Glycolysis
Glycolysis is the first stage in the oxidation of glucose, a pivotal metabolic pathway that is active in almost all living cells. Think of it as the gateway for glucose metabolism. During glycolysis, a single glucose molecule, which is a six-carbon sugar, is broken down into two three-carbon molecules called pyruvate. This process occurs in the cytoplasm and does not require oxygen, making it anaerobic.
Throughout the ten enzymatic reactions of glycolysis, energy is both invested and harvested. Initially, two ATP molecules are used to activate glucose and its six-carbon derivative, and later, a net gain of two ATP molecules is achieved, resulting in a payoff for the cell. This net gain is crucial for cellular processes, especially when oxygen is scarce. Furthermore, two molecules of NADH, a carrier of electrons, are produced here; these will play a significant role in the later stages of cellular respiration.
Krebs Cycle
Following glycolysis, pyruvate enters the mitochondria, where it is prepared to undergo the Krebs Cycle, also known as the Citric Acid Cycle or TCA Cycle. This aerobic process only happens when oxygen is present, unlike glycolysis. Here's where the majority of energy is harvested from glucose.
Each pyruvate is converted into Acetyl-CoA, which is then funneled into the Krebs Cycle. Through a series of reactions, the Krebs Cycle liberates electrons, which are carried away by NADH and FADH2, and produces ATP. Carbon dioxide, which we breathe out, is also a byproduct of these reactions. Moreover, the various steps in the Krebs Cycle provide substrates for other biochemical reactions, highlighting its central role in cellular metabolism.
Electron Transport Chain
The Electron Transport Chain (ETC) is the final destination for the electrons carried by NADH and FADH2. Located on the inner mitochondrial membrane, the ETC comprises a series of protein complexes that function like an electron relay race.
As electrons are passed down the chain, energy is released, which is used to pump protons across the mitochondrial membrane, creating a gradient. This process, often compared to a dam holding back water, is essential for the synthesis of a large amount of ATP in the next stage. Oxygen, the final electron acceptor, couples with these electrons and protons to form water, which is key in maintaining the flow of electrons through the chain. Without oxygen, this process would grind to a halt, as seen when organisms are deprived of oxygen (anoxic conditions).
ATP Synthesis
ATP Synthesis is the process that converts the energy from the Electron Transport Chain into a form that the cell can easily use. The enzyme responsible for this conversion is ATP synthase, a remarkable molecular machine.
The proton gradient created by the ETC leads to high proton concentration outside the mitochondrial inner membrane. These protons flow back into the mitochondrial matrix through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate. This process is known as oxidative phosphorylation, and it's central to cellular respiration. Each NADH and FADH2 that enter the ETC can result in the production of multiple ATPs, showcasing the efficiency of the cell's ability to harvest energy from glucose oxidation.
