Breathing is an essential process for all living organisms, as it provides the necessary energy to carry out daily life activities. This energy is derived from the oxidation of macromolecules we commonly refer to as ‘food.’ While green plants and cyanobacteria can produce their food through photosynthesis, other organisms, including animals and microbes, rely on various sources to obtain their energy. In this article, we’ll explore the interconnectedness of breathing and energy release from food, focusing on cellular respirationthe mechanism through which complex molecules are broken down to release energy within the cell. The energy released in cellular respiration is trapped in the high-energy phosphoester bonds of ATP for later use. The respiratory reaction is written as:

C6H12O6 + 6O2 —> 6CO2 + 6H2O + Energy

Plants and Animals breathe differently

The Oxygen required for respiration is taken up from the air by the animals. Plants produce most of their Oxygen by themselves during photosynthesis.

  • Plants require oxygen (O2) for respiration and release carbon dioxide (CO2) in the process.
  • Unlike animals, plants lack specialized respiratory organs, relying on stomata and lenticels for gas exchange.
  • Thin layers of living cells beneath the bark and lenticels in woody stems allow for gas exchange.
  • Each plant part handles its own gas exchange needs with minimal transport of gases between different parts. Animals have circulatory systems for the transport of gases.
  • Roots, stems, and leaves respire at much lower rates than animals, with significant gas exchange occurring during photosynthesis.
  • Plant cells employ glucose catabolism to release energy in several small steps, preserving some energy for ATP synthesis.
  • Respiration involves the utilization of oxygen, resulting in the production of carbon dioxide, water, and energy (in the form of ATP).
  • Some cells and organisms live in anaerobic conditions, where oxygen may not be available.
  • The first cells on Earth likely existed in an atmosphere without oxygen, and many present-day organisms are adapted to anaerobic conditions.
  • All living organisms retain the enzymatic machinery for glycolysis, the breakdown of glucose to pyruvic acid, even in the absence of oxygen.

The Glycolysis

  • Glycolysis is a process of partial oxidation of glucose to form pyruvic acid.
  • The term “glycolysis” originates from the Greek words “glycos” for sugar and “lysis” for splitting.
  • The scheme of glycolysis was proposed by Gustav Embden, Otto Meyerhof, and J. Parnas; therefore, the glycolytic pathway is also known as the EMP pathway.
  • Glycolysis occurs in the cytoplasm of all living organisms and is the only respiration process in anaerobic organisms.
  • Aerobic respiration (Krebs’ cycle) requires O2 supply and leads to the complete oxidation of glucose to CO2 and H2 Fermentation is an anaerobic process.
  • The glycolytic pathway is shown in the following flow chart.
  • Fructose-1,6-bisphosphate is broken down into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3P). DHAP and G3P are isomers and interconverted into each other.
  • The pathway proceeds via G3P.
  • For each molecule of glucose entering the glycolytic pathway, two ATPs are used in two different steps.
  • In the second half of the pathway, two G3P molecules are converted into the final product (pyruvate).
  • For two molecules of G3P, 2 NADH and four ATPs are produced.
  • Thus, one molecule of glucose yields two pyruvates, two NADHs, and two ATPs.
  • Each NADH molecule later yields three ATPs.
  • The net reaction can be written as:

Glucose (6 carbons) + 2 NAD+ + 2 ATP + 4 ADP + 2 Pi  —> 2 Pyruvate (3 carbons) + 2 NADH + 2 H+ + 4 ATP + 2 H2O

  • Thus, a net of 2 ATPs and 2 NADHs were produced.
  • Later, NADHs are transferred to mitochondria at the expense of two ATPs.
Glycolysis: first stage of sugar breakdown

The Fates of Pyruvic Acid

  • The Pyruvic acid produced in the glycolysis of glucose can enter one of the three pathways.
  • Lactic acid fermentation: Pyruvate is converted into lactic acid in the muscle cells. It is then transported into the liver where it is converted back into glucose (Cori cycle).
  • Alcoholic fermentation: It is driven by yeast, converts Pyruvic Acid into acetaldehyde through pyruvate decarboxylase and further transforms it into ethanol via alcohol dehydrogenase.
  • Notably, this process has a maximum ethanol production capacity of 13%. Beyond this threshold, the yeast’s ability to produce ethanol is self-limiting. Both processes are NADH-consuming.
  • Aerobic respiration: pyruvic acid is converted into acetyl CoA by the enzyme pyruvate dehydrogenase complex. The Acetyl CoA then enters into the TCA cycle. It produces energy-rich ATP and NADH.
  • All these three pathways are depicted in the following flow chart.
Fate of Pyruvic Acid

Kreb’s Cycle/Citric Acid Cycle/Tricarboxylic Acid (TCA) Cycle

  • The Kreb’s cycle is named after its discoverer, Sir Hans Krebs, in 1937.
  • The name Citric Acid Cycle is derived from the first compound involved in the cycle, citric acid.
  • The name TCA cycle stands for citric acid which is a Tricarboxylic Acid.
  • Site: Mitochondria
  • The cycle takes place as follows:
TCA Cycle: breakdown of glycolytic product
  • For one molecule of Acetyl CoA entering the TCA cycle the following molecules are produced.
  • 2 CO2, 1 ATP (from substrate-level phosphorylation of GTP), 3 NADH, and 1 FADH2
  • The overall reaction can be written as:

1 Pyruvate + 4 NAD+ + 1 FAD+ + ADP + Pi + 2 H2O —> 3 CO2 + 4NADH + 4 H+ + FADH2 + ATP

  • In this cycle, carbon entered as acetyl CoA is released as CO2 in two steps of decarboxylation (isocitrate à α-ketoglutarate à succinyl CoA).
  • For the complete oxidation of one molecule of glucose, 3 TCA cycles are required.

The Electron Transport System (ETS) and ATP Synthesis in Cellular Respiration

  • NADH and FADH2 are produced in the sugar oxidation (glycolysis and TCA cycle) used to synthesize ATPs.
  • They transfer their electrons to oxygen via ETS.
  • The ETS is located in the inner mitochondrial membrane.
  • The flow of electrons through the Electron Transport System (ETS) leads to the buildup of protons in the peri-mitochondrial space, establishing an electrochemical gradient across the inner mitochondrial membrane.
  • In the ETC, electrons pass through four protein complexes (complex I – IV).
  • These protein complexes are:
    • Complex I (NADH dehydrogenase or NADH-ubiquinone oxidoreductase): NADH generated during the TCA cycle is oxidized by complex I and electrons are transferred to ubiquinone within the inner membrane. For each pair of electrons transferred from NADH via complex I, four protons are also transferred from the matrix to the peri-mitochondrial space.
    • Complex II (succinate dehydrogenase): It is involved in both the TCA cycle and the ETC. It accepts electrons from FADH2, which is generated during the oxidation of succinate to fumarate and transfers them to ubiquinone.
    • Complex III (cytochrome bc1 complex or ubiquinol-cytochrome c oxidoreductase): It receives electrons from reduced ubiquinone also called ubiquinol and passes them to cytochrome c (Cyt c). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for the transfer of electrons between complex III and IV. It also transfers four protons from the matrix to the peri-mitochondrial space.
    • Complex IV (cytochrome c oxidase complex): It contains cytochromes-a and a3 and two copper centres. It transfers electrons to oxygen, forming water (H2O). As the electrons are finally transferred to Oxygen, the entire process is called aerobic respiration. It also transfers two protons into the peri-mitochondrial space.
    • Complex V (F0-F1 complex/ ATP synthase): It does not pass electrons but utilizes the proton gradient to produce ATP from ADP and Pi. Protons flow through F0, down their concentration gradient. For each pair of protons passed through F0, one molecule of ATP is synthesized by F1.
ETC: Schematic representation of electron flow in the inner mitochondrial membrane (Aqua arrows show electron flow, and broken red arrows represent proton flow)

Respiratory balance and net worth of glucose

  • The respiratory balance sheet summarizes the overall result of cellular respiration, indicating the products and reactants involved. Cellular respiration is the process by which cells generate ATP (adenosine triphosphate), the primary energy currency of the cell, through the breakdown of glucose and other organic molecules. The balanced equation for aerobic respiration is:

                                         C6H12O6 + 6O2 —> 6CO2 + 6H2O + ATP

  • During aerobic respiration of glucose, NADH and FADH2 are generated as reduced molecules. These electron carriers transfer their electrons to oxygen, leading to the synthesis of ATP. Each NADH molecule, when transferring its electrons through the Electron Transport Chain (ETC), produces 3 ATP molecules, while one FADH2 molecule generates 2 ATP molecules.
ATP Yield from Complete Glucose Oxidation
  • Thus, a total of 36 ATP is produced from the complete oxidation of one glucose molecule.

Amphibolic pathway

  • Glucose is the preferred substrate for respiration, and other carbohydrates are converted into glucose before entering the respiratory pathway.
  • Fats are broken down into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA which enters into the respiratory pathway. Glycerol enters the pathway after being converted to 3-phosphoglycerate (3-PG).
  • Proteins are degraded by proteases, and individual amino acids, after deamination, may enter the pathway at different stages within the Kreb cycle or as pyruvate or acetyl CoA.
  • The respiratory pathway is also involved in anabolic processes, as the same compounds used as substrates in respiration may be withdrawn from the pathway for synthesizing other compounds. Therefore, the respiratory pathway is described as an amphibolic pathway.
Amphibolic pathway, an interlinking of metabolic pathways

Substrate-level phosphorylation (SLP)

Substrate-level phosphorylation (SLP) is a process in which a phosphate group is transferred from high-energy molecules to ADP, converting it into ATP. In glycolysis, ADP gets phosphorylated through the transfer of phosphate groups from 1,3-bisphosphoglycerate (1,3-BPG to 3-PG) and from 2-phosphoenol pyruvate (2-PEP to pyruvate). Another SLP event occurs in the Krebs cycle during the conversion of Succinyl CoA to Succinate.

In aerobic respiration of glucose, there are two stages in glycolysis (1,3-BPG to 3-PG and 2-PEP to pyruvate) that produce 4 ATP molecules, and during the Kreb’s cycle, 2 ATP molecules are produced through SLP for each glucose molecule.


During cellular respiration, glucose and oxygen are consumed as substrates. Through a series of metabolic pathways, they are broken down, releasing energy. Carbon dioxide and water are produced as by-products. The released energy is captured in the form of ATP, which serves as the primary energy source for cellular activities.

                                  C6H12O6 + 6O2 –> 6CO2 + 6H2O + ATP

It is important to note that the actual yield of ATP may vary slightly depending on the cell type, the efficiency of the cellular respiration process, and other factors. The above equation represents the balanced overall reaction, but the actual process is more complex and occurs in multiple stages, including glycolysis, the citric acid cycle, and oxidative phosphorylation.


  • Nelson, D. L., & Cox, M. M. (2012). Lehninger principles of biochemistry (6th ed.). New York, NY: W.H. Freeman.
  • Karp, G., Iwasa, J., & Marshall, W. (2015). Karp’s Cell and Molecular Biology: Concepts and Experiments (8th ed.). Wiley.
  • Ferrier, D. R. (2013). Biochemistry (6th ed.). Philadelphia, PA: Lippincott Williams and Wilkins.
  • Biology: Text book for class xi. (2006).

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