Cellular Respiration

Cellular respiration is a crucial metabolic process through which cells obtain energy from organic substances, mainly glucose, to produce adenosine triphosphate (ATP). This process includes the oxidation of substrates and the transfer of electrons to a separate electron acceptor, facilitating ATP synthesis through a sequence of biochemical reactions.

Glycolysis: The Initial Step

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm of both prokaryotic and eukaryotic cells. It involves the breakdown of one glucose (C₆H₁₂O₆) molecule into two molecules of pyruvate (C₃H₄O₃), producing a net gain of 2 ATP and 2 NADH. This process does not require oxygen and serves as a preparatory step for both aerobic and anaerobic respiration.

The Krebs Cycle: Generating Electron Carriers

Pyruvate undergoes a crucial process known as decarboxylation, during which it is converted into acetyl-CoA. This newly formed acetyl-CoA then enters the Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle. This intricate series of biochemical reactions occurs in the cytoplasm of prokaryotes and in the mitochondrial matrix of eukaryotes.

As acetyl-CoA progresses through the Krebs cycle, it plays a vital role in energy production. For each molecule of acetyl-CoA processed, the cycle generates three molecules of NADH, one molecule of FADH₂, and one GTP. These electron carriers, NADH and FADH₂, are essential for driving ATP synthesis in subsequent reactions. Additionally, carbon dioxide (CO₂) is released as a byproduct during this process, contributing to the overall metabolic activities of the cell.

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is placed in the plasma membrane of prokaryotes, while in eukaryotes, it is placed in the inner mitochondrial membrane. Electrons from NADH and FADH₂ are transferred through a series of protein complexes, creating a proton gradient across the membrane. The return flow of protons through ATP synthase drives the phosphorylation of ADP to ATP. In aerobic respiration, oxygen serves as the terminal electron acceptor, forming water (H₂O).

ATP Yield and Alternative Electron Acceptors

Aerobic respiration can yield up to 38 ATP molecules per glucose in prokaryotes, while in eukaryotes, the net ATP production is about 30–32 due to energy costs associated with NADH transport into mitochondria. In anaerobic respiration, prokaryotes and some eukaryotes utilize alternative electron acceptors like nitrate (NO₃⁻), sulfate (SO₄²⁻), or carbon dioxide (CO₂), yielding significantly less ATP—typically around 2 ATP per glucose—due to the lower efficiency of these electron acceptors in generating a proton gradient.

Cellular respiration is essential for energy metabolism, allowing organisms to sustain cellular functions through ATP production. The efficiency and choice of the electron acceptor determine the energy yield and metabolic adaptations of different organisms in various environments.