PHOTOSYNTHESIS LAB
AP BIOLOGY
DR WEINER
BACKGROUND
Photosynthesis fuels ecosystems and replenishes the Earth’s atmosphere with oxygen.
Like all enzyme-driven reactions, the rate of photosynthesis can be measured by either
the disappearance of substrate or the accumulation of product (or by-products).
The general summary equation for photosynthesis is:
2 H2O + CO2 + light → carbohydrate (CH2O) + O2 + H2O
Because the spongy mesophyll layer of leaves is normally infused with gases (O2 and CO2), leaves —
or disks cut from leaves — normally float in water. If the leaf disk is placed in a solution with an
alternate source of carbon dioxide in the form of bicarbonate ions, then photosynthesis can occur in a
sunken leaf disk. As photosynthesis proceeds, oxygen accumulates in the air spaces of the spongy
mesophyll, and the leaf disk will once again become buoyant and rise in a column of water. Therefore,
the rate of photosynthesis can be indirectly measured by the rate of rise of the leaf disks. However,
there’s more going on in the leaf than that! You must also remember that cellular respiration is taking
place at the same time as photosynthesis in plant leaves. (Remember that plant cells have mitochondria,
too!) What else could be going on that might affect this process? Aerobic respiration will consume
oxygen that has accumulated in spongy mesophyll. Consequently, the two processes counter each other
with respect to the accumulation of oxygen in the air spaces of the spongy mesophyll. So now you have
a more robust measurement tool — the buoyancy of the leaf disks is actually an indirect measurement of
the net rate of photosynthesis occurring in the leaf tissue.
When immersed in water, oxygen bubbles are usually trapped in the air spaces of the spongy
mesophyll in the plant leaf. By creating a vacuum in this experimental procedure, the air bubbles can be
drawn out of the spongy mesophyll, and the space is refilled by the surrounding solution. This allows the
leaf disks to sink in the experimental solution. If the solution has bicarbonate ions and enough light, the
leaf disk will begin to produce sugars and oxygen through the process of photosynthesis.
Oxygen collects in the leaf as photosynthesis progresses, causing the leaf disks to float again. The length
of time it takes for leaf disks to float again is a measure of the net rate of photosynthesis.
You will be testing the effect of one factor on the rate of photosynthesis.
What is your hypothesis?
Materials
0.2% Bicarbonate solution
Dilute liquid soap (5 ml dish soap in 250 ml water)
3 30 ml syringes
Leaves
Hole punch
3 250 ml beakers
Timer
Light source
Procedure
1. Label the 2 beakers with CO2 and1 beaker without CO2.
2. Pour about 150 ml of water into the without CO2 beaker and about 150 ml bicarbonate into the with
CO2 beakers.
3. Add 2 to 3 drops of dish detergent to the beakers. Avoid generating suds. If suds form dilute it with
more water or bicarbonate. The soap acts as a surfactant, wetting the hydrophobic waxy cuticle of the
leaf allowing the water or bicarbonate to enter the leaf.
4. Using the hole punch, punch out 10 leaf disks for each cup. Avoid major leaf veins. 10 of the disks
will come from a treated set of leaves.
5. To draw out the gases from the spongy mesophyll and replace it with liquid from either cup perform
the following steps:
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Remove the plunger from the syringes, add 10 leaf discs to each, replace the plunger and push it
all the way down till very little space is left. DO NOT CRUSH THE DISCS!
Pull a small volume, about 5 ml of bicarbonate or water into the syringes. Do not let any air
enter. Make sure you know which syringe has what solution. Tap the syringes to suspend the leaf
disks. Make sure no air remains by holding the syringe with plunger facing down and pressing
up till the air is gone.
Create a vacuum in the syringe by holding your thumb over the narrow syringe opening and then
pulling back on the plunger. Hold this for 10 seconds. During this holding time swirl the disks to
suspend them.
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
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Release the plunger with your thumb still on the opening. If the plunger springs back you have
created a vacuum and the solution will infiltrate the spongy mesophyll and the leaf disks will
sink.
If the plunger doesn’t spring back, you may need to repeat this a few times BUT NO MORE
THAN 3.
If the leaf disks don’t sink then add a few more drops of soap to the beakers and step 5.
6. Drop the disks and the solution into the appropriate beakers and place them in front of the light source
7. Every minute record the number of disks floating.
Lab Writeup:
Abstract
Introduction including hypothesis
Summary of what you did
Table of time vs number of floating disks for all treatments
Graph of time vs number of floating disks for all treatments
Identify the ET50 (the time where 50% of the disks float) for all treatments
Discuss results