Glycolysis is the first step of cellular respiration where glucose is broken down down into pyruvate. It occurs in the cytosol. In the presence of oxygen, pyruvate is exported to the mitochondria where it is further oxidized. In the absence of oxygen, pyruvate is converted into ethyl alcohol or lactic acid (fermentation) in the cytoplasm.

Pyruvic acid is the three-carbon compound produced in glycolysis, the first stage in cellular respiration. Acetyl-CoA is a two-carbon molecule that is the result of the pyruvate dehydrogenase complex that follows glycolysis. Glucose is broken down in respiration, and ribose is a five-carbon sugar in RNA.

In glycolysis, the net gain of ATP molecules is 2. Two ATP per glucose molecule are required to initiate the process, then a total of four ATP are produced per molecule of glucose.

Glycogen is the polysaccharide stored in the liver and muscle cells of animals that can be broken down into glucose. Sucrose and fructose are sugars. Starch is a polysaccharide found in plants.

Lactic acid is produced in animal cells when no oxygen is present in order to keep making ATP. Alcohol is produced in yeast cells in fermentation. Glucose is broken down in the entire cycle of respiration, and sucrose is a disaccharide.

Glycolysis occurs in the cytosol. Recall that glycolysis is the process by which glucose is broken down into two molecules of pyruvate, which, under aerobic conditions, is further oxidized in the Krebs cycle and electron transport. The mitochondria is the site of the Krebs cycle and electron transport in eukaryotes, while these processes occur in the cytosol and across the cell membrane in prokaryotes.

Glycolysis is the first step taken in cellular respiration and takes place in the cytoplasm while pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation take place inside the mitochondria.

Glycolysis creates ATP and NADH through substrate level phosphorylation. The net products are 2 ATP, 2 NADH, and 2 pyruvate molecules. More ATP and high energy electron carriers are produced in the subsequent stages of the metabolic pathway such as pyruvate processing and the citric acid cycle. 

Glycolysis starts with the break down of one molecule of glucose via energy input from  to form 2 molecules of , 4 molecules of , 2 pyruvate molecules, and  molecules. This step will continue regardless of whether oxygen is present or not.

Without oxygen the final two steps of cellular respiration (Citric acid cycle and oxidative phosphorylation) can not be performed because it is used as the final electron acceptor. However, the first step, glycolysis produces a small amount of energy in the form of ATP as well as pyruvate and NADH. NADH can be recycle (to be used again in another round of glycolysis) by converting pyruvate to lactic acid. This process is known as lactic acid fermentation.