Cellular respiration commonly refers to a four-fold biochemical process through which the cells break down organic glucose molecules to form energy carriers in the form of ATP. This process aids in the conservation of energy for activities requiring ATP influxes, such as physical exercises. The process involves four critical processes: glycolysis, oxidation of pyruvate, the electron transport chain, and the tricarboxylic acid cycle. Glycolysis primarily facilitates the breakdown of two glucose molecules forming pyruvic acid. Afterward, the acid is oxidized to form acetyl CoA, a precursor for the Krebs cycle. The citric acid cycle completes the division of glucose forming carbon (II) oxide and electron carriers. The enzymes used exist in the mitochondrial matrix. These initial phases yield minimal ATP molecules. However, the eventual electron transport chain converts electron carriers (NADH and FADH2) to 34 ATP molecules. This stage comprises numerous protein complexes embedded in the mitochondrial intermembrane space.
| Cycle Involved | Location of Reaction | Electron Carrier Made | ATP Formed |
| Glycolysis | Cytoplasm | 2 NADH | 2 |
| Respiration per initial glucose amount | |||
| Pyruvate Oxidation | Mitochondrial matrix | 2 NADH | 0 |
| Tricarboxylic Acid Cycle | Mitochondrial matrix | 6 NADH
2 FADH2 |
2 |
| Electron Transport Chain | Intermebrane space of Mitochondria | 10 NADH
2 FADH2 |
30
4 |
| Subtract electron carriers and ATP used in transportation | |||
| Total ATP yielded per unit of glucose | 32 to 34 ATP | ||
Table 1: Four processes involved in energy production during cellular respiration.
Glycolysis
Glycolysis is the first pathway involved in the harnessing of electron carriers and ATP molecules. The pathway occurs in the cytosol in two different set-ups (Chaudhry & Varacallo, 2020, p. 1). The initial phase, investment, incorporates the use of two ATP molecules. This stage is followed by a payoff phase during which the metabolites are broken down, forming precursors of the next cycle. Notably, glycolysis occurs in aerobic and anaerobic environments. In aerobic reactions, the pyruvate joins the Krebs cycle and undergoes oxidative phosphorylation to yield 32 ATPs. In anaerobic conditions, the pyruvate is converted to lactate, given a total yield of 2 ATPs. This process commences with the uptake of glucose in the diet.
The glucose undergoes phosphorylation, forming glucose-6-phosphate by a glucokinase enzyme. Chaudhry & Varacallo (2020) postulate that glucokinase solely exists in the pancreas of human beings whereas hexokinase is available in plants. The resultant compound then undergoes isomerization forming fructose-6-phosphate. A phosphoglucose isomerase enzyme catalyzes the reaction. Afterward, the second ATP molecule assists in creating a fructose-1, 6-bisphosphate molecule, under the help of a kinase enzyme. The resultant compound dissociates into two sugars, a step catalyzed by a bisphosphate aldolase enzyme. The dihydroxyacetone phosphate is further isomerized into glyceraldehyde 3-phosphate yielding two similar molecules.
The two G-3-P molecules undergo oxidation yielding 1, 3-bisphosphoglycerate following the reduction of an NAD+ molecule. Chaudhry & Varacallo (2020) assert that this compound undergoes further reaction forming 3-phosphoglycerate under the influence of a kinase enzyme. This reaction facilitates the production of the first ATP molecule in the payoff phase. The resultant molecule is further reduced into 2-phosphoglycerate, which eventually forms phosphoenolpyruvate (PEP) under the influence of an enolase enzyme. The resultant PEP is relatively unstable. Hence, it loses a phosphate group in the form of a second ATP molecule forming pyruvate.
Pre-Krebs Cycle
The Krebs cycle commences with an acetyl CoA molecule. Hence, to bridge the two processes, the pyruvate formed in glycolysis is oxidized, forming acetyl CoA. Next, the pyruvate undergoes decarboxylation reactions yielding carbon (II) oxide under the influence of a pyruvate dehydrogenase enzyme (Minikel, 2013). Afterward, an acetyl group is added, forming acetyl CoA. Finally, the carbon (II) oxide formed is released as waste. Notably, the oxidation process releases electrons in the form of NADH. However, these electron carriers do not form ATP at this stage.
Krebs Cycle
The cyclic series is commonly identified as the tricarboxylic acid cycle or the citric acid system. This phase commences and ends within the same media. The process serves as the central hub in biosynthetic pathways by providing metabolites to build essential molecules such as amino acids (Minikel, 2013). The eight-step cycle commences by the addition of the acetyl group to oxaloacetate. Notably, the sophisticated reaction provides the electrons driving the oxidative phosphorylation reactions in the mitochondria. The electron carriers yielded are directly channeled to complex I in the electron transport chain.
The initial step involves the formation of citrate by combining acetyl CoA and oxaloacetate. This step is facilitated by a citrate synthase in the presence of a water molecule. Afterward, the aconitase converts the citrate to isocitrate. In this step, a water molecule is lost from the citric acid at the 3’ position and replaced at the 4’ position (Minikel, 2013). The resultant compound undergoes oxidative decarboxylation forming an alpha-ketoglutarate molecule. The step results in the formation of an NADH molecule under the influence of an isocitrate dehydrogenase enzyme. In the fourth step, the ketoglutarate is oxidized by an alpha-ketoglutarate dehydrogenase enzyme in a reaction characterized by the addition of coenzyme A and removal of carbon (II) oxide to form succinyl CoA. In this reaction, NADH + H+ results from the NAD+.
The sequential reaction involves succinate from succinyl CoA by removing the coenzyme A by a synthetase enzyme. This step leads to the release of energy in the form of GTP from a phosphorylated GDP molecule. The succinate is then converted to fumarate by a dehydrogenase enzyme leading to the formation of FADH2. Step seven involves the addition of oxygen and hydrogen to fumarate, forming malate. Lastly, the malate is oxidized to oxaloacetate, the initial compound of the cycle. This reaction is catalyzed by malate dehydrogenase, facilitating the reduction of NAD+ to NADH + H+.
Electron Transport Chain
This phase of cellular respiration gives rise to the greatest proportion of ATPs. The pathway consists of four protein complexes that facilitate redox reactions creating an electrochemical difference (Ahmad et al., 2020, p. 1). Subsequently, ATP molecules are formed in a complete oxidative phosphorylation cycle. The pathway occurs in the mitochondria. The external membrane is primarily porous, making the intermediate space share an ionic concentration with the cytosol. Thus, the path establishes an H+ gradient within the internal membrane.
The pathway commences by a transfer of reducing intermediates from the cytoplasm into the mitochondrion through the malate-aspartate shunt. Minikel (2013) asserts that the adenine nucleotide translocase can similarly move ATP from the matrix, depositing ADP instead. Next, the electrons flow through variable complexes including complexes I to IV, cytochrome C, and complex Q. This protein complex chain exchanges the electrons by allowing an upward migration to a more significant reduction potential. Finally the redox reaction involving oxygen to form water ensues. Notably, the generation of energy within some complexes releases protons into the intermembrane chamber. Ahmad et al. (2020) affirm that each NADH molecule transported upwards yields 3 ATP molecules while an FADH2 carrier gives 2 ATPs. Thus, at least 10 NADH and 2 FADH molecules are traded in, giving rise to about 32 to 34 ATP molecules.
Conclusion
The interplay between the four cycles depicts a perfect theme of biochemical system interaction aimed at maintaining homeostasis and metabolism. The cell makes at least 32 ATPs per molecule of glucose ingested. This number varies depending on the organism and molecules involved. Both glycolysis and citric acid cycles yield two ATPs. The majority of ATP molecules formed facilitate the transport of intermediates within the electron transport chain. This last phase produces the most ATP molecules with the help of the ATP synthetase in a ‘fall of electrons’ model. Each NADH molecule yields 3 ATPs, while an FADH2 molecule gives 2 ATPs. Conclusively, energy flows from glucose to electron carrier molecules and eventually ATP.
