Chapter 19: Cellular Respiration
Although most bakers don’t realize it, they work with explosive material. Flour is highly flammable and under appropriate conditions flour dust can explode. On several occasions flour mills have exploded, perhaps the most famous being near Minneapolis in 1878 when a recently built mill, one that at the time was the largest in the world, was totally destroyed and 18 workers killed (Fig. 1). More recently, in 2008, a sugar mill in Georgia exploded, killing 14 and injuring 40 more. These examples demonstrate that there is energy present in carbohydrates, chemicals with the general formula of CH2O, i.e., a ‘hydrated’ carbon. Often the carbohydrates are polymers of six carbon sugars, molecules with a formula of C 6H12O6. Cane sugar, what exploded in the Georgia mill, is made up of two such ‘hexose’ (six-carbon) sugars, glucose and fructose, bound together. Starch, the main component of flour, is made up of long chains of glucose molecules bound together. Glucose, fructose and starch are all carbohydrates and like all molecules they ‘contain’ energy. If carbohydrates react with oxygen to form carbon dioxide and water, energy is released.
The energy of carbohydrates and its release when interacting with oxygen is central to the biology of most organisms. And understanding how the energy is obtained and utilized is significant not only because the energy released is essential for the functioning of organisms but also because it represents a unifying feature of all living things, every living organism carries out this process, or part of this process, or something similar to this process.
Organisms need energy for growth, maintenance, and for the performance of work such as the motion of the whole organism, e.g., swimming, or internal motion, e.g., pumping materials within the organism or moving materials within a cell, that are essential for the organism’s livelihood. Many, but certainly not all, of these energy-requiring processes ‘run’ on energy ‘supplied by’ adenosine triphosphate, ATP, and most of an organism’s supply of ATP is provided by cellular respiration, a process that synthesizes ATP while carrying out a chemical reaction that utilizes carbohydrates. Exactly how ATP participates in metabolism varies and its action is often not direct and obvious in the way that the energy of falling water allows a mill to ‘do work.’ The action of ATP often involves ‘coupling’ different chemical reactions (examples below) with the consequence being that the participation of ATP makes unlikely events more likely to happen and/or events that occur slowly more likely to proceed rapidly. ATP’s participation in cellular activities causes the molecule to lose one or two of its three phosphate groups, forming either adenosine diphosphate (ADP) or adenosine monophosphate (AMP). Obviously, the regeneration of ATP is significant to an organism’s functioning and for most organisms, this regeneration is the result of a group of reactions d