Photosynthesis Lab

Olivia Lopez
Fabiriza, Isabella, Jenna
Mr.Bryan
AP Biology
19 November 2015

Introduction
Living systems require free energy to conduct lifes important
processes, such as growth, repair, and reproduction. Energy
deficiencies can be detrimental to individual organisms, leading to a
disruption of homeostasis, and potentially death. These deficiencies at
the individual level could also cause disruptions in populations and
even in their ecosystems. Through the processes of evolution through
natural selection, processes needed to capture, store, and use free
energy have been conserved in organisms as these processes are
essential to survival. Autotrophic organisms capture free energy from
the environment through photosynthesis and chemosynthesis, whereas
heterotrophic organisms harvest free energy from carbon compounds,
such as carbohydrates, produced by these autotrophs. In multicellular
plants, photosynthesis occurs in the chloroplasts within cells. The
process of photosynthesis occurs in a series of enzyme-mediated steps
that capture light energy to build energy-rich carbohydrates. The
process is summarized by the following reaction:
2 H2O + CO2 + light carbohydrate (CH2O) + O2 + H2O
The net rate of photosynthesis can be measured in two ways:
1. The consumption of carbon dioxide (CO2)
2. The production of oxygen (O2)
Measuring the consumption of carbon dioxide to investigate
answer questions about photosynthetic rate is ideal because both
gross and net consumption of carbon dioxide can be accurately
measured for the system using one procedure and the proper
instruments. However, equipment and procedures to do this are
generally beyond the reach of most introductory laboratories.
Measuring the production of oxygen can also be used to analyze

the effects of a multitude of variables on photosynthetic rates, and

equipment for this method is more easily obtained. However, this
method of measuring photosynthetic rates involves two data gathering
procedures, as opposed to the one used when measuring carbon
dioxide consumption. This is because the consumption of oxygen by
plants for use in aerobic respiration makes it difficult to measure the
gross production of oxygen without considering its effects on the
system. Therefore, measuring oxygen production alone during this
investigation is only equivalent to measuring net photosynthetic rates.
To estimate the gross production of oxygen in photosynthesis,
measuring the rate of oxygen consumption will also be necessary. This
is how you will be measuring photosynthetic rate in todays lab.
The photosynthetic production of oxygen does allow us to easily
construct a system that can be used to experimentally investigate
many of the photosynthetic variables though. Constructing this system
revolves around three key effects of photosynthesis and respiration on
the buoyancy of leaves in water:
1. First, the extracellular spaces that exist within plant leaves
continuously store air for purposes of gas exchange, causing
leaves to float on the surface of bodies of water. But what would
happen if all the air is forced out of the air spaces in the leaf?
How would the leaf behave? Sink of course! We must first force
out this stored air to ensure that any floating of the leaves is a
direct consequence of photosynthetic oxygen production during
your investigation.
2. Second, initiating photosynthesis by supplying the leaf with the
necessary requirements after it has sunk will result in the
production of oxygen. The oxygen the leaf produces will form
gas bubbles and the leaf will re-float. The speed at which this
process occurs will be used as your measurement of
photosynthetic rate.
3. Third, depriving the leaf of the necessary requirements for
photosynthesis will halt the production of oxygen, forcing the leaf
to turn to the reserve stores of oxygen in its extracellular spaces.
As a result, the leaf will sink once it has used up its oxygen
stores.
It is these processes that will be the basis for your experimental
design. But to conserve space and better control the environment, you
will model an entire leafs photosynthetic abilities using small disks cut
from leaves, rather than whole leaves, to perform a floating leaf disk
assay (FLDA).

This assay of photosynthesis may be used to answer many

questions, including: What factors affect the rate of photosynthesis?
How do changes in light intensity, wavelength, CO2 concentration,
structural adaptations, physiological adaptations, respiration, and
chlorophyll content change the rate of photosynthesis? In this lab, you
will be measuring the net production of oxygen, as it is affected by
your choice of one of the following experimental conditions:
Light Intensity (Distance from the light source)
Light Wavelength (various colors of light using colored
cellophane to cover the beakers or cups)
Concentration of bicarbonate solution
Type of Leaf (spinach, romaine lettuce, iceberg lettuce ,swiss
chard, kale, red leaf lettuce)
Color of Leaf
Age of leaves used
In lab, the light intensity's effect on photosynthesis of a leaf is
observed and measured. I believe the highest light intensity
will cause the highest rate of photosynthesis. If it takes the
least amount of time for the leaf disks to float under the
highest light intensity, then oxygen from photosynthesis was
produced the fastest.

Materials

0.2% sodium bicarbonate solution

liquid soap
10 cc plastic syringe (can be found in the pharmacies of most
major drug stores)
forceps
Fresh spinach leaves
Hole punchers
400 ml beakers, clear plastic cups, clear glass jars
Timer
Desk lamp or similar lamp whose light source can be adjusted to
point in one general direction (Bulb should be at least 60W)

Procedures
1. Choose an experimental condition above and determine how
many variations of the condition you will test. State your
hypothesis for this investigation. This experiment tests light
intensity's effect on spinach leaves photosynthesis.
I believe the highest light intensity will cause the highest rate of
photosynthesis. If it takes the least amount of time for the leaf
disks to float under the highest light intensity, then oxygen

from photosynthesis was produced the fastest. If testing Light

Intensity:
o Place the light source in a so that it is at the same
height of the beakers and facing them as well. Align the
beakers so that they are all in the direct path of the
light.
o Place a heat barrier, in the form of a beaker or clear
plastic cup filled with water, between the light source
and the closest beaker of disks.
2. For each variation, prepare 100ml of 0.2% sodium bicarbonate
solution.
3. Label a 400 ml beaker or clear plastic cup for each variation,
pour 100ml of 0.2% sodium bicarbonate solution into each cup,
add 1 small drop of liquid dish soap to each cup, and stir.
4. Using the hole punchers, punch 10 leaf disks from fresh spinach
leaves. Do not cut into the vein of the leaf.
5. Remove the plunger from a clean 10-ml syringe. Carefully blow
(for disks still in the straw) or use forceps to place the 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 in the process.
6. Draw about 8 ml of 0.2% sodium bicarbonate solution into the
syringe. The leaf disks should be floating at this time.
7. Hold the syringe tip upward and expel the air by depressing the
plunger carefully until no air bubbles are present. Be careful not
to squash the disks.
8. Seal the tip of the syringe using the index finger and hold the
body of the syringe with the other fingers of the same hand.
Then pull back on the plunger with the other hand, creating a
partial vacuum within the syringe. If your seal is good, it should
be hard to pull on the plunger and air bubbles will be seen
coming from the edges of the leaf disks. This is how you will
eliminate the air stored in the extracellular spaces of the leaf
disks so that they will sink.

Step 8
9. Release your index finger and the plunger at the same time.
Some of the leaf disks should start to sink.
10.
Repeat steps 7 and 8 until all the disks sink. Tap the body
of the syringe if disks seem to be stuck to the sides. Be careful
not to overdo these steps though, this could possibly damage the
cells of the disks and negatively affect your investigation.
11.
Pour the disks and solution into one of the 400 ml beakers
or clear plastic cups that has been filled with 100 mL of
bicarbonate solution
12.
Repeat steps 3-10 until you have added 10 disks to each of
the beakers or clear plastic cups.
13. Make a control group by preparing disks (steps 3-9) for a cup
with a solution of only water with a drop of soapno bicarbonate
added. Be sure to use this solution when drawing water into the
syringe as well.
14. Place one of the three beakers under high light intensity,
another under medium light intensity, and the last one under low
light intensity. Measure how long it takes for half of the disks to
rise and record data.
15. Place the control group under the highest light intensity and
record the time it takes for half of the leaves to rise.
16. Wait for all of the leave disks to rise then remove from light and
cover beaker. Wait until half of the leaves have sunk again and
record the times. Repeat for all four containers.

Results/Data Collection/Analysis

Low light intensity had the longest elapsed time before half of the
disks sunk, therefore it had the slowest photosynthetic time.
High intensity had the fastest elapsed time before half of the
disks sunk, therefore it had the fastest photosynthetic time. The
control group was ineffective due to it not having a carbohydrate
source to fuel the calvin cycle.

Discussion/Conclusions
The test results agreed with my hypothesis that high light
intensity had the
fastest rate of photosynthesis. This can be shown with it having the
least amount of time for the leaf disks to float due to the oxygen
production from the light reaction. Medium intensity is the
second most effective and low intensity is the least effective.
Plants use light to fuel the light dependant reaction, causing the
production of ATP, NADPH, and oxygen as a biproduct. By
measuring how long it takes for the leaves to float, or the

production of oxygen, the rate of photosynthesis is being

measured. By measuring how long it takes for the leaves to sink
again, or use of oxygen, the rate respiration is being measured.
The unexpected result is there being no rate of respiration. This
could be because the container was not completely covered,
therefore it was not in complete darkness. Without complete
darkness, some photosynthesis is still able to occur and oxygen
is the only variable being absorbed. This could be fixed by
putting the container into a dark room to insure no light entering
it when measuring reaction. A weakness to this expirement is
that all of the disks come from different sections of different
leaves which can sway results.
Experimental Design Questions
1. The independent variable for an experiment is the factor that
you change on purpose in an experiment. What is the
independent variable for your experiment?
Light intensity (high, medium, low)
2. The dependent variable is the variable that may change as a
result of a change in the independent variable. It is the data that
is observed and measured in an experiment. What is the
dependent variable for this experiment?
Time it takes for leaf disks to rise and time it takes for leaf disks to sink
3. In a controlled experiment all other factors should be kept the
same so that you can fairly compare the results of the
experiment. List three controlled variables - factors that were
kept constant in all of the setups for this experiment.
Time of leaf, amount of soap, amount of water in beaker
4. In this experiment, why did you set up a control group that was
lacking in sodium bicarbonate solution? Explain.
Sodium bicarbonate is the fuel source for the calvin cycle. Without
the Calvin cycle, photosynthesis is not complete.
5. Why is it important to use the average rate of photosynthesis (or
respiration)? Some leaf disks rise faster or slower than others
6. Why must we also consider respiration when performing this
investigation?
To measure gross oxygen production
Data Collection and Analysis Questions
1. What relationship do you see between the PS ET-50 and your

independent variable?
The higher the light intensity, the faster the PS ET-50.
2. What relationship do you see between the RS ET-50 and your
independent variable?
No RS ET-50 was observed.
3. What relationship do you see between the PS ET-50 and the RS
ET-50 in this investigation?
No RS ET-50 was observed.
4. State one claim you can make from the results of this
experiment.
The higher the light intensity, the faster the rate of photosynthesis.
5. What is the evidence that supports your claim?
The leaf disks rose the fastest under the highest light intensity and
second fastest under the second highest light intensity.
6. What is the scientific reasoning that supports your claim? (What
is the science behind your evidence?)
The lowest PS ET-50 occured in leaf disks under the highest light
intensity and the highest PS ET-50 occured in leaf disks under the
lowest light intensity.
7. Do your results support your hypothesis? Why or Why not?
Explain.
It does support my hypothesis because I predicts the highest light
intensity would have the fasted photosythesis rate.
8. What are the limitations of this investigation?
Oxygen production is not measured completely accurately.
9. Describe two specific things that you could have done to
improve the reliability of this experiment.
Used more variaties of light intestity and measured oxygen
better.
10.
Why is it important to study photosynthetic rate of plants?
To study what light intensitys effect is on the oxygen production of a
plant.
11.
How might knowledge of the effects of your variable on
photosynthetic rate benefit those in the agricultural (farmers)?
When growing crops, they should place crops under high light

intensity.