The question
Name: ______________________ Date: ____________ Floating leaf disks (9 points)
Experimental setup
Floating leaf disks
Students punched small disks from spinach leaves and used a syringe to pull the air out of the spaces inside them and replace it with a solution, so the disks sank. Each cup held 10 disks in 0.2% sodium bicarbonate solution, which releases CO₂ into the water. Cups were placed under a lamp at five light intensities in a 25 °C room. For each cup the students recorded ET50, the time until 5 of the 10 disks floated. Each light intensity was tested with 5 cups. Two extra cups were also set up: one at the highest light intensity with water instead of bicarbonate solution, and one with bicarbonate solution kept in the dark. In both extra cups, no disks floated within 30 minutes.
| Light intensity (µmol photons per m² per s) | ET50 (min) |
|---|---|
| 50 | 24.5 ± 2.1 |
| 100 | 15.2 ± 1.6 |
| 200 | 9.8 ± 1.0 |
| 400 | 7.6 ± 0.9 |
| 800 | 7.4 ± 1.1 |
(a) Describe the process in the leaf cells that makes the disks float. Explain why disks in water without bicarbonate do not float, even in bright light. [2 points]
(b) Identify the independent variable in this experiment. Identify what the cup kept in the dark controls for. Justify the students' decision to test 5 cups at each light intensity. [3 points]
(c) Calculate the percent change in mean ET50 when light intensity increases from 50 to 200 µmol photons per m² per s. Using the data, determine whether the mean ET50 at 400 and at 800 µmol photons per m² per s are significantly different, using ± 2 standard errors of the mean (SE = SD ÷ √n). [2 points]
(d) The students repeat the 800 µmol photons per m² per s trial with 0.4% bicarbonate instead of 0.2%. Predict the effect on ET50. Justify your prediction using the data. [2 points]