Cellular Respiration

What is Cellular Respiration

Cellular respiration is the process by which the cell obtains energy from sugars or other organic molecules, by reacting carbons and hydrogens in the presence of oxygen.

The result of cellular respiration is carbon dioxide, water and ATP ATP. Carbon dioxide is removed from the cell and ATP is the molecule that the cell uses as chemical energy to perform its functions.

Cellular respiration consists of several chemical reactions that take place in the cytoplasm and in the mitochondria. Chemical reactions are carried out by enzymes, proteins specialized in each reaction.

simple diagram of cellular respiration

Stages of cellular respiration

Cellular respiration consists of a series of reactions that can be grouped into three stages.

1. Glycolysis or glycolysis is the first step of respiration. It is produced in the cytoplasm of most cells. It consists of the conversion of a molecule of glucose, with six carbons, into two molecules of pyruvate, each with three carbons.

Glycolysis consists of ten reactions, each catalyzed by an enzyme, in which two molecules of ATP are consumed and four ATPs are produced. Additionally, two hydrogen-carrying molecules are generated, nicotinamide adenine dinucleotide (NADH), which will be used in the last step.

2. Citric Acid Cycle It is the second stage of cellular respiration that takes place in the mitochondria of eukaryotic cells. It is also known as the tricarboxylic acid cycle or Krebs cycle. This produces carbon dioxide COof them which is eliminated and molecules carrying electrons (NADH and flavin adenine dinucleotide FADHof them) that go to the next step.

The cycle consists of 8 steps, where oxaloacetate (a 4 carbon molecule) combines with acetyl (2 carbons) of acetyl-coenzyme A (acetyl-CoA) to form citrate (6 carbons).

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From citrate, carbon dioxide (1 carbon) is released until it forms oxaloacetate again and starts the cycle, as shown below:

simplified diagram of the Krebs cycle
Simplified diagram of the Krebs cycle. NADH: nicotinamide adenine dinucleotide; H+: proton; GTP: guanosine triphosphate; FADHof them: reduced flavin adenine dinucleotide; Acetyl-CoA: acetyl coenzyme A; 6C: 6-carbon compound; 5C: compound with 5 carbons; 4C: compound with 4 carbons.

3. Oxidative phosphorylation It is the last stage of cellular respiration where oxygen participates directly. Electron transporters such as NADH and flavin adenine dinucleotide (FADH)of them), deposit electrons in a sequence of proteins embedded in the inner membrane of mitochondria.

Electrons pass to oxygen molecules Oof them and combines with hydrogen H+ to produce water Hof themO. At the same time, a phosphate group is added to adenosine diphosphate ADP to form adenosine triphosphate ATP. This is called phosphorylation.

Complete oxidation of one molecule of glucose produces 36-38 molecules of ATP.

See also Krebs cycle

Types of cellular respiration

Cellular respiration processes depend on the participation of oxygen.

Aerobic respiration

In aerobic respiration, pyruvate, which has been produced by glycolysis from sugars in the cytosol, is transported to the mitochondria of eukaryotic cells. Here, pyruvate is converted into carbon dioxide, which is eliminated, and acetyl-CoA, which enters the Krebs cycle.

Anaerobic respiration

This is cellular respiration in the absence of oxygen. Anaerobic respiration begins with the transformation of glucose by glycolysis, just like aerobic respiration. However, pyruvate is then transformed into other compounds by fermentation.

Pyruvate can be converted to lactate in muscle cells or to ethanol and carbon dioxide in alcoholic fermentation.

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Anaerobic respiration produces much less energy than respiration in the presence of oxygen.

See also:


Alberts, B., Johnson, A., Lewis, J, Raff, M., Roberts, K., Walter, P. (2008) Molecular Biology of the Cell 5th ed. Garland Science. UK.
Hardin J, Bertoni G, Kleinsmith LJ (2012) Becker’s World of the cell. 8th ed. Pearson Ed. NY.