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Photosynthesis (Team 4)

Page history last edited by Karen Huang 10 years, 7 months ago

A.  Learning Objectives

In this lab, students will:

• use a spectroscope to observe the component wavelengths of white light.

• determine which wavelengths are absorbed or transmitted by filters and plant

pigments.

• determine under which conditions a plant leaf produces starch or utilizes starch stores.

• determine which pigments are most efficient in supporting photosynthesis

 

B. Textbook Correlation: 

Please review Chapter 8: Photosynthesis when preparing for the lab.

 

C.  Introduction

Write a three paragraph introduction to photsysnthesis.

In paragraph 1 discuss the difference between autotrophs and heterotrophs.  Give examples of each.

Heterotrophs must eat organic molecules in order to sustain life. Humans and mushrooms are good examples or this. If humans did not consume organic molecules they would die. Autotrophs make organic molecules from inorganic sources. Photoautotrophs such as most plants and algae are a good example because they use light as a source of energy.  

In paragraph 2, summarize the process of photosynthesis.  Include the major chemical reaction (reactants and products) and the change in free energy.  Discuss the different wavelengths of visible light, photons, and the different quantity of energy associated with the individual wavelengths of light.  Include an images/animations about the individual wavelengths of visible light.

The process of photosynthesis consists of two parts: the light-dependent reactions and the light-independent reactions, also known as the Calvin cycle. The overall equation of photosynthesis is:

6CO2 + 6H2O + light energy --> C6H12O6 + 6O2

During the light dependent reactions which occur in the thylakoid membrane, wavelengths of light energy hit pigment molecules located in the light-harvesting complex of photosystem II. (Depending on the pigment molecules, the wavelengths of light will either be reflected or absorbed.) The introduction of the light energy then initiates the light reactions which ultimately produces O2 as a byproduct and the energy intermediates ATP and NADPH to be used in the Calvin cycleThe Calvin cycle, which occurs in the stroma of the chloroplast, turns CO2 into G3P which will later produce glucose.

 


 

 

In paragraph 3, summarize the light-dependent and light-independent reactions of photosynthesis.  In your summary, include cellular location, discuss the role of pigments and excitable electrons in the reaction.  Also, discuss the energy intermediates that link the two processes and how a delay in either the light-dependent or the light-independent reactions affects the other set of reactions.  Include a useful image/animation for both the light-dependent and light-independent reactions.

Photosynthesis takes place in the chloroplast pf plant cells. It is divided into 2 parts: the light-dependent reactions and the light independent reactions. The light dependent reactions occur first and they take place in the thylakoid membrane of the chloroplast. As the name suggests, the light dependent reactions cannot take place with the presence of light. As the light strikes a leaf, it gets absorbed by the photosystems, more specifically, the pigment molecules. First, as the photons of light strike the pigment, it excites the electrons within pigment molecules in the light harvesting complex of photosystem II.  Since the electrons have gained energy, they are excited, and to become stable, they have to release the energy. To do this, the electrons travel down an electron transport chain (ETC). As they move down the ETC from molecule to molecule, they are releasing energy in small increments. This is used to pump H+ ions into the lumen of the thylakoid. This creates a H+ electrochemical gradient. This gradient will soon come into play. The electrons from photosystem II eventually reach the photosystem I where they are met with another input of light energy which boosts them up with energy yet again. To become stable the electrons transfer from the photosystem I to Ferredoxin and then to NADP+ Reductase, an enzyme that reduces NADPH. NADP+ Reductase takes 2 electrons and one H+ ion from the stroma and produces 1 NADPH molecule. To make up for these lost electrons, photosystem II oxidizes water to gain an electron. This also releases an H+ ion into the lumen of the thylakoid, again contributing to the formation of the H+ electrochemical gradient. Since there are more H+ ions inside the thylakoid (in the lumen) than outside (in the stroma), to reach equilibrium, the H+ ions must move out. This is made possible by an ion channel, ATP Synthase. ATP Synthase phosphorylates ADP and inorganic phosphate into ATP as the H+ ions move out of the lumen. During the light dependent reactions ATP and NADPH are made in equal amounts. The process just described is called non-cyclic electron flow. This occurs when the amount of NADP+ is high and NADPH is low, so the cell can generate more NADPH. Cyclic electron flow produces only ATP. The electrons circle back to photosystem I in cyclic electron flow, this gives more energy to form the H+ ion electrochemical gradient. Cyclic electron flow is favored when NADP+ is low and NADPH is high and also when ATP is low. Cyclic electron flow occurs because the Calvin Cycle (described next) uses more ATP than NADPH.

In the second part of photosynthesis known as light independent reactions or the Calvin Cycle, CO2 is incorporated into the 5 carbon molecule Ribulose-Bisphosphate(RuBP) through the action of the enzyme RuBisCO.  The addition of CO2 changes RuBP into phosphoglycerate. Through a series of phosphorylations and reductions, 12 G3P are made. Of the 12, two are used for energy catabolism and the rest are recycled and phosphorylated once more to return to their original form of RuBP. In total, the Calvin Cycle uses 18 ATP and 12 NADPH, and releases 2 G3P and regenerates RuBP, it also releases 18 ADP and 12 NADP+ and 12 inorganic phosphates. Since the noncyclic light dependent reactions produce ATP and NADPH in same amounts, this leads to a shortage of ATP in the Calvin Cycle since it requires more ATP than NADPH. This is the reason behind the cyclic electron flow. These two reactions are co-dependent since the products from one reaction are used in the other reaction and vice versa. For example, if the light dependent reactions are delayed, the production of ATP and NADPH will be slower. This will halt the Calvin Cycle since it cannot continue without either ATP or NADPH. Likewise, if the Calvin Cycle is slow or delayed, then it will not produce the NADP+, the inorganic phosphates or the ADP, without these crucial precursors, the light reactions cannot take place.

 

     

In the following series of exercises, you will learn how to use a spectroscope to view the different wavelengths composing white light, investigate how leaves kept in light and dark store and utilize glucose, study chloroplast structure, and investigate how Elodea leaves fix carbon.

 

D.  Light Absorption by Photosynthetic Pigments

Colored substances can either absorb particular wavelengths of light or reflect them. Wavelengths that are absorbed are taken up by the colored substance and therefore are not available for our eyes to detect them. On the other hand, wavelengths that are reflected by a colored surface are the ones that can enter our eyes and strike the retina. These are the colors that we see.  A filter absorbs a particular wavelength of light and transmits (allows to pass through) the rest.  This is important because plants must contain pigment molecules capable of absorbing light waves in order to use the energy they contain for photosynthesis.

 

In this exercise you will use a spectroscope to observe the component wavelengths of white light and then determine which wavelengths are absorbed or transmitted by filters and plant pigments such as chlorophyll.

 

Procedure

1. A spectroscope uses a prism to separate white light into its component wavelengths. This spectrum can be observed along with a scale indicating the wavelength associated with each color.

 

2. Use colored pencils to construct the spectrum of white light you observe using the spectroscope.

 

 

 

 

 

3. Now place a red filter over the spectroscope slit.

 

Which colors do you observe in the spectroscope? ______________________________

 

Which colors were absorbed by the filter? _____________________________________

 

4. Repeat using the blue and green filter.

 

Colors observed with the blue filter. _________________________________________

 

Colors absorbed by the blue filter. ___________________________________________

 

Colors observed with the green filter. ________________________________________

 

Colors absorbed by the green filter. __________________________________________

 

5. Now hold a test tube of plant pigment extract in front of the spectroscope. Some of the light waves will be reflected, giving the extract the color you perceive. Other wavelengths will be absorbed by the pigments. These are the wavelengths that will pass into the spectroscope where you can observe them. These are also the wavelengths available to the plant cells for photosynthesis.

 

6. Observe the spectrum in the spectroscope and record the colors and wavelengths you observe.

 

7. What difference do you note between the spectra produced by a green filter and by the plant extract?   Is green really the color of photosynthesis as is commonly characterized?  If not, explain your answer.   

 

E.  Requirement of Light for Starch Production

As noted in the introduction, plants produce glucose, using some immediately for cellular respiration and storing the rest as starch. This exercise will  measure the requirement for light energy in the production of starch.  We will work with Germanium leaves that have a small section covered so that light can not strike it. In this exercise you will remove all the pigments from these leaves and test each for the presence of starch.

 

Predictions

Before conducting this exercise, test your understanding of photosynthesis by predicting if starch will be present:

a) in an uncovered light-grown leaf

b) in a covered light-grown leaf

c) in an uncovered dark-grown leaf

d) in a covered dark-grown leaf

 

Procedure

1. Place the water beaker on the hot plate and bring to a boil.

2. After the water has boiled, place 150 mL of 95% ethanol onto the hot plate. The boiling point for ethanol if much lower than that of water, so it will come to a boil quickly.

3. Take the light-grown leaf, remove its covering and place it in the boiling water for one minute. This kills the plant cells and breaks open its membranes.

4. Remove the leaf from the boiling water using the tweezers and place it in the boiling ethanol. This removes the pigments from the leaf. Leave it in the ethanol until the leaf turns white.

5. Remove the leaf from the ethanol and return it to it’s the petri dish filled with water.

6.  Place the leaf into the pertri dish with iodine (Lugol’s solution).  This stain reacts with starch to produce an intense dark blue-black color. Allow the leaves to soak in the Lugol’s solution for 2-3 minutes.

7.  Return the leaf to the water dish. Use the tweezers to spread it out flat.

8.  Observe the staining pattern for the leaf. Draw what you observe in Figure 4 and answer the following questions.

 

 

 

 

 

   

Questions

1. When a section of the leaf stains blue-black, what does that mean?

When it turns blue/black it means that starch is present. 

 

2. When a section of the leaf fails to stain blue-black, what does that mean?

When a section of the leaf fails to stain blue-black, complex carbohydrates are not found in that part of the leaf. 

 

3. Is there a difference in the staining of the covered vs. uncovered sections? Explain why.

 

 

 

F.  Measuring the Efficiency of Different Pigments in Absorbing Light
Pigments of a leaf act as solar panels.  They absorb sunlight and convert that energy to the chemical energy of ATP and NADPH.  Some pigments are more efficient than others in supporting photosynthesis.   The main photosynthetic pigment is called chlorophyll and provides the green color observed in many leaves.  There are other leaf pigments, like carotenoids (yellow) and anthocyanin (pink) found in the leaves of the ornamental Coleus plant.  The goal of this experiment is to determine the efficiency of the different pigments in supporting the process of photosynthesis.

 

Procedure:

1.  Remove a leaf from the Coleus plant.  Sketch the distribution of colors within the leaf and note the pigment responsible.

 

 

   

        

                  Coleus leaf with intact pigment                                       Coleus leaf after staining with iodine

 

2. Place the water beaker on the hot plate and bring to a boil.

3. After the water has boiled, place 150 mL of 95% ethanol onto the hot plate. The boiling point for ethanol if much lower than that of water, so it will come to a boil quickly.

4. Take the Coleus leaf, remove its covering and place it in the boiling water for one minute. This kills the plant cells and breaks open its membranes.

5.  Remove the leaf from the boiling water using the tweezers and place it in the boiling ethanol. This removes the pigments from the leaf. Leave it in the ethanol until the leaf turns white.

6.   Remove the leaf from the ethanol and return it to it’s the petri dish filled with water.

7.  Place the leaf into the pertri dish with iodine (Lugol’s solution).  This stain reacts with starch to produce an intense dark blue-black color. Allow the leaves to soak in the Lugol’s solution for 2-3 minutes.

8.  Return the leaf to its dish the water dish. Use the tweezers to spread it out flat.

9.  Observe the staining pattern of each for the leaf. Draw what you observe and answer the following questions.


Questions:

1.  Which pigment was most efficient in supporting photosynthesis?  Explain.

 

 

 

2.  Which pigment was least efficient in supporting photosynthesis?  Explain.

 

 

 

   

G.  Post-Lab Questions

1. In order for photosynthesis to occur the following must be present (hint: review the photosynthesis equation):

a. ________________________________ as the energy source.

b. ________________________________ as the carbon source.

c. ________________________________ as the electron donor.

d. ________________________________ for the absorption of light energy.

 

2. Explain why grow lights for house plants are never green.

 

    

 

3. Explain why plants that have been kept in the dark for a week will still be able to conduct cellular respiration.

 

 

 

 

 

4. As you saw from your chromatogram, there are more pigments present in leaves than you can see. Propose an explanation for why leaves only appear green in the growing season, but turn bright colors in the autumn when the weather turns cold.

 

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