1. AP Biology Investigation 5: Photosynthesis

2. Lab Notes General summary equation for photosynthesis:
2H2O + CO2 + light  carbohydrate (CH2O) + O2 + H2O
Ways to measure the rate of photosynthesis:
Production of O2
Consumption of CO2
In this lab, we will use a system to measure the accumulation of oxygen

3. Spongy mesophyll layer of leaves normally infused with gases (O2 and CO2)
Leaves will normally float in water

4. Floating Leaf Disk Technique
Draw gases from leaves using a vacuum and replacing gases with water
Sunken leaves placed in solution with source of carbon dioxide (sodium bicarbonate)
Photosynthesis occurs in leaves  oxygen accumulates  leaves become buoyant and rise in the water
Rate of photosynthesis measured indirectly by rate of rise of the leaf disks

5. Materials

6. Procedures Label 2 cups: “With CO2” and “Without CO2”
With CO2: pour 0.2% sodium bicarbonate (NaHCO3) solution into plastic cup to a depth of about 3 cm
Without CO2: distilled water (control)

7. Procedures Add 1 drop of dilute liquid soap into each cup
Soap acts as a surfactant or “wetting agent”
Wets hydrophobic surface of leaf, allowing solution to be drawn into leaf enabling leaf disks to sink in fluid

8. Procedures
Using hole punch (or plastic straw), punch out 10 uniform leaf disks for each cup.
Avoid major leaf veins

9. Procedures
Draw gases out of leaves and infiltrate with solution (either with or without CO2)
Remove plunger from syringe. Place 10 leaf disks into each syringe barrel. Tap to put disks at bottom of syringe.
Carefully replace plunger. Push down close to leaf disks.
Draw small volume (~ 5 cc) of solution into syringe. Tap to suspend disks in solution. Invert and expel excess air from syringe.

10. Create a vacuum in plunger using finger to draw air out of leaf tissue.
Hold finger over syringe  draw back plunger 10 sec  swirl tube to resuspend disks  release
Repeat 2-3 times to get all disks to sink (No more than 3+ times! Damage to disks can occur.)

11. Pour disks and solution from syringe into appropriate cup.
Disks infiltrated with sodium bicarbonate go into “With CO2” cup
Disks infiltrated with distilled water go into “Without CO2” cup

12. Place both cups under light source and start timer.
Record # floating disks at the end of each minute.
Swirl disks to dislodge any that stick to sides of cups.
Continue until all disks are floating in “With CO2” cup

13. Analysis Need standard point of reference to make comparisons
ET50 = Estimated Time it takes 50% of leaf disks to float
Graph data  determine ET50 from graph
ET50 = time it takes for half the disks (5) to float
# floating leaf disks
Time (minutes)

14. Class Data

15. Comparison of ET50 across treatments
Inverse relationship: ET50 decreases as rate of photosynthesis increases (negative slope)
To correct this representation, graph the inverse (1/ET50) vs. rate of photosynthesis

16. What are some factors that could affect photosynthesis?
Brainstorm
What are some factors that could affect photosynthesis?

17. Inquiry Investigation
Problem (choose a specific factor to test)
Experimental Design (IV, DV, control, contants, etc.)
Hypothesis (If…, then…. because….)
Materials
Procedure (flowchart)
Data Table
Record # floating disks each minute until all disks are floating

18. Analysis and Conclusion
Determine ET50 (estimated time it takes 50% of disks to float)
Graph lab group data
Class data
Conclusion (use Explanation Tool)