Lab 6: Photosynthesis and Cellular Respiration
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Purpose
The goals of this lab are to (1) investigate photosynthesis and cellular respiration in plants and
(2) demonstrate the two-step process of photosynthesis – the light reaction and the Calvin Cycle.
Introduction
Photosynthesis, carried out in green plants, produces all the food for the world and all the O2
we breathe. Human life could not exist without photosynthesis. Photosynthesis is a two-step
process. First photosynthesis captures the energy of sunlight and converts into chemical energy in the form of ATP and reducing power (NADPH) (Light Reactions). These light reactions
also produce O2. Next this chemical energy is used to reduce CO2 to sugar (Calvin Cycle).
Chemically, the photosynthetic reaction looks like this:
6CO2 + 6H2O + solar energy à C6H12O6 + 6O2
The diagram on the left (thefullwiki.org) shows the location of the chloroplasts (green circles
lining the palisade and mesophyll cells). The vascular bundle to the right of the diagram is responsible for the supply of water and mineral salts and removal of sugar. Some leaves have
more than one layer of palisades cells, and all have a spongy mesophyll layer with spaces between the cells to permit free circulation of air, so that CO2, which enters via the stomata, can
be fixed in carbon assimilation. The image to the right (en.citizendium.org) represents the light
and dark reactions of photosynthesis that take place within the chloroplasts. In starch forming
C3 leaves, all of the chloroplasts will come to contain starch grains after illumination. Remember starch is a polysaccharide composed of many glucose molecules.
Notice that in photosynthesis, CO2 (carbon dioxide) is being used up as it is “fixed” into glucose molecules. Plants (and animals) can then use this stored energy (sugar/glucose), to fuel
their metabolic processes in a process called cellular respiration. As the plant releases the energy (ATP) stored in glucose by breaking it down, CO2 is being given off into the surrounding
water or atmosphere. Chemically, the respiratory process looks like this:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
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Portions of this lab have been adopted from http://cibt.bio.cornell.edu/labs/dl/PELO.PDF; Dr. Bill Eisinger, Santa Clara Univeristy; and http://www.hansatech-instruments.com/forum/uploads/david_walker/Starch%20Pictures.pdf.
BIO124 Plant Science – Lab 6 Photosynthesis and Cellular Respiration
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Lab 6: Photosynthesis and Cellular Respiration
Among other things, the converted energy from respiration is used to synthesize molecules,
move materials around within the organism, grow (create new cells) and reproduce.
Plants can carry out photosynthesis and respiration simultaneously.
However, plants can carry out respiration all the time, but they need
light to photosynthesize! The relationship between these two processes is special in that it allows plants to recycle some of their byproducts. While CO2 is being given off during respiration, it can
be re-utilized during photosynthesis. Consider this testable statement or hypothesis; we will explore it in more detail in the course of
the lab.
In this lab we will demonstrate the light reactions and the dark reactions of photosynthesis as well as respiration using Elodea, a common water plant that is often used as greenery in fresh water aquariums.
PROCEDURE:
1. Observe the light reaction of photosynthesis.
a. Obtain a healthy Elodea sprig and place it in a beaker with fresh water that contains a small amount of baking soda (NaHCO3), which provides a source of CO2
in the water.
b. Place the beaker in bright light and wait for a few minutes.
c. Because Elodea is a water plant, O2 bubbles often form on their leaves when
they photosynthesize. (See the photosynthesis equation above.) Watch the cut
ends as they release bubbles of O2. Count the number of bubbles per minute in
bright light.
d. Predict what would happen if you moved the Elodea from the light to dark?
e. After a few minutes of counting in the light, move the beaker to dim room light.
Repeat the bubble count under darkened conditions.
f. You may wish to return the beaker to bright light and measure how long it takes
to restore the level of bubble release previously seen in the light.
2. Observe the result of the Calvin Cycle of photosynthesis (i.e. production of sugar/starch).
a. Obtain a piece of cardboard and fold it in half. On one side and next to the fold
draw the image/shape you would like to use for your starch picture. Try to keep
your cutout less than ½ the size of a geranium leaf.
b. Keeping the two sides of cardboard connected by a small flap, cut out your design.
c. Write your name and lab section on the cardboard.
d. Carefully attach your cutout to a geranium leaf using a paper clip that is slightly
sprung. (Take care not to damage the leaf!)
e. Return your plant to a sunny spot on the windowsill or under the light bank.
f. During the next lab period, develop your starch picture.
i. Carefully drop the leaf into a solution of boiling alcohol (best over a hot
plate and in a fume hood to avoid fire risk).
ii. Use tongs to hold the petiole and gently turn the leaf until it has lost all of
its pigments. Boil GENTLY so as not to break the leaf.
BIO124 Plant Science – Lab 6 Photosynthesis and Cellular Respiration
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Lab 6: Photosynthesis and Cellular Respiration
3.
4.
5.
6.
iii. Once the chlorophyll has been bleached from the leaf (it takes about 15
minutes and it may help to change the alcohol), gently rinse/flood it well
in water in a small container. The leaf will be brittle.
iv. Add a few drops of iodine solution to the water and gently swirl to develop your picture (10-15 minutes).
v. Iodine solution, which is iodine dissolved in an aqueous solution of potassium iodide, reacts with starch in the chloroplasts to produce a purple-black color.
vi. Dry your leaves by laying them on a paper towel.
Observe how a pH indicator can be used to measure CO2 in solution.
a. In water, carbon dioxide dissolves to form a weak acid, carbonic acid. As a result, an acid-base indicator such as bromothymol blue can be used to indicate
the presence of carbon dioxide. In the absence of CO2, bromthymol is blue, but
turns yellow as CO2 is bubbled into the solution.
b. Prepare a 250 ml flask containing 20 ml of bromthymol blue.
c. CAUTION: bromthymol blue is toxic if ingested and will stain your clothes.
d. Remember that you are a living generator of CO2. Put on your safety goggles for
this part. Use a straw to gently exhale into the flask containing bromthymol
blue. Continue exhaling into bromthymol blue solution for about one minute.
e. Record your observations.
f. Predict what would happen if you left the solution open to the air overnight.
g. The CO2 in solution will eventually achieve equilibrium with atmospheric CO2.
The air around us contains relatively little CO2. Therefore most of the CO2 molecules bubbled into the solution should leave. When CO2 is removed from
bromthymol blue solution, the solution will turn back to its original color (blue).
h. To confirm this, label with your names or the group number the 250 ml flask into
which you have just exhaled CO2. Set it aside for 24 hours with the top uncovered.
You are now equipped to use bromthymol blue to explore the relationship between
photosynthesis and respiration. The hypothesis for this lab has already been developed: While CO2 is being given off during respiration, it can be re-utilized during
photosynthesis.
Design a controlled experiment to test your hypothesis. Your experiment should
demonstrate that (1) environmental CO2 is used during photosynthesis in Elodea and (2)
that there is net production of CO2 when Elodea respires in the absence of photosynthesis. Remember to consider replicates and controls.
Equipment at your disposal:
a. Elodea plants (vigorous stems, each with an end bud)
b. Aluminum foil (which is an excellent way to provide a plant with a totally dark
environment)
c. Large clean test tubes
d. Bromthymol blue working solution
e. Tape and marking pen or wax pencil
f. Straws
g. Large Test tube racks
h. Flasks: 250 ml (1/group)
i. 100 ml graduated cylinder
j. Safety goggles
k. light source
BIO124 Plant Science – Lab 6 Photosynthesis and Cellular Respiration
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Lab 6: Photosynthesis and Cellular Respiration
NOTE: The bromthymol blue will not interfere with respiration or photosynthesis
in Elodea.
m. HINT: You can use your own breath to normalize your observations by SLOWLY
blowing into the solution until the bromthymol is the color that you want. Try to
balance your experimental tubes for exactly the same threshold of blue or yellow
color that you would like to start with. By setting the starting pH/color just below or above the threshold for a color change you can limit the amount of time it
will take to observe a change in color due to your experimental variables.
7. Run the experiment and collect data. Set up your experiment. Remember to include
controls as necessary and take notes. Collect data about 2 days after you set up your
experiment.
8. Interpret the results and draw conclusions about the hypothesis. Were you able to
demonstrate both utilization and production of CO2?
9. If your hypothesis was not supported, speculate on possible sources of error or reasons
why the hypothesis was not supported.
l.
BIO124 Plant Science – Lab 6 Photosynthesis and Cellular Respiration
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Lab 6: Photosynthesis and Cellular Respiration
Name & Date ____________________________________________
1. What differences did you observe between the tubes with the Elodea in baking soda in the
light and in the dark? Why did this occur?
2. Explain the photosynthetic process that allowed you to create a starch image on the geranium leaf.
3. Which process of photosynthesis occurs only in the light? Which one occurs both in the
light and dark?
4. What happened when you exhaled into the bromthymol blue solution? Why?
5. One of the implications of this investigation is that plants can recycle some of their “waste”
products. CO2 is clearly an example of a material that can be recycled by plants. What
other gas might plants generate as a “waste” through one metabolic process, but re-use in
a subsequent process?
6. On the back of this page, create a chart of your results. You should include the conditions
for each treatment, the starting color for each replicate, the ending color for each replicate,
and the meaning of the color change in terms of CO2 uptake or release and the processes
that occurred (photosynthesis, respiration, or both).
7. Plants in the presence of light carry out both photosynthesis and respiration (see the equations for each). From the results of your investigation, which process is occurring more in a
plant that is being supplied with sunlight? What evidence did you use to come to this conclusion?
BIO124 Plant Science – Lab 6 Photosynthesis and Cellular Respiration
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