OCR GCSE Biology Practice Exam 2025 – Comprehensive All-in-One Guide to Exam Success

Anaerobic respiration typically occurs when there is little or no oxygen available. This process allows organisms to generate energy in environments where oxygen is scarce, which can be crucial for survival. For example, certain microorganisms thrive in anaerobic conditions, such as in deep water sediments or within the intestines of animals.

Anaerobic respiration is less efficient than aerobic respiration, leading to the production of by-products like lactic acid in animals or ethanol and carbon dioxide in yeast. This shift to anaerobic respiration can also happen in muscle cells during intense exercise when the demand for oxygen exceeds its supply.

The other options do not accurately describe the conditions under which anaerobic respiration occurs. When there is plenty of oxygen available, aerobic respiration is favored for its efficiency in energy production. The suggestion that anaerobic respiration occurs during the night only is too restrictive, as the process can happen at any time when oxygen levels are low. Lastly, while low temperatures can affect metabolic processes, they are not a direct trigger for anaerobic respiration, which is primarily determined by the availability of oxygen.