Glycolysis takes place in the cell's cytoplasm and can occur with or without oxygen. However, without oxygen present, the glycolysis phase of cellular respiration doesn't produce additional adenosine triphosphate, or ATP.
Glycolysis takes place in two ways: aerobic with oxygen and anaerobic without oxygen. Aerobic glycolysis is the first stage of cellular respiration. During this 10-step process, oxygen plays a role in breaking down a single molecule of glucose to produce three ATP, two pyruvic acid and two Nicotinamide adenine dinucleotide. To achieve this, oxygen oxidizes the glucose to produce the byproducts. When oxygen is present, the cell produces 34 more ATP, which it then uses for energy. During anaerobic glycolysis, fermentation takes place. In humans, this leads to the production of lactic acid but no additional ATP. In plants, fermentation results in alcohol.
Glycolysis needs to take place for cells to fuel their activities. Some cells can survive temporarily using aerobic glycolysis, such as the muscles during exercise. Others need a constant supply of ATP and glucose, such as the brain, which begins to shut down after a short period without them. The process takes place inside cells after insulin moves glucose into them. Glycolysis forms part of the broader citric acid cycle.