Name: _______________________________________________________ Period: _________ Date: _________

General Biology

Lab: Investigating the Effect of Light Intensity on the Rate of  Photosynthesis



Photosynthesis is the process in which the sun’s energy is converted into chemical energy stored in the bonds of glucose.  It occurs in chloroplasts in the leaves of plants.  The overall equation for photosynthesis is:


            6CO2   +    6H2O     +    sunlight                             C6H12O6    +   6O2


Photosynthesis consists of light-dependent reactions and light-independent reactions. In order to carry out photosynthesis, a plant must have light. Some plants need a lot of light and others do not. This lab will focus on the light-dependent reactions where sunlight energy is captured and stored in ATP and NADPH, and oxygen is released.  ATP and NADP both carry energy within the cell.   ATP (adenosine triphosphate) carries energy in the form of a high energy chemical bond when a phosphate is added to ADP.  The energy stored in ATP and NADPH is used to fuel the light-independent reactions in another part of the chloroplast.  During the light-dependent reactions, water molecules are split and oxygen is released as a byproduct from this reaction. 

Purpose: The purpose of this investigation is to determine the effect of light intensity (bright light and dim light) on the rate of oxygen production during the light-dependent reactions of photosynthesis.

Hypothesis: Write a hypothesis in the form of an If…then statement addressing what effect light intensity (dim light and bright light) will have on oxygen production during the light-dependent reactions of photosynthesis.




250 ml beaker                          One sprig of evergreen                        test tube                       plastic spoon

400 ml beaker                          1000 ml beaker                                    triple beam balance      forceps

Hand lens                                baking soda                                         stirring rod                                          


Procedure:  (For each table of students.)

1.       Obtain a 250 ml beaker and fill it with fresh tap water.

2.      Take the 250 ml beaker to the teacher lab desk.  Cut a fresh sprig of evergreen approximately 6 cm in length.   Cut the sprig under water. 

3.      Immediately place the sprig in the fresh tap water in the 250ml beaker and return to your lab station.

4.      Fill the 1000 ml beaker with 500 ml of tap water from the sink. 

5.      Place a piece of scrap paper on the triple beam balance at your lab station.  Mass out 35 grams of baking soda onto the scrap paper.

6.      Using the paper as a funnel, pour the baking soda into the 1000 ml beaker to make a sodium bicarbonate solution.

7.      Gently mix the sodium bicarbonate solution with a stirring rod.  Some will settle to the bottom.

8.      Pour the sodium bicarbonate solution into a 400ml beaker leaving about a centimeter at the top of the beaker.  Save the remaining sodium bicarbonate solution in the 1000ml beaker. 

9.      Obtain a glass test tube. 

10.  Fill the test tube with some of the remaining sodium bicarbonate from the 1000ml beaker.

11.  Hold the test tube over the 1000 ml beaker and use forceps to place the spring of evergreen into the test tube with the cut end of the sprig pointing downward in the test tube (towards the closed end).  If any of the sodium bicarbonate spills out pour more into the test tube.  It should be filled to the rim.




12.  Completely cover the mouth of the test tube with your thumb.

13.  Turn the test tube upside down and place the mouth of the test tube under the surface of the sodium bicarbonate solution in the 400ml beaker.  Remove your thumb from the mouth of the test tube once it is submerged in the sodium bicarbonate solution.  (Try to do this without letting any air bubbles into the test tube.)



14.  Gently lower the test tube inside the beaker so that the test tube leans against the side of the beaker.

15.  Plug in the desk lamp at your lab station, turn it on, and place it over the beaker.   Be sure the light is very close to the top of the beaker but not touching the water.

16.  Wait for 5 minutes.   While you wait, pour out any remaining sodium bicarbonate from the 1000 ml beaker and repeat steps 4-7 to make a new batch of solution.

17.  After five minutes, use the hand lens to count the number of bubbles that formed on the needles of the sprig.  There will be tiny bubbles floating up, but these are being produced from the sodium bicarbonate.  Only count the bigger bubbles on the needles.  

18.   Record the total number of bubbles formed during the 5 minute period in the Data Table.

19.  Remove the test tube from the beaker.

20.  Use forceps to remove the sprig from the test tube.  Place the sprig back into the water in the 250 ml beaker.

21.  Turn off the desk lamp and unplug it. 

22.  Repeat steps 8-20 with the same sprig of evergreen when the room is dark.  You will have to wait for the entire class to catch up to do this part.  (Be sure you use the fresh mixture of sodium bicarbonate.)

23.  Clean the test tube and beakers with soap and water and dry them.

24.  Throw the sprig of evergreen in the trash can. 

25.  Clean your lab table with water and dry it.


Name: _______________________________________________________________  Period:

Lab: Investigating the Effect of Light Intensity on the Rate of  Photosynthesis


Data Table:  The rate of photosynthesis in bright light VS dim light

Light Intensity

Number of Bubbles Produced in 5 minutes

Bright Light



Dark/Dim Light





1.      What is the manipulated variable in this investigation?



2.      What is the responding variable in this investigation?


3.       What gas being released by the sprig causes the bubbles to form on the needles?



Write a paragraph in the space below that addresses the following in the order listed below.

1.       Restate the original hypothesis.  Begin the paragraph with, “The original hypothesis was…”

2.      Accept or reject the hypothesis.

3.      Provide numeric data to support your acceptance or rejection. 

4.      Provide a scientific reason why the results came out as they did.  (Why was more oxygen produced in one circumstance over the others?) Please refer to your background for help.