OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What is the primary output of anaerobic respiration in animals?

1. Carbon Dioxide

2. Water

3. Lactic Acid

4. Ethanol

The correct answer is: Lactic Acid

The primary output of anaerobic respiration in animals is lactic acid. This process occurs when oxygen levels are low, typically during intense exercise when the demand for energy exceeds the supply of oxygen available for aerobic respiration. In the absence of oxygen, glucose is broken down through glycolysis, producing pyruvate, which is then converted into lactic acid to regenerate the NADH required for glycolysis to continue. Lactic acid can accumulate in muscles, leading to fatigue and discomfort, but it can later be recycled back into the liver or converted back to glucose when oxygen becomes available again, highlighting the importance of anaerobic respiration in energy production during oxygen deficits. Other outputs, such as carbon dioxide and water, are associated with aerobic respiration, while ethanol is typically produced during fermentation in yeast, representing a different pathway altogether.