USA Biology Olympiad (USABO) Practice Exam

In an experiment, if oxygen production levels off after increasing light intensity, what does that imply?

1. Light is the only limiting factor

2. Light is limiting until CO2 becomes a limiting factor

3. CO2 limits oxygen production

4. Oxygen levels inhibit light absorption

The correct answer is: Light is limiting until CO2 becomes a limiting factor

The observation that oxygen production levels off despite increasing light intensity suggests that other factors are limiting the photosynthesis process once the light reaches a certain threshold. At lower light intensities, light is indeed a primary limiting factor, boosting the rate of photosynthesis as its intensity increases. However, once oxygen production stabilizes, it typically indicates that the availability of carbon dioxide has become a limiting factor in the process. In photosynthesis, carbon dioxide is essential for the formation of glucose and the production of oxygen as a byproduct. When light intensity is high enough that light is no longer the limiting factor, the reaction rate is then constrained by CO2 levels in the environment. Therefore, as light intensity increases, one must consider that at some point, the carbon dioxide availability will limit the overall rate of photosynthesis, leading to the stabilization of oxygen production levels. This scenario highlights the interplay between multiple factors influencing the rate of photosynthesis, demonstrating that an increase in light intensity alone will not indefinitely increase oxygen production if carbon dioxide or other factors become limiting.