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Cellular respiration

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Cellular Respiration involves 3 stages: Glycolysis, the Citric Acid Cycle, and the Electron Transport Chain.
For the respiration better known as breathing, see respiratory system.

Cellular respiration is the metabolic process that occurs largely in the mitochondria of eukaryotes, whereby cells obtain energy from organic molecules. The process requires oxygen, and releases carbon dioxide] which are transported between the lungs and cells by the blood.

Certain organic molecules, such as carbohydrates and fats, are formed to store chemical potential energy. Cellular respiration is the reaction where a cell breaks down these molecules to obtain the energy required for cellular work, such as performing other chemical reactions or moving muscle. It involves the transfer of the chemical potential energy by the breakdown (catabolism) of the sugar molecules and the subsequent assemblage (anabolism) of another molecular called Adenosine Triphosphate (ATP). Once formed, ATP is then used directly to supply energy for cellular functions.

In a broad sense, the process is a controlled burn. It is the oxidation of organic material similar to how carbohydrates (wood) are consumed in a fire to release thermal energy. In cellular respiration, this oxidation process is controlled so that the energy is captured, and used to drive the other processes that go on inside the cell.

During these reactions oxygen is typically consumed, however, the process may be anaerobic (without oxygen) or aerobic (uses oxygen), depending on availability.

Metabolic pathway


The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Glycolysis occurs in the cytoplasm outside the mitochondria. During glycolysis, glucose is broken down into a molecule called pyruvate. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). However, only four ATP molecules can be made from one molecule of glucose in this pathway. In prokaryotes, glycolysis is the only method used for converting energy.[1]

During this stage the organism takes molecules of glucose, a six-carbon sugar, and splits it creating 2 molecules of pyruvate. Glycolysis is the first step in cellular respiration. Although it is not the final step of cellular respiration it still produces some of its own energy in the form of ATP. This can be shown by the following chemical equation.

Glucose + 2 ATP + 2 NAD+ + 2 Pi + 4 ADP → 2 pyruvate + 2 ADP + 2 NADH + 4 ATP + 2 H2O + 4 H+[2]

During this process only glucose is needed. Oxygen plays no role in the splitting of glucose. When glycolysis is finished with its operation the pyruvate created are set directly to the Mitochondria where the rest of cellular respiration takes place.(Purves 128-129)

Citric acid cycle

Citric acid cycle.png

The second pathway, called the Citric acid cycle (or Kreb's cycle), occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions. Once again, the cycle begins with a glucose molecule, which during the process of glycolysis is stripped of some of its hydrogen atoms, transforming the glucose into two molecules of pyruvic acid. Next, pyruvic acid is altered by the removal of a carbon and two oxygen atoms, which go on to form carbon dioxide. When the carbon dioxide is removed, energy is given off, and a molecule called NAD+ is converted into the higher energy form, NADH. Another molecule, coenzyme A (CoA), then attaches to the remaining acetyl unit, forming acetyl CoA.

Acetyl CoA enters the citric acid cycle by joining to a four-carbon molecule called oxaloacetate. Once the two molecules are joined, they make a six-carbon molecule called citric acid. Citric acid is then broken down and modified in a stepwise fashion. As this happens, hydrogen ions and carbon molecules are released. The carbon molecules are used to make more carbon dioxide. The hydrogen ions are picked up by NAD and another molecule called flavin-adenine dinucleotide (FAD). Eventually, the process produces the four-carbon oxaloacetate again, ending up where it started off. All in all, the citric acid cycle is capable of generating from 24 to 28 ATP molecules from one molecule of glucose converted to pyruvate. Therefore, it is easy to see how much more energy we can get from a molecule of glucose if our mitochondria are working properly and if we have oxygen.[3]

Electron transport chain

Aerobic respiration

Cellular respiration flowchart.png

Cellular respiration is an aerobic operation. This means that it requires oxygen to be performed. During this process the pyruvate from glycolysis goes through the Citric Acid Cycle. Before the pyruvate reaches the cycle it is diffused into the mitochondria, which is where cellular respiration takes place, and is oxidized into acetyl CoA (coenzyme A). The Citric Acid Cycle has eight major stages that it goes through.[4]

  1. The two-carbon acetyl group binds with the four-carbon sugar oxaloacetate. The result from this is a six-carbon sugar citrate. The CoA molecule from the acetyl is released, and reused.
  2. Citrate is then rearranged into its isomer counterpart Isocitrate.
  3. Isocitrate is oxidized and loses CO2 and yields a NADH + H+ transforming from Isocitrate to ά-Ketoglutarate which is a five-carbon molecule.
  4. ά-Ketoglutarate is oxidized making it a four-carbon molecule. In the process it loses a CO2 molecule releasing energy that is stored in a NADH + H+ this changes the ά-Ketogutrate into succinyl CoA.
  5. Succinyl CoA is then releases the CoA becoming just succinate. During this transformation releases energy, which causes surrounding GDP to GTP, which in turn converts ADP to ATP.
  6. Next succinate is oxidized to fumarate a four-carbon molecule. During the oxidation it loses to H atoms, which are transferred to the carrier enzyme FAD, which is then converted to FADH2.
  7. Then fumarate reacts with water in the cycle creating malate.
  8. Malate is then oxidized releasing a H+ that creates with NAD+, which creates a NADH+ H+. Due to the oxidation the malate is changed to Oxaloacetate, which reacts with a new incoming acetyl CoA starting the cycle all over again(Purves 130-133).

During this process cellular respiration yields 36 ATP molecules each time it runs through.[5]

This whole process is done in the mitochondria matrix. One of the major roles of the Mitochondria is to produce energy, generally in the form of ATP. Glycolysis takes place in the cytosol but the main reactions of cellular respiration are done in the Mitochondria. This organelle was specifically designed to accommodate this reaction.[6]


Enzyme inhibitors are very important in the process of cellular respiration. It is used during glycolosis and during the citric acid cycle. An inhibitor is an enzyme that stops a reaction for continuing or slows it down so that there is not a buildup of intermediates. The main enzyme used to slow the process or even bring to a stop of cellular respirations is isocitrate dehydrogenase. A regulating process called allosteric control controls this inhibitor. It works by taking a high concentration of the product of the reaction and using it to suppress the activating enzymes of the whole reaction that were catalyzed to create the products. This process can also take the over abundance of the product and use it in other relations near by to help speed up the synthesis of those reactions. (Purves 143)

Anaerobic respiration

Main Article: Anaerobic respiration

Anaerobic respiration happens in the absence of oxygen. This process is critical. When glycolysis splits glucose into pyruvate and there is no oxygen, the pyruvate does not go into the mitochondria. It also does not go through the citric acid cycle. It goes through a process of Fermentation. Fermentation takes place in the cytoplasm of the cell. Fermentation makes a small amount of energy. Enough to keep the body running but at a much lower rate. Fermentation allows for small amounts of energy production. The ATP that is produced is not produced during the actual reaction however. Only as much ATP is made as there can be gleaned from the substrate-level phophorylation. This does not produce as much as Aerobic respiration does, but it is sustainable for life. The total yield of ATP molecules from the completion of Fermentation is 2 ATP molecules for every molecule of glucose that goes through glycolysis. There are two types of fermentation that can take place, Lactic Acid and Alcoholic Fermentation. (Purves 137-139)

Lactic acid fermentation

Glycolysis produces pyruvate, ATP and NADH + H+ in the process of breaking down glucose. During this process the NADH + H+ acts as a reducing agent and breaks down the pyruvate into lactic acid (lactate). This process takes place most frequently in microorganisms and in muscle cells. Hard workouts can cause your body to build up lactic acid because of the lack of oxygen. (Purves 139)

Alcoholic fermentation

Alcoholic fermentation takes the pyruvate from glycolysis and converts it into acetaldehyde and CO2. The molecule NAHD + H+ acts like a reducing agent and takes the acetaldehyde and turns it into ethanol. This process is the basis for the entire brewing industry. This process happens naturally most often in yeast.