Anaerobic respiration primarily occurs in the cytoplasm of cells, not in mitochondria, and its location can vary by organism and tissue type. Key points

• In most cells, the initial stage of anaerobic respiration (glycolysis) takes place in the cytoplasm. This is the fluid part of the cell outside organelles where enzymes drive the breakdown of glucose without oxygen.

• In animals, the subsequent steps that use the products of glycolysis are limited or bypassed under anaerobic conditions, so energy yield remains low and the process mainly serves to regenerate NAD+ for glycolysis to continue. The activity is concentrated in the cytoplasm during intense exercise when oxygen is scarce.

• In many microorganisms and plants, fermentation pathways operate in the cytoplasm as well, producing either lactic acid (in animals and some bacteria) or ethanol and CO2 (in yeasts and some plant-associated microbes) without requiring mitochondrial involvement.

• Some sources describe anaerobic processes occurring in environments outside the cytoplasm (e.g., microbial ecosystems where anaerobic respiration can involve alternative electron acceptors). However, the classic, textbook location for anaerobic respiration in most cells is the cytoplasm, with mitochondria not participating in the anaerobic pathway.

Common contexts

• Human muscle cells can switch to anaerobic respiration during high-intensity, short-duration activities when oxygen delivery doesn’t meet demand, producing lactic acid as a byproduct.

• Yeasts and some bacteria perform fermentation (a form of anaerobic respiration) in the cytoplasm to regenerate NAD+, leading to ethanol and CO2 or lactic acid depending on the organism and pathway.

• Educational resources consistently place the location of anaerobic respiration in the cytoplasm, differentiating it from aerobic respiration, which largely involves mitochondrial processes.

If you’d like, I can tailor this to a specific organism or cell type (e.g., human muscle cells vs. yeast) and include a brief comparison with aerobic respiration.