The breakdown of glucose has taken the following pathway : Glucose $\xrightarrow{(a)} Pyruvate + Energy \xrightarrow{(b)} Lactic\ acid + Energy$ The sites 'a' and 'b' respectively are :

The breakdown of glucose to produce energy is a multi-step process. The first step, glycolysis, takes place in the cytoplasm and produces pyruvate and energy. The second step, which produces lactic acid and energy, takes place in oxygen-deficient muscle cells. This is because, in the absence of oxygen, the muscle cells cannot carry out aerobic respiration and instead produce lactic acid through anaerobic respiration.