Biol 102

Lab 10

Photosynthesis and Different Light Wavelengths

Pre-Lab Reading:  The ultimate source of energy that sustains life on earth is the sun.  Without constant influx of solar energy in the form of photons of light of different wavelengths or frequencies, life would end in short order.  The complex molecules that form living organisms require constant inputs of energy to maintain their order and structure. Unfortunately, light energy cannot be used directly to sustain life, even in photosynthetic organisms.  The energy from light must be trapped, then converted into a biochemical form of energy that is then stored or used.  Photosynthesis is the set of molecules that are responsible for capturing light and converting light energy to chemical energy.  Photosynthesis occurs in only a fraction of all organisms; they occur in plants, the cyanobacteria, and some one-celled organisms called protists.  The chemical reactions of photosynthesis occur within organelles that are called chloroplasts in all organisms but cyanobacteria.  These cyanobacteria are so small that the entire cell resembles a chloroplast.  Within chloroplasts are a series of highly folded membranes.  The surface of the membranes is covered with light trapping pigment molecules called chlorophylls.  Electrons within the chlorophylls are excited by captured photons of light.  This is the fundamental step in the conversion of light energy to chemical energy.  As the electrons return to their resting state they transfer captured energy to other molecules in the photosynthetic pathways.  These stepwise energy transfers ultimately cause the synthesis of simple sugars from carbon dioxide.  Sugars are a stable energy source that can be stored for long periods or used immediately to sustain life. 

Light energy is the key to photosynthesis.   A relatively narrow portion or the visible light spectrum is most active in photosynthesis.  By performing experiments that selectively filter out different wavelengths of light you can learn about which wavelengths are most active in driving photosynthesis.

6CO2 + 12H2O + sunlight à C6H12O6 + 6H2O + 6O2

Objectives:

1.      You should be able to describe how light sustains life through the process of photosynthesis.  Your understanding should include the ability to explain the general steps of photosynthesis and which organisms are capable of converting light energy into chemical energy.

2.      You should learn how variation in the quantity and quality of light affects photosynthesis.

3.      You should be able to relate objectives 1 and 2 to discuss how biological diversity reflects differences in conditions for photosynthesis among different regions of the earth. 

Introduction and Background Information:  So here is the problem with which you are faced. You have someone who appears to have found that plants don't grow as well (or die) under some types of light and he wants to know why, so he asks you and wants to know what evidence you can show him. So, first you have to decide what parameters can you truly manipulate or measure. What do we mean by "different types of light"? Well incandescent and flouresent lights are different, but how? Different heat output. But that doesn't fit the question at the top of the investigation. Aha, so maybe they differ in the colors of the light they emit. What does that mean? What about what you have learned in lecture about light?

OK, so let's say you think that infrared light is something to test (Why?). Do you shine it on the plant and measure its growth? Would you really see a measurable difference in plant growth in 2 hours? (Hint - No!). There is already evidence that the plants don't grow as well, we want to know WHY! Showing that plants grow better under different colors of light will not answer this question anyway.

Is there a parameter that is correlated with growth that you could measure? Growth requires energy. So, this should make you think about metabolic rate and cellular respiration, concepts that involve the production and use of ATP. But, is this type of ATP production dependent on light in any way that we have discussed? What process that is dependent on light and that only plants (and algae and cyanobacteria) do provides "food" and energy? (Hint - look at the question for this investigation!).

You have to come up with a hypothesis in the truest sense. What do we mean by that? Hypotheses are explanations for phenomena - What is the mechanism or cause for what is observed. Look at the following concept map, it shows a variety of relationships and posses questions about the process you should be investigating.

What is the relationship between the color of light and photosynthesis?

You should be testing some idea of why the plants do not grow well under different color lights not whether they grow better under different color light and you can't test growth but must measure something that contributes to growth.

Lab Activity
 

Question:  Which wavelength of light are most active in photosynthesis and how do they affect photosynthesis?

Based on the introduction and the procedures described below formulate a hypothesis that can be tested in this lab.

Some Weak Hypotheses

  • Light makes plants grow.
    • Yes... but How? Why?
  • Different colored lights make plants grow differently.
    • Too general. Which color(s) of lights? What is "different" growth?
  • Red light affects photosynthesis.
    • Again, too general. How does red light affect photosynthesis? Why?
  • Plants grow best in bright light.
    • How do you define bright light? How do you define the "best" growth?
  • Plants grow good in green and blue light.
    • How will you test "good" growth?
  • Plants on an average will grow an extra 2 inches in blue light than red light.
    • How will you test this?

Hypothesis: 

 

 

Prediction A prediction is based on the hypothesis, and states in advance the result that is expected to be obtained from testing the hypothesis. Most predictions are written in the form of if/then statements: "If the hypothesis is true, then the results of the experiment will be…"

Prediction: If                                                                                      

 

then  

 

 

Reasoning (Why do you think you'll see these results?): 

 

 

When evaluating the results of an investigation, you should revisit your prediction. If the results match the prediction, then your hypothesis is supported. If the results do not match your prediction then your hypothesis is falsified.

Variables:  Independent (Manipulated) Variable - You will choose one variable, or experimental condition to manipulate in the testing of your hypothesis. This variable is called the independent or manipulated variable.

 Independent Variable: 

 

Dependent (Responding) Variable:  Within the experiment one variable will be measured or counted or observed in response to the experimental conditions. This variable is the dependent or responding variable.

 Dependent Variable: 

 

Controlled Variables: There may be a variety of other variables that could significantly affect the dependent variable and the outcome of the experiment. These variables, called the controlled variables, must be kept constant during the course of the experiment. There is an assumption in the experimental design, that the selected independent variable is the one affecting the dependent variable. This assumption is correct only if all other variables are controlled.

Controlled variables: 

1. 

2. 

3. 


METHODS

Part A:  Measurement of photosynthesis

Materials

Supplies

Elodea (or other aquarium plant) in pond water

Test tube

3% sodium bicarbonate solution

Rubber stopper fit with bent glass tubing

Test tube rack

500 mL beaker filled with water

 

100 watt lamp

 

Black Sharpie marker

Procedure:

1.      Place a generous quantity of aquarium plant with the cut side up in a test tube with a rubber stopper fitted with a piece of bent glass tubing.

2.      Add 3% sodium bicarbonate solution, enough so that when the stopper is fitted onto the tube the solution fills the tubing to about 1/4 of the length of the horizontal portion.  Mark the meniscus on the tube with a marker.

3.      Place a beaker of plain water next to the plant tube to serve as a heat absorber.  Place a lamp next to the beaker.  The tube, beaker, and lamp should be as close to one another as possible.  See picture below.
 

 

4.      Turn on the lamp.  As soon as the edge of the solution in the tubing begins to move, time the reaction for 10 minutes.  Be careful not to bump the tubing or to readjust the stopper, or your readings will be altered.

5.      After 10 minutes, mark the new meniscus point, and measure in millimeters the distance the meniscus moved.

6.      Calculate the net photosynthesis in mm/min.  (Divide the mm of movement by 10 minutes.)

7.      Clean the glassware and repeat the experiment.
 

Part B.  Effect of Different Light Wavelengths
 

Materials

Supplies

Elodea (or other aquarium plant) in pond water

Test tube

3% sodium bicarbonate solution

Rubber stopper fit with bent glass tubing

Record other materials used

500 mL beaker filled with water

 

100 watt lamp

 

Black Sharpie marker


Procedure:

1.      Using a similar experimental set-up as described in Part A, design an experiment to test your hypothesis.  

2.      In your lab notebook, record the specifics of the protocol you use to test your hypothesis, the data you collect and whether it supports your hypothesis.  Be sure to include a control!  How many replicates (what is a replicate?) will you conduct?

 RESULTS

Part A.
 

Trial

Distance moved (mm)

Photosynthetic Rate (mm/min)

1



 

2



 

Mean

 

 

SD

 

 

1.      Calculate the mean and standard deviation of your individual group photosynthetic rate.

2.      Now pool the class data and recalculate the mean and standard deviation. 

 

Part B.  Record your results in an appropriate table

 

1.      Create a graph that indicates the overall result of your experiment.  Be sure that the graph you include directly addresses the hypothesis.

 

DISCUSSION and CONCLUSIONS

 

Part A.

1.      How does the mean/standard deviation of photosynthetic rate compare between group and class data?  Which calculation do you have more confidence in?  Why?

2.      Why is sodium bicarbonate (NaHCO3) used as the solution for this experiment?

3.      Explain how the measure of distance indicates photosynthetic rate.

 

Part B.

1.      What was your hypothesis and how did your experimental design directly address this hypothesis?

2.      Was your hypothesis supported or rejected?

3.      How would you refine your experiment in the future?

4.      If you had access to any equipment and had no time limit, what question dealing with photosynthesis would you like to explore?

 

Assignment:

  1. Each student will write a lab report on Part B of this lab activity.  This will be due at the beginning of class on June 24 (Lab 12)

 

 

Modified from Colin Henderson, Okalahoma State University’s Introductory Biology Web Site and S. Mader, Biology Laboratory Manual, 5th ed.