Biological oxidation, in the context of biochemistry, refers to the process by which living organisms extract energy from organic molecules.
This is a crucial part of metabolism, which encompasses all chemical reactions within cells, including those that break down nutrients to produce energy and those that use energy to construct cell components.
Biological oxidation involves the controlled release of energy from organic compounds, which is then used to form ATP (adenosine triphosphate), the primary energy currency of the cell.
The process occurs in several steps and pathways, with the most notable being glycolysis, the citric acid cycle (also known as the Krebs cycle or TCA cycle), and oxidative phosphorylation.
These processes are interconnected and take place in different cellular locations, such as the cytoplasm for glycolysis and the mitochondria for the citric acid cycle and oxidative phosphorylation in eukaryotic cells.
1. Glycolysis:
This is the first stage of cellular respiration, taking place in the cytoplasm.
It converts glucose, a six-carbon sugar, into two molecules of pyruvate, generating a small amount of ATP and NADH (nicotinamide adenine dinucleotide + hydrogen) in the process.
2. Citric Acid Cycle:
Pyruvate enters the mitochondria, where it is converted to acetyl-CoA, which then enters the citric acid cycle.
This cycle completes the oxidation of substrates and produces NADH and FADH2 (flavin adenine dinucleotide in its reduced form) along with ATP and carbon dioxide as by-products.
3. Oxidative Phosphorylation:
This process takes place in the inner mitochondrial membrane, where NADH and FADH2 produced in previous steps donate electrons to the electron transport chain.
As electrons move through this chain, energy is released and used to pump protons across the mitochondrial membrane, creating a proton gradient.
This gradient drives the synthesis of ATP from ADP (adenosine diphosphate) and inorganic phosphate, catalyzed by the enzyme ATP synthase.
Biological oxidation is distinguished from simple combustion because it occurs in a series of small, controlled steps, allowing organisms to efficiently capture and use energy.
The process is also tightly regulated to meet the energy needs of the cell while minimizing the production of reactive oxygen species (ROS), which can cause cellular damage.