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Photosynthesis in Leaf Disks     Introduction   This lab is…

Photosynthesis in Leaf Disks

 

 

Introduction
 

This lab is performed to measure the amount of oxygen produced by spinach leaves during
photosynthesis when the plants are under different conditions of light and carbon dioxide. Sugar is
produced as a main product, and oxygen is produced as a waste product. The more sugar that is
produced from increased levels of photosynthesis, the more oxygen that is produced as well.
To supply carbon to the leaves, a solution of sodium bicarbonate (baking soda) is maked. Sodium
bicarbonate dissociates (dissolves) into sodium ions and bicarbonate ions, and the bicarbonate ions
then form carbon dioxide and hydroxide.
NaHCO3 (in water) → Na+ + HCO3- → Na+ + OH- + CO2
Leaf disks float, normally, because the spongy mesophyll has air spaces that are infused with gases,
CO2 and O2 (refer to leaf anatomy in Chapter 7). When the air spaces in the spongy mesophyll are filled
with solution, the overall density of the leaf disk increases and the disks sink. 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 and consumes oxygen, the rate that
the disks rise in an indirect measurement of the NET rate of photosynthesis (Net = O2 produced – O2 consumed)

 

 

 

 

Form Your Hypotheses Before Reviewing Data
You should address the three specific factors this lab is testing:
• Presence vs absence of light
• Presence vs absence of CO2
• Concentration of CO2
Materials
Spinach leaves
Drinking Straw
Water
Plastic Syringes
Aluminum Foil
Sodium Bicarbonate Solutions

 

Procedure
 

1. Obtain six syringes. Label #1 to #6. Remove the plunger from the syringes.
2. Using a straw as a punch, cut 10 leaf discs for each syringe. When cutting the spinach discs,
twist the straw to make clean cuts, and avoid major veins in the leaf. Also, allow the discs
to stack up in the straw; do not empty it until all 10 discs are cut.
3. Blow the 10 discs into a syringe.
4. Repeat steps 2-3 until you have 10 spinach discs in each syringe.
5. Push a plunger into each syringe, being careful not to crush the discs, until only a small
volume of air and the discs remain.
6. For syringes 1 and 2, pull 8 ml of 0.4% bicarbonate solution into the syringe.
7. For syringes 3 and 4, pull 8 ml of 2.0% bicarbonate solution into the syringe.
8. For syringes 5 and 6, pull 8 ml of water into the syringe.
9. Hold a syringe upright and tap it to move any air bubbles and to help settle the leaf discs to
the bottom. Gently push the plunger just enough to squeeze out any remaining air.
10. Place a finger over the syringe opening. Pull back on the plunger to make a vacuum in the
syringe. Hold the vacuum for 10 seconds.
11. Release the plunger, then release your finger. (Plunger first, finger second.) Remove any
new air bubbles, then tap the syringe to help settle the discs.
12. Repeat steps 8-10 until all the discs settle to the bottom.
13. Repeat for all syringes.
14. Wrap syringes 2,4, and 6 with aluminum foil.
15. Place each syringe, tip up, in a test tube rack. Place all syringes under the grow lights.
16. Observe each syringe every 3 minutes for 30 minutes (or until all 10 disks are floating).
Count the number of discs floating at each interval and record in a data table. Swirl to
dislodge any stuck discs and return syringes under grow lights.
17. Remove the syringes after 30 minutes and count the number of floating leaf discs in each.

 

Data
 

1. Use the provided data to make your own table and graph(s). These go into the results
section. Also, you need to calculate and include the ET50 in your report.
2. The ET50 stands for “Effective time for 50% of the treatment to float.” Remember each
syringe has 10 disks in it, so to calculate your ET50 is when 50% of 10 leaf disks float. If this
does not occur at a precise time point that was recorded, you need to average between the
time points. E.g., If 40% of the disks were floating at 30 minutes and 60% at 35 minutes,
then the ET50 would be 32.5 minutes. Using the ET50 increases reliability and repeatability of
the experiment. Note: Some treatments may never achieve 50% of the disks floating. These
treatments do not have an ET50.
3. You will need to include a table, graph(s) and the ET50 in the results section of your report.

 

Post-lab Write-up – 50 points
 

• Use the rubric in D2L. Working with a biology tutor from the CAS can earn bonus points.
• Must be written in past tense, third person, active voice. Refer to “Writing Lab Reports”
module on D2L for tips and information.
• When writing the Materials & Methods. Do not list the materials, do not use bullets, and do
not include the given protocol word for word. You should rephrase everything into your
own words and write in past tense. Example below:
o Present tense: “Cut 10 leaf disks for each syringe.”
o Past tense: “Ten leaf disks were cut and placed in separate syringes.”
• Remember to teach the reader about photosynthesis. This is done in the introduction. Be
sure to include your hypothesis in the last paragraph of your introduction. Don’t forget you
need a minimum of 2 APA formatted style in-text citations in the introduction.
• In the discussion, state whether the hypothesis was supported, rejected, or revised. Draw
conclusions about the requirements of photosynthesis based on your experiment. Discuss
how environmental factors affect photosynthetic rates. Don’t forget to include APA
formatted style in-text citations
 

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