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Cellular respiration

The cellular respiration is the set of biochemical reactions why particular organic compounds are completely degraded by oxidation to become inorganic substances, a process that provides useful energy by the cell (mainly in the form of ATP ).

As seen in the introduction to the topic, there are two types of cellular respiration:

  • Aerobic respiration. The final electron acceptor is molecular oxygen, which is reduced to water. It is carried out by the vast majority of living beings, and they are called aerobic organisms.
  • Anaerobic respiration. The final electron acceptor is an inorganic molecule other than oxygen. It is a very common type of metabolism in many microorganisms, especially prokaryotesIt should not be confused with fermentation, also an anaerobic process, but in which nothing like an electron transport chain is involved.

We will focus on aerobic cellular respiration, by which the pyruvic acid obtained in glycolysis is completely oxidized to CO2 and H2O, in the presence of oxygen. Cellular respiration occurs in two successive stages:

  • The Krebs cycle : It takes place in the mitochondrial matrix, which contains all the enzymes, water, phosphates and other molecules necessary for respiration. It consists of eight chemical reactions in which, from acetyl-coenzyme A from oxidation reactions of different molecules (especially pyruvic acid, fatty acids and amino acids), ATP, CO2 and  other molecules with reducing power are obtained ( NADH and FADH2).
  • The electron transport chain: the molecules with reducing power obtained in the Krebs cycleNADH and FADH2, enter the electron transport chain or respiratory chain, where the electrons go from a reduced molecule to an oxidized one until they reach to the final acceptor, which is oxygen, which will be reduced to form water. The energy obtained in this process, called oxidative phosphorylation, is invested in the synthesis of ATP and is explained by Mitchell's chemosmotic hypothesis. For every NADH that enters the chain, three ATP, and for each FADH2  two ATPs.


FADH 2 → 2 ATP

The respiratory chain occurs in the mitochondrial ridges, where specific enzymes are found, which are grouped in such a way as to facilitate energetic coupling and electron transfer. For this process, the presence of oxygen in the cell is essential .  

In aerobic prokaryotic cells, cellular respiration occurs in the cytoplasm and mesosomes .

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon.  September 2017, option A, question 2 .

Differences: (2.5 points)

d) Indicate three differences between internal respiration and photosynthesis . (0.75 points)

Aragon. June 2004, option A. Question 4 .- (2 points)

Indicate in which organelles and in which structures the following metabolic processes take place and give a brief overview of the processes indicated in sections c and d.

a) glycolysis

b) Krebs cycle

c) photosynthetic electronic transport chain

d) respiratory chain and phosphorylates tion oxidative

e) CO2 fixation

La Rioja, June 2021, question 9

Reasonably indicate whether cellular respiration is an anabolic or catabolic process. Briefly describe each of its stages. State where each of these stages occurs in the cell and organelle. In each of the stages, list the substrates that enter and the products that are obtained.

La Rioja, July 2021, question 10

List the stages (phases) of cellular respiration. In each of them, name the substrates and the resulting products. Point out where in the cell each stage occurs.