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HASPI Medical Biology Lab 06
Background/Introduction
The Purpose of Photosynthesis
Date:
Energy Transformations
Photosynthesis is a process that takes light energy
and converts it into chemical energy. Why this is
important: Because LIFE on Earth would not be
possible without this process! Our bodies need
energy to perform even the simplest functions, like
breathing, but we are unable to use light energy
from the sun as an energy source. Photoautotrophs,
including plants, algae, and some bacteria, are
organisms capable of performing photosynthesis,
and thus they convert/provide energy for us.
http://www.dstudy.org/wp-content/uploads/2013/01/013013_1233_Conservatio1.jpg
It is important to remember that energy cannot be created or destroyed. As light energy is
absorbed by a photoautotroph, some of the energy is converted and saved in a chemical
form. It remains in this form until it is released. For example, when we consume broccoli that
has stored light energy in molecules, we are able to break down those molecules in the
process of digestion, which releases the energy. Our bodies are able to use this chemical
energy to perform basic functions. The energy does not disappear when we use it, but
instead it changes form again to be released from our bodies as heat.
The Process of Photosynthesis
Photosynthesis is actually a series of chemical reactions. There are three components
needed for a photoautotroph to perform photosynthesis: Carbon dioxide, water, and light.
These items come together and are rearranged by the photoautotroph to produce a sugar,
called glucose, and oxygen. The following equation summarizes the reactants (what goes
into the reaction) and the products (what comes out of a reaction) of photosynthesis:
Reactants
Products
6 CO2 + 6 H2O
carbon dioxide
water
light
6 O2 + C6H12O6
oxygen
glucose
Notice from this equation that the same amount of reactants are found in the products. For
example, there are 6 total carbon dioxide molecules, which means there are 6 carbon
atoms going into the reaction. These 6 carbons can be found in the glucose molecule as a
product. The same can be seen for oxygen and hydrogen. Light energy is needed to make
this reaction occur and to create the bonds between the carbon, hydrogen, and oxygen of
glucose. In this way, the light energy is held in the bonds holding glucose together. When
plants, or organisms that consume plants, break the bonds in glucose, they release (and are
able to use) the energy.
Why is it that only photoautotrophs can perform photosynthesis? They contain specialized
proteins that contain chlorophyll, a pigment capable of absorbing the energy in light. These
proteins are found in an organelle called the chloroplast. The energy absorbed by
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chlorophyll is used to drive the chemical reaction that creates and stores that energy in
glucose. Oxygen is actually just a by-product of photosynthesis and is released by the
photoautotroph into the atmosphere.
Photosynthesis and Medicine
Hopefully it has been made clear how important photosynthesis is for life, but the process
itself has medical application. Most obviously, there are many plants that have medicinal
properties. Mint has been reported to calm the stomach, soothe headaches, and fight
nausea. Aloe vera can aid in wound healing, treat burns, and improve circulation. Sage is
reported to be anti-inflammatory, anti-fungal, and can relieve digestive distress. These are
only a few examples of how plants can improve our health. In addition to these obvious
medical connections of plants, photosynthesis can also be used to synthesize medications
and exists in a new treatment called photodynamic therapy. Photosynthesis to Produce Medications
Through recent research, it has been discovered that it is possible to re-engineer the
photosynthetic process to produce more than just glucose. In the future, plant cells may be
used to manufacture drug components for medications, fabrics, and even fuels using
photosynthesis. In this process, light energy would be transferred into the bonds to create
these products.
Photodynamic Therapy
Photodynamic therapy is a treatment that uses a specific
wavelength of light to destroy cancer cells. A drug called a
photosensitizer is injected into a patient’s bloodstream. The
photosensitizer would normally be eliminated from the body by the
kidneys in 24-72 hours, but remains in cancer cells for a longer
period of time. When the photosensitizer in the cancer cells is
exposed to a specific wavelength of light, it produces an
abundance of oxygen that builds up and kills the cancer cells
surrounding the photosensitizer. Because the light must pass
through the skin, it can only be used to treat cancer that is less than
1 cm under the skin or body lining. To date, this therapy has only
been used to treat esophageal and non-small cell lung cancer.
Photodynamic therapy can also be used to treat acne.
http://www.moderncancerhospital.com/system/templets/green/static/treatment/Photodynamic-therapy.gif
Review Questions – answer questions on a separate sheet of paper
1. What is the purpose of photosynthesis?
2. Why is photosynthesis important to us?
3. What are photoautotrophs? Give 3 examples.
4. Explain how light energy is converted into chemical energy in photosynthesis.
5. What are the reactants and products of photosynthesis?
6. How many hydrogen atoms go into photosynthesis? How many come out of photosynthesis?
7. How many carbon atoms go into photosynthesis? How many come out of photosynthesis?
8. How many oxygen atoms go into photosynthesis? How many come out of photosynthesis?
9. How can photosynthesis be used to produce medications?
10. What is photodynamic therapy and how can it be used to treat cancer?
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HASPI Medical Biology Lab 06
Adapted from “The Floating Leaf Disk Assay for
Investigating Photosynthesis” by Brad Williamson.
Scenario
You are a medical research scientist team for HASPI Pharmaceuticals. You have been
tasked with determining which plant leaves will be the most effective in producing a new
medication to treat diabetes. The most productive plant leaves will undergo photosynthesis
the fastest. Your team will use the leaf disk assay procedure to determine the rate of
photosynthesis of different leaves. An assay is analysis, often of a drug, to determine the
presence, absence, or quantity of one or more components.
Materials
0.2% CO2 solution
Straw
3 Plastic cups
Syringe
Timer
Leaf A
Leaf B
Leaf C
Light source
Paper towels
Marker
Graduated cylinder
Procedure/Directions
Your lab team will be given tasks, or directions, to perform on the left. Record your questions,
observations, or required response to each task on the right.
Part A: Set-up
Task
Obtain the following supplies for your lab team:
1
2
3
4
•
•
•
•
•
•
Response
a. Hypothesis
300 ml of 0.2% CO2 solution (in a beaker)
3 plastic cups
straw
syringe
paper towels
timer
Your teacher has provided samples of different types
of leaves from a variety of plants. Choose and collect
leaves from 3 of the plant options. Hypothesize which
leaf will undergo photosynthesis the fastest and record
the type of leaf.
Label the plastic cups A, B, and C. Measure and pour
100 ml of CO2 solution into each cup.
Leaf A: _______________________________
Leaf B: _______________________________
Leaf C: _______________________________
Place the leaves on paper towels. Use the end of the
straw to cut out 10 small circles from each leaf. Try to
stay away from any large veins in the leaf. Make sure
each circle is whole and keep them in separate piles
for each leaf type.
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Part B: How Is This Going to Work?
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Answer the following questions before proceeding
1. Each leaf disk contains air spaces and
plant cells capable of performing
photosynthesis.
2. Plant cells need carbon dioxide (CO2),
water, and light to produce oxygen
(O2) and glucose through
photosynthesis.
3. You will be providing the leaf disks with
carbon dioxide and water in the CO2
solution. The light source will be a lamp
provided by your instructor, or direct
sunlight.
4. The leaf disks are saturated with the
CO2 solution, which causes them to sink
to the bottom. (Do not add leaves yet.)
5. As the leaf disks are exposed to light
they will perform photosynthesis, and
therefore start producing oxygen and
glucose.
6. As the oxygen is produced, it will fill in
the spaces in the leaf disk. Once
enough oxygen has been produced, it
will cause the leaf disk to float to the
surface of the solution.
7. Different types of plants have different
rates of photosynthesis. By comparing
how quickly the leaf disks float to the
surface, we can infer which plant would
have the faster rate of photosynthesis.
a. What does a plant need for photosynthesis?
b. What do plants produce through photosynthesis?
c. Explain how you are providing the following for the
leaf disks in this experiment:
i. carbon dioxide
ii. water
iii. light
d. Explain how you are able to compare the rate of
photosynthesis between the three leaves in this
experiment.
Plant cells Leaf disk http://www.elbiology.com/Images/basis.jpg
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Part C: Forcing CO2 Solution Into the Leaf Disks
1
2
3
4
5
6
The first thing your team needs to do is fill those air spaces with the CO2 solution.
Remove the plunger from the syringe and gently place
the 10 disks from leaf A into the syringe. Replace the
plunger gently and push it until only a small amount of
air and the leaf disks are left. DO NOT SMASH THE LEAF
DISKS!
From cup A, pull approximately 3-5 ml of CO2 solution
into the syringe with the leaf disks.
Cover the opening of the syringe and lightly pull back
the plunger to create a vacuum. Hold the vacuum for
10 seconds and then release it.
The vacuum will force the CO2 solution into the air
spaces of the leaf disks and the disks will float to the
bottom of the syringe when they are full of CO2
solution. You will have to repeat the vacuum and
release a few times until all of the leaf disks remain at
the bottom of the syringe.
IF all of the disks have not sunk to the bottom after 4
vacuum attempts, add one drop of liquid soap to the
solution in the syringe and repeat steps 4 and 5. Soap
coats the surface of the leaf, allowing the CO2 solution
to be more easily pulled into the leaf.
Part D: Measuring Photosynthesis
1
2
3
4
5
6
Remove the plunger from the syringe and pour the leaf disks and
solution into cup A.
The plant cells within each leaf disk are capable of performing
photosynthesis and producing oxygen. The only things they need
are CO2, water, and light. You have provided CO2 and water in
the CO2 solution. The only item you need now is the light source.
a. What does the leaf disk need to perform
Your teacher may provide a lamp as the light
source, or you can place the cup in direct sunlight. photosynthesis?
Take cup A to the light source. As soon as the cup
is placed in the light source, start the timer.
As the plant cells perform photosynthesis, they
b. What do the leaf disks produce through
photosynthesis?
produce sugar (glucose) and oxygen (O2). As the
O2 is produced, it will push the CO2 solution out of
the spaces in the leaf. Once enough O2 has been
produced, it will make the leaf float to the top of
c. Why do the leaf disks float to the surface?
the cup. The faster the leaf disks float to the top,
the more photosynthesis is occurring.
Every minute, record the number of leaf disks that have floated to the top in Table 1. Stop
recording when either 15 min. has elapsed, or all of the leaf disks have floated to the top.
After each minute, gently swirl the cup to make sure none of the leaf disks are sticking to
the sides or bottom of the cup.
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Repeat Part C and Part D for leaf disks B and C. If you are in a team, one member can
watch and record the times for Cup A, while the other members prepare and run the lab
for leaves B and C.
Analysis & Interpretation
Answer the following questions using data from your lab AND internet research if needed.
Table 1. Leaf Disk Assay
Time (min) Leaf A:
Leaf B:
Leaf C:
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Create a graph comparing and contrasting the time it took the leaf disks to float between
leaves A, B, and C. Do not forget a title and to label your axes.
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Analysis Questions – answer questions on a separate sheet of paper
1. Which leaf disks had the fastest rate of photosynthesis? Explain your answer.
2. Which leaf disks had the slowest rate of photosynthesis? Explain your answer.
3. Which plant would your research team recommend be used to produce the new
medication? Why?
4. List and explain at least two variables that could have impacted your results.
5. What three components were needed to make the leaf disks perform
photosynthesis?
6. What two components were produced by the leaf disks during photosynthesis?
7. Which component produced were you observing in this lab? How were you able to
observe this?
8. What is the chemical reaction that occurred in the leaf disks? (HINT: What are the
reactants and products?)
9. What do you think would happen if you removed the leaf disks from the light source?
Explain your answer. NOTE: Plants are capable of both photosynthesis AND cellular
respiration.
10. Research and find at least one plant example that can be used medically. Describe
what part of the plant can be used as a “drug,” what it is used to treat, and how it
works.
Connections & Applications
Your instructor may assign or allow you to choose any of the following activities. As per
NGSS/CCSS, these extensions allow students to explore outside activities recommended by
the standards.
1. RESEARCH AND CREATE A WEBSITE: Go to the following site and register to create a free
website: www.webs.com (if you have another free website resource you would rather
use, you may use that instead). Create a website with 5 pages. Each page will have the
following topic:
a. Page 1: Outline the process of photosynthesis, focusing on what is needed for
photosynthesis to occur and what is created in photosynthesis.
b. Page 2: List of 5 plants/herbs commonly used in medicine, and what conditions they
are used to treat.
c. Page 3: How can photosynthesis be used to create medicines?
d. Page 4: How can photodynamic therapy treat cancer?
e. Page 5: How can photodynamic therapy treat acne?
A minimum of 2 paragraphs (4-5 sentences each) is needed for each page. Use at least
1 digital media resource for each page (table, graph, audio, video, or interactive
animation) to enhance your presentation and support the page topic. ALL of the
information must be in your own words and resources correctly cited at the bottom of
each page, including the digital media resources.
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2. CALCULATING THE RATE OF PHOTOSYNTHESIS: Use the following information to calculate
the rate of photosynthesis. One of your research associates conducted a leaf disk assay
on a rare species of orchid that is believed to have a very high rate of photosynthesis.
This plant is also rumored to have a high rate of cellular respiration, which could negate
the high rate of photosynthesis. Analyze the following graphs and answer the questions
below to determine the gross rate of photosynthesis.
Graph 2. Dark Exposure (Respiration)
12 12 10 10 8 8 Percent (%) Percent (%) Graph 1. Light Exposure (Photosynthesis)
6 4 6 4 2 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Time (min) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Time (min) Graph 1 has the results for the percentage of
leaf disks that floated to the surface each
minute. In order to calculate the rate of
photosynthesis, you need to know the time
(min) at which 50% of the leaf disks rose to
the surface. This is called the Estimated Time
50-light, or ET50-light.
As the plant cells undergo respiration, they
use the oxygen that was created in
photosynthesis, causing the disks to drop.
Graph 2 has the results for the percentage of
leaf disks that dropped to the bottom each
minute. The ET50-dark is the time (min) at which
50% of the leaf disks dropped to the bottom.
What is the ET50-light?________________________
What is the ET50-dark?_______________________
a. Use the ET50-light and ET50-dark to complete the following equation and determine the rates
of photosynthesis and respiration.
1
Rate of respiration =
=
ET50-dark
=
min-1
Your calculations
Rate of photosynthesis =
1
ET50-light
=
=
Your calculations
212
Answer
min-1
Answer
Photosynthesis: How Is It Important to Medicine? HASPI Medical Biology Lab 06
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b. Use the following equation to determine the relative and gross rate of photosynthesis. The
gross rate of photosynthesis takes into account any cellular respiration that may have
occurred. This is important since the products of photosynthesis (oxygen and glucose)
are used in cellular respiration.
Relative rate of =
photosynthesis
min-1
Rate of respiration
+
min-1
=
Rate of photosynthesis
min-1
Answer
min-1
Gross rate of photosynthesis =
Relative rate of photosynthesis
min-1
=
Answer
Rate of photosynthesis
c. A gross rate of photosynthesis that is 2, for example, would signify that photosynthesis
occurred at 2 times the rate of respiration. If the gross rate of photosynthesis is negative,
it would mean that respiration is occurring at a greater rate than photosynthesis. What
does the gross rate say about the photosynthesis rate of the plant that was tested?
d. The researcher also collected data from a young leaf and an old leaf.
Young Leaf
Old Leaf
ET50-light
ET50-dark
ET50-light
ET50-dark
0.193
0.064
0.178
0.023
Determine the gross rate of photosynthesis for the young and old leaf. Does the young
leaf or the old leaf have a greater gross rate of photosynthesis? Explain your answer.
3. EXTENDING THE EXPERIMENT: Choose one of the following questions to create your own
experiment:
a. Does the amount or type of light affect the rate of photosynthesis?
b. Does the age of the leaf affect the rate of photosynthesis?
c. Does the amount of carbon dioxide in the solution affect the rate of photosynthesis?
Using the same leaf disk assay model, create and perform an experiment to test one of
the questions above. Create a lab report that includes the question, your hypothesis,
materials, procedure (step-by-step), and a table summarizing your results.
NOTE: You may need to check out supplies (such as a syringe) from your instructor. To make the 0.2% CO2
solution at home, add 0.6 grams (approximately 1/8 tsp) and 1 drop of liquid soap to 300 ml of water.
Resources & References
• Armstrong, J. E. 1995. Investigation of Photosynthesis Using the Floating Leaf Disk Assay.
http://www.bio.ilstu.edu/Armstrong/biolab/cellbio/psynex1.htm.
• Dolmans, D.E., Eukumura, D., and Jain, R.K. 2003. Photodynamic Therapy for Cancer. Nature Reviews
Cancer; 3(5); 380-387.
• Williamson, B. 2010. The Floating Leaf Disk Assay for Investigating Photosynthesis. Exploring Life Community,
http://www.elbiology.com/labtools/Leafdisk.html.
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Photosynthesis: How Is It Important to Medicine? HASPI Medical Biology Lab 06