Cellular respiration is the process the body uses to convert glucose to ATP, which can be used as energy. Anaerobic respiration does not require oxygen to occur. Aerobic respiration requires oxygen and most of the body's ATP is produced aerobically. There are three processes involved in cellular respiration. Each process is carried out in specific regions of the cell.
Glycolysis is anaerobic and occurs in the cytoplasm. The cytoplasm surrounds the cell's nucleus and houses the organelles. It is made of mostly water, so diffusion can take place. Glycolysis splits the 6-carbon glucose molecule into two 3-carbon pyruvic acids. The reaction requires 2 ATP to proceed, and provides a net gain of 2 ATP. The 3-carbon pyruvic acids are broken down into lactic acid (carbon dioxide and ethanol). Glycolysis requires no oxygen and makes hydrogen available. If oxygen is present, the pyruvic acid is broken down into different carbon compounds by a process called the Kreb's Cycle.
The Kreb's Cycle
The Kreb's Cycle requires oxygen and is a series of eight chemical reactions. Pyruvic acid from glycolosis enters the mitochondria by the process of diffusion. The mitochondria contain an extensive series of inner folds, providing the necessary substrate (surface area) for the reactions to occur. Pyruvic acid is broken down by enzymes and coenzymes into 2-carbon compounds. The 2-carbon derivative of pyruvic acid bonds to the 4-carbon compounds always circulating in the Kreb's Cycle. Oxygen combines with the 4-carbon compounds and releases carbon dioxide. The newly made carbon compounds are oxidized and hydrogen is released. With each complete cycle, two carbon dioxide molecules and eight hydrogen atoms are released. The carbon dioxide by-product is expelled by the lungs, and the hydrogen can be transported to the Cytochrome System, where a much larger quantity of ATP can be produced.
The Cytochrome System
The Cytochrome System, also known as the hydrogen carrier system, or electron transport system, occurs in the cristae. Cristea are tiny stalks located on the mitochondrial membrane. This system is the most productive and produces most of the ATP in the body. Utilizing the hydrogens released from glycolysis or the Kreb's Cycle, this system transforms enzymes into other enzymes that can be oxidized. During oxidation, hydrogen is made available and energy is released. The hydrogen released in this process bonds to oxygen; thus, 38 ATP are produced and water is the by-product of these reactions.