Read

Photosynthesis is the process by which autotrophs turn light energy into chemical energy. The balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light absorbed by chlorophyll.

In plants it takes place mainly in the leaves, in chloroplasts in the mesophyll layer. Gas exchange happens through stomata, which the plant can open and close.

The glucose produced is the starting point for the plant's other molecules and is broken down in respiration to release energy. Heterotrophs, including humans, depend on it through food chains.

Experiment: Pondweed bubbles and light

1. Add a small pinch of sodium hydrogencarbonate to the water and stir. Put the pondweed in the tube, cut end up, held down by the paper clip.
2. Place the lamp 10 cm from the tube. Wait 2 minutes for the plant to adjust.
3. Count the bubbles leaving the cut end in 1 minute. Repeat twice and take the mean.
4. Move the lamp to 20, 30, 40 and 50 cm and repeat. Keep the room lights off or dimmed.
5. Plot the mean number of bubbles per minute against the distance. Describe the trend.

Distance (cm)	Bubbles/min 1	Bubbles/min 2	Bubbles/min 3	Mean
10	Predict the shape of the graph of bubbles per minute against distance.
20	Predict the shape of the graph of bubbles per minute against distance.
30	Predict the shape of the graph of bubbles per minute against distance.
40	Predict the shape of the graph of bubbles per minute against distance.
50	Predict the shape of the graph of bubbles per minute against distance.

Questions

1. Write the balanced equation for photosynthesis and label the reactants and products.

2. Explain how transpiration moves water from the roots to the leaves.

3. Sketch your graph of bubbles per minute against lamp distance and describe the trend.

   🍎

4. Give two limitations of counting bubbles as a measure of the rate, and one improvement.

Sources for every fact on this sheet: echoesof.science/school/plants/sources/

🖨 Print this sheet ← back to the lesson