PHOTOSYNTHESIS EXPERIMENT
(Adapted from Citrus College Lab Manual)
Introduction:
The first Law of Thermodynamics states that energy can neither be created nor destroyed,
but its form can be changed. In the process of photosynthesis, light energy is captured by
plant cells and then transformed into chemical energy. This chemical energy can be
stored in the chemical bonds of sugar molecules.
In general terms we can describe photosynthesis by the following chemical equation:
Light energy + CO2 + H2O
→ (CH2O)n + O2
In this experiment we will assay different aspects of this equation to monitor the rate of
photosynthesis as well as modify different aspects to determine the effect on the rate of
photosynthesis.
Experiment 1: CO2 utilization in photosynthesis
Carbon dioxide lowers the pH of water once it dissolves in it. This is because it forms a
weak acid upon dissolving and that acid frees up H+ ions.
CO2
+
H2O
↔
H2CO3 (Carbonic Acid)
↔
H+ + HCO3-
For this experiment, we will utilize a pH indicator called Phenol Red Dye to monitor
relative CO2 levels. Phenol Red changes color based on the concentration of H+ ions
present in the test sample (liquid). In acidic conditions phenol red is yellow, neutral
conditions will change phenol red to orange and basic or alkaline conditions will change
phenol red to pink.
For land plants, CO2 enters mainly via small openings on the undersides of the leaves
called stomata. However for aquatic plants like elodea, the CO2 just dissolves in from the
surrounding media.
Experiment 1 Set-up
1.
2.
3.
4.
Fill five test tubes 2/3 full with tap water add about 15 drops of phenol red to
each tube. Mix well.
Turn the tubes slightly acidic by GENTLY blowing into them through a straw,
your goal is to get all five tubes the same uniform orange color.
Obtain three equally sized pieces of Elodea, about 4-5 inches long for each
piece
For one of your Elodea pieces, grease the surface of the leaves gently with
vaseline. This is to prevent the entry of CO2.
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Photosynthesis
5.
Finally set up your tubes as follows with the cut side of the Elodea up:
6.
Let the reactions proceed for about one hour. The dark environment can be in
your microscope cabinet, the light environment will be on your benchtop.
After one hour compare the color of the liquid in tubes with Elodea to that of the
the control tube without Elodea.
7.
Data Table for Experiment 1: CO2 utilization in photosynthesis
Tube Predicted
Reason for
Observed
Does the prediction match
color after
prediction
color after
the observation? If not, what
one hour
one hour
is your explanation?
1
2
3
4
5
Questions to answer:
How did each condition affect the utilization of CO2 in photosynthesis?
Did anything else besides photosynthesis happen in the tubes? How can you tell?
2
Photosynthesis
Experiment 2: Light Intensity and photosynthesis
During photosynthesis, light energy absorbed by chlorophyll molecule, water from the soil,
and carbon dioxide from the atmosphere are essential to the formation glucose molecules.
However, not all of the energy associated with visible light is used equally by the plant for
photosynthesis.
In this experiment we will look at how the usable light energy affects the rate of
photosynthetic activity. To measure the rate of photosynthesis we will measure the rate of
oxygen production. Each group will have the choice of using either white light, or light
filtered through a green, blue or red screen. Class data will then be combined.
Experiment 2 Set-up
1.
Obtain two test tube set ups (test tube, rubber
stopper with syringe and bent glass tube) BE
CAREFUL: SYRINGES ARE SHARP
2. Obtain a piece of Elodea five to six inches long
and place in one of the test tubes with the cut end
up.
3. Completely fill both test tubes with sodium
bicarbonate (NaHCO3) solution. What will this
solution be supplying to the plant?
4. Seal test tubes with the rubber stoppers with
syringe and bent glass tube as shown. Make sure
there are no bubbles in your system.
5. Place both test tube set ups in a beaker of room
temperature water and position them 50 cm away
from a light source. The light source will get very hot over the course of the
experiment, so to protect the Elodea from the intense heat, place a beaker of
tap water between your test tube set up and your light source to serve as a heat
trap.
6. Once you have your experiment set up, let it equilibrate for five minutes
7. At the end of the five-minute equilibration period, use the syringes adjust the
sodium bicarbonate level in both bent glass pipettes to read 0.2 ml.
8. Record the position of the meniscus in each tube every five minutes for 20
minutes and enter data in Table 1
9. Move the lamp so that it is 25 cm away from test tubes, be sure to still include
your heat trap. Again let the system equilibrate for five minutes
10. At the end of the five-minute equilibration period, again adjust glass pipettes to
read 0.2 ml. and record the position of the meniscus in each tube every five
minutes for 20 minutes and enter data in Table 2
3
Photosynthesis
Table 1: Data for 50 cm away from light source
Time 0
5 min
10 min
15 min
20 min
10 min
15 min
20 min
Elodea
Control
(Velodea - Vcontrol)
Table 2: Data for 25 cm away from light source
Time 0
5 min
Elodea
Control
(Velodea - Vcontrol)
The gas volume changes observed in the test tubes are the result of simultaneous
happenings. First, each tube is filled with sodium bicarbonate (NaHCO 3), which produces
CO2 gas. Second, oxygen production from Elodea will increase the volume of gas
produced in the test tube. Third, there are constant fluctuations in temperature and
pressure that can either increase or decrease the volume of gas produced in the test tube.
So how can the amount of oxygen produced by the Elodea be distinguished from the
other gases? This is why we needed a control. The difference between the Elodea tube
and the Control tube should be the volume of oxygen produced by Elodea.
Graph O2 production (Velodea - Vcontrol) versus time for both the 50 cm and 25 cm
measurements, so it is easy to compare oxygen production from both distances. Include
a best fit line for each set of data.
Questions to answer:
What does your data indicate? How does light intensity affect photosynthesis? How
does time affect light intensity affect photosynthesis?
Reference:
Citrus Community College District. (2007).
Hayden-McNeil Publishing, Inc.
Biology 105 Laboratory Manual. Plymouth:
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