PLAYING WITH PHOTOSYNTHESIS: THOUGHT LABS AND VIRTUAL LABS
In biology, labs do not have to involve actual materials. In fact, many great scientists will
perform thought labs and models to develop ideas and hypotheses before performing
experiments with expensive lab equipment. Today you will explore 4 such “dry” labs.
You do not have to write a formal lab write-up. However, all answers and recordings should
be neatly written/drawn on a separate piece of paper with a clear separation between each part.
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Dry Lab 1 – Chromatography
Background:
One way to separate pigments in a plant leaf sample is to use chromatography. Basically, a plant leaf
sample is dissolved in a liquid solvent. The solvent containing the plant leaf sample then travels up a
piece of absorbent paper. Any pigments dissolved in the solvent will travel up the paper also. However,
how far they travel up the paper depends on how soluble they are in the solvent and how big they are.
Generally, the pigments that travel the farthest up the paper are the smallest and most soluble in the
solvent. If you use a different solvent, use a different type and length of absorbent paper, or use a
different plant sample, the results will look very different. Pigments that are similar in size may create
only one band on a short piece of paper, but may actually separate into two bands if you use a longer
piece of paper and a longer chromatography time.
Procedure:
1. Go to http://www.phschool.com/science/biology_place/labbench
2. Click on Lab 4: Plant Pigments & Photosynthesis
3. Click on 4.1 – Chromatography
4. Read through Key Concepts I: Plant Pigment Chromatography & click next when done.
5. Read through the Design of the Experiment 1 pages and click next when done. Be sure to wait on
the pigment separation (time-lapse) page as it takes a few seconds to start the animation.
6. Under Analysis of Results 1, calculate the Rf value for the green pigment (note: when you return to
the black ink chromatogram, you will then have to click through the experiment again until you return
to the Analysis of Results 1 page). If you do not have a calculator, there is one on the computer under
the start menu.
7. When you enter an Rf value, click the check answer button. Retry the calculation until you have it
correct.
***8. Perform the lab quiz and print your answers. Collect your printout, write your name(s) on the
sheet and include with your write-up.
9. Using the chromatogram shown below, provide an answer to the questions below:
Chromatogram of
boiled spinach
sample.
Which pigment is a) most soluble and b) least soluble in the
solvent you used? Explain how you made your decision.
Do you think additional pigments could still be present? How could you find out?
Is there an advantage for a chloroplast to contain more than one pigment?
Part 2: Photosynthesis
What is the relationship between light intensity and the rate of photosynthesis? In this activity,
you will measure the rate of photosynthesis at different light intensities. Since oxygen is
produced during photosynthesis, you can use the rate of Oxygen production as an indicator of
the rate of photosynthesis.
Go to http://learnalberta.ca Then search for “photosynthesis gizmo”
1.
In the Gizmo, observe the aquatic plant. During photosynthesis, the plant produces
oxygen, which is visible as bubbles leaving the plant. Set the light intensity to 0%, the
temperature slider at 30.0 °C and the CO2 level at 5.0%.
a. Slowly drag the light intensity slider to the right.
b. What happens to the rate of oxygen production as the light intensity increases?
c. What does this tell you about the rate of photosynthesis as the light intensity
increases?
2. During photosynthesis, plants use carbon dioxide as a source of carbon, which is used
to build glucose molecules. Does the amount of CO2 available influence the rate of
photosynthesis? Set CO2 level at 0%, light intensity to 90%, and temperature to 30
°C.
a. Slowly drag the CO2 level slider to the right.
b. What happens to the rate of oxygen production as the CO2 level increases?
3. The production of sugars in photosynthesis is the result of a sequence of chemical
reactions. Each reaction is sensitive to the temperature at which it takes place.
Depending on their environments, different plants will undergo photosynthesis most
efficiently at different temperatures. Set light intensity to 90%, CO2 level to 10%,
Temperature to 0 °C.
a. What happens to the rate of oxygen production as temperature is increased?
b. What is the maximum temperature for optimal oxygen production?
4. White light is a combination of different colours of light. The specific colour of light
we see is determined by its wavelength. Select the COLOUR tab. Set the light
intensity to 90%, CO2 level to 5%, temperature to 30 °C, and light wavelength to
400nm.
a. Slowly drag the light wavelength slider to the right.
b. Describe what happens to the rate of oxygen production as you do so.
c. Which two colours resulted in the highest rates of photosynthesis?
d. What colour led to the lowest rate of photosynthesis?
e. How do your observations explain why plants appear green?
5. Experiment with the Gizmo to find the ideal values for temperature, light intensity,
CO2 level, and wavelength.
Part 3: Oxygen Modeling
Background: You have all seen chemical reactions before, for example, the chemical
reaction for photosynthesis is:
6H2O(l) + 6CO2(g)
Light, Chlorophyll
C6H12O6(aq)+ 6O2(g)
Scientists are able to put radioactive tags on each separate atom on the reactants’ side of the
reaction and figure out where that atom will end up in the products’ side of the reaction by
looking at the tag.
In this thought lab, you will work with a similar reaction for photosynthesis as the one shown
above. Using the information provided, you will predict where the oxygen atoms from the
reactant side will show up on the product side.
Procedure:
Read through the background information in Thought Lab 5.1 on page 175 of your text.
1. Examine the following general equation that resulted from the van Niel experiment:
CO2(g) + 2H2O*( l ) → CH2O(s) + H2O( l ) + O2*(g)
2. Radioactive water tagged with an isotope of oxygen as a tracer (shown by the *) was used.
Note where the tagged oxygen ends up on the right side of the equation.
3. Assume that the experiment was repeated, but this time a radioactive tag was put on the
oxygen in CO2.
4. Using two-different color pens or markers, rewrite the equation above. Color the oxygen in
CO2 with one color. Predict where the oxygen from CO2 ends up on the product side and
color the atom(s) using the same color. Color everything else in the reaction a different
color.
5. Using the model above, predict what happens to all the hydrogen atoms that began as water
on the reactant side.
Part 4 – Action Spectrum of Green Algae.
Background:
 Recall that an action spectrum shows the rate of photosynthesis of an organism by
measuring oxygen production (a byproduct of photosynthesis).
 Aerobic bacteria need oxygen. Therefore, on an action spectrum, aerobic bacteria will
only live on the wavelengths where a photosynthetic plant is producing oxygen.
 In the experiment you are about to look at, Englemann wanted to measure the oxygen
production of green algae. Because he did not have an oxygen-measuring device, he
used the growth of aerobic bacteria as a way to see where along the action spectrum the
green algae produced the most oxygen.
 A good hypothesis is a two part statement. The first part states an assumption, and the
second part makes a guess as to what will happen in the experiment based on the
assumption. For example, if I wanted to predict what will happen if I drop a ball in the
presence of gravity, my hypothesis would look like this:
o “If gravity is a force that causes two objects to be attracted to each other, than a
ball dropped from a height will be attracted to the floor and will fall towards it.”
Use this format when you write your hypothesis.
Procedure:
1. Read the background information to question 14 on page 179 and answer the questions
in your write-up. I expect at least two or three sentences in answer 14a.