Our cells breathe and digest, as does the organism as a whole. They indeed use oxygen to draw the energy contained in the nutrients they ingest, before discarding the waste, as carbon dioxide and water. Glucose is a preferred nutrient for the cells. Its digestion occurs in the cytoplasm, in the absence of oxygen, and leads to the formation of pyruvate and a small amount of energy. Pyruvate is then carried into mitochondria, the cell's power plants, for a complete burning, thus providing a maximal energetic yield.
A mediocre energetic yield in tumor cells