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.
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.
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)
Small beaker x2
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.
1. 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 2. Add 1 drop of dilute liquid soap to this solution.
3. 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. 4. 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. 5. Pull a small volume of sodium bicarbonate solution into the syringe. Tap the syringe to suspend the leaf discs in the solution. 6. 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. 7. Pour the discs and solution into a small beaker. Add bicarbonate solution to a depth of about 3 centimetres. 8. 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. 9. In a new beaker, repeat steps 4-7.
10. 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. 11. In a third beaker, repeat steps 4 – 7.
12. 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...