}
• 1833
  The nucleus recognized as a constant component
  The nucleus was observed by the Scottish botanist Robert Brown in 1833 as a constant component of plant cells.^{4Cell theory | Definition, History, Importance, Scientists, First Proposed, & Facts · Encyclopaedia Britannica ↗}

The reach of any microscope is bounded by two distinct limits: how much it can enlarge an object and how fine a detail it can separate. A simple microscope can resolve below 1 micrometre, while a compound microscope can resolve down to about 0.2 μm. In terms of enlargement, single-lensed simple microscopes can magnify up to 300 times and are capable of revealing bacteria, whereas compound microscopes can magnify up to 2,000 times.

Antonie van Leeuwenhoek learned to grind lenses before 1668 and used them to make simple hand-held microscopes that, with appropriate lighting, allowed him to magnify objects over 275 times. His instruments employed lenses ranging from 70x to over 250x.

The transmission electron microscope has magnifying powers of more than 1,000,000 times and forms images of thin specimens, typically sections, in a near vacuum. The instrument was first developed by Ernst Ruska in 1933.

The chief component of prokaryotic cell walls is peptidoglycan, while the major organic molecule in the plant cell wall is cellulose, a polysaccharide comprised of glucose units. Many bacteria are further enclosed by a polysaccharide capsule, and some prokaryotes bear flagella, pili or fimbriae. These envelopes and appendages distinguish the prokaryotic surface from the cellulose-based wall of the plant cell.

Eukaryotic cells are distinguished by a membrane-bound nucleus and by numerous membrane-bound organelles, among them the endoplasmic reticulum, the Golgi apparatus, chloroplasts and mitochondria, and they carry several rod-shaped chromosomes. The nucleus is the largest and most prominent of the cell's organelles, measuring about 5 μm in diameter, and it houses the cell's DNA while directing the synthesis of ribosomes and proteins. It also acts as the command center of the cell, sending directions for the cell to grow, mature, divide or die.

Mitochondria are the powerhouses, or energy factories, of the cell, making ATP, the main energy-carrying molecule; they are found in almost all eukaryotic cells and generate most of the cell's supply of ATP from the breakdown of organic molecules. Chloroplasts are the sites of photosynthesis and are found only in plants and green algae. The endoplasmic reticulum is involved in the processing and transport of proteins and in the synthesis of lipids, and proteins are carried from it in small membrane vesicles to the Golgi apparatus for further processing and sorting.

The cytoskeleton is a network of protein filaments extending throughout the cytoplasm, providing the cell with a structural framework and responsible for the movements of entire cells and for the positioning of organelles, including the movements of chromosomes during cell division. Lysosomes serve as the cell's garbage disposal, their enzymes breaking down proteins, polysaccharides, lipids, nucleic acids and worn-out organelles. Ribosomes are the structures responsible for protein synthesis, appearing as clusters or as single tiny dots floating freely in the cytoplasm.

Animal cells each have a centrosome and lysosomes, whereas most plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole.

Cells and their structures span an enormous range of sizes, and the units used to describe them are correspondingly small. A typical human red blood cell is about eight micrometers in diameter, so that about 250 red blood cells could fit on a pinhead ^{12Biology 2e, 4.1 Studying Cells · OpenStax, Rice University ↗}. Most bacterial cells are spherical, rod-shaped or spiral, with diameters of 1 to 10 μm; their DNA contents range from about 0.6 million to 5 million base pairs, enough to encode about 5000 different proteins ^{13The Origin and Evolution of Cells (The Cell: A Molecular Approach, 2nd edition) · NCBI Bookshelf / National Center for Biotechnology Information (Geoffrey M. Cooper) ↗}.

Every cell carries its genetic material in the form of DNA, and the quantity of that DNA varies widely across cell types. In most bacteria, the genome spans roughly 0.6 million to 5 million base pairs, a complement sufficient to encode about 5000 different proteins ^{13The Origin and Evolution of Cells (The Cell: A Molecular Approach, 2nd edition) · NCBI Bookshelf / National Center for Biotechnology Information (Geoffrey M. Cooper) ↗}. DNA is one of the four components common to all cells, alongside the plasma membrane, the cytoplasm, and the ribosomes that synthesize proteins ^{14Biology 2e, 4.2 Prokaryotic Cells · OpenStax, Rice University ↗}. The scale of the genome is matched by the scale of the machinery that reads it: a single E. coli cell contains approximately 30,000 ribosomes.

The contrast between bacterial and human genomes is instructive. Human body (somatic) cells have 46 chromosomes, whereas human gametes — sperm or eggs — carry 23 chromosomes each ^{15Biology 2e, 10.1 Cell Division · OpenStax, Rice University ↗}. Between these extremes sits the yeast Saccharomyces cerevisiae, whose genome comprises 12 million base pairs of DNA.

The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, when Stanley Miller showed that electric sparks discharged into a mixture of hydrogen, methane and ammonia in the presence of water led to the formation of organic molecules including several amino acids.

A very early event in evolution appears to have been the divergence of three lines of descent from a common ancestor, giving rise to present-day archaebacteria, eubacteria and eukaryotes. Based upon the chloroplasts' ability to reproduce independently, the Russian botanist Konstantin Mereschkowski suggested in 1905 that chloroplasts may have originated from ancestral photosynthetic bacteria living symbiotically inside a eukaryotic cell.

A cell reproduces by performing an orderly sequence of events in which it duplicates its contents and then divides in two, a cycle of duplication and division that is the essential mechanism by which all living things reproduce. In unicellular species such as bacteria and yeasts, each cell division produces a complete new organism, while in multicellular species long and complex sequences of cell divisions are required to produce a functioning organism.

The period between one M phase and the next is called interphase, and in most rapidly proliferating cells it is divided into three phases: S phase, in which DNA is replicated, and two gap phases (G1 and G2) that provide additional time for the cell to grow.

The cell-cycle control system monitors progression through the cell cycle and delays later events until earlier events have been completed. When the system malfunctions, excessive cell divisions can result in cancer.

Progression through G1 and G2 is delayed by braking mechanisms if the DNA in the chromosomes is damaged by radiation or chemicals. Delays at these DNA damage checkpoints provide time for the damaged DNA to be repaired.

There are four classes of cyclins, each defined by the stage of the cell cycle at which they bind Cdks and function. Three of these classes are required in all eucaryotic cells.

Sexual reproduction requires the union of two specialized cells called gametes, each of which contains one set of chromosomes; when gametes unite they form a zygote that contains two sets of chromosomes. Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same cellular mechanisms as mitosis.It is a two-step process that reduces the chromosome number by half to form sperm and egg cells, and it also allows genetic variation through a process of gene shuffling.