Observing Photosynthesis In Leaf Discs
Photosynthesis is an essential process in all green plants: using chlorophyll and sunlight energy to convert carbon dioxide and water into glucose which is then stored in the leaves for the cells to use in respiration. Without this ongoing process plants would die and there would be a huge increase in the carbon dioxide levels in the atmosphere. The products of photosynthesis also include oxygen, imperative for life to exist.
However, photosynthesis itself, like respiration, is a hard process to study. We can look at the effects of it rather than the process itself, to know that it is happening:
Carbon dioxide + water → glucose + oxygen
We can investigate the production of oxygen, and this is routinely done in schools, looking at pondweed and counting the bubbles of oxygen given off, however this is a fiddly practical to set up and the correct pond weed is proving harder and harder to find.
Here, we show you a different way to look at photosynthesis, using leaf discs.
- Leaf discs (about 10 per group), punched from a fast-growing plant
- Hole punch/jumbo straw/cork borer to punch the leaf discs
- Syringe ,10 mL
- Sodium carbonate/bicarbonate solution 0.2 moldm-ᶟ
- Light source
- Remove the plunger from the syringe.
- Punch out lots of leaf discs using the punch/borer or straw.
- Hold the syringe (point down) with your thumb over the pointed end, and fill with carbonate solution (5 – 6 mL), with a drop of detergent added*.
- Still holding the syringe, add the leaf discs to the solution.
- Keeping your thumb on the end, add the plunger and turn the syringe upside down (pointed end up) you can remove your thumb.
- Expel the air from the syringe.
- Replace your thumb and pull the plunger back (it will be hard; you are creating a vacuum).
- Hold the plunger back for a few seconds whilst gently shaking the syringe, to release the air from the air sacs in the leaf discs.
- Keeping your thumb over the end, allow the plunger to return to the start position to where there was no air in the syringe (i.e., gently hold it as it slides back into position) and see if any discs have sunk.
- Repeat points 7 and 8 a few times to expel all the air from the discs (degassing), by which time the discs should have sunk to the bottom of the syringe.
- Take your thumb off the end and place the syringe in the dark (pointed end up) until you are ready to begin.
- Set up a bright light/turn on a light bank and place the syringe in front of it. (You can add the contents of the syringe to a beaker with the same solution before placing it under the light source, this gives more room for the discs to move.)
- Watch as the discs rise.
*Detergent stops the discs from sticking to the sides of the beaker but doesn’t affect the experiment
Saps have a downloadable resource pack for this practical which can be found here
The science bit:
In the setup of the practical, the oxygen in the airspaces of the leaf disc is replaced with a solution containing carbon dioxide, which is heavier and makes them sink.
Once in the light the leaf will start photosynthesizing, using the carbon dioxide in the leaf, and producing oxygen which accumulates in bubbles on the outside of the leaf. This will lighten the disc and allow it to rise in the solution.
If the leaf is photosynthesising quickly (and producing more oxygen) the leaf will rise faster, and vice versa.
So, we can use this method as a way of indirectly studying the effects of temperature or light on photosynthesis.
Compare carbonate solution to distilled water alone.
Compare the rate of photosynthesis in the light or in the dark.
Look at distance from the light (this is a direct comparison to the elodea photosynthesis experiment).