Elena investigated how light intensity affects photosynthesis and respiration in pondweed sprigs.
She placed 5 cm3 of green bromothymol blue indicator solution into each of three glass vials, labelled A, B, and C.
All three vials were placed in a water bath maintained at 20∘C20^\circ\text{C}20∘C for two hours.
Bromothymol blue is green in normal atmospheric levels of carbon dioxide. It turns blue as carbon dioxide levels decrease, and yellow as carbon dioxide levels increase.
The results at the end of the experiment are shown below:
| Vial | Colour of indicator at start | Colour of indicator at end |
|---|---|---|
| A | green | yellow |
| B | green | green |
| C | green | blue |
Explain the colour change observed in vial C.
Explain the purpose of vial B.
Elena set up a fourth vial, vial D, containing a pondweed sprig in the indicator solution, and placed it under a mesh filter that allowed only extremely faint light to pass through. After two hours, the colour of the indicator solution in this vial remained green. Explain why.
Elena carried out a further investigation to examine the effect of temperature on the rate of photosynthesis in pondweed.
Suggest how she could vary her independent variable.
Suggest how she could measure her dependent variable.
Suggest how she could ensure that her results were reliable.
