Leaf discs float normally. When the air spaces are infiltrated with the solution the overall density of the leaf disc increases and the disc sinks. The infiltration solution includes a small amount of sodium bicarbonate. The bicarbonate ions serve as the carbon source for photosynthesis. As photosynthesis proceeds oxygen is released into the interior of the leaf which changes the buoyancy – causing the discs to rise. Since cellular respiration is taking place at the same time, consuming oxygen, the rate of which the discs rise is an indirect measurement of the net rate of photosynthesis.
To find out if light affects the rate of photosynthesis through an investigation with leaf discs of a spinach leaf. Hypothesis: If the light source is removed or becomes limited, then it will take much longer for the leaf disc to rise as cellular respiration will no longer be taking place as part of the equation for photosynthesis will be missing; light. Indirectly, this means that the rate of which the discs rise will take longer.
Materials: 3 Leaves Hole punch Lamp Diluted soap (one drop) x2 1 X 1 metre Aluminium foil x2 1 X 1 metre Glad wrap x2 300mL water x2 1g of sodium bicarbonate x2
Plastic syringe (10cc or larger) Stopwatch Small beaker x2 Variables: Dependent variable: Spinach leaf disks as it is changed by not only the bicarbonate soda but also possibly light and many other factors. Independent variable: Time as we are measuring how long the photosynthesis will take to make the leaves rise. Time is also not influenced by any other factors.
- Prepare a solution: using scales, measure 1 gram of sodium bicarbonate. Then, using a measuring cylinder, measure 300ml of water. Place both materials into a beaker
- Add 1 drop of dilute liquid soap to this solution.
- Using a hole punch, cut 10 uniform leaf discs for each trial. Be careful to not cut yourself in this process. Do not use a knife in replace of a hole punch as this is extremely dangerous.
- Remove the piston or plunger and place the leaf discs into the syringe barrel. Replace the plunger being careful not to crush the leaf discs. Push on the plunger until only a small volume of air and leaf disc remain in the barrel.
- Pull a small volume of sodium bicarbonate solution into the syringe. Tap the syringe to suspend the leaf discs in the solution.
- Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold this vacuum for about 10 seconds. While holding the vacuum, swirl the lead discs to suspend them in the solution. Let off the vacuum. The bicarbonate solution will infiltrate the air spaces in the leaf causing the discs to sink. You will probably have to repeat this procedure 2 or 3 times to get the discs to sink. During this procedure, make sure to be conscious of who and what is around you as it is possible to elbow someone or possibly something dangerous during this.
- Pour the discs and solution into a small beaker. Add bicarbonate solution to a depth of about 3 centimetres.
- Place under the light source and start the timer. At the end of each minute, record the number of floating discs. Then swirl the discs to dislodge any that are stuck against the sides of the beaker. Continue until all of the discs are floating.
- In a new beaker, repeat steps 4-7.
- Cover this second beaker with aluminium foil. Then, place it under the light source and start the timer. At the end of each minute, record the number of floating discs. Then swirl the discs to dislodge any that are stuck against the sides of the beaker. Continue until all of the discs are floating.
- In a third beaker, repeat steps 4 – 7.
- Cover this third beaker with glad wrap, limiting the amount of light the can go into the solution with the discs. Then, place it under the light source and start the timer. At the end of each minute, record the number of floating discs. Then swirl the discs to dislodge any that are stuck against the sides of the beaker. Continue until all of the discs are floating. When cutting the glad wrap or aluminium foil, make sure not to cut yourself. Never touch the light bulb when the lamp is turned on as this is extremely dangerous and you are very likely to hurt yourself.
Do not eat anything in the lab as it is not sanitary and you do not know how damaging chemicals can be to your digestive system. Always make sure you wear your lab coat as solutions can stain some clothes.
As the results show, the control variable test showed that it would usually take ten leaf disks to float in a sodium bicarbonate, water and soap solution 8 minutes and 11 seconds.
The leaves float because of cellular respiration, or photosynthesis. In these processes, 12H2O + 6C02 —–(light)—– > C6H12O6 + 6O2 + 6H2O. In other words, twelve molecules of water and six molecules of carbon dioxide which are catalysed by light produce glucose, oxygen and water. In the experiment, the sodium bicarbonate serves as the carbon molecules for photosynthesis. Then, when the light is applied to the solution and discs, the reaction is complete and so the disks float. With the aluminium foil covering a solution identical to the control, however, the outcome is completely different.
It showed that no matter how long we waited, the discs never began to float in the sodium bicarbonate, water and soap solution. This is because photosynthesis uses light for the reaction to take place. When the beaker was covered with the aluminium foil, we were not allowing the light into the leaves and the leaves therefore did not float as photosynthesis could not take place. As interesting as these results were, the most interesting results lies with the glad wrap. Considering that the glad wrap was not completely covering the light from the disks, there were only 3 possible options for how the discs will react.
They could rise faster, they could take the same amount of time or they could take longer. As the amount of light that could have flowed into the beaker was limited, however, it can be expected that due to this fact, the leaves would take longer to float. This was not the case, though, as the time it took for ten leaves to rise was one minute and 31 seconds quicker than the control trial. That’s 6 minutes and 40 seconds for all ten leaves to rise. As nearly impossible as this seems, there is a very likely reason for this to have happened.
Due to time restraints, we decided that doing two experiments at once would be the best option for us so what would usually take 20 minutes only took 10. When we did this, we had the beaker wrapped with aluminium on the right and the beaker wrapped with glad wrap on the left, both under the same lamp. Because light from a lamp is not concentrated on one spot and also because aluminium has a very reflective surface, when the light was directed onto both beakers, some light reflected from the aluminium onto the glad wrapped beaker.
This meant that light was more concentrated for the glad wrapped beaker which would have altered the results in comparison to the control trial to be quicker for the ten leaves to float. Brad Williamson from elbiology. com also trialled this experiment and he too came up with approximately the same results. Discrepancies and Errors: Our biggest discrepancy during our experiment was trials 4, 5 and 6. Due to lack of organisation, we only had half an hour to do all of these trials. Naturally, due to the fact that it is impossible for the leaves to rise anyway, trial 5 (aluminium covered) worked as it did in trial 2.
However, trials 4 and 6 did not work at all. This is most likely because we accidentally added in more than one drop of soap, as “The soap wets the hydrophobic surface of the leaf allowing the solution to be drawn into the leaf. ” according to Brad Williamson. However, if too much soap is added, making suds, the sodium bicarbonate becomes overpowered in the solution, rendering it useless and therefore not making the experiment work. Our next biggest discrepancy was placing the aluminium foil next to one of our trials. This made our results for the glad wrap very doubtful to be true.
This means that it is an unreliable result. If I were to do this experiment again, I would make sure to have more than enough time to complete the required tasks. When this experiment is rushed, it seems that so many more processes can go wrong, therefore making your results unreliable, as was seen from our errors. You need to make sure you have at least 40 minutes to do the observations and at least 20 minutes to set up the experiment. Without this, it is likely that at least one of your results will be incorrect.
As predicted, if the light source is removed or becomes limited, then it will take much longer for the leaf disc to rise. However, not as predicted, if there is no light source at all, the leaves will never move. This is because as cellular respiration will no longer be taking place as part of the equation for photosynthesis will be missing; light. My investigation demonstrated this well as shown by the graph that with a light source, the leaves float, but without a light source, the leaves do not float.
- Wilson, B. (no date). “The Floating Leaf Disk Assay for Investigating Photosynthesis” Retrieved on the 27th of July, 2011.
Cite this Photosynthesis Investigation and Light
Photosynthesis Investigation and Light. (2019, May 01). Retrieved from https://graduateway.com/photosynthesis-investigation-and-light/