The Power Plant in the Microcosmos: The ATP Synthesis

The cell is a dynamic environment that involves constant formation, breakdown and repair of its constituents. At every second, its genetic code is read, tens of thousands of enzymes are produced from this code, and other proteins and enzymes are broken down elsewhere. In addition, cells are furnished with many complex systems for survival. These include transportation of internal cargoes from one place to another; transfer of various signals from and to the command center, development of defense mechanisms, and replication and repair of DNA. These vital functions require thousands of different enzymes in order to take place. Most enzyme reactions that occur in the cell are "energetically unfavorable," meaning that they require energy.

For that reason, these reactions are coupled to the breakdown of the ATP molecule that supplies sufficient energy for them. The ATP (adenosine triphosphate) molecule acts as the "energy currency" of all known cells. Strikingly, many enzymes can bind to this molecule, rip off a phosphate ion from ATP, and utilise the energy generated by this chemical reaction to produce force, motion, voltage, torque and other mechanical features to drive biological functions. In tliis article, I tell the story of the making of a single ATP molecule by describing the energy conversion ability given to the cell and other biological processes taking place simultaneously, which provides indisputable evidence to the complexity of lite.

The ATP required by the cell is synthesized from food sources and light. Through a set of reactions that occur in the cytoplasm, cells can derive 10% of the energy available from the partial degradation of sugars and fats. However, life demands much more than that. Highly specialized organelles – mitochondria and chloroplasts – have a more efficient way of obtaining biologically useful energy. Mitochondria, which are present in virtually all organisms (animals, fungi, plants), act as a "power plant" that uses oxygen to completely bum sugars and fats. Chloroplasts, on the other hand, are only present in plant cells; they act as "solar panels" that utilize the energy of light. This is a "clean" way of obtaining energy, so to speak, where carbon dioxide (CO₂) is used to produce oxygen and ATP. In this way, plants not only produce die energy they require, but also play an important role in the cycling of C0₂ and oxygen in the atmosphere.

The aforementioned facts of metabolism were discovered through the research and efforts of many scientists in die early twentiedi century. Aldiough diis information is already fascinating enough to force us to think about how tiny cells and organelles can achieve the production of energy and know how to make use of it for diousands of different purposes, the most fascinating phenomenon awaits us in die smaller scale. How ATP is produced in Mitochondria The story starts with the intake of sugars and fats from our daily meal, compounds that are rich in Carbon (C), Oxygen (O) and Hydrogen (H) atoms. In the cells, their hydrogen ions (H⁺) are ripped apart and the remaining carbon and oxygen are converted to CO₂ gas through sets of chemical reactions within the mitochondria (Figure 1). Most importantly, for every H⁺, one highly energetic electron is also obtained in these reactions. By carrier molecules, H⁺ ions along widi electrons are brought into contact with the electron transport chain (ETC) complex that is embedded in mitochondrial membrane (Figure I).

As shown in Figure 1, the electrons flow from one protein to another in the ETC by a finely tuned mechanism that is still obscure to the scientific world. The complex consists of four proteins, each one firmly held to the inner membrane. Each protein has a greater affinity for an electron than has the one before so that the electrons tend to "swim" from one to another. Finally, the electrons are transferred to the oxygen molecule, which has the highest affinity for electrons. This is the only place where the oxygen we inhaled from the air is used to burn our food intake in the cells, simply to take electrons out of the chain. More importantly, as the electrons are taken by a member of the complex, one H' is also taken from the carrier molecules, and released to the spacing between the inner and the outer membrane of the mitochondria, while the electron continues to travel along the ETC.