Question

Muscle cells engaged in vigorous activity build up a high concentration
a. Lactic acid
b. Pyruvic acid
c. Alcohol
d. Cholesterol

Answer 1

Hint: Anaerobic respiration takes place in the muscles. It uses electron acceptors other than the molecular oxygen. But oxygen is not the final acceptor. The energy produced during the process is less than that of the aerobic respiration. Glucose gets converted into lactic acid which leads to a lack of oxygen in the body due to which glycogen reserves in the muscles become low. During vigorous exercise or activity, several changes take place in the body, the breathing rate increases to bring more oxygen to the body.

Complete answer:
- Anaerobic respiration in muscles takes place during vigorous activity, glucose gets converted into lactic acid which leads to a lack of oxygen in muscles. Glucose $({C_6}{H_{12}}{O_6} \to 2{C_3}{H_6}{O_3})$
- Muscles need the energy to contract. While exercising, the muscles need additional energy.
- Changes taking place in the body to enable this are:
- The breathing rate and volume of each breath increase to bring more oxygen into the body and remove the carbon dioxide produced.
- The heart rate increases, to supply the muscles with extra oxygen and remove the carbon dioxide produced
- Glycolysis is the metabolic reaction that produces two molecules of ATP through the conversion of glucose into pyruvate, water, and NADH.
- When the body has oxygen in plenty, pyruvate is shuttled to an aerobic pathway to be further broken down for more energy. But when oxygen is limited, the body temporarily converts pyruvate into lactate, which allows glucose breakdown and leads to energy production
- Pyruvate is continually processed into lactic acid. With pyruvate accumulation, the amount of lactic acid produced increases.
- During exercise when muscles do not get enough oxygen, lactate is produced, glycogen reserves in the muscles become low as more glucose is used for respiration, and additional glucose is transported from the liver.
- If insufficient oxygen is available to the muscles, the exercise is vigorous and prolonged, the heart and lungs are unable to supply sufficient oxygen. Muscles begin to respire anaerobically. Lactic acid is produced from glucose, instead of carbon dioxide and water. Muscles continue to contract, but less efficiently.
- So, Muscle cells engaged in vigorous activity build up a high concentration of Lactic acid.

Hence, the correct answer is option (A).

Note: The lactic acid that accumulates in the muscle tissue reduces the pH, causing it in making it more acidic and producing the stinging feeling in muscles when exercising. It inhibits anaerobic respiration. Pyruvate is the intermediate molecule formed during glycolysis. The aerobic processes produce up to 38 ATP molecules per glucose.