AP Biology Investigation 5:
Photosynthesis
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
• Spongy mesophyll layer of leaves normally
infused with gases (O2 and CO2)
• Leaves will normally float in water
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
Materials
Procedures
1. 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)
Procedures
2. 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
Procedures
3. Using hole punch (or plastic straw), punch
out 10 uniform leaf disks for each cup.
– Avoid major leaf veins
Procedures
4. Draw gases out of leaves and infiltrate with
solution (either with or without CO2)
a) Remove plunger from syringe. Place 10 leaf disks
into each syringe barrel. Tap to put disks at
bottom of syringe.
b) Carefully replace plunger. Push down close to
leaf disks.
c) Draw small volume (~ 5 cc) of solution into
syringe. Tap to suspend disks in solution. Invert
and expel excess air from syringe.
d) 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.)
5. 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
5. Place both cups under light source and start
timer.
6. 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
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)
Class Data
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
Brainstorm
What are some factors that could affect
photosynthesis?
Inquiry Investigation
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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
Analysis and Conclusion
• Analysis
– Determine ET50 (estimated time it takes 50% of
disks to float)
– Graph lab group data
– Class data
• Conclusion (use Explanation Tool)