The Effect of Light Intensity and Temperature on Photosynthetic Rate
Integrated Science 3 Honors
Name:
Per:
Background
The human species is dependent on photosynthesis for virtually all of our food/energy needs.
Whether we eat plants directly or indirectly (by eating animals), it is the photosynthesis conducted in the
leaves of those plants that limits the food resources and ultimately, the population of our species.
The overall equation for photosynthesis is written as:
6 CO2 + 6 H2O "enzymes,cholorphyll,light
" " " " " ""→ C6H12O6 + 6 O2
In words, this says that carbon dioxide combines with water to form glucose and oxygen. This
chemical change will take place as long as chlorophyll, certain enzymes, and light energy are present. The
plant uses the glucose produced in photosynthesis as an energy source to grow and reproduce. If people (or
€ then they will use glucose that the plant has not used as an energy source for their
other animals) eat a plant
own growth and reproduction. Oxygen produced in photosynthesis is a waste product and is given off as a
gas. If a lot of oxygen is being given off, photosynthesis is occurring rapidly. If little oxygen is being given
off, photosynthesis is occurring slowly.
Since photosynthesis is so important to the human population, it is important to understand the
factors that might limit the rate of this chemical reaction. In this lab, we will examine the effect of increased
temperature and light energy on the production of glucose in the leaves of spinach plants. We will assess
this reaction rate by a method called the floating leaf disk assay (FLDA). This technique utilizes a procedure
that causes small disks of spinach leaves, which normally float to sink. As these leaves photosynthesize,
they produce oxygen that causes the leaves to become more buoyant and ultimately float. The rate at which
the leaves float gives a measure of the overall rate of photosynthesis.
Based on what we understand of photosynthesis, increased light intensity should increase the
photosynthetic rate by making more energy available to excite electrons in the light dependent reactions.
Additionally, higher temperatures should also increase the photosynthetic rate by increasing the overall
movement of the molecules involved in both the light dependent and light independent reactions. We will
attempt to answer which of the variables, light or temperature, has a greater effect on photosynthesis. We
will also be able to look for the optimal value of each variable’s effect on photosynthetic rate.
Procedures
1. Gather the following materials: timer; 12 cc syringe; hole puncher; leaves; diluted dish soap; dropper
pipette; 100 ml graduated cylinder; sodium bicarbonate (NaHCO3); 250 ml beaker; 400 ml beaker;
stirring rod; goose-neck lamp; 40 W, 100 W and 200 W light bulbs; water supply, at target temperature.
2. Your lab team will be assigned a particular temperature variable, based on your table number: Tables 1
and 8 = 10º C; Tables 2 and 7 = 20º C; Tables 3 and 6 = 30º C; Tables 4 and 5 = 40º C. Each lab team will
also test all three levels of light intensity, using 40 W, 100 W and 200W light bulbs.
3. Using one of your spinach leaves, carefully punch and collect 12 leaf disks. In doing this, you should
avoid major leaf veins.
4. Using the 400 ml beaker, mix 300 ml of water – at your team’s correct temperature – with small scoop
(demonstrated in class) of sodium bicarbonate and one drop of the dilute soap solution. Gently stir this
mixture, without making bubbles, until the sodium bicarbonate is dissolved.
5. Remove the plunger of the syringe and place your leaf disks in the syringe barrel. Replace the plunger
and, without breaking any disks, push the plunger down until only a small volume of air remains.
6. Draw a small volume (about 5 cc) of sodium bicarbonate solution into the syringe. Tap on the syringe to
suspend the leaf disks into the solution.
7. Place your finger over the end of the syringe and draw back on the plunger to create a vacuum. Hold
this for 10 seconds. Follow this by gently pushing down on the plunger for 5 seconds.
8. Continue to repeat the vacuum procedure, from above, until all of the leaf disks sink to the bottom.
9. Remove the plunger from the syringe and empty the leaf disks and solution into the 250 ml beaker. If
necessary use more of the sodium bicarbonate solution to flush all the disks from the syringe. Add an
additional 150 ml of the sodium bicarbonate solution to the 250 ml beaker.
10. Place the beaker underneath a 40 W light bulb located 7.5 cm above the top edge of the beaker.
11. Turn on the light and start the timer. Record the time, in seconds, for the 6th disk to float to the surface.
12. Repeat the procedure (2) times for each of the three light bulbs (40W, 100W, 200W). You will need to remix the sodium bicarbonate solution each time, to make sure your target temperature is correct.
13. Record your team’s data in the class data table, located on the classroom computer. You will receive a
copy of this class data for your data analysis.
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Design Outline and Data Tables
After reading the experimental procedures above, write a design outline for this laboratory activity
on a separate sheet of paper. It will be used to help write your laboratory report, but the outline itself will
not be a component of your formal laboratory report. You will also need to construct appropriate data
tables to collect you team’s data. We will share class data using a common data table set-up.
Laboratory Report Assignment
The purpose of this activity is to model the scientific process by conducting a laboratory experiment.
You will be responsible for writing a formal lab report using the format/information from the RHS Lab
Report Format document.
Introduction
Rough Draft Due:
Use information from the introduction of this lab handout, your textbook and other research materials to
write an Introduction to the experiment. You must include appropriate reference citations.
Hypothesis
Rough Draft Due:
Make specific predictions about how both light intensity and temperature will affect the rate of
photosynthesis in spinach. You may, but are not required to, use the standard ‘if… , then …’ format.
Experimental Design
Rough Draft Due:
Using your Design Outline and the procedures from this handout, write an Experimental Design section for
this experiment. Your experimental design should include the design of the experiment and procedures to
carry out that design. It must be written in paragraph form, not done as a list.
Data and Results
Rough Draft Due:
Data Table(s): Design and complete the appropriate data table(s) for this experiment. You should have
tables for both the raw data and the statistical calculations that summarize the raw data. Be sure to include
all appropriate data analysis calculations.
Graph(s): Create graph(s) that most accurately represent the data collected in this experiment.
Discussion
Rough Draft Due:
Using the data and results, write a Discussion for this experiment. Your Discussion should discuss and
explain the results of the experiment, an analysis and critique of the experimental variables and procedures,
a comparison of your results to other experimenters, and suggestions for further studies relating to the
experimental variables.
Consider the following questions as you explain the results of the experiment:
 How does light intensity influence photosynthetic rate? What might be an implication of reduced
sunlight to the earth’s surface? Under what circumstances might this occur?
 How does temperature influence photosynthetic rate. What might be an implication of increased or
decreased temperature in photosynthesis? Under what circumstances might this occur?
 Discuss how rate of photosynthesis might play a role in predicting and determining the carrying
capacity for humans on earth.
Conclusion
Rough Draft Due:
Write a Conclusion that summarizes the purpose and results for this experiment.
References Cited/Bibliography
Rough Draft Due:
Cite sources according to the format presented in the Guidelines.
Final Draft Due:
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