Leaf Cutouts
This experiment will allow you to indirectly show the process of photosynthesis and cellular respiration in a plant’s leaves. This helps to show that plants use energy from cellular respiration to do work (like humans), but also have this additional process of creating energy from sunlight.
Objectives
- Plants perform work like humans (e.g. transporting materials to and from cells and maintaining cell functions) and have to perform cellular respiration to do that.
- You can use indirect measures to quantify invisible processes like photosynthesis
- The difference between making a statement from an observation and from an experimental process
Procedure
- Cut leaf discs
- Single hole punches work well but so do plastic straws.
- Choice of leaf material is critical. Leaf surface should be smooth and not too thick. Avoid plants with hairy leaves.
- Avoid major veins
- Get discs prepared
- 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 leaf disks to suspend them in the solution. Let off the vacuum. The bicarbonate solution will infiltrate the air spaces in the leaf causing the disks to sink. You will probably have to repeat this procedure 3‐4 times in order to get the disks to sink. If you have difficulty getting your disks to sink after about 4 tries, it is usually because there is not enough soap in the solution. Add a few more drops of soap.
- Create a testing cup
- Pour the disks and solution into a clear plastic cup. Add bicarbonate solution to a depth of about 3 centimeters. Shallower depths work just as well.
- Place under the light source and start the timer. At the end of each minute, record the number of floating disks. Then swirl the disks to dislodge any that are stuck against the sides of the cups. Continue until all of the disks are floating.
- Note the point at which 50% of the leaf disks are floating (the median)
- Explanation of the process
- Leaf disks float, normally. When the air spaces are filled with solution the disks sink. The infiltration solution includes a small amount of sodium bicarbonate. The bicarbonate serves as the carbon source for photosynthesis (instead of carbon dioxide which it normally uses). As photosynthesis proceeds oxygen is released into the interior of the leaf, which changes the buoyancy‐‐causing the disks to rise. Since cellular respiration is taking place at the same time, consuming oxygen, the rate that the disks rise is an indirect measurement of the net rate of photosynthesis (photosynthesis – cellular respiration
- Note that after a while the disks begin to sink. Why? Cellular respiration removes the oxygen from the cell spaces. The rate that the disks sink is an indirect measure of the rate of cellular respiration. Can you think of a way to how you might measure the gross rate of photosynthesis with this technique?
Questions to Think About...
- Why do you take the measurement from the disc that is from the halfway (1/2) mark? Why not record the first or last disc to float?
- Would you trust the results from the whole class over just your own observation? Why?
- How could you improve the design of this experience to get a more conclusive result?