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An Ode to PSII.


As instructors of introductory biology, one of the challenges we face is to motivate and interest our students who may be wondering why they have to sit through a required course such as ours to begin with. This becomes especially challenging when the topic focuses on the "less exciting" subject of plants, as most students have little interest in plants and their physiological processes and approach any topic in plant physiology with not .just a lack of enthusiasm, but a phobia of the biophysical and biochemical steps involved.

Many instructors of biology have noted the usefulness of hands-on exercises that require building and using a model (for example, see Stavroulakis, 2005) or role-playing (Chinnici et al., 2004) in helping students to visualize and understand abstract concepts better. In my introductory courses, I have resorted to role-playing and biological "plays" to help students visualize more abstract subjects in a manner that is easier for most students to understand. An example is the exercise described here, in which students trace the photochenrical steps of photosynthesis around Photosystem Il (PSII). Most introductory biology and plant biology texts provide beautiful diagrams showing these steps. However, these diagrams are .just the first step in my classroom, where the students use reading, seeing, hearing, and physical participation in activities such as that described here all as part of the learning process. In addition, I have chosen to include some of the physical detail about the process that is typically left out of the introductory texts that I consider important to understanding the process better. An example is the role of the oxygen evolving complex (OEC) and its interaction with two water molecules, even though the actual mechanism through which it acts is still not well understood.

The activity I describe here was inspired by my participation as a graduate teaching assistant at Cornell University in Dick Ecklund's photosynthesis play. Carol Reiss, through a special grant, made a special film of this play, in which the plant biology faculty at Cornell University (including notables such a Andre Jagendorf whose experiments in the 1960s provided support for the chemiosmotic hypothesis) were included as participants in the play. This video may still be available through the section of Plant Biology at Cornell University. In the play, Dick Ecklund explained and walked the several hundred introductory students in the class through the steps of the photochemical reactions. The actors on the stage (TAs and faculty alike), each wearing a sign identifying them as parts of the system, acted their parts to show the reactions and the interactions between the various components of the thylakoid and stroma of the chloroplast. In my small class of 24 students, I chose to perform the play with my students as the characters in the play rather than the audience watching the play, a necessity in Dick Ecklund's large class. Moreover, I wrote the Ode to PSII that I present here to both help my students learn what happens around Photosystem II (PSII) that leads to the splitting of water, and to make the whole exercise more interesting for them.

* Learning Objectives

To understand:

* the basics of the photochemical reactions in Photosystem II

* the difference between the stroma, the thylakoid membrane, and the lumen

* the topology of the photochemical reactions and their relation to the development of the pH gradient across the thylakoid membrane

* the significance of light absorption, excitation of electrons, energy transfer, and electron transfer

* the significance of the pigments in the reaction center as well as those in the antenna complex

* Materials Required

This exercise requires the willingness of the students to participate in the play, and some simple party supplies.

* simple printout labels with large print pinned to each character's shirt to designate as a pigment or other participant

* different color party hats worn by each character, with colors that designate the character's identity, e.g., yellow and orange for carotenoids

* party balloons as electrons. A character may hold on to its electrons, share them with another, shake and excite them, pass them on, grab them from a neighbor, or extract them from a nearby "compound" such as water

* flashlight used by "the sun" to provide photons of light

* magic wand used by the OEC during the splitting of water to designate the as-of-yet poorly-understood process

* rope used to demarcate the lipid bilayer of the thylakoid membrane, and lumen versus stroma

* The Cast of Characters

The following list engaged all of the students in my small class (up to 24):

* the instructor or one student as the narrator to read the Ode

* two students as the two chlorophyll a molecules in the PSII reaction center

* several students, depending on the size of the class, designated as chlorophylls a and b, and carotenes and xanthophylls in the antenna complex

* at least two water molecules: two students act as the oxygen atoms, four act as the hydrogens, and each hydrogen shares a balloon electron with the oxygen in its water molecule

* one student with a flashlight acts as the sun dispensing photons of light

* one student acts as the first electron acceptor, the grey pheophytin

* one student initially holding four balloon electrons acts as the OEC

* several students, depending on the class size, can act as various members of the electron transport chain (ETC), whisking the excited electrons away from PSII

* The Play

The text of the Ode to PSII is presented below as are general directions for the play. As we perform this play, the narrator reads the Ode and the following events are acted out.

Scene Script (in regular text) Set directions (in italics)

1 This is the story of the little pigment that could

If only the sunlight would hit it where it should.

The reaction center is awaiting the sun after a dark night But how are the two chlorophyll a's to capture the light?

We meet the two chlorophyll a molecules in the reaction center, surrounded by the antenna complex pigments, and other characters in the play. Their heads are bowed down. As their characters are called, they raise their heads and act their parts to highlight their roles in the process.

2 There are the antenna complex pigments to the rescue

Each showing off its unique, lovely hue:

The orange and yellow carotene and xanthophyll,

The blue- or grass-green, b and a versions of chlorophyll.

They surround the reaction center dutifully

Working with it in harmony beautifully.

The sun shines light on various points in the class, eventually hitting one of the pigments in the antenna complex.

3 When an antenna complex pigment in plain sight Absorbs a photon of light,

It "gets so excited

And it just can't hide it!"

As the pigment gets excited, the refrain from the" song "I'm So Excited" by the Pointer Sisters plays in the background. The balloon electron is raised from ground state to a higher energy level.

4 The excitation is contagious:

Pigment to pigment, carotene to xanthophyll.

Finally reaching the reaction center Chlorophyll.

Through resonance energy transfer; the excitation is passed on until it reaches the reaction center and one of the chlorophyll a molecules there is excited.

5 But what to do with all that energy and excitation?

After all, what is the use of all that pigmentation?

Pheophytin is lurking by the chlorophyll a

Ready to take away its excited electron, you don't say.

Is pheophytin up to no good?

Or doing for the plant what it should?

The excited electron is passed on to the pheophytin, the first electron acceptor, thus carrying out the first photochemical reaction.

6 It passes the electron to the electron transport chain

Where its work will not be in vain.

Pheophytin donates the excited electron to the first member of the electron transport chain (ETC). The excited electron is passed down the chain.

7 But the chlorophyll a is now incomplete

Feeling empty inside with pain replete.

The oxygen evolving complex comes to the rescue.

It gives of itself lovingly to the reaction center in full view.

The oxygen evolving complex, sitting on the lumen side, donates one electron to the chlorophyll a molecule that had donated its excited electron to pheophytin.

8 Another photon of light and the antenna complex is excited.

The energy is passed on to the chlorophyll a who is delighted.

Again pheophytin takes away the excited electron

Leaving chlorophyll a to think of the fleeting delight

That is now gone.

The altruistic OEC gives of itself as it is able

And once again chlorophyll a is safe and stable.

One more photon of light

And the antenna complex is again excited.

The energy is passed on to the chlorophyll a

Who is easily delighted.

Pheophytin takes away the electron again

And chlorophyll a is in a fury.

The OEC gives of itself once more

And chlorophyll a is safe, not to worry.

But it's sunny and another photon of light

And the antenna complex is excited.

The energy is passed on to the chlorophyll a

Who is again delighted.

Pheophytin takes the excited electron again

And now chlorophyll a is in despair.

The OEC gives of itself once again

Altruism beyond compare.

Repeat Steps 2-7 three more times. Each time the sun may excite a different antenna pigment, randomly.

9 But four electrons have been taken from the OEC

Who is now shaking with instability.

What to do, what to do ...

The magic wand to the rescue. In the lumen water molecules are prancing around

And it's with electrons that protons and oxygen are bound!

Come on OEC, do do that voodoo

That you do so well.

Grab the electrons, release the protons

The shaking you shall thus quell.

But each oxygen atom is now just one

No longer able to hold on to either proton.

Finally the oxygens are joined as one

And they leave the lumen when all is said and done.

When the OEC (oxygen evolving complex) has donated all four electrons, it will grab four electrons from two molecules of water in the lumen, thus splitting the two to release four protons and one oxygen molecule. This process is not well understood, hence the reference to "the magic wand." It is important for the two water molecules to be split together, so that one molecule of oxygen ([O.sub.2]) is formed. The oxygen molecule passes through the membrane (ropes) and diffuses away.

10 Now all is happy in the thylakoids of the chloroplast.

But the story is not over, as long as the sun is shining

And it's not overcast.

Pigments are excited and electrons will flow.

Taken by the ETC, where will they all go?

This is not the end of the process. As long as there is sunshine, there will be excitation of the pigments and electron flow. Also, the electron transport chain is carrying excited electrons away from PSII, and the fate of these electrons should be an element of curiosity.

* Review

Following the play, there is a quick review of the major steps and their relation to establishing the pH gradient across the thylakoid membrane. We perform and review the play during one 50-minute class period. Sample review questions may include:
 What is the role of the antenna complex versus the reaction center
 pigments? What is the difference between the energy transfer and
 the redox reactions in the thylakoid membrane? What steps help to
 establish a pH gradient across the thylakoid membrane? What is the
 significance of the OEC (oxygen evolving complex) in the process?

Such topics and questions can be used for an in-class written assignment or as part of a quiz or exam to assess student learning. I cannot provide quantitative analysis of differences in performance by students who learned about the photochemical reactions using this activity with those who learned the subject without this activity, as I always taught the topic to my introductory students with this activity and thus have no point of comparison among my students.

* Further Suggestions

The exercise presented here is meant as an active exercise that engages the students in the process. Such plays have served to show my students my enthusiasm for biology, and have helped to get the students more interested in the topic. This exercise has also helped to visualize the biochemical and biophysical steps of the photochemistry around PSII for students who may have difficulty in learning these concepts.

I have not extended the Ode to PSII to include what happens in PSI (Photosystem I), although other instructors may choose to do so following this format. Nevertheless, I usually follow this play with other in-class activities that demonstrate the reactions leading to the final products of both the photochemical reactions and the Calvin Cycle (Firooznia, 2007). Thus, the connection between the photochemical and biochemical reactions through ATP and NADPH becomes an easy point of discussion. In a larger class with enough participants for all of the roles, such activities can be easily combined as a final "recap" for photosynthesis to emphasize the interconnection between all of the reactions involved.

Exercises such as the one described in this article are inexpensive ways to involve students in learning the photochemistry of photosynthesis in plants, and similar exercises can be developed for other molecular reactions in cells. Examples may include mitochondrial respiration, meiosis and mitosis, neuronal and hormonal signal transduction, and transcription and translation. Such activities help to supplement the traditional methods where the instructor simply describes and reviews the steps involved as presented in a given textbook. Such activities can be easily and cheaply implemented in any introductory biology course at the high school or college level, and would add both entertainment and substance to the course.

* Acknowledgments

I thank my students for participating in the biological plays I have incorporated in my teaching, especially the Ode to PSII. I also thank the Department of Biology at Denison University for the funds to purchase the few party supplies needed for this exercise. And I thank Dick Ecklund and Carol Reiss for the original inspiration for this exercise.


Chinnici, J.P., Yue, J.W. & Kierom, M.T. (2004). Students as "human chromosomes" in roleplaying mitosis and meiosis. The American Biology Teacher, 66(1), 35-39.

Firooznia, F. (2007). The story of the Calvin Cycle: Bringing carbon fixation to life. The American Biology Teacher, 69(6), 364-367.

Mauseth, J.D. (2003). Botany: An Introduction to Plant Biology, 3rd Ed. Sudbury, MA: Jones and Bartlett Publishers.

Stavroulakis, AM+ (2005). Meio-socks & other genetic yarns. The American Biology Teacher, 67(4), 233-238.

Taiz, L. & Zeiger, E. (2002). Plant Physiology, 3rd Ed. Sunderland, MA: Sinauer Associates, Inc.


FARDAD FIROOZNIA (fardadfirooznia is a former Assistant Professor of Biology at Den/son University in Granville, OH. He currently works as an Instructional Designer at PTi International in New York, NY, a firm providing compliance training for the pharmaceutical industry.
Table 1. Pertinent Vocabulary/Background (Mauseth, 2003; Taiz &
Zeiger, 2002)

Accessory Pigment A pigment with an absorption spectrum that
 differs from that of chlorophyll a, and
 that transfers its absorbed energy to the
 chlorophyll a in the reaction center

Antenna Complex A group of pigments that absorbs energy and
 passes it to the reaction center

Carotenes A class of carotenoids found in the antenna

Electron Transport Chain A series of electron carriers that transfer
 electrons from a donor, which becomes
 oxidized, to an acceptor, which becomes

Excited State When a quantum of light is absorbed by a
 pigment, one electron is raised from its
 stable or ground state to a higher energy
 orbital; that is, the electron and thus the
 pigment are said to be "excited."

Lumen The space inside the thylakoid membranes

Oxygen Evolving Complex A special protein complex attached to PSII
 on the lumen side, which provides electrons
 to the oxidized reaction center
 chlorophyll, and in response will split two
 molecules of water to replenish the donated
 electrons, and will release one molecule of
 oxygen and four protons into the lumen of
 the thylakoid.

Pheophytin A special chlorophyll molecule that is
 missing the central [Mg.sup.2+] ion. It is
 the first electron acceptor in PSII.

PSII Photosystem II: A complex of pigments and
 proteins that transfer electrons from water
 to Photosystem I

PSI Photosystem I: A complex of pigments and
 proteins that transfer electrons to NADPH

Reaction Center Special chlorophyll a molecules in a
 photosystem that are actually involved in
 the transfer of elec trons, and thus
 chemical reactions, in photosynthesis

Redox Reactions Chemical reactions involving simultaneous
 reduction and oxidation of chemical

Resonance The rapid transfer of energy between
 pigments, possible through the close
 proximity of the electron orbitals of the
 pigments that are tightly packed in the
 thylakoid membrane

Stroma The solution surrounding the thylakoid

Thylakoid The photosynthetic membranes inside the

Xanthophylls A class of carotenoids found in the antenna
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Author:Firooznia, Fardad
Publication:The American Biology Teacher
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2009
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