DESCRIPTION OF INTERACTIVE
Some reactions occur in cycles and so they are called coupled reactions. A coupled reaction is a pair of reactions where the products of one reaction are used as reactants for the other reaction. ATP and ADP are coupled by the addition and removal of water. In this case, though, we don’t use the term dehydration synthesis for the synthesis of ATP because one of the reactants, phosphate, isn’t a molecule but an ion. Instead, the synthesis of ATP is called phosphorylation.
Alt tag: A cyclical reaction with ATP at the top of the circle and ADP at the bottom. Above ATP is a picture of a charged battery. Below ADP is a dead battery. Clockwise from the ATP the reaction arrow shows water entering and it is noted that the reaction releases energy. Continuing clockwise from ADP the reaction arrow shows water leaving and it is noted that this reaction requires energy.
Caption: ADP and ATP are coupled by hydrolysis (on the right) and phosphorylation (on the left). The phosphate ion in this picture is written as Pi and is pronounced “inorganic phosphate” because it’s not part of a carbon molecule yet.
Phosphate groups aren’t the only functional group that can be added or removed to a molecule. Other functional groups that do this include amination/deamination (of an amino group), acetylation/deacetylation (of an acetyl group, CH3CO-), methylation/demethylation (of a methyl group, CH3-), and carboxylation/decarboxylation (of a carboxyl group).
Alt tag: The reactant is a three-carbon molecule called pyruvic acid. Pyruvic acid is made of a carboxyl group connected to a carbonyl group which is then connected to a carbon with three hydrogens. The reaction arrow shows carbon dioxide leaving. The product is a two-carbon molecule called acetaldehyde. Acetaldehyde is made of a carbonyl connect to a carbon with three hydrogens.
Caption: the decarboxylation of a molecule of pyruvic acid is an important reaction in alcoholic fermentation.
Alt tag: The reactant is a three-carbon molecule called 3-phosphoglycerate. 3-phosphoglycerate is made of an anion form of a carboxyl functional group connected to a second carbon. This second carbon is connected to a hydrogen, a hydroxyl group and the last carbon. The last carbon is connected to two hydrogens and a phosphate group. Above the reaction arrow is written “phosphoglycerate kinase”. Below the arrow is written “ATP donates phosphate group (phosphorylation)”. The product is a three-carbon molecule called 1,3-bisphosphoglycerate. 1,3-bisphosphoglycerate is made of a carboxyl group attached by its single-bonded oxygen to a red coloured phosphate group. The carboxyl group is also bonded to a second carbon which is connected to a hydrogen, a hydroxyl group and a third carbon. This last carbon is connected to two hydrogen and a phosphate group.
Caption: the phosphorylation of a molecule of 3-phosphoglycerate is an important reaction in photosynthesis.
Another pair of coupled molecules are NAD+ and NADH. The reactions that couple them are oxidation and reduction. You may have heard of these reactions before. Sometimes they’re also called redox reactions. They are common reactions in Biology and we can describe them in terms of a molecule gaining or losing hydrogen or oxygen.
Alt tag: A cyclical reaction with NAD+ at the top of the circle and NADH+H+ at the bottom. Clockwise from the NAD+ the reaction arrow shows molecule B entering and molecule BH2 leaving. It is noted that B is reduced while NADH + H+ is oxidized. Continuing clockwise from NADH + H+ the reaction arrow shows molecule AH2 entering and molecule A leaving. It is noted that AH2 is oxidized and NAD+ is reduced.