# Characteristic Chemical Reactions

All chemical reactions begin with a reactant, the general term for the one or more substances that enter into the reaction. Sodium and chloride ions, for example, are the reactants in the production of table salt. The one or more substances produced by a chemical reaction are called the product. **Note that there is some "hidden" excitement in the story about table salt involving water that we'll see soon.**

In chemical reactions, the atoms and elements present in the reactant(s) must all also be present in the product(s). Similarly, there can be nothing present in the products that was not present in the reactants. This is because chemical reactions are governed by the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. This means when you examine a chemical reaction you must try to account for everything that goes in AND make sure you can find it all in the stuff the comes out!

Just as you can express mathematical calculations in equations such as 2 + 7 = 9, you can use chemical equations to show how reactants become products. By convention, chemical equations are typically read or written from left to right. Reactants on the left are separated form products on the right by an single or double-headed arrow indicating the direction in which the chemical reaction proceeds. For example, the chemical reaction in which one atom of nitrogen and three atoms of hydrogen produce ammonia would be written as

$N + 3H→NH_3.$

Correspondingly, the breakdown of ammonia into its components would be written as:

$NH3→N + 3H.$

Note that in either direction, you find 1 N and 3 Hs on both sides of the equation.

#### Reversability

In theory, any chemical reaction can proceed in either direction under the right conditions. Reactants may synthesize into a product that is later revert back to a reactant. Reversibility is also a quality of exchange reactions. For instance, A+BC→AB+C could then reverse to AB+C→A+BC. This reversibility of a chemical reaction is indicated with a double arrow: A+BC⇄AB+C.

#### Synthesis Reactions

Many macromolecules are made from smaller subunits, or building blocks, called monomers. Monomers covalently link to form larger molecules known as polymers. Often the synthesis of polymers from monomers will also produce water molecules as products of the reaction. This type of reaction is known as dehydration synthesis or condensation reaction.

Figure 1: In the dehydration synthesis reaction depicted above, two molecules of glucose are linked together to form the disaccharide maltose. In the process, a water molecule is formed.

Attribution: Marc T. Facciotti (own work)

In a dehydration synthesis reaction (Figure 1), the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water. At the same time, the monomers share electrons and form covalent bonds. As additional monomers join, this chain of repeating monomers forms a polymer. Different types of monomers can combine in many configurations, giving rise to a diverse group of macromolecules. Even one kind of monomer can combine in a variety of ways to form several different polymers: for example, glucose monomers are the constituents of starch, glycogen, and cellulose.

In the carbohydrate monomer example above the polymer is formed by a dehydration reaction, this type of reaction is also used to add amino acids to a growing peptide chain, and nucleotides to the growing DNA or RNA polymer. Visit the modules on Amino Acids, Lipids, and Nucleic Acids to see if you can identify the water molecules that are removed when a monomer is added to the growing polymer.

Figure 2: This depicts using words (decorated with functional groups colored in red) a generic dehydration synthesis/condensation reaction.

Attribution: Marc T. Facciotti (own work)

#### Hydrolysis Reactions

Polymers are broken down into monomers in a reaction known as hydrolysis. A hydrolysis reaction includes a water molecule as a reactant(Figure 3). During these reactions, a polymer can be broken into two components: one product carries a hydrogen ion (H+) from the water while the second product carries the water's remaining hydroxide (OH–).

Figure 3: In the hydrolysis reaction shown here, the disaccharide maltose is broken down to form two glucose monomers with the addition of a water molecule. Note that this reaction is the reverse of the synthesis reaction shown in figure 1 above.

Attribution: Marc T. Facciotti (own work)

Figure 4: This depicts using words (decorated with functional groups colored in red) a generic hydrolysis reaction.

Attribution: Marc T. Facciotti (own work)

Dehydration synthesis and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes. Note that both dehydration synthesis and hydrolysis reactions involve the making and breaking of bonds between the reactants - a reorganization of how the atoms in the reactants are bonded together. In biological systems (our bodies included), food in the form of molecular polymers is hydrolyzed into smaller molecules by water via enzyme catalyzed reactions in the digestive system. This allows for the smaller nutrients to be absorbed and reused for a variety of purposes. In the cell, monomers derived from food may then be reassembled into larger polymers that serve new functions.