Author: Hannah Kwon
Editors: Kira Tian
Artist: Jenny Liu
Cellular respiration is a crucial part of our lives that occurs in our cells’ mitochondria: it allows humans and animals to convert the glucose we consume into energy. Its chemical equation can be represented as: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. There are two types of cellular respiration: aerobic and anaerobic. Aerobic cellular respiration occurs when oxygen is present: the process begins with glycolysis, then moves on to the citric acid cycle, followed by the electron transport chain, and finally oxidative phosphorylation. If oxygen is not present—anaerobic cellular respiration—glycolysis is followed by either alcoholic fermentation or lactic acid fermentation. In this article, I will focus on aerobic respiration.
To start, glycolysis, which occurs in the cytoplasm, breaks down one molecule of glucose into two three-carbon molecules of pyruvate, which is needed for the Krebs cycle that follows. Glycolysis also results in a net gain of two ATP that is produced by substrate-level phosphorylation, where a phosphate group is directly transferred to ADP to form ATP.
Moving forward, there is the citric acid cycle (referred to as the Krebs Cycle). Just before it enters the Krebs cycle, pyruvate first combines with coenzyme A to form acetyl-CoA; this conversion also produces two molecules of NADH. During the actual citric acid cycle, oxidation of glucose to CO2 is completed. The cycle also turns twice for each pyruvate molecule and generates one ATP per turn by substrate-level phosphorylation. In addition, the cycle also reduces coenzymes NAD+ and FAD to NADH and FADH2. To sum up, each turn of the Krebs cycle releases three NADH, one ATP, and one FADH, with CO2 being exhaled.
After the Krebs cycle, when the electron transport chain embeds in the cristae membrane, NADH and FADH are oxidized, releasing hydrogen ions and electrons. The hydrogen ions (H+) are pumped to the outer compartment of the mitochondria, creating a steep proton gradient. Then, they flow down the gradient into a matrix through the ATP synthase channel, generating energy for phosphorylation of ADP to ATP. This process is called chemiosmosis or oxidative phosphorylation. As for electrons, through a series of redox reactions, they are carried by electron transport chains to a highly electronegative oxygen, the final electron acceptor. The oxygen combines half an oxygen molecule with two electrons and two protons, forming water.
To conclude, this is how cellular respiration is performed each day in our bodies. Glucose goes through glycolysis and Krebs cycle to produce CO2 and some energy while, during oxidative phosphorylation, lots of energy is produced and water is formed.
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