Name: __________________________________________
Pd: ____________
Introduction
Plants transform sunlight to chemical energy by means of photosynthesis (photo=light,
synthesis=putting together). During the process, plants fix carbon dioxide from the atmosphere
and release oxygen and water (called transpiration). Measurements of photosynthesis are needed
for comparing and understanding productivity of vegatation and their response to environmental
stresses.
Photosynthesis, generally, is the synthesis of sugar from light, carbon dioxide and water, with
oxygen as a waste product. It is the most important biochemical pathway known; nearly all life
depends on it. It is a complex process, comprised of many coordinated biochemical reactions. It
occurs in most plants, as well as algae, some bacteria, and some protists, organisms
collectively referred to as photoautotrophs.
Photosynthesis uses the energy of light to make the sugar, glucose. A simple general equation
for photosynthesis follows.
6CO2 + 6H2O + light → C6H12O6 + 6O2
Carbon dioxide + water + light energy → glucose + oxygen
Photosynthesis occurs in two stages. In the first phase light-dependent reactions or
photosynthetic reactions (also called the Light reactions) capture the energy of light and use it
to make high-energy molecules. During the second phase, the light-independent reactions (also
called the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the highenergy molecules to capture carbon dioxide (CO2) and make the precursors of glucose.
In the light-dependent reactions the pigment chlorophyll absorbs light and loses an electron
that travels down an electron transport chain producing the high energy molecules NADPH
and ATP. The chlorophyll molecule regains its electron by taking one from a water molecule
through a process called photolysis that releases oxygen gas as a byproduct.
In the Light-independent or dark reactions the enzyme RuBisCO captures CO2 from the
atmosphere and in a complex process called the Calvin-Benson cycle releases 3-carbon sugars
which are later combined to form glucose.
Photosynthesis may simply be defined as the conversion of light energy into chemical energy by
living organisms. It is affected by its surroundings and the rate of photosynthesis is affected by
the concentration of carbon dioxide, light intensity and the temperature.
There are three main factors that affect photosynthesis. These factors are: Light irradiance and
wavelength, carbon dioxide concentration, and temperature.
Photosynthesis and respiration: Virtually all oxygen in the atmosphere is thought to have been
generated through the process of photosynthesis. All cellular respiration based organisms
(including plants) utilize this oxygen and produce CO2. Thus, photosynthesis and respiration are
interlinked, with each process depending on the products of the other. The global amount of
photosynthesis is on the order of a trillion kg of dry organic matter produced per day, and
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respiratory processes convert about the same amount of organic matter to CO2. A large part
(probably the majority) of photosynthetic productivity occurs in open oceans, mostly by
oxygenic prokaryotes. Without photosynthesis, the oxygen in the atmosphere would be depleted
within several thousand years. It should be emphasized that plants respire just like any other
higher organism, and that during the day this respiration is masked by a higher rate of
photosynthesis.
Objectives:
1. To test the ability of plants to synthesis their food (sugars).
2. To demonstrate the effect of light, carbon dioxide and chlorophyll on photosynthesis.
Procedure:
1. You will be conducting several experiments to test the role of light, carbon dioxide and
chlorophyll on the photosynthesis process.
2. Record the results and answer all the questions in the spaces provided.
EXPERIMENTS:
Experiment 1: Testing for starch using iodine as an indicator (every student should do this
experiment).
1. Make a solution of starch and water. Note that starch is not soluble in water.
2. Transfer few drops of the starch/water solution in to a Petri dish and add a drop of
iodine to the mixture. Record the change of the color.
3. Put a drop of iodine on a piece of white printer paper. Record the change of the color.
4. Explain the changes.
Results:
Mixture of Starch/Water:
Paper:
Interpretation of the results:
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Experiment II: Importance of sun light for photosynthesis:
1.
2.
3.
Place one corn plant in the dark for at least twenty four hours (Plant 1).
Keep another corn plant exposed to sunlight (growth lamps) for 2 (Plant 2).
Test Plant 1 and plant 2 for the presence of starch as outlined below:
4. Detach a leaf from each of Plant 1 and Plant 2.
5. Plunge each leach into boiling water, in order to kill the leaf, and then place it in a warm
ethanol which gradually dissolves out the chlorophyll.
6. After all the chlorophyll has been extracted; the leaf appears a dirty cream color (off
white color).
7. Carefully take the leave out and place it on a white sheet of paper or in a Petri dish.
8. Apply a drop of iodine on the leave and record any changes in the color of the leaves. If
the color changes to blue-black or dark brown, this indicates the presence of starch.
9.
Record your results? Discuss the role of sunlight in photosynthesis?
Results of experiment II:
Role of light in photosynthesis:
Experiment III: Role of atmospheric Carbon dioxide in photosynthesis.
1.
Incubate a corn plant in a sealed container/Bell jar (Plant A). Deprive the incoming air in
the container from carbon dioxide by passing it through the soda lime. Place in the jar
caustic potash which will absorb any carbon dioxide which might be in the jar. (* see bar
jar setup below)
2.
Keep another corn plant (Plant B) exposed to sunlight. This is considered as a positive
control plant.
3.
Test the leaves of plant A and plant B for the presence of starch as explained in
Experiment II.
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4.
Record your results and explain your findings?
5.
Discuss the role of carbon dioxide in photosynthesis?
Results of experiment III:
Role of carbon dioxide in photosynthesis:
Experiment 3 Bell Jar Setup:
1. Air Pump: fish tank or any other simple air pump will work. You want the air to be
pumped toward the bell jar (to the right in the image above).
2. Rubber Stopper: Rubber stopper with 2 holes in it for two tubes (one in and one out
flow of air). Each flask will need one of these.
3. Soda Lime: Soda lime in the first flask. This will pull the carbon dioxide out of the air.
4. Water in 2nd Flask: this will remove any of the soda lime in the air from the 1st flask.
The “in tube” should be below the water level so air bubbles up.
5. Empty Flask: any water vapor will collect here.
6. Rubberf stopper for bell jar: this rubber stopper needs only 1 hole for an input tube.
The air coming into the bell jar should now be free of carbon dioxide.
7. Plant and Caustic potash: in the bell jar you should place the corn plant and a small
beaker of potash. The potash will absorb any carbon dioxide that the plant breathes out.
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