In this article, I will discuss methods for the generation of energy in the body that are used by the various body tissues. These methods include aerobic respiration and anaerobic respiration in addition to the generation of energy in muscle tissue which is different than the other methods of generating energy in the body.
The energy currency in the body is the ATP molecule except in the muscle tissue in which it is creatine phosphate that is the energy currency. ATP is an energy rich molecule due to the existence of phosphoanhydride bonds in its molecular structure. ATP is composed of one DNA base called adenosine and three phosphate groups connected to the DNA base.
The hydrolysis of the ATP molecule to generate ADP and a phosphate group releases energy that is used or is consumed in other biochemical reactions which otherwise do not proceed without the ATP molecule. ATP stands for adenosine triphosphate.
The first method or mechanism to generate energy and which is used by the body tissues is the process in the muscle cells which is entirely different than the mechanisms of energy generation in all other body tissues. Instead of the molecule ATP as a source of energy the muscle tissue uses creatine phosphate molecules.
Creatine phosphate is also an energy rich molecule which has also energy rich bonds that upon hydrolysis releases energy. It is hydrolyzed to a creatine molecule and a phosphate ion. This process releases energy which is used to induce or drive reactions to completion.
Muscle cells usually normally produce excess amount of ATP molecules which are used to generate creatine phosphate. For this purpose an enzyme that is called creatine kinase usually phophorylates a creatine molecule forming creatine phosphate. Creatine is a molecule which is synthesized in the various tissues of the body such as the liver and the kidney.
Oxygen is abundant in muscle tissue which is bound there to the protein myoglobin. This is in contrast to hemoglobin which is the oxygen carrier in the other tissues of the body. Creatine molecule is converted in the body to the molecule creatinine. This molecule has clinical significance in which it functions as a marker for kidney function. Increased amount of creatinine in the blood signifies a kidney dysfunction or renal failure.
In other tissues in the body the glucose molecule is used as the source to generate energy rich molecules in addition to the process of oxidation of fatty acids which occurs in the liver and produce much more energy than the process of glucose oxidation. Fatty acids are obtained from triglycerides using the enzyme lipase that is usually found in the pancreatic secretions in the small intestine.
Glucose molecules are used by the various body tissues to generate ATP molecules which in turn are used to drive chemical reactions in the body to completion such as ions transport across the cellular membrane. Glucose is used in a process which does not use or require oxygen to proceed. This process is called glycolysis.
Glycolysis is a general mechanism that is used by many cells that possess mitochondria. The end product of the glycolytic process is the molecule pyruvic acid. The net result of glycolysis is the conversion of glucose to pyruvic acid with the production of few ATP molecules.
Pyruvic acid then can undergo one of two processes which can utilize oxygen or not. The process which does not use or consume oxygen is called anaerobic respiration. In this process pyruvic acid is converted to lactic acid causing metabolic acidosis with increased pulmonary respiration rate. This process is inefficient at producing ATP molecules. This process occurs often in strenuous excercise.
Glucose enter the muscle fibers through a process which is called cotransport in which its transport is coupled to the movement of sodium ions along its concentration gradient on the cellular membrane. In addition to this muscle fibers can obtain glucose from the degradation of the glycogen polymer.
Anaerobic respiration can provide energy for a short period of time only. In aerobic respiration pyruvic acid is consumed in a process which utilizes oxygen and which is called krebs cycle and oxidative phosphorylation. In these processes many ATP molecules are produced than by the process of anaerobic respiration or by glycolysis.
Oxidative phosphorylation in particular is liable to generate most of the ATP molecules in the cellular respiration process. Cytochromes are used in these processes and which contain Fe-S clusters that can be oxidized and reduced in this process. Thus generating energy rich ATP molecules.
