The purpose of glycolysis is to generate pyruvate and NADH from glucose. The pyruvate will enter the citric acid cycle and the NADH will be used to donate a proton and electron to the electron transport chain, helping to generate the proton gradient for ATP synthesis. The first step of the citric acid cycle is the conversion of pyruvate to acetyl-CoA.

Lactic acid fermentation occurs in the absence of oxygen, and involves an enzyme that converts pyruvate from glycolysis to lactic acid via the transfer of two hydrogen atoms from NADH and H⁺. The NADH is oxidized to form NAD⁺, a required component to catalyze glycolysis.

During anaerobic respiration the Krebs cycle and electron transport chain are not functional, leaving all cell metabolism reliant on glycolysis. Without NAD⁺, glycolysis would also become non-functional and cell metabolism would completely stop. Both alcoholic fermentation and lactic acid fermentation serve this purpose or replenishing glycolysis reactants, but occur in different organisms. Lactic acid fermentation is most common in animals while alcoholic fermentation occurs in unicellular organisms, such as yeast.

Cellular respiration is the metabolic process used to generate energy, in the form of ATP, that can power cellular functions. During cellular respiration, glucose is broken down and used to generate NADP and FADH₂. These molecules then donate electrons to the electron transport chain, power the proton gradient that is responsible for producing ATP through ATP synthase.

Glucose and oxygen are consumed during this process, while water and carbon dioxide are produced, along with ATP. Sulfur dioxide and carbon monoxide are not involved in the processes of cellular respiration.

Glycolysis will split a glucose molecule into two molecules of pyruvate. This process will result in a net gain of 2 molecules of both ATP and NADH. Lactate is not a product of glycolysis, and will only be made in the event that oxygen is not available and fermentation must be used to reduce pyruvate into lactate.

Glycolysis is the first step of cellular respiration, and takes place in the cytosol. The citric acid cycle and electron transport chain are both located in the mitochondria.

In glycolysis, which takes place in the cytoplasm, 4 ATP are made and 2 ATP are spent. This means that there is a net gain of 2 ATP, which are produced via substrate level phosphorylation, which involves adding a phosphate to ADP to yield ATP.

The overall process of glycolysis is:

The net gain of pyruvate, NADH, and ATP during glycolysis is 2 pyruvate, 2 NADH, and 2 ATP per molecule of glucose. Although the process of glycolysis yields 4 ATP, the early steps of glycolysis use 2 ATP to convert glucose into 2 phosphoglyceraldehydes (note: phosphoglyceraldehyde is a 3 carbon molecule) leading to a net gain of 2 ATP. 

The process of lactic acid fermentation is:

Pyruvate + NADH +   Lactate + 

(under hypoxic or partially anaerobic conditions)

Lactic acid fermentation is a biological process where pyruvate is converted into  and lactate under hypoxic conditions. It occurs in some bacterial cells, and some animal cells such as muscles. This process is catalyzed by the enzyme lactate dehydrogenase. If oxygen is present in the cell, many organisms will bypass fermentation and undergo cellular respiration.  (Fun fact: lactic acid fermentation is utilized to produce kimchi, sauerkraut, and yogurt). Animal muscle cells will undergo lactic acid fermentation, when starved of oxygen. This process is a last resort for energy and cannot be tolerated for long periods of time.

The process of glycolysis (glucose to pyruvate) occurs in the cytosol.

Once formed, pyruvate can have numerous fates. In yeasts it can remain in the cytosol and undergo alcoholic fermentation. Pyruvate can also undergo lactic acid fermentation (under hypoxic conditions) within the cytosol of red blood cells and active muscles. Additionally, pyruvate can undergo cellular respiration in the mitochondria where it is oxidized completely into carbon dioxide and water. 

Energy from catabolism—exergonic or energy-yielding processes—is used to drive the formation of ATP from ADP and inorganic phosphate because ATP formation requires energy to be made. Catabolism is a pathway, which breaks down a larger molecule into smaller ones. Glycolysis is an example of a catabolic pathway.