USA Biology Olympiad (USABO) Practice Exam

What limit might apply in a scenario with increased light intensity and constant CO2 levels?

1. Water availability

2. Nutrient availability

3. Light saturation

4. CO2 saturation

The correct answer is: Light saturation

In a scenario where light intensity is increased while carbon dioxide levels remain constant, light saturation is the limiting factor that would apply. Photosynthesis in plants is driven by light energy, and as light intensity increases, the rate of photosynthesis also increases until it reaches a maximum point. This maximum point is referred to as light saturation. At this stage, all available pigments in the plant’s chloroplasts are absorbing light as efficiently as possible, and further increases in light intensity do not lead to a corresponding increase in the rate of photosynthesis. Instead, other factors become limiting, such as CO2 availability, water, and nutrients, but since CO2 levels are held constant in this scenario, light saturation becomes the primary concern. This concept illustrates that light is one component among many that affect the rate of photosynthesis, and it's crucial to understand how each factor interacts within the process. Therefore, in the context of increased light intensity with constant CO2 levels, light saturation is indeed the limiting factor that defines how effectively a plant can perform photosynthesis, despite the availability of other resources.