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An underlying principle of chemical reactions are the law of conversation of mass (the mass of the reactants equals the mass of the products) and the law of conversation of energy (energy cannot be created or destroyed; only its form can be changed). Energy exists in two forms: as potential energy and as kinetic energy. Chemical energy is a form of potential energy stored in a chemical bond.
The sum of all chemical reactions in the body is referred to as metabolism. Exergonic reactions release energy; endergonic reactions absorb energy. The body's coupling of exergonic reactions with endergonic reactions is a key feature of metabolism. Exergonic reactions occur as nutrients, e.g. glucose, are broken down. Some of the released energy is used in the formation of adenosine triphosphate. ATP.
When a chemical reaction takes place, it will first require activation energy (the energy, in the form of forceful collisions among molecules, that is needed to start a chemical reaction). Both the concentration and the temperature of a compound will influence the forcefulness of the collisions. Thus, applying pressure or applying heat both can start chemical reactions.
Catalysts lower the activation energy needed for certain chemical reactions. A catalyst changes the orientation at which reactants collide with each other, thus facilitating chemical reactions that depend not only on the force with which reactant molecules collide, but also on the precise spot at which they hit each other. Enzymes are catalysts, and the body uses a large number of enzymes to make chemical reaction happen in a controlled fashion.
Metabolism can be divided into chemical reactions that combine or fracture molecules. Synthesis reactions are named anabolism. Decomposition reactions are referred to as catabolism. Exchange reactions consist of both a decomposition (catabolic) and a synthesis (anabolic) reaction. The reactants just switch chemical partners. Reversible reactions can go in either way, decomposition (for example when water is added) or synthesis (for example when heat is added).
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Inorganic compounds and solutions
A salient characteristic of water is its polarity. In the covalent bond between 1 oxygen atom and 2 hydrogen atoms, the shared electrons spend more time with the oxygen than they do with the hydrogens. The oxygen side of the molecule therefore has a partial negative charge, while the hydrogens have a partial positive charge.
Ionic compounds dissolve well in water (and form an electrolyte) because of the polarity of the water molecule. Hydrophilic compounds are those that dissolve easily in water; hydrophobic are those that don't. The breaking of a chemical bond by adding a water molecule is called hydrolysis. The joining of molecules by removing a molecule of water is called dehydration synthesis.
Water has a high heat capacity because a lot of the heat that is supplied is used to break hydrogen bonds; it is therefore not available to speed up the molecules and thus raise the temperature of the water. The high heat capacity of water also means that sweating (and the transformation of water to vapor) removes a lot of heat from the body. Water is essential for the temperature homeostasis of the body. Because it is a major part of mucus and other lubricants, water also plays an important role as lubricant.
Water, in the human body and in many other applications, often is involved in liquid mixtures. Liquid mixtures can either be solutions, colloids, or suspensions. Colloids are solutes that are large enough to scatter light. The solutes of solutions and colloids do not dissociate from the solvent. In a suspension, however, the suspended materials do settle out.
An example of a suspension is blood. In liquid blood, the red blood cells are in suspension. They separate from the mixture and settle at the bottom of the test tube. The blood plasma is both a colloid (because of the plasma proteins) and a solution (because of dissolved ions and other small particles).
The solute content of solutions can be expressed in a variety of ways. 1. As mass per volume percentage: the mass of solute per volume of solution (grams per 100 milliliter). To make a 10 % NaCl solution, take 10 gram NaCl and fill container until 100 mL. 2. As molarity: mol per liter. To make a 1 mol NaCl solution, take 1 mol NaCl (58.44 gram) and fill container until 1 L.
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