A Sealed Plant Chamber
A class team seals a leafy potted plant inside a clear 2 L chamber fitted with a carbon dioxide sensor and a lamp. In bright light, CO₂ falls from 800 ppm to about 420 ppm over 20 minutes, then holds steady near 420 ppm even though the lamp stays on. The team then wraps the chamber in foil; over the next 20 minutes CO₂ climbs from 420 ppm to 610 ppm. When they uncover the chamber with the lamp on, CO₂ falls again. A second identical chamber containing only moist potting soil and no plant shows no measurable CO₂ change during either period. A classmate concludes, "This proves plants only release oxygen and never use it."
1. Explain the fall and the rise in CO₂ in terms of plant processes, and state what the plateau near 420 ppm represents.
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Two processes run simultaneously in the plant:
- Photosynthesis - light-driven; takes in CO₂ and releases O₂.
- Cellular respiration - runs continuously, light or dark; takes in O₂ and releases CO₂.
What the sensor detects is the net result:
- Light on: photosynthesis outpaces respiration, so CO₂ is removed from the air faster than it is added - the reading falls.
- Foil on: photosynthesis stops, respiration continues - CO₂ accumulates - the reading rises.
The plateau near 420 ppm is the point where the two rates have become equal, so net exchange is zero. In a sealed chamber, CO₂ is a finite resource; as it drops, photosynthesis slows until it exactly matches respiration. The lamp is still on, so light is not what limits the plant at the plateau - CO₂ supply is.
2. Use the team's observations to evaluate the classmate's claim.
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The claim is wrong, and the foil phase is the observation that refutes it.
- If the plant only released oxygen and never used it, then in darkness it should have no effect on CO₂. Instead CO₂ rose by about 190 ppm.
- A rise in CO₂ means the plant was taking in O₂ and releasing CO₂ - respiration. So the plant certainly uses oxygen.
- The light phase does not show "only oxygen released"; it shows net removal of CO₂, meaning photosynthesis exceeded respiration. Both were still happening.
The soil control matters here: a plant-free chamber of moist soil produced no change, so soil microorganisms are not the source of the CO₂ rise. The plant itself is responsible. "Net effect" and "only effect" are not the same thing, and the classmate has confused them.
3. Suppose the chamber had a small slow leak. Predict how the foil-phase result would differ, and explain.
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The rise would be smaller and slower, and the reading would tend to level off near the outside air's CO₂ concentration rather than climbing steadily.
Reasoning: with a leak, CO₂ produced by the plant escapes to the room while room air (near ambient, roughly 420 ppm) seeps in. The chamber starts the dark phase at about 420 ppm, essentially the same as the surrounding air, so there is little concentration difference to drive net movement - the accumulated CO₂ is continually diluted away.
The same leak would also flatten the light-phase dip: outside air would keep replenishing the CO₂ the plant removes, so the reading would not drop as far before plateauing. The sealed design is what makes the plant's net exchange visible at all.