INVESTIGATING PHOTOSYNTHESIS AND "DISKUSSING" THE RESULTS

(Adapted from: DARLENE LANGHOFF BIOLOGY 288 SUMMER 1995 (MU Biology HHMI Precollege Outreach)

INTRODUCTION

In this activity you will investigate some of the factors that influence photosynthesis. To do this you will use small round disks of  leaves. If you force the air out of the intercellular spaces of these disks they will sink in water. If you allow these disks to go through photosynthesis the air spaces will eventually fill with oxygen and the disks will float again. The faster the rate of photosynthesis, the sooner the disks will float.

Read over the entire procedure for part one and write a hypothesis for this experiment.

NOTE: the lab write up rubric is located at the end of the lab instructions.

Part One : Light Intensity and Photosynthesis

Materials:

Basic Procedure:

  1. Adjust the height of the lamps according to the chart below. Group number Light Intensity Distance (cm) from table top to rim of light

    1. 100%         13 cm  (Tables 1 & 2)

    2. 50%         18 cm  (Tables 3& 4)

    3. 25%         26 cm  (Tables 5 & 6)

    4. 12.5%     35 cm  (Tables 7 & 8)

  2. Label the 250 ml beakers

  3. Punch out about 40 disks form the bean leaves, do not punch through the large veins. Place the leaf disks into the water in one of the small culture dishes as you punch them out.

  4. Fill the beakers with the 0.4% sodium bicarbonate solution (fill to the 0/250 mL mark) and add one drop of the 10% soap solution

  5. Remove the plunger from the syringe. Place ½ of the leaf discs in the empty syringe, put the plunger back in and push out most of the air without crushing the discs.

  6. Pull the sodium bicarbonate + soap solution into the syringe.

  7. Invert the syringe. the plunger in now at the bottom, and evacuate the rest of the air

  8. Cover the tip of the syringe with your finger.

  9. Pull back on the plunger with your other hand to create a vacuum in the syringe.

  10. Hold the plunger back for 10 seconds. Gently shake the syringe during this time.

  11. Keeping your finger over the tip of the syringe, let go of the plunger and allow it to return to its original position.

  12. After the plunger has returned to position force out any air bubbles that may have arisen by gently pushing up on the plunger.

  13. Repeat two or three times until the disks sink.

  14. Remove the plunger from the syringe.

  15. Lower the syringe into the beaker and tip it up so the disks are released into the beaker. The disks should sink. Do not use any that float.

  16. Repeat for the 2nd beaker.

  17. Place each beaker under the correct lamp. (match beaker numbers with distance from the chart)

  18. Turn on the lamp and start timers.

  19. Note the amount of time it takes for each of the 10 disks to rise to the surface in each beaker.

  20. Record your results below.

DAY 2: Inquiry activity (may be an optional assignment).

What other factors might effect the rate of photosynthesis?
Design a new procedure to investigate the rate of photosynthesis.
Let teacher know what materials you will need.
Do the experiment.

DATA:

Make a data table that records the GROUP DISTANCE (CM) LIGHT INTENSITY (%) TIME FOR 10 DISKS TO RISE (MIN)

Part Two : Your Turn (May or may not be assigned)

Now that you know one way to measure the rate of photosynthesis you and your group must design an experiment to test the influence of a different variable one the rate of photosynthesis.

Procedure: (Write this out in your journal)

  1. Select a variable from your list in part one question three to be your experimental variable. Check with your teacher before you proceed.

  2. Write a hypothesis indicating how the variable you selected will influence photosynthesis. Then write a one sentence rationale for your hypothesis.

  3. Write out the steps you will use to test your hypothesis. Be sure to include controls in your design. Be sure to indicate what materials you will need, what type of data you will collect and how you will present the data. You do not have to rewrite all the steps used to remove air form the disks. Assume that you will start with disks with the air removed.

  4. Show your procedure to the teacher for approval.

  5. Perform your experiment and record data. (Prepare spinach or ivy leaf disks using the procedure in part one)

Lab Write up Rubric (what does a well written lab contain... see AdaptedGeneric_Lab_Rubric2013.htm not all categories may apply):

1.  Title

2. Graph time vs. light intensity for each distance (all four trend lines can be on one summary graph
X-Y scatter, include the equations, label axes with units and don't forget a title).  (DAY 2: include a graph of your results)
(if this lab is assigned to only be done in your journal, either do an accurate graph by hand or do a computer graph and paste the graph into your journal) 

2. Summarize the results of this experiment in the form of a paragraph (also called a trend, results or observation paragraph). 
This paragraph needs to include statistics (compare slopes (rates) using the "M" in the equation y=mx+b, R2 equation or if graph is done by hand pick the steepest part of the graph and calculate your slope or rate by hand m=y2-y1/x2-x1)

(DAY 2 if assigned : include a discussion of your findings in question 3).

3. Write a brief discussion of what was learned about photosynthesis (what is photosynthesis, how does photosynthesis work, why did the leaf disks rise, the effect of light on photosynthesis,etc.) List several other variables besides light intensity that can influence the rate of photosynthesis. If you wish to use the power conclusion format here is a link: https://docs.google.com/document/pub?id=1L3bwXf5AYMMPSzjdauLLx45haI06295I1000plllG8E. 

4. References (APA style www.bibme.org)