Why does cell respiration have three steps that occur in three different places?
Cell respiration is a complex metabolic process that breaks down glucose in order to release energy. It occurs in three main steps, each taking place in a different location within the cell, to efficiently extract the maximum amount of energy available from glucose.
The three steps of cell respiration are glycolysis, the Krebs cycle (also known as the citric acid cycle or the tricarboxylic acid cycle), and the electron transport chain.
1. Glycolysis: This is the first step of cell respiration and it takes place in the cytoplasm of the cell. During glycolysis, a molecule of glucose is converted into two molecules of pyruvate. This step is anaerobic, meaning it does not require oxygen. Glycolysis releases a small amount of energy in the form of ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).
2. Krebs Cycle: After glycolysis, if oxygen is present, the pyruvate molecules produced in the cytoplasm move into the mitochondria, where the Krebs cycle occurs. The Krebs cycle is a series of chemical reactions that take place in the matrix of the mitochondria. In this cycle, each pyruvate molecule is further broken down and completely oxidized, producing carbon dioxide, NADH, FADHâ‚‚ (flavin adenine dinucleotide), and a small amount of ATP. The Krebs cycle generates more energy-rich molecules, which will be used in the final step of cell respiration.
3. Electron Transport Chain (ETC): The final step of cell respiration is the electron transport chain, which occurs in the inner mitochondrial membrane. The ETC is composed of a series of proteins and molecules embedded in the membrane. The NADH and FADHâ‚‚ generated from glycolysis and the Krebs cycle donate their electrons to the ETC. As the electrons move along the chain, they release energy that is used to pump protons across the membrane, creating an electrochemical gradient. This gradient is utilized by ATP synthase to produce ATP, the energy currency of the cell. At the end of the ETC, oxygen acts as the final electron acceptor, combining with hydrogen to form water. This step is why cell respiration is considered aerobic.
The reason these steps occur in different locations is to facilitate the efficient production of ATP and extract the maximum amount of energy from glucose. Glycolysis occurs in the cytoplasm because glucose enters the cell through the cell membrane. Moving to the mitochondria for the Krebs cycle allows for the complete breakdown of glucose and the generation of energy-rich molecules. Finally, the inner mitochondrial membrane provides the necessary conditions for the electron transport chain to occur effectively and efficiently produce ATP.