Question

A flask of duckweed in water is placed under lights on a 12-hour light, 12-hour dark cycle, next to a control flask containing only water. Assuming there is no change in water temperature, which flask will have the lowest dissolved oxygen (DO) concentration, at what time? A. DO will always be lower in the flask with duckweed compared to the control. B. DO will always be higher in the flask with duckweed compared to the control. C. The flask with duckweed will have the lowest DO at the end of the dark cycle and the highest DO at the end of the light cycle. D. DO will be the same between the control and the duckweed-containing flask.

Answer 1

Duckweed performs photosynthesis. Photosynthesis produces oxygen. They take in carbon dioxide and produce oxygen. So, when it is kept in the light cycle it will produce oxygen which will get dissolved in the water.

But the flask with water does not contain dissolved oxygen. So, at the end of light cycle the flask containing the plant will have more DO.

Now, in dark the plant does not produce oxygen. Instead it uses it to break the glucose and produce carbon dioxide. While this is not the case with the flask without the weed.

So, the flask without the plant will have more DO then the flask with the plant.

So, at the end of light cycle the flask with the weed will have more oxygen while at the end of dark cycle the flask with water will have more DO.