Overview Of Cellular Respiration Equation, Types, Stages & Products

Cellular Respiration Equation (C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 38*ATP)
Cellular Respiration Equation: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 38*ATP

Cellular Respiration Equation: Every machine needs specific parts and fuel in order to function. Likewise, “biological machines” also require well engineered parts and good energy source in order to work. Perhaps the second most important molecule (DNA is the first) is adenosine triphosphate (also known as ATP)[1]. Basically, ATP serves as the main energy currency of the cell.

What is Cellular Respiration?

Living organisms, including plants, animals, and microorganisms, generate their own energy in a process called Cellular Respiration. Interestingly depending on the type of precursor for ATP production, organisms can be classified into two:

  • A) Organisms that utilize oxygen in the process are called as “aerobic“.
  • B) And those who do not are described as “anaerobic“.

Why is Cellular Respiration Important?

As alluded to earlier, cellular respiration (regardless whether it is aerobic or anaerobic) provides the required amount of ATP for living organisms. The energy present in the form of ATP can then be utilized to drive various intra-cellular physiological processes like the transport of molecules across cell membranes and the synthesis of bio-molecules.

Aerobic Respiration

Aerobic Respiration
Source: Wikimedia
Aerobic respiration is the type of cellular respiration that requires the presence of oxygen. Among all the types of cellular respiration it is the most efficient. Plants and animals carry out this kind of respiration; plants obtain the precursor molecules from photosynthesis while animals obtain them from the food they eat (i.e. plants/animals).

Photosynthesis Equation

The equation for photosynthesis of plant cells is:

6CO2 + 6 H2O + Sunlight → C6H12O6 + 6O2
( 6 Carbon Dioxide + 6 Water + Sunlight → Glucose + 6 Oxygen )

Cellular Respiration Equation

It is important to note that cellular respiration in general is not a single process but is in fact a set of metabolic reactions. Hence, the locations where occur in the cell vary from pathway to pathway. Its overall chemical reaction of cellular respiration equation is simplified as:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 38*ATP
( Glucose + 6 Oxygen → 6 Carbon Dioxide + 6 Water + ATP )

* Value is not constant for all aerobic organisms. May range from 34 to 38 net ATP.

The above cellular respiration formula is formulated by combining the three following processes into a single one. Such processes are explained below.

Stages of Cellular Respiration

1. Glycolysis

Cellular Respiration Pathway
Diagrammatic illustration of the Cellular Respiration Pathway (Source: Slide Share)
The first metabolic pathway during cellular respiration is glycolysis. Coming from the Greek word “glyk” which means “sweet” and “lysis” which means “dissolution“, glycolysis is the breakdown of one molecule of glucose (sugar) into two molecules of pyruvate[2].

  • As shown in the above diagram, glycolysis takes place in the cytosol.
  • Glycolysis is referred to as a “ten enzyme-catalyzed reaction” but the overall simplified equation is[5]:
    C6H12O6 + 2 NAD+ + 2 ADP + 2 P → 2 pyruvic acid, (CH3(C=O)COOH + 2 ATP + 2 NADH + 2 H+
  • While the above equation shows that glycolysis produce two ATP molecules, four molecules are actually produced during the entire process. However, two molecules are consumed during the preparatory phase, hence, resulting to a net of just two ATP molecules.
  • After glycolysis, there is a so-called “link reaction” that occurs. Such reaction is the oxidative decarboxylation of pyruvate by the Pyruvate dehydrogenase complex (PDC).
  • In simpler terms, the pyruvate from glycolysis is oxidized (converted) to acetyl coA, one molecule of NADH (nicotinamide adenine dinucleotide), and one molecule of carbon dioxide.

2. Krebs Cycle

Krebs Cycle
The Krebs Cycle (Source: Wikimedia)
Also called as the Tricarboxylic Acid (TCA) cycle, or simply the Citric Acid cycle, the Krebs cycle (identified by Hans Adolf Krebs) is an 8-step process that involves 18 different enzymes.

  • The Krebs cycle, which occurs in the matrix of the mitochondrion, includes a series of oxidation-reduction reactions that result in the oxidation of the acetyl group to two carbon dioxide molecules[3].
  • During one cycle, there is a net of 3 NADH, 1 FADH2 (flavin adenine dinucleotide), and GTP (guanosine triphosphate, may be alternatively used to produce ATP).
  • Hence, from one glucose molecule (that formed 2 pyruvate), a total of 6 NADH, 2 FADH2 and 2 ATP molecules are produced.
  • Note that the goal of the Krebs cycle is to generate high energy electrons from carbon sources. Also notice that the process itself does not generate huge amounts of ATP and does not use oxygen as a precursor molecule.
  • Instead it uses the electrons from acetyl coA to form NADH and FADH2.

3. Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain
Electron Transport Chain (Source: Wikimedia)
The final pathway in the cellular respiration (PDF) is comprised of the electron transport chain and oxidative phosphorylation which both occur in the inner membrane of the mitochondrion.

  • This former, which is a part of the latter, establishes the chemiosmotic gradient (proton gradient) across the inner membrane of the mitochondrion by oxidizing the NADH from the Krebs cycle whereas the latter manages the pathway in which the electrons from the donors are transferred to the acceptors in redox reactions.
  • In ETC, electrons are transferred from one complex to next where the electrons reduce oxygen to produce water. Such reactions produce the majority of ATP during cellular respiration.
  • Overall ETC produces water, NAD and FAD (which are both recycled back to glycolysis and Krebs cycle), and up to 34 ATP per one molecule of glucose!

In total the resulting product of aerobic cellular respiration from a single glucose molecule can be up to 38 ATP. However some organisms can only produce 34 to 36 because they have a different precursor molecule.

What is the role of Oxygen in the process?

Oxygen is an essential molecule in cellular respiration. But where does it exactly fit in the picture? Basically, oxygen can be found at the end of the ETC (during aerobic respiration) where it accepts electrons while picking up protons in order to produce water molecules[4].

  • Because of this, oxygen is also called as the “final electron acceptor”. When oxygen levels are depleted, electrons will be simply dispersed and the electron transport chain will discontinue.
  • Of course no ATP will be produced, causing the cease of some physiological functions in the cell.

Fermentation

Lactic Acid Fermentation
Lactic Acid Fermentation (Source: Wikimedia)
Alternatively during a depletion of oxygen, the cell may undergo a process called fermentation and utilize an alternative pathway at the end of glycolysis. And instead of oxidative phosphorylation, it uses substrate level phosphorylation that does not require oxygen in the process (this is not to be confused with anaerobic respiration).

  • In animal cells, this process is called the lactic acid fermentation. It is almost the same with aerobic respiration except that it produces lactic acid in the process. It can be simplified in the equation:
    C6H12O6 → 2 CH3CH(OH)COOH + 2 CO2 + 2 ATP
  • On the other hand, microorganisms like yeast produce ethanol and carbon dioxide. Such process is referred to as the ethanol or alcohol fermentation.
    C6H12O6 → 2 C2H5OH + 2 CO2 + 2 ATP
  • In both types of fermentation process, only 2 ATP are produced from a glucose molecule.

Anaerobic Respiration

Anaerobic Respiration
Anaeribic Respiration (Source: Wikimedia)
This process occurs just like the typical cellular reaction (same glycolytic and Krebs cycle pathway) but only differs because it is used by organisms like bacteria and archaea where oxygen is not the final electron acceptor. Rather, these organisms use sulfates or nitrates instead.

  • It is important to note that while both fermentation and anaerobic happen in the absence of oxygen, the former is only an alternative and extends glycolysis to produce energy whereas the latter uses other molecules to complete the cycle as the organism will die in the presence of oxygen.
  • Unlike aerobic respiration that occurs in the mitochondria, aerobic respiration happens in the cytosol.
  • The process of anaerobic respiration generates only 2 ATP per glucose molecule.

Looking back at the overall process, it will be apparent that living things should produce ATP which in turn powers every metabolic and activity of organisms. Also the whole pathway of cellular respiration equation is so precise that it cannot proceed if a single molecule or enzyme is missing. Just imagine the metabolic confusion if they are not so.

Life is so complex, isn’t it?

Cite this article as: "Overview Of Cellular Respiration Equation, Types, Stages & Products," in Bio Explorer, December 5, 2016, https://www.bioexplorer.net/cellular-respiration-equation.html/.

References

  • [1] – “ATP: The Perfect Energy Currency for the Cell.” ATP: The Perfect Energy Currency for the Cell. Accessed December 05, 2016. Link.
  • [2] – Berg, Jeremy M. “Glycolysis and Gluconeogenesis.” Biochemistry. 5th Edition. 1970. Accessed December 05, 2016. Link.
  • [3] – Berg, Jeremy M. “The Citric Acid Cycle.” Biochemistry. 5th Edition. 1970. Accessed December 05, 2016. Link.
  • [4] – “Oxidative phosphorylation – Khan Academy.” Khan Academy. Accessed December 05, 2016. Link.
  • [5] – “Glycolysis Reactions.” Glycolysis Reactions. Accessed December 05, 2016. Link.
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