Experiment #5: Photosynthesis
Objective
Can we measure how fast plants work?
In this experiment, we are going to measure the photosynthesis rate of a water plant under
different intensities and colors (wavelengths) of light.
Theory
Photosynthesis
Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy
from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by
all living things. The conversion of sunlight energy into usable chemical energy, is associated
with the actions of the green pigment chlorophyll.
Figure 1: The inputs and outputs of the photosynthetic process.
We can write the overall reaction of photosynthesis as:
Light
6H2O + 6CO2 
 C6H12O6+ 6O2
The above chemical equation can be written in words as:
Light
Water + Carbondioxide 
 Sugar + Oxygen
The primary molecule involved in photosynthesis, chlorophyll, can absorb red and blue light. The
energy that falls to the plant as sunlight is white light. It contains the mixture of all colors in the
visible spectrum. When the leaf absorbs blue and red colors of light from the Sun, green light is
reflected. This is the reason why plants appear green. Number of secondary molecules involved
in photosynthesis tend to absorb blue light, so they appear red and orange. The colors of
secondary molecules are masked as the amount of chlorophll molecule in plant leaves is much
greater. However, during autumn, when the plant dies and the chlorophyll is no longer produced,
the secondary colors can be seen.
Photosynthesis and Respiration
Some of the sugar produced during photosynthesis is used by the plant for its life processes (such
as growing and reproducing); the excess is converted mainly to starch and stored in various plant
parts which may be used as food by animals and humans.
The reaction of respiration:
C6H12O6 + 6O2 = 6CO2 + 6H2O + energy.
Sugar + Oxygen Carbon dioxide + Water + Energy
Figure 2: The cycle of Photosynthesis and respiration
Oxygen produced during photosynthesis replenishes the oxygen that was used up by living things
during respiration. This cycle of photosynthesis and respiration maintains the balance of carbon
dioxide and oxygen on earth.
Rate of Photosynthesis
The rate of photosynthesis means how fast photosynthesis takes place. This can be measured by
the amount of glucose or oxygen produced by a plant over a given time. By understanding the
factors that affect the rate of photosynthesis scientists can do work to try and increase the rate of
photosynthesis to increase the yield of a crop.
The three main things affecting the rate of photosynthesis are, Light, Temperature and Carbon
dioxide.
The rate of photosynthesis increases linearly with increasing light intensity. Gradually the rate
falls of and at a certain light intensity the rate of photosynthesis stay constant. After that point,
the rise in light intensity has no effect on the rate of photosynthesis as the other factors such as
temperature and carbon dioxide become limiting.
Photosynthesis is a chemical reaction and the rate of most chemical reactions increases with
temperature. However, for photosynthesis at temperatures above 40°C the rate slows down. This
is because the enzymes involved in the chemical reactions of photosynthesis are temperature
sensitive and destroyed at higher temperatures.
The rate of photosynthesis increases linearly with increasing carbon dioxide concentration.
Gradually the rate falls of and at a certain carbon dioxide concentration the rate of photosynthesis
stays constant. After that point, the rise in carbon dioxide levels has no effect on the rate of
photosynthesis.
In our experiment, we are going to determine the rate of photosynthesis by measuring oxygen
production of a water plant, elodea under bright light, different colors of light and dim light.
We are going to use two different method in measuring the oxygen production. The first method
is to count the oxygen bubbles prodused in a given time. Second method is to measure the chance
in the water volume in the capillary tube.
Safety Considerations:
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Wear goggles
Be cautious with glassware; report breakage to your lab instructor immediately!
The spotlights and the metal shades become extremely hot and can cause serious burns
Equipment
-Elodea (green water plant)
-250 ml beaker
-Funnel
-1 ml measuring pipette
-Tubing
- Syringe
-Desk lamp
-Red and green filters
-Stopwatch
Figure 3: Experimental Set up
Procedure
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Obtain a sprig of elodea. Remove several leaves from around the top end of the stem.
Cut off a piece of the stem near the top end at an angle and lightly crush around the cut.
Add about 100 ml water and 5 ml of baking soda to a beaker.
Place the sprig in the beaker, the cut side up.
Place a funnel on top of the plant in the beaker.
Make sure that all the plant is covered with the funnel and there is no air bubble left in the
funnel.
There is a 1 ml pipette and tubing connected to the funnel. Connect a syringe to the open
end of the tube.
Slowly increase the inner volume of the syringe. This will create less pressure in the
tubing and the atmopheric pressure will push the water in the funnel up.
Let the water level rise to middle of the pipette.
Place a desk lamp next to the beaker.
The water in the beaker will help to absorb the heat from the light, helping to keep the
temperature constant.
10- Turn on the desk lamp, the source of bright white light.
11- As soon as you see small bubbles coming from the cut end of the stem. Start timing the
photosynthesis reaction for 4 minutes.
If you do not see bubbles for two minutes, cut the stem again and recrush the plant.
12- One group member will observe the volume change in the pipette as with the increasing
gas volume in the tubing, the water level will drop.
13- Read the initial volume in the pipette and record it in table 1.
14- Read final volume in the pipette after 4 minutes and record it in table 1.
15- One group member will count the number of oxygen bubbles given of by the plant for 4
minutes and record the result in table 2.
16- Place a red filer between the plant and the desk lamp for a source bright red light. Repeat
steps 13-15 and record your results in table 1 and table 2.
17- Place a green filter. Repeat the steps 13-15 and record your results in table 1 and table 2.
18- Trun off the dek lamp. Repeat the steps 13-15 and record your results in table1 and table2.
Table 1
Initial volume
in the pipette
(ml)
Final volume in
the pipette after
4 minutes (ml)
Change in
volume in 4
minutes (ml)
Bright white light
Bright red light
Bright green light
Dim light
Table 2
Bubbles in 4 minutes
Bright white light
Bright red light
Bright green light
Dim light
Bubbles per minute
Change in
volume per
minute (ml)
Data Analysis
1- Calculate the oxygen production of the plant under white light, red light, green light and dim
light in volume change per minute. Record your results in table 1.
2- Calculate the oxygen production of the plant under white light, red light, green light and dim
light in bubbles per minute. Record your results in table 2.
3- On a graph paper, draw a graph to illustrate the effect of different light sources on the oxygen
production of the plant.
Questions
1- Compare the two methods you used to measure the rate of photosynthesis?
2- How did the rate of photosynthesis change with the light intensity? Did the result agree with
your prediction?
3- How did the photosynthesis rate change by changing the color of the incident light? Did the
result agree with your prediction?
4- What are the variables in this experiment and what did you do to eliminate them?
Reference
1- Trefil, J., M. Hazen, R., Sciences, An Integrated Approach, 6. Ed, 2010, ISBN: 978-0-47050581-6