Chemical reactions are typically accompanied by energy changes. The equation for the synthesis of ammonia from its elements is N2 + 3 H2 2 NH3, but that reaction takes place only under very special conditionsnamely at a high temperature and pressure and in the presence of a catalyst. Energy changes that occur during chemical reactions are the subject of a field of science known as thermodynamics.
Most chemical reactions can be categorized into one of about five general types: synthesis, decomposition, single replacement, double replacement, and oxidation-reduction. A miscellaneous category is also needed for reactions that do not fit into one of these five categories.
Several aspects or subdisciplines of chemistry are brought to bear in the study of chemical reactions. One is stoichiometry (stoy-kee-AH-muh-tree), which is concerned with the relationships among the amounts of reactants and products in a chemical reaction. The balancing of the chemical equation for water earlier in this essay is an example of basic stoichiometry.
A synthesis, or combination, reaction is one in which a compound is formed from simpler materialswhether those materials be elements or simple compounds. A basic example of this is the reaction described earlier in relation to chemical equations, when hydrogen and oxygen combine to form water. On the other hand, some extremely complex substances, such as the polymers in plastics and synthetic fabrics such as nylon, also involve synthesis reactions.
A chemical reaction in which a compound is broken down into simpler compounds, or even into elements. This is the opposite of a synthesis or combination reaction.
In addition, chemical reactions are often a good deal more complex than a chemical equation might lead one to believe. For example, one can write the equation for the synthesis of hydrogen iodide from its elements, as follows: H2 + I2 2 HI. In fact, chemists know that this reaction does not take place in a single step. Instead, it occurs in a series of reactions in which hydrogen and iodine atoms react with each other one at a time. The final equation, H2 + I2 2 HI, is actually no more than a summary of the net result of all those reactions. The field of chemistry that deals with the details of chemical reactions is known as chemical kinetics.
Chemical reactions are represented by means of chemical equations. A chemical equation is a symbolic statement that represents the changes that occur during a chemical reaction. The statement consists of the symbols of the elements and the formulas of the products and reactants, along with other symbols that represent certain conditions present in the reaction. For example, the arrow (or yields) sign, *, separates the reactants from the products in a reaction. The chemical equation that represents the electrolysis of water is 2 H2O 2 H2 + O2.
Metabolism is a word often used interchangeably with biotransformation. It denotes chemical breakdown or synthesis reactions catalyzed by enzymes in the body. Nutrients from food, endogenous compounds, and xenobiotics are all metabolized in the body.
Then the method of how to construct a fully balanced ionic redox symbol equation from half–cell reactions (often given as the reduction half–reaction) is explained for many examples.
This is obviously the case regarding important antioxidants such as vitamin C and glutathione (GSH), which are essential for maintaining redox equilibrium and which have a protective role against the adverse effects of the oxygen- or xenobiotic-derived free radicals which are involved in a variety of pathological conditions (Kehrer 1993). Humans cannot auto-synthesize vitamin C, contrary to the rat, and levels as well as the turnover rate of erythrocyte GSH in humans are considerably lower than that in the rat. Humans also lack some of the protective antioxidant enzymes, compared to the rat or other mammals (e.g., GSH- peroxidase is considered to be poorly active in human sperm). These examples illustrate the potentially greater vulnerability to oxidative stress in humans (particularly in sensitive cells, e.g., apparently greater vulnerability of the human sperm to toxic influences than that of the rat), which can result in different response or greater susceptibility to the influence of various factors in humans compared to other mammals (Teliman 1995).
For example, theoxidation of propane releases heat and light, and a rapid reactionis an explosion,A balanced equation also shows a macroscopic quantitative relationship.
Dietary habits have an important influence on susceptibility to chemical toxicity, mostly because adequate nutrition is essential for the functioning of the bodys chemical defence system in maintaining good health. Adequate intake of essential metals (including metalloids) and proteins, especially the sulphur-containing amino acids, is necessary for the biosynthesis of various detoxificating enzymes and the provision of glycine and glutathione for conjugation reactions with endogenous and exogenous compounds. Lipids, especially phospholipids, and lipotropes (methyl group donors) are necessary for the synthesis of biological membranes. Carbohydrates provide the energy required for various detoxification processes and provide glucuronic acid for conjugation of toxic chemicals and their metabolites. Selenium (an essential metalloid), glutathione, and vitamins such as vitamin C (water soluble), vitamin E and vitamin A (lipid soluble), have an important role as antioxidants (e.g., in controlling lipid peroxidation and maintaining integrity of cellular membranes) and free-radical scavengers for protection against toxic chemicals.
Given the apparent lack of physiological substrates, it is possible that P450 enzymes in families CYP1, CYP2, CYP3 and CYP4 that have appeared in the past several hundred million years have evolved as a means of detoxifying foreign chemicals encountered in the environment and diet. Clearly, evolution of the xenobiotic-metabolizing P450s would have occurred over a time period which far precedes the synthesis of most of the synthetic chemicals to which humans are now exposed. The genes in these four gene families may have evolved and diverged in animals due to their exposure to plant metabolites during the last 1.2 billion yearsa process descriptively termed animal-plant warfare (Gonzalez and Nebert 1990). Animal-plant warfare is the phenomenon in which plants developed new chemicals (phytoalexins) as a defence mechanism in order to prevent ingestion by animals, and animals, in turn, responded by developing new P450 genes to accommodate the diversifying substrates. Providing further impetus to this proposal are the recently described examples of plant-insect and plant-fungus chemical warfare involving P450 detoxification of toxic substrates (Nebert 1994).