Name ______________________________________________________________ Period _______

Objective

How can we tell if photosynthesis is occurring? Will the leaf discs in this lab sink or rise when exposed to

CO2 solution?

Background Information

Leaf disks float, normally. When the air spaces are infiltrated with solution, the overall density of the leaf

disk increases and the disk sinks. The infiltration solution includes a small amount of Sodium bicarbonate

(NaHCO3). In this lab, the Bicarbonate ion (HCO3-) serves as the carbon source for photosynthesis. 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.

Hypothesis

If ___________________________, then leaf discs should ___________ because _______________

Materials

Sodium bicarbonate (Baking soda)

Liquid Soap

Large plastic syringe

Leaf material

Hole punch

Beakers

Timer

Light Source

Measuring the Rate of Photosynthesis Lab 7th Grade Science

Unit 8

Procedure

1. Prepare 100 ml of bicarbonate solution for each trial.

a. The bicarbonate serves as an alternate dissolved source of carbon dioxide for photosynthesis.

Prepare a 0.2% solution. (This is not very much it is only about 1 gram of baking soda in 100 ml

of water.)

b. Add 1 drop of dilute liquid soap to this solution. The soap wets the hydrophobic surface of the

leaf allowing the solution to be drawn into the leaf. It’s difficult to quantify this since liquid

soaps vary in concentration. Avoid suds. If your solution generates suds then dilute it with

more bicarbonate solution.

2. Cut 10 or more uniform leaf disks for each trial.

a. Single hole punches work well for this but stout plastic straws will work as well.

b. Choice of the leaf material is perhaps the most critical aspect of this procedure. The leaf

surface should be smooth and not too thick. Avoid plants with hairy leaves. We will use fresh

spinach leaves.

c. Avoid major veins.

3. Infiltrate the leaf disks with sodium bicarbonate solution.

a. Remove the piston or plunger and place the leaf disks into the syringe barrel.

b. Replace the plunger being careful not to crush the leaf disks.

c. Push on the plunger until only 1mL of air and leaf disk remain in the barrel (< 10%).

d. Pull 9mL of sodium bicarbonate solution into the syringe. Tap the syringe to suspend the leaf

disks in the solution. Push any remaining air out of the syringe.

e. Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold

this vacuum for about 10 seconds.

f. While holding the vacuum, swirl the leaf disks to suspend them in the solution. Continue to

hold your finger over the end of the syringe and push back in on the plunger of the syringe.

The bicarbonate solution will infiltrate the air spaces in the leaf causing the disks to sink.

4. Pour the disks and solution into the solution remaining in the beaker.

5. For a control infiltrate leaf disks with a solution of only water with a drop of soap--no bicarbonate or no light.

6. 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 beakers.

Video of the Experiment

http://www.youtube.com/results?search_query=photosynthesis+leaf+discs

Time

(minutes)

Number of discs floating

Leaf Discs with Light

(Experimental Group)

Leaf Discs Without Light

(Control Group)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Graph

Title

Conclusion

1. Did the results confirm or reject your initial hypothesis? Why or why not?

2. What gasses are being consumed and produced in photosynthesis?

3. How do the gasses consumed and produced in photosynthesis affect the buoyancy of the leaf?

4. Identify the following elements of experimental design in this experiment:

a. Independent variable:

b. Dependent variable:

c. Control group:

d. Experimental group:

e. Identify three constants:

5. Possible errors:

a. What possible errors may have affected the results? (Identify at least one.)

b. How would the results be different had these errors not occurred?

6. Briefly explain how this experiment could be changed to test a different variable that may impact the

rate of photosynthesis.