A Sealed Chamber: Separating Photosynthesis from Respiration
A student seals a healthy sprig of Elodea in a 2 L clear chamber fitted with a CO2 monitor and a controllable light bank. Temperature is held at 25 °C throughout.
- 0-30 min, dark: CO2 rises from 400 to 520 ppm.
- At 30 min the light is switched on at 1,000 lux: CO2 falls to 260 ppm by 60 min, 250 ppm by 90 min, and 248 ppm by 120 min.
- At 120 min the light is raised to 4,000 lux: CO2 reaches 245 ppm by 150 min.
- At 150 min, 5 mL of 0.5 M sodium bicarbonate solution is injected into the chamber: CO2 falls to 180 ppm by 180 min.
- At 180 min the chamber is wrapped in foil: CO2 climbs to 300 ppm by 210 min.
The student must decide which factor limits the plant at each stage.
1. Calculate the CO2 production rate in the dark (0-30 min) and after foil is applied (180-210 min). What does the agreement mean?
Read the worked answer
+120 ppm over 30 min = 4 ppm/min in both intervals. Respiration therefore ran at the same rate in light and in darkness; light does not switch respiration off.
That matters because during the illuminated intervals the monitor reads only the net exchange: gross photosynthesis minus respiration. The 4 ppm/min figure is the respiratory baseline that has to be added back to every light reading to obtain gross photosynthesis. Without the two dark intervals, every light reading would be misinterpreted as gross carbon fixation when it is actually a difference between two opposing processes.
2. CO2 fell from 400 to 260 ppm and then nearly stopped changing between 60 and 120 min. Doubling the light changed little, but bicarbonate produced a renewed fall. Interpret this.
Read the worked answer
Between 60 and 120 min the chamber is close to the light compensation point: net uptake drops to 10 ppm in 30 min, then 2 ppm in 30 min, roughly balancing the 4 ppm/min of respiration.
Light was not limiting. If it had been, 4,000 lux would have increased uptake, and instead CO2 moved only from 248 to 245 ppm, within noise. Bicarbonate restarted the fall, which identifies CO2 as the limiting factor - the carbon source was scarce. "Limiting" means precisely this: adding the scarce factor restarts the process, which the non-limiting control did not. Temperature was held at 25 °C and the injection itself introduced no CO2 beyond the bicarbonate, so neither of those explains the change.
3. Estimate gross photosynthesis between 150 and 180 min and explain why that value cannot be read directly off the monitor.
Read the worked answer
Net uptake was 245 → 180 ppm, or 65 ppm in 30 min, about 2.2 ppm/min. Adding the respiration rate of 4 ppm/min gives a gross rate of roughly 6.2 ppm/min of CO2 fixed.
The monitor cannot be read directly because it sees only the difference between two processes running simultaneously: chloroplasts fixing CO2 and mitochondria releasing it. The same two rates explain all three phases of this experiment - CO2 rising in the dark, falling under bright light, and hovering near the compensation point at low light. They are not different forms of metabolism, just different balances between the same two.