Aerobic and anaerobic respiration

Aerobic and anaerobic respiration are types of cellular respiration, that is, ways that some cells have to obtain energy from what they consume. They differ in that to perform aerobic respiration oxygen is required, while in anaerobic respiration oxygen is absent.

Also, aerobic respiration produces more energy compared to anaerobic respiration; however, it has more chemical steps, so it takes longer to complete.

Aerobic and anaerobic respiration share in their first phase glycolysis or glycolysis, a chain of chemical reactions that transform glucose into smaller molecules.

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Breathing aerobic	Breathing anaerobic
Definition	Cellular process of transformation of glucose to obtain energy with the participation of oxygen.	Process of obtaining energy that some cells use in the absence of oxygen.
Electron acceptor	Oxygen	Nitrate Sulfate Carbon dioxide Metal ions
Stages or phases	Glycolysis Citric acid cycle Oxidative phosphorylation	Glycolysis Fermentation: a) lactic b) ethanolic
Products	Water Carbon dioxide 32 ATP / glucose	Lactic acid Ethyl alcohol Methane 2 ATP / glucose
Examples	Liver cells Kidney cells	Bread yeasts Erythrocytes or red blood cells Bacteria Arches

Aerobic respiration

Aerobic cellular respiration is the process that transforms the chemical energy of food into ATP in the presence of oxygen. Oxygen receives electrons at the end of a chain of chemical reactions, generating water, carbon dioxide, and energy.

First phase of aerobic respiration: glycolysis

The first step in aerobic respiration is the breakdown of glucose or glycolysis. This occurs in the cytoplasm of cells. As a result of glycolysis, two ATP and two electrons are obtained in the form of NADH and two pyruvate molecules:

Second phase of aerobic respiration: citric acid cycle

The second step of aerobic respiration is the citric acid cycle or Krebs cycle. This is a series of eight chemical reactions that take place in the mitochondria of eukaryotic cells.

A pyruvate from glycolysis enters the cycle and results in three NADH, three carbon dioxide, one GTP and one FADH₂:

Third phase of aerobic respiration: oxidative phosphorylation

The third and final step of aerobic respiration is the process of oxidative phosphorylation. This process takes place in the electron transport chain, a set of proteins in the membrane of the mitochondria that transfer electrons from NADH from the Krebs.

The end result of aerobic respiration is 32 ATP per glucose molecule:

scheme of ATP production in the mitochondria by oxidative phosphorylation

Anaerobic respiration

Anaerobic cellular respiration is the way prokaryotic cells and some eukaryotic cells obtain energy from glucose, without the need for oxygen. It takes place in the cytoplasm of the cell.

Anaerobic respiration is used for the production of energy in cells that do not have mitochondria, such as bacteria, archaea, and red blood cells. In rapid muscle contraction, muscle cells can resort to anaerobic respiration, producing lactic acid.

First phase of anaerobic respiration: glycolysis

Anaerobic respiration begins with glycolysis, the process of glucose breakdown, as in aerobic respiration. In this step two energetic molecules or ATP are produced.

Second phase of anaerobic respiration: fermentation

The next step can be fermentation, of which there are two types:

Lactic fermentation: where pyruvate is transformed into lactate, as it happens in yoghurt bacteria.
Ethanolic fermentation: in this case, pyruvate gives rise to ethanol and carbon dioxide, a process carried out by the yeasts in wine and beer.

Some animals can switch to anaerobic respiration, such as carp or goldfish. When the surface of lakes freezes in the winter, the oxygen in the water decreases. These fish, which normally have aerobic respiration, can survive thanks to their ability to maintain anaerobic respiration.