As explained at Sparknotes.com, respiration is the process of organisms burning food to produce energy. There are two kinds of cellular respiration--aerobic and anaerobic. In aerobic respiration, glucose is broken down into two pyruvate molecules--with each molecule made of three carbons-- to form energy molecules of adenosine triphosphate (ATP). ATP is a nucleic acid that has three phosphate groups. As burning coal produces heat and energy in the form of electricity, ATP is like electricity. In anaerobic respiration, glucose in animals is broken down into two pyruvate molecules, but converts pyruvate to lactic acid.
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Both aerobic and anaerobic respiration occur in the cytoplasm of a cell. Aerobic respiration occurs in the presence of oxygen, in which glucose and oxygen produces energy (ATP), carbon dioxide and water. Anaerobic respiration occurs without oxygen. During anaerobic respiration in humans, the Royal Society of Chemistry (England) explains, glucose is broken down to produce lactic acid and energy.
Aerobic Cell Functions
According to rsc.org, there are four stages of aerobic respiration: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. Sparknotes.com lists three stages: glycolysis, the Krebs cycle and the Electron Transport Chain (oxidative phosphorylation).
Glycolysis occurs without oxygen in the cytoplasm of a cell, and two molecules of ATP are "invested" to start the glycolysis process. ATP splits glucose into two compounds called pyruvates. Other end products are two NADH molecules (coenzymes) and four ATP. Thus, the net gain of ATP is two. The two NADH molecules travel to the mitochondria of a cell, later creating more ATP.
In the link reaction, pyruvate sugars are transported to the mitochondria. During this transport, pyruvate is converted into the two-carbon molecule called acetate. "The extra carbon from the pyruvate is released as carbon dioxide, producing another NADH molecule that heads off to the electron transport chain to help create more ATP. The acetate attaches to a coenzyme called coenzyme A to form the compound acetyl-CoA." This compound is needed to start the Krebs cycle.
Occurring in the mitochondrial matrix, the Krebs cycle begins when acetyl-CoA and oxaloacetate interact to form a six-carbon compound citric acid--which is why it is also sometimes called the citric acid cycle. Energy is stored in ATP, NADH, and FADH2. The Krebs cycle sends NADH and FADH2 on to the electron transport chain.
According to sparknotes.com, the electron transport chain occurs in the inner membrane of the mitochondria--powered by FADH2 and NADH--in a process called oxidative phosphorylation. Hydrogen ions flow back into the matrix through a membrane protein called an ATP synthase, producing 34 ATP molecules. Staff.jccc.net explains that the electron transport chain produces 32 ATP.
Anaerobic Cell Functions
In anaerobic respiration, energy is not released at one time. It is released gradually with much of it stored in chemical form as opposed to being released as heat.
Even though anaerobic respiration occurs in the absence of oxygen, the glycolysis stage--which does not require oxygen--still requires NAD+. In anaerobic respiration, cells must turn NADH back to NAD+ through a process called "fermentation." The goal of fermentation is not to produce additional energy, but to enable pyruvates to replenish NAD+ so that glycolysis can continue to slowly produce ATP. After glycolysis occurs, pyruvate is converted to a three-carbon compound called lactic acid. Anaerboic respiration only produces two ATP in glycolysis.
Though the Krebs cycle does not directly require oxygen, it can only take place when oxygen is present because it relies on byproducts from the electron transport chain, which requires oxygen. The Krebs cycle is therefore an aerobic process which produces two ATP.
In the electron transport chain, waste products combine with oxygen to produce water molecules. By accepting these waste products, oxygen frees NAD+ and FAD to play their roles in the Krebs cycle and the electron transport chain. Without oxygen, these vital energy-carrier molecules would not perform their roles and the processes of aerobic respiration could not occur.
Anaerobic respiration takes place in humans when muscles undergo contraction during exercise. Muscle contraction requires energy. This energy is normally released from glucose through aerobic respiration; however in vigorous exercise, cells do not contain enough oxygen. They must obtain energy from glucose in anaerobic respiration. An "incomplete breakdown of glucose" occurs in anaerobic respiration and less energy is released than during aerobic respiration, so the "resulting oxygen debt has to be repaid when exercise stops." Because lactic acid--produced in anaerobic respiration--is a toxic substance, its build-up in the muscles produces fatigue and soreness.
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