What do human and animal cells obtain through lactic-acid fermentation during exercise?
Animal cells obtain lactic acid and NADH.
The first half of the diagram is glycolysis, the process used by most life to obtain ATP (energy currency of the cell). In the process of making ATP, there are two byproducts: 2 pyruvates and 2 NADH.
The pyruvates need to be turned into something that the body can dispel of. Lactic acid fermentation turns the two pyruvates into 2 lactate molecules. These molecules can be dispelled by the body (liver).
Side-note: lactic acid is what makes our muscles sore after a good work-out
The second byproduct that needs to be taken care of is NADH. Glycolysis makes NADH from NAD+, and if the body does not have NAD+, it would not be able to do glycolysis. All types of fermentation take care of this issue (alcohol or lactic acid). Fermentation turns NADH back into NAD+ so that the cell can continue to perform glycolysis.