The study of bioenergetics was a start to your study of energy metabolism. Prior to focusing in on specific chemical reactions or connected sequences of multiple reactions (metabolic pathways), it is worthwhile to step back and look at the big picture, which includes context of the spatial aspects of cellular energy metabolism, along with the chemical structures of key families of molecules involved in these reactions. This is the purpose of this Topic. The reactions of muscle energy metabolism are distributed throughout the cytosol, as well as within a specialized organelle called the mitochondrion. Depending on the type of muscle, there could be few, or many mitochondria. The image to the left is an electron micrograph of numerous mitochondria between myofibrils within heart muscle (myocardium). Myocardium has a prolific distibrution of mitochondria, for as you can imagine, such muscle contracts repeatedly through our life and the erengy demands of this work are derived from the use of oxygen in mitochondrial respiration, which of course occur within mitochondria. In contrast to mitocondrial respiration, the reactions of the phosphagen and glycolytic energy systems are randomly (though some are chemically linked to structural proteins within the matrix, as well as to organelles such as the mitochondria) distributed throughout the cytosol, much closer to the contractile proteins that form the myofibrils of the sarcomeres and where the origin of ATP breakdown (hydrolysis) and demand occurs. These locations, and the distances between the contractile proteins and the mitochondria, are the playing field of cellular energy metabolism. As you will learn, the cell and in fact total function of the multiple systems of energy metabolism have adapted well to minimize the constraints imposed by the different locations of specific chemical reactions and pathways, and the barriers (mitochondrial membranes, as well as the cell membrane) to efficient (time and quantity) transfer of these molecules throughout the cell.
Recommended sequence of topics:
Atoms, electrons, charge and chemical reactions
Overview of the macronutrients
Enzyme control and metabolic design
The three energy systems