The Effect of Light Intensity on Photosynthetic Rate ...

The Effect of Light Intensity on Photosynthetic Rate

Integrated Science 3

Redwood High School

Name:

10/12

Per:

n 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 "e"nzym"es,"chol"orp"hyll,"lig"ht C6H12O6 + 6 O2

In words, this says that carbon dioxide combines with water to form glucose and oxygen. This chemical change will take place aslong 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 ivy 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 leaf disks, 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

1. The set-up for your lab group includes following materials: timer, 20 ml syringe, 250 ml beaker, ruler, goose-neck lamp with a 100W light bulbs, leaves and a hole puncher.

2. After using the initial leaf disks provided you will need to hole punch more leaf disks as needed. 3. Remove the plunger of the syringe and place 7 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 5ml) of sodium bicarbonate solution into the syringe (to be passed around just

before you need it ? small amount in 250ml beaker). Tap on the syringe to suspend the leaf disks into the solution.

* A VIDEO IS AVAILABLE TO DEMONSTRATE THE EVACUATION OF GASES FROM THE DISCS

5. Evacuation of Gases from the Discs. Place a cap or finger over the end of the syringe, point the end of the syringe at the floor and draw back on the plunger to create a vacuum. Hold this for 10-15 seconds. Bring the syringe back to an upright position and check to see if the leaf disks have sunk to the bottom of the chamber.

6. If necessary, repeat the vacuum procedure 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; fill to the 200ml line. 8. Place the beaker underneath the 100W light bulb located 5 cm above the top edge of the beaker. 9. Turn on the light and start the timer. Watch for leaf disks to flip over/float upwards. Record the time, in seconds, when the 5th disk flips over. 10. Repeat the procedure a total of 3 times, with new leaf disks each time for each of the three light bulb distances: 5 cm, 10 cm, 15 cm *Replace half the NaHCO3 solution for trials 2 and 3, gently swirling each time. 11. After recoding your data in your group data tables, record your group's data in the class data tables located on the classroom computer. You will receive a copy of this class data for your data analysis.

n Lab Write-Up [Typed]

*refer to your guidelines packet

Design Outline Experimental Design Data and Results Discussion Conclusion Lab Notebook Entry

Group Data Table

Quantitative Data Light Intensity

(distance in centimeters) 5

10

15

Time For All Leaf Disks To Float (seconds) Trial

Light Intensity (distance in centimeters)

Qualitative Data Pace (slow, med. rapid)

Trial

5

10

15

2

Photosynthetic Rate Lab

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