Conjugate focal plane
In optics, a conjugate plane or conjugate focal plane of a given plane P, is the plane P` such that points on P are imaged on P`. If an object is moved to the point occupied by its image the moved object's new image will appear at the point where the object originated. In other words, the object and its image are interchangeable; this comes from the principle of reversibility which states light rays will travel along the originating path if the light's direction is reversed. The points that span conjugate planes are called conjugate points. In a telescope, the subject focal plane is at infinity and the conjugate image plane, at which the image sensor is placed, is said to be an infinite conjugate. In microscopy and macro photography, the subject is close to the lens, so the plane at which the image sensor is placed is said to be a finite conjugate. Within a system with relay lenses or eyepieces, there may be planes that are conjugate to the aperture
Brønsted–Lowry acid–base theory
The Brønsted–Lowry theory is an acid–base reaction theory, proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, the base forms its conjugate acid by exchange of a proton; this theory is a generalization of the Arrhenius theory. In the Arrhenius theory acids are defined as substances that dissociate in aqueous solution to give H+, bases are defined as substances that dissociate in aqueous solution to give OH−. In 1923 physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in England both independently proposed the theory that carries their names. In the Brønsted–Lowry theory acids and bases are defined by the way they react with each other, which allows for greater generality; the definition is expressed in terms of an equilibrium expression acid + base ⇌ conjugate base + conjugate acid. With an acid, HA, the equation can be written symbolically as: HA + B ⇌ A− + HB+The equilibrium sign, ⇌, is used because the reaction can occur in both forward and backward directions.
The acid, HA, can lose a proton to become its conjugate base, A−. The base, B, can accept a proton to become its conjugate acid, HB+. Most acid-base reactions are fast so that the components of the reaction are in dynamic equilibrium with each other. Consider the following acid–base reaction: CH3COOH + H2O ⇌ CH3COO− + H3O+Acetic acid, CH3COOH, is an acid because it donates a proton to water and becomes its conjugate base, the acetate ion. H2O is a base because it accepts a proton from CH3COOH and becomes its conjugate acid, the hydronium ion; the reverse of an acid-base reaction is an acid-base reaction, between the conjugate acid of the base in the first reaction and the conjugate base of the acid. In the above example, acetate is the base of the reverse reaction and hydronium ion is the acid. H3O+ + CH3COO− ⇌ CH3COOH + H2OThe power of the Brønsted–Lowry theory is that, in contrast to Arrhenius theory, it does not require an acid to dissociate; the essence of Brønsted–Lowry theory is that an acid only exists as such in relation to a base, vice versa.
Water is amphoteric as it can act as a base. In the image shown at the right one molecule of H2O acts as a base and gains H+ to become H3O+while the other acts as an acid and loses H+ to become OH−. Another example is furnished by substances like aluminium hydroxide, Al3. Al3 + OH− ⇌ Al−4, acting as an acid 3H+ + Al3 ⇌ 3H2O + Al3+, acting as a base The hydrogen ion, or hydronium ion, is a Brønsted–Lowry acid in aqueous solutions, the hydroxide ion is a base, by virtue of the self-dissociation reaction H2O + H2O ⇌ H3O+ + OH−An analogous reaction occurs in liquid ammonia NH3 + NH3 ⇌ NH+4 + NH−2Thus, the ammonium ion, NH+4, plays the same role in liquid ammonia as does the hydronium ion in water and the amide ion, NH−2, is analogous to the hydroxide ion. Ammonium salts behave as acids, amides behave as bases; some non-aqueous solvents can behave as bases, that is, proton acceptors, in relation to Brønsted–Lowry acids. HA + S ⇌ A − + SH +; the most important such solvents are dimethylsulfoxide, DMSO, acetonitrile, CH3CN, as these solvents has been used to measure the acid dissociation constants of organic molecules.
Because DMSO is a stronger proton acceptor than H2O the acid becomes a stronger acid in this solvent than in water. Indeed, many molecules behave as acids in non-aqueous solution that do not do so in aqueous solution. An extreme case occurs with carbon acids; some non-aqueous solvents can behave as acids. An acidic solvent will increase basicity of substances dissolved in it. For example, the compound CH3COOH is known as acetic acid because of its acidic behaviour in water; however it behaves as a base in a much more acidic solvent. HCl + CH3COOH ⇌ Cl− + CH3C+2 In the same year that Brønsted and Lowry published their theory, G. N. Lewis proposed an alternative theory of acid–base reactions; the Lewis theory is based on electronic structure. A Lewis base is defined as a compound that can donate an electron pair to a Lewis acid, a compound that can accept an electron pair. Lewis's proposal gives an explanation to the Brønsted–Lowry classification in terms of electronic structure. HA + B: ⇌ A:− + BH+In this representation both the base, B, the conjugate base, A−, are shown carrying a lone pair of electrons and the proton, a Lewis acid, is transferred between them.
Lewis wrote in "To restrict the group of acids to those substances that contain hydrogen interferes as with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen." In Lewis theory an acid, A, a base, B:, form an adduct, AB, in which the electron pair is used to form a dative covalent bond between A and B. This is illustrated with the formation of the adduct H3N−BF3 from ammonia and boron trifluoride, a reaction that cannot occur in aqueous solution because boron trifluoride reacts violently with water in a hydrolysis reaction. BF3 + 3H2O → B3 + 3HF HF ⇌ H+ + F−These reactions illustrate that BF3 is an acid in both Lewis and Brønsted–Lowry classifications and emphasizes the consistency between both theories. Boric acid is recognized as a Lewis acid by virtue of the reaction B3 + H2O ⇌ B−4 + H+In this case the acid does not dissociate, it is the base, H2O that dissociates. A solution of B3 is acidic. There is strong evidence that dilute aqueous solutions of ammonia contain negligible amoun
Conjugate vaccines combine a weak antigen with a strong antigen so that the immune system has a stronger response to the weak antigen. Vaccines are used to prevent diseases by invoking an immune response to an antigen, the foreign part of a bacteria or virus that the immune system recognizes; this is accomplished with an attenuated or dead version of a pathogenic bacterium or virus in the vaccine, so that the immune system can recognize the antigen in life. Many vaccines contain a single antigen. However, the antigen of some pathogenic bacteria does not elicit a strong response from the immune system, so a vaccination against this weak antigen would not protect the person in life. In this case, a conjugate vaccine is used in order to invoke an immune system response against the weak antigen. In a conjugate vaccine, the weak antigen is covalently attached to a strong antigen, thereby eliciting a stronger immunological response to the weak antigen. Most the weak antigen is a polysaccharide, attached to strong protein antigen.
However, peptide/protein and protein/protein conjugates have been developed. The idea of a conjugate vaccine first appeared in experiments involving rabbits in 1927, when the immune response to the Streptococcus pneumoniae type 3 polysaccharide antigen was increased by combining the polysaccharide antigen with a protein carrier; the first conjugate vaccine used in humans became available in 1987. This was the Haemophilus influenzae type b conjugate; the vaccine was soon incorporated with the schedule for infant immunization in the United States. The Hib conjugate vaccine is combined with one of several different carrier proteins, such as the diphtheria toxoid or the tetanus toxoid. Soon after the vaccine was made available the rates of Hib infection dropped, with a decrease of 90.7% between 1987 and 1991. Infection rates diminished more once the vaccine was made available for infants. Vaccines evoke an immune response to an antigen, the immune system reacts by producing T cells and antibodies.
The T cells remember the antigen so that if the body encounters it antibodies can be produced by B cells to break down the antigen. For bacteria with a polysaccharide coating, the immune response creates B cells independent of T cell stimulation. By conjugating the polysaccharide to a protein carrier, a T cell response can be induced. Polysaccharides by themselves cannot be loaded onto the major histocompatibility complex of antigen presenting cells because MHC can only bind peptides. In the case of a conjugate vaccine, the carrier peptide linked to the polysaccharide target antigen is able to be presented on the MHC molecule and the T cell can be activated; this improves the vaccine as T cells stimulate a more vigorous immune response and promote a more rapid and long-lasting immunologic memory. The conjugation of polysaccharide target antigen to the carrier protein increases efficiency of the vaccine as a non conjugated vaccine against the polysaccharide antigen is not effective in young children.
The immune systems of young children are not able to recognize the antigen as the polysaccharide covering disguises the antigen. By combining the bacterial polysaccharide with another antigen, the immune system is able to respond; the most used conjugate vaccine is the Hib conjugate vaccine. Other pathogens that are combined in a conjugate vaccine to increase an immune response are Streptococcus pneumoniae and Neisseria meningitidis', both of which are conjugated to protein carriers like those used in the Hib conjugate vaccine. Both Streptococcus pneumoniae and Neisseria meningitidis are similar to Hib in that infection can lead to meningitis. Vaccine T cell B cell Haemophilus influenzae type b Hib vaccine Immunogenicity Meningococcal vaccine Pneumococcal vaccine Pneumococcal conjugate vaccine Pneumococcal polysaccharide vaccine Immune system Immune response Vaccines, Conjugate at the US National Library of Medicine Medical Subject Headings "Conjugate Vaccines Against Enteric Pathogens". Archived from the original on 2006-09-30.
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Isogamy is a form of sexual reproduction that involves gametes of similar morphology, differing in general only in allele expression in one or more mating-type regions. Because both gametes look alike, they cannot be classified as "male" or "female". Instead, organisms undergoing isogamy are said to have different mating types, most noted as "+" and "−" strains, although in some species of Basidiomycota there are more than two mating types. In all cases, fertilization occurs, it appears. In several lineages, this form of reproduction independently evolved to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is much larger than the male and has no ability to move. There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction. In Ascomycetes, anisogamy evolved from isogamy before mating types. There are several types of isogamy. Both gametes may be flagellated and thus motile.
This type occurs for example in algae such as some but not all species of Chlamydomonas. In another type, neither of the gametes is flagellated; this is the case for example in the mating of yeast. Yeast mating types are noted as "a" and "α" instead of "+" and "-". Another, more complex form, is conjugation; this occurs in some the Zygnematophyceae, e.g. Spirogyra; these algae grow as filaments of cells. When two filaments of opposing mating types come close together, the cells form conjugation tubes between the filaments. Once the tubes are formed, one cell balls up and crawls through the tube into the other cell to fuse with it, forming a zygote. In ciliates, cell fission may follow self-fertilization. In zygomycetes fungi, two hyphae of opposing mating types form special structures called gametangia where the hyphae touch; the gametangia fuse into a zygosporangium. In other fungi, cells from two hyphae with opposing mating types fuse, but only the cytoplasm is fused; the two nuclei do not fuse, leading to the formation of a dikaryon cell that gives rise to a mycelium consisting of dikaryons.
Karyogamy eventually occurs in sporangia, leads to the formation of diploid cells that undergo meiosis to form spores. In many cases, isogamous fertilization is used by organisms that can reproduce asexually through binary fission, budding, or asexual spore formation; the switch to sexual reproduction mode is triggered by a change from favorable to unfavorable growing conditions. Fertilization leads to the formation of a thick-walled zygotic resting spore that can withstand harsh environments and will germinate once growing conditions turn favorable again. Anisogamy Evolution of sexual reproduction Gamete Mating in fungi Meiosis Oogamy Sex Hypergamy Hypogamy Sa Geng. "Evolution of Sexes from an Ancestral Mating-Type Specification Parthway". PLOS Biology. Doi:10.137/journal.pbio.10001904