Practice a College Board-style free response question on Cellular Energetics. Write your response, then reveal the model answer to see exactly what earns each point.
A student investigates how light intensity affects the rate of photosynthesis in the aquatic plant Elodea. She places identical sprigs of Elodea in beakers of water containing dissolved CO₂. Each beaker is exposed to a different light intensity for one hour, and the student measures the rate of oxygen production (mL O₂ / hr). All other conditions (temperature, CO₂ concentration, water volume) are held constant. Her results are shown below.
| Light intensity (lux) | Rate of O₂ production (mL/hr) |
|---|---|
| 0 | 0 |
| 1,000 | 6 |
| 2,000 | 12 |
| 4,000 | 22 |
| 8,000 | 28 |
| 16,000 | 28 |
As light intensity increases from 0 to 8,000 lux, the rate of photosynthesis (measured by O₂ production) increases proportionally. From 8,000 to 16,000 lux, the rate plateaus at 28 mL/hr, showing no further increase. At low light intensities, light is the limiting factor — the rate is restricted by how much light energy the chlorophyll can absorb. At high light intensities, some other factor (such as CO₂ concentration or the maximum activity of rubisco) becomes the limiting factor, so adding more light no longer increases the rate.
The light reactions occur in the thylakoid membranes of the chloroplast. The thylakoid membranes are stacked into grana, which dramatically increase the surface area available to hold chlorophyll and the proteins of the electron transport chain. The thylakoid membrane also encloses an internal space (the thylakoid lumen), allowing H⁺ to be pumped into a sealed compartment — creating the proton gradient that drives ATP synthase. So the membrane's structure (folded for surface area, sealed for compartmentalization) directly supports its function (capturing light, running the ETC, making ATP via chemiosmosis).
The rate of photosynthesis would most likely increase above 28 mL/hr. At 16,000 lux, light is no longer the limiting factor — the data shows the plateau began at 8,000 lux. The most probable limiting factor at high light is CO₂ availability, since CO₂ is required by the Calvin cycle (specifically as a substrate for rubisco). Increasing CO₂ would allow the Calvin cycle to run faster, which would consume the ATP and NADPH made by the light reactions more quickly, allowing the light reactions to run faster as well, thereby increasing overall O₂ production.