The respiration of mature erythrocytes is mainly anaerobic glycolysis because mature erythrocytes show a double-sided concave disc shape. Mature erythrocytes have only an intact cell membrane, no nucleus, and no organelles, so in order to play a greater role in transporting hemoglobin, mature erythrocytes have evolved into a structure without organelles. The energy of the red blood cell itself is not achieved through aerobic respiration, but through anaerobic glycolysis. However, if anaerobic glycolytic enzymes are defective and the patient suffers from pyruvate kinase deficiency, the patient will have insufficient energy produced by anaerobic glycolysis and the patient will be prone to symptoms of hemolytic anemia. In addition to the anaerobic glycolytic pathway, erythrocytes can also pass through the pentose phosphate pathway. If glucose-6-phosphate dehydrogenase is deficient, the pentose phosphate pathway also appears to produce insufficient energy and is also prone to hemolytic anemia, which is clinically known as serum sickness.