Plant measurements during day and night: results

This experiment was run over 24 hours, starting just before 19:00 in the evening at the end of April. Sunset was given as 20:18. The top set of graphs clearly show that the oxygen level drops overnight in a sealed bag with a tray of growing grass turf. Light is shown in the top graph. Oxygen levels rose a few hours after sunrise, and showed an upward trend, until starting to drop again as light levels began to fade. The humidity graph is rather more complicated, showing a gradual drop at night followed by sharp dips. There were water droplets condensing on the plastic bag during the day, which presumably explain the sharp dips. DrDAQ measures relative humidity, so it is to be expected that this will fall as the temperature rises, as warm air will hold more water vapour. The high temperatures during the day can be attributed to light falling directly onto the sensors. The room temperature was 20°C rather than 30°C.

The change in the oxygen level may not seem very large, but the lowest level of 17.9% is considered to be very oxygen deficient. The oxygen content of air is about 20.9%, and safety monitors are generally set to respond at 20.5% oxygen. Below 18% is considered hazardous.

A control experiment was run over the same time frame a day later, with damp soil in the same tray, and no grass. This showed a drop in humidity as the temperature rose (see above for comment on relative humidity). The oxygen level also dropped as the light level and temperature rose, and then returned to nearly the original level as the temperature and light levels fell in the evening. There is no obvious reason for this change in oxygen level, but it was much smaller and showed an opposing trend to the experiment with the growing grass. It may have been an artefact of having the oxygen sensor in direct light.

Answers to questions

1. Photosynthesis takes place in leaf cells. These contain chloroplasts, which contain chlorophyll. Chlorophyll is the name for a few green pigments that absorb visible light strongly. They mostly absorb red and blue light wavelengths. The green wavelengths are poorly absorbed, and so leaves appear green. The name chlorophyll comes from the Greek words for ‘green’ and ‘leaf’.
2. Some of the glucose is used in photosynthesis to produce energy for the plant. Some is stored for later use in the form of starch, fats and oils. Glucose is used in making cellulose which strengthens cell walls and in making proteins.
3. Low temperature and inadequate supply of carbon dioxide will slow and eventually stop photosynthesis. (Lack of water, pollution and mineral deficiency leading to a lack of chlorophyll will also have an effect.)
4. Leaves usually have a large surface area, frequently with a rough underside, giving an even greater surface area. They contain chlorophyll in chloroplasts to absorb light. They have stomata for gas exchange. These are tiny holes which can open and close allowing gases to diffuse in and out.
5. Net oxygen is produced during the day and consumed at night. There are seasonal variations with more photosynthesis during the summer months. Overall, growing plants will produce more oxygen than they consume, but in a mature forest, this will be balanced by decomposition, where microorganisms consume oxygen. (See the Decomposition of grass cuttings experiment).
6. The same answer as above applies, but in reverse. Overall, growing trees will consume carbon dioxide, but microorganisms will release it as part of the decay process.

Teachers’ notes

This experiment should be considered with decomposition to explain why mature plant systems have no net oxygen or carbon dioxide emission to the atmosphere.

This experiment can be carried out with a pot plant, but better results were obtained with a piece of overgrown turf.  The setup must be right by a window, and carried out between April and September when plants are actively growing. Low night-time temperatures outside were not a problem in an insulated building.

Look at the combined heat and power system used to grow tomatoes by a nursery near a giant sugar refinery, described at britishsugar.co.uk/tomatoes.aspx. As well as using waste heat, carbon dioxide is pumped into the greenhouses to boost plant growth. .

Credits, comments and further info

This experiment was written by Susan Hammond of Woking College.