Overview Of Cellular Respiration Equation, Types, Stages & Products

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.

Photosynthesis Equation

The equation for photosynthesis of plant cells is:

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:

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

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

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

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.

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?

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.