Spinach Leaf Disk Mystery…Why, why, why, why, why?
Procedure & research questions (bold)
1. Using a punch made from a small diameter soda straw, cut 10 leaf disks from young actively growing spinach leaves by supporting the leaf with your index finger while pressing and using a twisting motion of the straw. (A)
2. Remove the plunger from a clean 10-ml syringe. Blow the 10 disks into the body of the syringe. Be sure the leaf disks are near the tip of the syringe as you re-insert the plunger so as not to damage the disks. (B)
3. Insert the tip of the syringe into a beaker of 0.1% sodium bicarbonate solution and draw about 8 ml into the syringe. The leaf disks should be floating at this time.
(C) Why are they floating?
4. Hold the syringe tip upward and expel the air by depressing the plunger carefully.
5. Seal the tip of the syringe using the index finger of your left hand. Pull back on the plunger, creating a partial vacuum within the syringe. If you have a good seal, it should be hard to pull on the plunger and you should see bubbles coming from the edge of the leaf disks. (D) Why are there bubbles leaving the disks?
6. Simultaneously, release your index finger and the plunger. Some of the leaf disks should start to sink. Tap the side of the tube to dislodge bubbles on the edges of the disks. (E)
7. Repeat steps 5 and 6 until all disks sink. Do not overdo these steps!! You have been successful if the disks sink to the bottom. Don't repeat "just to be sure" as
it is possible to damage the cells of the leaves. Why do the disks sink?
8. Set the syringes, tip up, in front of the flood lights or other light source selected, making sure
to place a jar of water between the syringes and the light as a heat absorber. The lights are best
held in a clamp on a ring stand. (F)
9. As photosynthesis proceeds, the disks will start to float. At 2 minute intervals, invert the syringes to agitate the leaf disks and immediately return them to their positions in front of the lights. Count the number of disks floating after each interval until all disks are floating. Why do
the disks float?
10. Using the supplies available, design an experiment to see what variables will affect this phenomenon. You will need at least two syringes in your experimental design. Why? Write
your research question, hypothesis and prediction in your lab notebook, and construct a date
table for your experiment.
11. Proceed with your experimental treatment. Make sure all syringes used are filled to the same level by adding or expelling bicarbonate solution. If colored filters are used, they are wrapped around the body of the syringe. Record your data, then write a conclusion – was your
hypothesis supported, did you answer your question? Why do you say so? Provide evidence
and reasoning ! (answer all bold print lab questions in the research section of lab write up)
; 100 mL of 0.1 % sodium bicarbonate ; (2) 10-ml syringes
solution. ; wax pencil
; 150 watt flood light in fixture ; soda straw punch
; ring stand and clamps ; young spinach leaves
; large jar for heat absorber ; 50 mL beaker for sodium bicarbonate solution.
Some possible experiments: Design protocols to investigate the following
; Light wavelengths
; Differing concentrations of sodium bicarbonate
; Differing light intensity
; Different pH values
Adapted from Steucek,G.L. and Hill,R.J. (1982) Am. Biol. Teacher, 471:96-99, as modified by
Linda Holler, Susquehanna University.
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