The Effect of Light Intensity on Photosynthetic Rate
Integrated Science 3
Name:
10/10
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
, light

 C6H12O6 + 6 O 2
written as: 6 CO 2 + 6 H 2O enzymes,cholorphyll
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 other animals) eat a
plant then they will use glucose that the plant has not used as an energy source for their own growth and
reproduction. Oxygen produced in photosynthesis is a waste product and is given off as a gas. The rate of
photosynthesis is directly related to the production of O2 [ O2 produced = photosynthesis rate ].
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 light intensity on the
production of glucose in the leaves of 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 plant leaves, which normally float
to sink. As leaves photosynthesize under water the leaves produce oxygen causing 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.
 Procedures
Read the following procedures and complete the Experimental Organizer for this experiment.
1. The set-up for your lab group includes following materials: timer; 20 ml syringe; 100 ml graduated cylinder;
sodium bicarbonate (NaHCO3) solution with diluted dish soap; 250 ml beaker; 400 ml beaker; ruler; gooseneck lamp with a 200 W light bulb, and plant leaf disks.
2. Add 300ml of the NaHCO3- soap solution to your the 400 ml beaker. Gently swirl this mixture; avoid making
bubbles.
3. Remove the plunger of the syringe and place 10 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.
4. Draw a small volume (about 5 ml) of sodium bicarbonate solution into the syringe. Tap on the syringe to
suspend the leaf disks into the solution.
5. Place a cap (or your finger) over the end of the syringe and draw back on the plunger to create a vacuum. Hold
this for 10 seconds. While doing this, swirl the syringe to continue suspending the leaf disks in solution.
6. If necessary, repeat the vacuum procedure 2 or 3 times until all of the leaf disks sink to the bottom.
7. 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 additional
sodium bicarbonate solution to the 250 ml
beaker to make the total volume of 100 ml.
8. Place the beaker underneath the 100W light
bulb located 5, 10 or 15 cm above the top
edge of the beaker. Measure from the bottom of the bulb.
9. Turn on the light and start the timer. Record the time, in seconds, when the 5th disk floats to the surface. Also
record how the disks rise using the following ranking systems:
Type of Disk Rising
Ranking
Steady rise of disks
1
Sporadic rise of disks
2
All disks rise together
3
10. Repeat the procedure 3 times, each with new leaf disks and with new sodium bicarbonate/soap solution, for the
following three distances: 5 cm, 10 cm, 15 cm
11. Record data in your individual group data table and in the class data table on the classroom data table.
12. Create a Laboratory Notebook Entry on a separate sheet of paper.
Individual Group Data Table
Light Intensity
Quantitative Data
(distance in centimeters)
Time (seconds)
Light Intensity
(distance in centimeters)
5
5
10
10
15
15
Qualitative Data
Rising of Disks (1, 2, 3)
 Data and Results
1. Using class data values, calculate all data analysis values for quantitative and qualitative data.
2. Construct graphs for quantitative and qualitative data, displaying central tendency and variation values.
 Discussion Questions
Type answers to the below questions.
 What was the effect of light on oxygen production. Make references to specific data values (central
tendency and variation) to support your conclusions.
 Based on the comparison of standard deviation and frequency distribution values, are the results reliable?
Explain.
 Using your knowledge of photosynthesis, discuss how variable environmental conditions, such as reduced
sunlight to the earth’s surface, might play a role in determining the carrying capacity for humans on earth.
 Design an experiment using the same set up to investigate a different variable in the rate of photosynthesis.
Include an explanation of why you chose this variable to test and what you would expect the results to be.
2
Photosynthetic Rate Lab
Experimental Organizer
Title:
Hypothesis:
Independent Variable:
Continuous
Discontinuous
Treatments of I.V.
(indicate control treatment):
# of trials you will conduct
for each treatment:
Dependent Variable (s): describe quantitative and qualitative data
Quantitative Measurements (include units):
Qualitative Measurements:
Constants:
3
Photosynthetic Rate Lab