Photosynthesis
Question: What factors affect the rate of photosynthesis in living leaves?
Photosynthesis fuels ecosystems and replenishes the Earth’s atmosphere with oxygen. Organisms known as autotrophs
can use energy from the sun to make glucose, which is the goal of photosynthesis. Like all enzyme-driven reactions, the
rate of photosynthesis can be measured by either the disappearance of the reactants (or substrates) or the
accumulation of products (or by-products) The general summary equation for photosynthesis is
Prelab
1. Label the reactants and products above.
2. Use arrows to describe which portions of the reactions would decrease when measuring photosynthesis and which
would increase. Assume the measurements are taking place in a closed system.
3. Summarize the equation in words:
Investigation Use a system that measures the accumulation of oxygen to determine the rate of photosynthesis. The
sodium bicarbonate solution provides a carbon dioxide source, which is a reactant necessary for photosynthesis.
Materials
*baking soda
*liquid soap
*2 plastic syringes without the needle.
*living leaves (spinach, ivy)
*hole punch
*2 clear plastic cups
*timer
*light source
Safety Precautions
*wear safety goggles, glasses, and aprons (optional but recommended)
*keep the solutions away from electrical cords of your light source.
Procedure
1. Using a hole punch, cut 10 or more uniform leaf disks for each cup. Avoid major leaf veins.
Note: When immersed in water, oxygen bubbles are usually trapped in the air spaces of the spongy mesophyll in
the plant leaf. If these bubbles are not released, the leaf disk will float skewing results. A vacuum can be created
using a syringe to pull out the oxygen from the mesophyll in order to measure the rate of photosynthesis.
2. Once 20-25 spinach chads are obtained, remove the plunger or piston from both syringes.
3. Place the 10 leaf disks into each syringe barrel and replace the plunger-be careful not to crush the leaf disks.
4. Push in the plunger until only a small volume of air and leaf disk remains in the barrel (<10%)
5. Pull a small volume (5-6 cubic centimeters (cc) of sodium bicarbonate plus soap solution from the prepared beaker
into the syringe. Be careful not to make suds.
6. Tap each syringe to suspend the leaf disks in solution. Make sure that, with the plunger inverted, the disks are
suspended in the solution. Hold the syringe vertically and move the plunger up to push the air from the syringe.
7. 10. Create a vacuum: This step needs to be done 2 or 3 times. Hold a finger over the narrow syringe opening
while drawing back the plunger. Hold the vacuum for about 10 seconds. While holding the vacuum, swirl the leaf
8.
9.
10.
11.
disk to suspend them in the solution. Now release the vacuum by letting the plunger spring back. The solution will
infiltrate the air spaces in the leaf disk, causing the leaf disk to sink in the syringe. If the plunger does not spring
back a good vacuum was not created. Repeat again until a good vacuum is made.
Release the vacuum and tap the syringe to let the leaf disks sink.
Once most the leaf disks have sunk, empty the contents into one beaker and place the beaker under a piece of
paper. One partner will begin a timer to determine the time it takes for each leaf disk to rise to the surface. At the
end of each minute, record the number of floating disks. Then swirl the disks to dislodge any that stuck against the
side of the cup. Continue until all the disks are floating.
While one partner is timing, the second partner will repeat steps 3-9.
Empty the non-floating leaf disks into the second beaker provided. Place this beaker under the light source and
begin timing the time it takes for each leaf disk to rise to the surface. Record the data in a data table.
Experimental Design
Write a hypothesis- what variable was changed? Use an if/then statement.
Experimental Group
Independent Variable
Control Variables
Control Group:
Dependent variable
Data Table: Title
Graph – Remember Tails – T-Title A-Axis, I-interval, L-labels, S-Scale/ The dependent variable is on the y axis –
remember to label and the independent variable is on the x axis.
Analysis Question: No credit if not RSQ’d
1. What is the overall goal of photosynthesis?
2. Describe how the rate of photosynthesis was measured in this lab. Be detailed.
3. Which beaker photosynthesized at a faster rate? Use evidence (data) to describe the spinach disks that where
photosynthesizing at a faster rate.
4. If the experiment was changed to include a beaker with sodium bicarbonate and one without, which would you
expect to photosynthesize faster and why?
5. What factors can slow the rate of photosynthesis down?
6. What factors can speed the rate of photosynthesis up?
7. Use information from question 4 or 5 to design an experiment.
Questions:
Hypothesis:
Experimental Group
Independent variable
Control variables
Control Group
Dependent variable