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Respiratory system in humans involves the organs responsible for breathing, i.e. exchange of gases (taking in oxygen and breathing out carbon dioxide) between the organism and the environment.

There are two basic types of respiration: external and internal. External respiration is a physical process of exchange of O2 and CO2 between the organisms and the surroundings. Internal respiration, on the other hand, is a chemical process of food oxidation that occurs within the cell. Internal respiration leads to release of energy, carbon dioxide, and water. It is also known as cellular or tissue respiration.

Respiration efficiency (in percentage) is mathematically represented as:

Respiration Efficiency = 100* Energy captured in ATP per mole of substrate oxidized/Total energy released per mole of substrate oxidized

Types of Respiration

Sachs classified cellular respiration into two major types: aerobic and anaerobic, depending on the availability of oxygen (Figure 1).

Figure 1. Types of respiration
Figure 1. Types of respiration

Aerobic respiration

Aerobic respiration (Figure 2) uses oxygen and oxidizes the organic food completely to carbon dioxide and water. The organisms which carry on this respiration are called aerobes.

C6H12O6 + 6O2 → 6CO2+ 6H2O + ATP (Energy)

Following are the important stages (shown below) of aerobic respiration.

  1.  Glycolysis,
  2.  Krebs Cycle, and
  3.  Electron Transport Chain (ETC).
Figure 2. Aerobic Respiration
Figure 2. Aerobic Respiration

Anaerobic respiration

Anaerobic respiration does not use molecular oxygen and incompletely oxidizes the organic food with or without production of carbon dioxide and it releases small amounts of energy. The organisms which carry on this respiration are called anaerobes. The common products of anaerobic respiration are carbon dioxide  ethyl alcohol, and lactic acid.

Anaerobic respiration is divided into facultative and obligate. Organisms which normally require oxygen but can live without it when grown on suitable media are called facultative anaerobes. For example, lactic acid bacteria (Figure 3). Obligate anaerobes live in the absence or negligible concentration of oxygen.

Figure 3. Bacteria Lactobacillus, gram-positive rod-shaped lactic acid bacteria which are part of normal flora of human intestine
Figure 3. Lactobacillus, gram-positive rod-shaped lactic acid bacteria which are part of normal flora of human intestine

Anaerobic respiration also occurs in the human body muscles. It happens during strenuous exercise when there is not enough oxygen reaching the cells to convert glucose into carbon dioxide and water completely.

Figure 4. The three key enzymatic steps of the dissimilatory sulfate reduction pathway, when sulfate serves as terminal electron acceptor to anaerobic respiration by sulfate-reducing microrganisms
Figure 4. The three key enzymatic steps of the dissimilatory sulfate reduction pathway