Cellular respiration produces energy from food molecules and can be aerobic or anaerobic. Mitochondria are organelles where aerobic respiration occurs, starting with glycolysis outside the mitochondrion. The Krebs cycle and electron transport chain then produce ATP. The process creates waste products of water and carbon dioxide, which play a role in maintaining blood pH.
Cellular respiration is the process in which food molecules are used to produce cellular energy. It can be aerobic, where oxygen is present, or anaerobic, where oxygen is absent and a sugar such as glucose is needed to fuel the process. Aerobic cellular respiration typically occurs in eukaryotic cells, the cells found in plants and animals. The metabolic processes involved take place in tiny structures within the cell known as mitochondria. Starting with glucose and continuing through a series of chemical reactions, cellular respiration enables the production of a form of biochemical energy called adenosine triphosphate (ATP).
Mitochondria, the tiny cellular organs, or organelles, where aerobic cellular respiration takes place, are found within nearly all eukaryotic cells. Cells that have higher energy requirements, such as brain cells, contain higher numbers of mitochondria. Before cellular respiration can occur, an initial phase, known as glycolysis, takes place outside the mitochondrion in the cell cytoplasm. The cytoplasm is a gelatinous substance that fills the cell and in which organelles such as the mitochondria are found.
Glycolysis is a metabolic reaction in which glucose is broken down, forming two molecules of pyruvic acid and two molecules of reduced nicotinamide adenine dinucleotide (NADH). This process is the initial stage that occurs in cells prior to anaerobic or aerobic cellular respiration. Glycolysis does not require oxygen, and while the process uses two ATP molecules, it creates four, resulting in a net gain of two ATP molecules. The pyruvic acid and NADH then enter the mitochondrion, where the pyruvic acid is converted into a substance called acetyl CoA. Energy is required to transport NADH into the mitochondrion, and this results in the loss of two ATP.
Two phases of aerobic cellular respiration then occur, known as the Krebs cycle, or citric acid cycle, and the electron transport chain. Acetyl CoA enters the Krebs cycle which produces reduced flavin adenine dinucleotide (FADH2) and NADH, along with ATP. FADH2 and NADH then carry the electrons up the electron transport chain, where they are oxidized and more ATP is created. Overall, taking into account the initial loss of two ATP, the reactions that take place within the mitochondria produce 36 ATP molecules.
Water and carbon dioxide are the waste products of aerobic cellular respiration. Carbon dioxide combines with water to create carbonic acid, making blood more acidic. This plays an important role in maintaining blood pH. Breathing continually removes carbon dioxide from the body, preventing the blood from becoming too acidic.
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