Respirometer Virtual Lab: Oxygen Uptake by Germinating Peas

This respirometer virtual lab follows the AP Biology cellular respiration investigation. Three tubes stand in a water bath: germinating peas, dry peas plus glass beads, and glass beads only. Students read the coloured drop in each capillary every 5 minutes and correct the readings with the bead tube. They plot O₂ uptake against time and take the rate from the slope. Then they repeat the run at other temperatures and calculate Q₁₀. Runs that take a class period in the lab take seconds here, and nobody handles KOH. Every number below was read from the simulation.

Curriculum links

• AP Biology: Unit 3 (Cellular Energetics), topic 3.6 on cellular respiration (ENE-1), and the AP cellular respiration investigation, which compares germinating and non-germinating peas at two temperatures.
• NGSS: HS-LS1-7, cellular respiration as a chemical process that breaks the bonds in food molecules and forms new compounds, releasing energy.
• Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Ask: "The water bath is raised from 15 °C to 25 °C. How will the rate of O₂ uptake by germinating peas change?" Students choose: about double, about the same, about half, or about ten times faster. They write one reason. On a class link this is question 1; the simulation unlocks after they answer.

Method in the simulation

The simulation measures the rate in mL of O₂ per minute and in µL per gram per minute. Use the per-gram rate to compare runs with different numbers of seeds.

1. Keep the starting values: Peas, 25 seeds per tube, 25 °C, 30 min, KOH, Equilibrate 10 min. Set Speed: 5 min per second and press Start.
2. During the run, tap a tube to zoom in on its scale. Each 5 minutes, copy the readings, Δ and corrected values from the table.
3. At the end, record the tube 1 and tube 2 rates from the Runs table.
4. Change only the temperature and press Run again, at 20, 15 and 10 °C. For 30 °C and 35 °C, set Seeds per tube to 10. With 25 seeds the drop is drawn into the tube before 30 minutes.
5. Read the Q₁₀ line under the Runs table. It compares the latest run with an earlier one at least 5 °C away.
6. At 25 °C, run once with No KOH, then once with Close clip, start now.

Temperature (°C)	Seeds	Tube 1 rate (mL/min)	Tube 1 rate (µL/g/min)	Tube 2 rate (mL/min)
10	25
15	25
20	25
25	25
30	10
35	10

Expected results

The rate varies by a few percent between runs, so the values below are ranges. The ranges come from 20,000 runs of the simulation's model and were checked in the browser.

• 25 °C, 25 peas: tube 1 takes up 0.019–0.021 mL/min, which is 1.49–1.70 µL/g/min. Corrected uptake after 30 minutes is about 0.56–0.64 mL. One browser run gave 0.0204 mL/min (1.63 µL/g/min).
• Rates at other temperatures: about 0.59 µL/g/min at 10 °C, 0.81 at 15 °C, 1.14 at 20 °C, 2.2 at 30 °C and 3.1 at 35 °C (30 and 35 °C with 10 seeds).
• Q₁₀: from runs at 15 and 25 °C, between 1.73 and 2.21. The browser run at 15 °C gave 0.83 µL/g/min, so Q₁₀ = 1.63 ÷ 0.83 = 1.97.
• Dry peas (tube 2): between −0.0003 and 0.0005 mL/min, close to zero.
• No KOH: tube 1 falls to between −0.0004 and 0.0003 mL/min. The seeds still respire, but they give out as much CO₂ as the O₂ they take in.
• No equilibration at 25 °C: the gas warms and expands. In most runs the drops in tubes 2 and 3 are pushed out of the capillary within the first 5 minutes. At 15 °C the gas cools instead, and the drops are drawn into the tubes.

Questions for students

1. Prediction: from 15 °C to 25 °C, how will the rate of O₂ uptake by germinating peas change?
2. Tube 3 holds only glass beads. What is it for?
3. Run 25 peas at 25 °C for 30 min with KOH. What is the tube 1 rate in µL/g/min?
4. Run again at 15 °C. Calculate Q₁₀ from the two rates.
5. Remove the KOH and run at 25 °C. Explain why the drop hardly moves even though the seeds are respiring.

Answers for teachers:

1. It roughly doubles.
2. It records changes in gas volume caused only by temperature and air pressure. These are subtracted from tubes 1 and 2.
3. Accept 1.45–1.75 µL/g/min.
4. Accept 1.65–2.30; most runs give about 1.9–2.0.
5. KOH absorbs CO₂. Without it, the CO₂ given out replaces the O₂ taken in: RQ is about 1 for starch. The gas volume stays the same.

Common misconceptions

• "Dry seeds are dead." They are alive but dormant. Their rate is about 1.5 % of the germinating rate, which the simulation shows as almost zero.
• "The drop moves because the seeds give out gas." With KOH the drop moves towards the tube because O₂ is used up and the CO₂ is absorbed.
• "The bead tube is not needed in a water bath." Small temperature and pressure drifts still move every drop. Tube 3 measures them.
• "Q₁₀ of 2 means the rate doubles every degree." It doubles for each 10 °C rise.

Extension

• RQ. Run at 25 °C with KOH, then without KOH. The simulation calculates the respiratory quotient from the two runs.
• Peas or maize. Compare µL/g/min for peas and maize at 25 °C. Ask why per-gram rates are needed when the seeds differ in size.

FAQ

Why use µL/g/min and not mL/min?

mL/min depends on how many seeds are in the tube. Dividing by the seed mass gives the same rate for 10 or 25 seeds, so runs with different numbers can be compared.

Why does my rate differ from my neighbour's?

The simulation adds a few percent of natural variation between seed batches, plus small temperature and pressure drifts. That is why the questions accept a range.

Can I pin the settings for my class?

Yes. On the Share page, create a link and set the seeds, number, temperature, run time, KOH and equilibration in the starting values.

Related

Fermentation – lactic acid and ethanol pathways Photosynthesis rate – light, CO₂, temperature and the compensation point

For the other side of the carbon cycle, see the photosynthesis virtual lab. Temperature affects respiration through enzymes: see the enzyme activity virtual lab.