Light Dependent Reaction Photosynthesis Lab




     Is light important for photosynthesis?  Photosynthesis is the process by which plants take carbon dioxide from the atmosphere, add water, and use the energy of sunlight to produce sugar. Photosynthesis occurs in the chloroplast, an organelle in plant cells that contains the molecule chlorophyll. Chlorophyll absorbs the energy of sunlight. That light energy is converted to chemical energy through the steps of photosynthesis.
      The reactions of photosynthesis can be divided into two major types: light-dependent reactions and light-independent reactions. The light-dependent reactions convert energy from the sun into a form that the chloroplast can then use to make sugar from carbon dioxide, in the process producing oxygen as a waste product. The light-independent reactions use that energy to make glucose from carbon dioxide and water.

     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. Bicarbonate ion 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. The light depended reaction of photosynthesis also uses water in conjunction with two photosystems (PII / PI) to create ATP and NADPH using H+’s and e-’s. The hypothesis is that plant disks will form oxygen as a by-product and thus float when they are exposed to CO2 , water and light during the photosynthesis process.







per Class / Stock Table


Fresh Bok Choy    

Sodium Bicarbonate (baking soda)

Liquid Soap (Dawn); w/ Beaker and Eye Dropper(s)

Clean Drinking Water (not distilled)  

Electronic Scale and Boat

per Group / Partners


400 mL beaker

Transparent (not opaque) Plastic Cup

Plastic Syringe (ie. 10+ cc) – no needle

Plastic Straw

Needle Probe

Stirring Rod

Light Assemble (60+ watts)

Timer (clock)

White Notecard

Dark Room  











1.      Gather all necessary materials in order to conduct the lab.  


2.      Prepare 300 mL of bicarbonate solution:


-          The bicarbonate will serve as an alternate dissolved source of carbon dioxide, which is introduced in the Calvin cycle to help provide carbon for glucose, for photosynthesis.


-          Using your 400 mL beaker, prepare .2% solution by measuring about 2 g of sodium bicarbonate (NaHCO3) into 300 mL of clean drinking water and stir until dissolved.


-          Add 4 “drops” of liquid soap to your NaHCO3 solution and stir gently.  The soap will wet the hydrophobic surface of the leaf allowing the solution to be drawn into the leaf.


3.      Obtain a leaf and place it on your thumb.  Avoiding major veins and using a plastic straw drill and press out 20 uniform leaf disks by pressing the leaf against your thumb which will cushion the process.   The leaf disks should have accumulated in the straw. 


-          Remove the piston or plunger and carefully place the leaf disks into the syringe barrel near the tip; note, you may have to blow them out of the straw into your syringe.


-          Replace the plunger being careful not crush the leaf disks.


4.      Infiltrate the leaf disks by removing the air in the leaf and replacing the air with the sodium bicarbonate / soap solution:


-          Pull on the plunger to draw in a small volume of sodium bicarbonate solution enough to suspend the leaf disks (~1/4th of the syringe full).


-          Tap the syringe to suspend the leaf disks in the solution; they should be floating.


-          Slowly push on the plunger until no air remains in the barrel or nozzle; stop before any solution comes out. Again, leaf disks should be floating in the barrel of the syringe.


-          Holding a finger over the syringe opening, draw back on the plunger “gently” until you feel a slight press on your finger in order to create a vacuum for ~10 seconds.  Very small bubbles may be coming off the leaves if you are doing this correctly.


-          Let off the vacuum (remove your finger) and swirl the barrel with the leaf disks around to see if they will sink toward the plunger of the syringe because of air space infiltration.


Note: You may have to repeat the vacuum creating steps 2-4 times in order to get the disks to sink. Do not repeat too often because you could cause damage to the leaf.  If most of the disks sank just do not count those that are still floating in the next step. If necessary, use the needle probe to gentle push any disks out of the nozzle.


5.      Remove the plunger over your clear plastic cup and allow all the solution and disks to fall into your cup.  Add more bicarbonate solution from your beaker to a depth of about ¾ your cups total volume. 


6.      Place your cup directly near your light source, on a white notecard and start a timer. 


7.      Count and record the number of disks (see Data/Result Sheet) that float to the top within 10 minutes.  Note: Some disks may stick to the cup or each other; you may swirl the cup with your stirring rod to release them but “only” count them if they rise to the top of your cup after swirling.


8.      Clean, dispose, and dry all necessary materials.


9.      Repeat steps 3-8 however do not use any light source but instead place your experiment in a dark room.                                       


10.  Clean and return all materials.  Complete the Data/Result Sheet.

















Data/Result Sheet

Table: 10 points




No Light

Number of





Questions: 2 point each


 1. Calculate the rate of photosynthesis for each test:  Rate of Photosynthesis = Number of Disks / Time





No Light

Number of

Disks per Minute






 2. What’s the general formula for “photosynthesis” in order to make “glucose”?



 3. What gas came from the “splitting of the water” which allowed the disks to float upward?



 4. What was the NaHCO3 used for?



 5. Water would have been used to provide what two subatomic particles, which are used to make ATP and NADPH via photosystem II and I?



 6. Which photosystem(s) was/were used inside the thylakoid membrane for this experiment?



 7. If one were to prevent wavelengths of red from coming in contact with the leaf disks; do you feel that there would be a significant change in the number of raised disks?



 8. If one were to prevent wavelengths of green from coming in contact with the leaf disks; do you feel that there would be a significant change in the number of raised disks?



9. If one were to prevent wavelengths of violet from coming in contact with the leaf disks; do you feel that there would be a significant change in the number of raised disks?



10. Which photophosphorylation would have been present during the experiment: non-cyclic, cyclic, both, or neither?



Conclusion Paragraph:  10 pts


Do not include use any personal pronouns: “I, we, our, etc…” in the paragraph

Do not include any proper names “ie Jack or Jill, etc…” in the paragraph

Do not include the word “it” anywhere in the paragraph.

Do not have fragmented sentences in the paragraph.

State the following in your paragraph:

- ie Start off by writing… The problem question was ….

- ie Start off by writing… The hypothesis was that …

- ie Start off by writing… The hypothesis was …

 - ie Start off by writing… An unforeseen event was …

- ie Start off by writing…  An improvement that could be made for this lab would be …


               * A springboard question is a “question” that you make up based on the problem question or this activity. 

                 Do not answer this question; but simply state it as the last sentence in your conclusion paragraph with a

                 question mark (?) at the end.  The question is meant to “springboard” other thoughts that the reader

                 could investigate having done this lab which is an inventive yet effective way to end a lab activity.)


Conclusion Paragraph Here: (write clearly)