5n cell biology, the nucleus (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel) is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cells genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are the cells nuclear genome. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nucleoskeleton (which includes nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole. Because the nuclear membrane is impermeable to large molecules, nuclear pores are required that regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. The interior of the nucleus does not contain any membrane-bound sub compartments, its contents are not uniform, and a number of sub-nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the nucleolus, which is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA. 4The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons, each with mass approximately one atomic mass unit, constitute the nucleus of an atom, and they are collectively referred to as nucleons.[4] Their properties and interactions are described by nuclear physics. The nucleus consists of a number of protons, or atomic number with symbol Z, and a number of neutrons, or neutron number with symbol N. The atomic number defines the chemical properties of the atom, and the neutron number determines the isotope or nuclide.[5] The terms isotope or nuclide are often used synonymously, but they refer to chemical or nuclear properties, respectively. The atomic mass number, symbol A, equals Z+N. For example, carbon has atomic number 6, and its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine (see stable nuclide). Other elements occur as many stable isotopes, such as tin with ten stable isotopes. Even though it is not a chemical element, the neutron is included in the table of nuclides.[6] Within the nucleus, protons and neutrons are bound together through the nuclear force, and neutrons are required for the stability of nuclei. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes. The neutron is essential to the production of nuclear power. The neutron was discovered in 1932,[7] and it was realized as early as 1933 that neutrons might act to form a nuclear chain reaction. In the 1930s, neutrons were used to produce many different types of nuclear transmutations. When nuclear fission was discovered in 1938,[8] it became clear that, if a fission event produced neutrons, these neutrons might cause more fission events, etc., in a cascade known as a chain reaction.[5] These events and findings led to the first self-sustaining nuclear reactor (Chicago Pile-1, 1942) and the first nuclear weapon (Trinity, 1945). Free neutrons, or individual neutrons free of the nucleus, are effectively a form of ionizing radiation, and as such, are a biological hazard, depending upon dose.[5] A small natural neutron background flux of free neutrons exists on Earth, caused by cosmic ray muons, and by the natural radioactivity of certain fissile elements in the Earths crust.[9] Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. 2The proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with mass approximately one atomic mass unit, are collectively referred to as nucleons. One or more protons are present in the nucleus of an atom. The number of protons in the nucleus is referred to as its atomic number. Since each element has a unique number of protons, each element has its own unique atomic number. The word proton is Greek for first, and this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by collision. The proton was therefore a candidate to be a fundamental particle and a building block of nitrogen and all other heavier atomic nuclei. In the modern Standard Model of particle physics, the proton is a hadron, and like the neutron, the other nucleon (particle present in atomic nuclei), is composed of three quarks. Although the proton was originally considered a fundamental particle, a proton is now understood to be composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks are thought to contribute only about 1% of the protons mass. The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together. Because the proton is not a fundamental particle, it possesses a physical size—although this is not perfectly well-defined since the surface of a proton is somewhat fuzzy, due to being defined by the influence of forces that do not come to an abrupt end. The proton is about 0.84–0.87 fm in radius.[2] The free proton (a proton not bound to nucleons or electrons) is a stable particle that has not been observed to break down spontaneously to other particles. Free protons are found naturally in a number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay. Protons also result (along with electrons and antineutrinos) from the radioactive decay of free neutrons, which are unstable. At sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom. The result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, which is chemically a free radical. Such free hydrogen atoms tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H2), which are the most common molecular component of molecular clouds in interstellar space. Such molecules of hydrogen on Earth may then serve (among many other uses) as a convenient source of protons for accelerators (as used in proton therapy) and other hadron particle physics experiments that require protons to accelerate, with the most powerful and noted example being the Large Hadron Collider. 1The atom is the smallest unit that defines the chemical elements and their isotopes. Most matter on Earth is made up of atoms. The size of atoms is measured in picometers (trillionths of a meter). A single strand of human hair is about one million carbon atoms wide.[1] Every atom is composed of a nucleus made of protons and neutrons (hydrogen-1 has no neutrons). The nucleus is surrounded by a cloud of electrons. The electrons in an atom are bound to the atom by the electromagnetic force, and the protons and neutrons in the nucleus are bound to each other by the nuclear force. Over 99% of the atoms mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. Normally, an atoms electrons balance out the positive charge of its protons to make it electrically neutral. If an atom has a surplus or deficit of electrons, then it will have an overall charge, and is called an ion. The number of protons in the nucleus determines what chemical element the atom belongs to (e.g. all copper atoms contain 29 protons). The number of neutrons determines what isotope of the element it is.[2] The electron cloud of the atom determines the atoms chemical properties and strongly influences its magnetic properties. Atoms can attach themselves to each other by chemical bonds to form molecules, network solids, metal alloys, crystals, and other solid solutions. The tendency for atoms to bond and break apart is responsible for most of the physical changes we observe in nature, and this is studied by the science of chemistry. Atoms and sub-atomic particles behave in peculiar ways that cannot be explained through the classical laws of physics. The field of quantum mechanics was developed to explain the structure and behavior of atoms. Not all matter is made up of atoms, but atoms do comprise all the types of matter than can be seen and touched. Astronomical observations indicate that most of the Universes matter is dark matter, composed of particles of a currently unknown type. An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electronic engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice. It is different from the positron, which is an actual particle of antimatter. 3The electron hole was introduced into calculations for the following situations: If an electron is excited into a higher state it leaves a hole in its old state. This meaning is used in Auger electron spectroscopy (and other x-ray techniques), in computational chemistry, and to explain the low electron-electron scattering-rate in crystals (metals, semiconductors). In crystals, electronic band structure calculations lead to an effective mass for the electrons, which typically is negative at the top of a band. The negative mass is an unintuitive concept,[1] and in these situations a more familiar picture is found by considering a positive charge with a positive mass.
Posted on: Thu, 13 Nov 2014 02:55:36 +0000