Bio H - Energy
Experimental Design: Photosynthesis
Objective: To design and experiment to test the effect of one variable on the rate of photosynthesis. To perform the
experiment and write a complete lab report.
Phase I: Experiment Design
-
Pick a variable from the list and design a controlled experiment to test the effects of that variable on PS.
Basic set up:
As we discussed in class, the spongy mesophyll layer of a leaf contains air pockets to facilitate diffusion of gases in and
out of the cells. Because these pockets are filled with CO2 and O2, leaves float on water. But, if the leaves are placed in
water and exposed to suction, the gases can be sucked out of the air pockets and water can fill up the pockets instead.
This causes the leaf disks to sink.
As leaves photosynthesize, they produce oxygen gas which will force the water out of the mesophyll’s air pockets and
refill the pockets with gas causing the disks to float to the top of the beaker over time. How quickly they rise can be
equated to how fast the plant is producing oxygen and therefore the rate of photosynthesis.
As a control we’ll set up optimal conditions for photosynthesis: 10 spinach disks will be submerged in 40mL of 0.2%
Sodium bicarbonate solution (basically CO2 water!) in a 50mL beaker. The beaker will then be exposed to a 60W white
light bulb and kept at room temperature. A beaker of water (called a heat sink) will be placed between the light and the
beaker containing the leaf disks to absorb the heat coming off the light bulb. All groups will set this up as their control
to demonstrate optimal conditions.
Variables: - There are several variables that can be changed to affect photosynthesis – list some below
Your job:
1. Begin by choosing a variable and deciding how you will change the experiment to examine that variable.
2. Write out a purpose for your experiment
3. Write out a detailed procedure describing how you would perform your experiment.
a. You will run 1 variation IN ADDITION to your control
b. Your procedure must provide ALL necessary details
c. Your procedure must include specific numbers, quantities, directions for data collection, etc.
d. Pictures speak a thousand words so if you can make a diagram of the set up rather than explain it that
would be better.
4. Write out your predictions. Explain what data you expect to receive from your experiment and include a
scientific basis for your predictions.
5. Provide me with a materials list for anything OTHER than the control materials.
Bio H - Energy
Phase II: Performing the Experiment
1. I will have your materials ready and provide pre-suctioned leaf disks (hole punched leaves).
2. You will perform your experiment according to your procedure BUT should recognize when something has been
left out of your procedure AND ADD IT IN. This is your experiment. You are in charge.
3. You must create an appropriate and efficient data chart and record your data in it.
4. At each 1 min mark, record how many disks are floating. You’ll need the ET50, which is the time at which 50% of
your leaf disks are floating. We’ll use this time to calculate the rate. Who remembers how to calculate the
rate?? (think back to the catalase lab!!)
5. This data will need to be graphed for your final report.
Phase III: Writing a complete lab report - Complete lab reports consist of the following sections in the following order.
Purpose: Should describe the scientific purpose of your experiment NOT the educational purpose of the assignment.
Introduction: Should provide RELEVANT background so that the reader can read what they need to understand the
experiment. Take a look at past labs we’ve done and read the introduction as a guide. This is not the place to write
EVERYTHING you know about photosynthesis. IE: if you are testing temperature, a detailed discussion of pigments and
absorption spectra is pretty irrelevant. Your intro should include:
a.
Basics of photosynthesis AS THEY ARE RELEVANT TO YOUR EXPERIMENT. In other words you should have
enough detail for the person to understand how each of your variables/constants fits in and that’s it.
b. Description of how the experiment reflects the theory. In other words, how do floating spinach leaves
reflect photosynthesis?
c. Your experimental variable: why do you THINK changing your experimental variable will have any effect?
d. Hypothesis: A single clear statement explaining the relationship between your variable and PS.
Procedure: Revise your procedure as you go through the experiment. Remember: any relevant details must be spelled
out. Do not assume that your readers will know what you are thinking. If you want beakers a certain distance apart, you
need to tell them. If you want a data recorded, you have to tell them what to record. Clear, labeled, pictures are great!
Data: your data presented neatly in titled chart AND a graph with all the proper graph components. Charts and graphs
should contain all relevant information, titles, labels and units and should be properly organized. Yes, there are multiple
ways to make a data chart, but efficiency will be taken into account. (meaning some ways are not practical) This is the
time to learn to use MS Excel or other graphing program. For this assignment your graph must be done on the
computer.
Data analysis/Conclusion: Finally you want to analyze your data and draw some conclusions. REMEMBER data analysis
is different from data repetition! You do NOT need to recite your data again. BUT here is where you interpret the
numbers. Saying “6 disks floated under blue light and 4 disks floated under red light” is reciting data where as “the fact
that more disks were floating under blue light than red light suggests that photosynthesis happens at a faster rate when
the leaves are exposed to blue light” is data analysis. This should also be supported with a rational for WHY
photosynthesis happens better under blue light.
Next you should address any inconsistencies with your data. For example, if your theory says PS happens better in cold
but your data says it does better in heat, you should hypothesize why you didn’t get the theoretically accurate results.
Bio H - Energy
And no, “human error” is not sufficient. For starters, if you messed up the experiment and are aware of what you did
wrong, you should repeat the experiment. Secondly, it doesn’t actually tell us what you did wrong. What errors were
made? Did your experimental design neglect to take something into account? How might you change the experiment in
the future to rule out this problem? How might this problem have skewed the data?
Finally you discuss the significance of your findings. In other words, why is the relationship between your variable and
photosynthesis important in the “wild.” How does it benefit the plant?
Grading rubric for Photosynthesis lab report: 40pts
a. General – 5pts total –
i. Report is typed, 12pt font
______/1
ii. Proper formatting: Paragraphs; Sections headings, proper order
iii. Proper spelling and grammar; No more than three errors
______/2
______/2
b. 2pts. Purpose – 2pts
i. Single statement exploring the effect of the variable on the rate of photosynthesis ______/2
c. 12pts Introduction:
i. Overview of photosynthesis including definition, reactants products, purpose, relationship to light and types
of organisms _____________/3
ii. Explanation of technique and how it demonstrates photosynthesis including basics of leaf structure and how
“floating” reflects photosynthesis happening ____________/3
iii. Elaboration of specific variable that you investigated ________/3
iv. Hypothesis – a single clear statement – _________/2
v. Concise, well organized without extraneous information ____________/1
d. 5pts. Procedure:
i. Complete and clear. No missing steps or descriptions
__________/2
ii. Data to be recorded is clearly outlined.
_________/1
iii. Reflects what group actually did
_________/2
e. 6pts. Data:
i. Data chart – complete with Title, labels units; Logically organized ___________/3
ii. Graph of data – complete with all components of the graphing rules sheet _________/3
iii. Comparison of ET50 calculated properly
___________/2
f.
8pts. Data analysis/Conclusion:
i. What does the data tell you about the variations you tested and their effect on photosynthesis?
____________/2
ii. Explanation of why you got the data you did __________/2
iii. Analysis of experiments including possible errors ________/2
iv. Significance of findings _________/2