It's basically a metabolic reaction which results in formation of GTP or ATP by the transfer of a phosphoryl to GDP or ADP from another phosphorylated compound.

Substrate Level Phosphorylation:

Substrate level phosphorylation is a metabolic process in which a phosphate group is transferred from a substrate molecule to ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). This process occurs during glycolysis and the citric acid cycle in cellular respiration. Unlike oxidative phosphorylation, which occurs in the electron transport chain, substrate level phosphorylation directly synthesizes ATP without the involvement of an electron transport system.

Process:

During substrate level phosphorylation, a high-energy phosphate group is transferred from a phosphorylated substrate molecule to ADP. This transfer is catalyzed by specific enzymes known as kinases. The phosphate group is usually attached to an organic molecule, such as a sugar or an intermediate compound in a metabolic pathway.

Glycolysis:

In glycolysis, substrate level phosphorylation occurs twice. In the first step, a phosphate group is transferred from a phosphorylated glucose molecule to ADP, forming ATP and a phosphorylated intermediate compound. In the final step, a phosphate group is transferred from a phosphorylated intermediate compound to ADP, again forming ATP.

Citric Acid Cycle:

In the citric acid cycle, substrate level phosphorylation occurs in two steps. During the conversion of succinyl-CoA to succinate, a phosphate group is transferred to ADP, forming ATP. Similarly, during the conversion of phosphoenolpyruvate to pyruvate, another phosphate group is transferred to ADP, resulting in ATP synthesis.

Significance:

Substrate level phosphorylation is essential for cellular energy production as it directly generates ATP. It plays a crucial role in sustaining various metabolic processes, including muscle contraction, active transport, and biosynthesis. Additionally, substrate level phosphorylation contributes to the regulation of metabolic pathways by controlling the availability of ATP.

Differences from Oxidative Phosphorylation:

While oxidative phosphorylation occurs in the mitochondria and involves the electron transport chain, substrate level phosphorylation occurs in the cytoplasm and mitochondrial matrix. Oxidative phosphorylation relies on the transfer of electrons from NADH and FADH2 to generate a proton gradient, whereas substrate level phosphorylation directly transfers a phosphate group.

In conclusion, substrate level phosphorylation is a process in which ATP is synthesized by transferring a phosphate group from a phosphorylated substrate molecule to ADP. This process occurs during glycolysis and the citric acid cycle. It is an important mechanism for ATP production and regulation of metabolic pathways.