Advanced Placement (AP) Biology Practice Exam

The correct definition of oxygen concentration refers specifically to the amount of oxygen present in a given environment. This can be measured in various contexts, such as the concentration of oxygen in the atmosphere, in water bodies, or even in the tissues of organisms. Oxygen concentration is crucial for understanding various biological processes, including cellular respiration, in which organisms utilize oxygen to produce energy. High or low levels of oxygen can significantly impact ecosystems and the organisms that inhabit them.

The other choices, while related to oxygen, do not accurately capture the essence of "oxygen concentration." The distribution of oxygen in the atmosphere only addresses the spatial aspects rather than the actual quantity present in a specific environment. The number of organisms that require oxygen provides insight into biodiversity but does not quantify the oxygen itself. Lastly, measurement of oxygen exchange in photosynthesis pertains to a specific process rather than defining the overall concentration of oxygen in an environment.