Photosynthesis (Practical) Flashcards
What equipment do you need for this practical?
Boiling tube, metre stick, light source, stand to hold the boiling tube, pondweed
How would you set up this practical?
-Take a boiling tube and place it 10cm away from the light source (use an LED as too much heat could damage)
-Fill the boiling tube with sodium hydrogen carbonate solution, to help with photosynthesis
-Place the pondweed into the boiling tube with the cut end at the top
-Leave for 5 mins to acclimatise
-You should see bubbles being produced from the cut end of the pondweed
-The bubbles are oxygen, produced by photosynthesis
-Start a stop-watch and count the number of bubbles produced in 1 min
-Repeat 2x and calc the mean of bubbles produced in 1 min
-Repeat this experiment from 20cm, then 30cm, etc
What are some problems with the practical?
-The number of bubbles can be too fast to count accurately
-The bubbles are not always the same size, and a large bubble will count for a small one
How can you solve these issues?
Measuring the volume of oxygen produced instead of counting bubbles
What extra equipment would you need for this?
-Measuring cylinder
-Funnel
How would you complete this?
-Place the pondweed under a funnel and catch the bubbles in a measuring cylinder
-Use the measuring cylinder to measure the volume of the oxygen gas produced
If we plot the mean number of bubbles per min/vol of oxygen per min against the distance from the lamp to the pondweed, then you get…
A graph with a curved line of best fit
If we double the distance, then the number of bubbles per min falls by a factor of…
4
Because of this, going from a distance of 10cm to a distance of 20cm, causes the number of bubbles per min to…
Fall by a factor of 4
Going from 20cm to 40cm, the number of bubbles per min again falls by…
4 times
This is called…
The Inverse Square Law
If we double the distance, the light intensity falls by…
4 times
Because we need light for photosynthesis, when the light intensity falls by 4 times, so does…
the oxygen produced
