PHOTOSYNTHESIS

WITHOUT IT, WE WOULD HAVE

- NO FOOD

- NO FUEL

AND

- NO OXYGEN

A guide to interpreting the Photosynthesis poster produced by The Huntington Botanical Gardens, San Marino, CA. 1995. This project made possible through a generous gift from Dr. and Mrs. Peter S. Bing. The poster and guide are copyright property of The Huntington and the artist.

"It's Elemental My Dear Watson"

INDEX

Introduction

The Real Scoop

(Suggested grade levels: 8 - 12)

Missing Elements

(Suggested grade levels: 5 - 7)

Word Search

(Suggested grade levels: 2 - 5)

A Fungus Among Us

(Suggested grade levels: 6 - 8)

O Two, Brute

(Suggested for any grade level)

Name Your First Child Phloem

(Suggested grade levels: 6 - 12)

It's Not that Easy Being Green

(Suggested grade levels: 7 - 12)

The Dark Side

(Suggested grade levels: 8 - 12)

The Charge of the Light Brigade

(Suggested grade levels: 6-8)

INTRODUCTION:

Every student should have some understanding of the interactions between plants and the elements. Not the weather! That's ecology. In this PHOTOSYNTHESIS poster we are talking about physiology, the study of life's chemistry. The elements of great concern are therefore atoms - building blocks of all matter - and their roles and reactions in a living plant.

In analyzing the make-up of plants and in studying the various elements plants require for normal growth, scientists have determined that green plants require only a few of the many known elements. There is a simple device to help students of all ages remember which elements constitute necessary nutrients.

Just picture a small-town restaurant, the Hopkins Cafe, managed by earthy patroness, Mozel. It's the kind of place you might go to get a great hamburger or a bad cup of coffee. A hangout for locals, grease-encrusted plaques near the cash register suggest minor pretensions as a tourist attraction along the backwater of old Route 66. Because Mozel supports the Science Club at the local school, students there decided to raise a billboard advertising her restaurant. Of course there had to be more to their project than pure announcement. The message was to have other meanings. It would utilize only the symbols for elements - and only those elements that might be found in a leaf of wilted lettuce on Mozel's unfortunate salad bar. The product of their brainstorming would become a ray of hope for tens of thousands of students facing examinations on plant nutrition. You also will find this advertisement in scientific shorthand very useful:

In spoken English, one pronounces the phrase: "See Hopkins Cafe, Managed by my cousin Mozel."

This mnemonic tells any interested person with fairly basic knowledge concerning the atomic elements a lot about plants. It is a comprehensive listing of all required plant nutrients: Carbon, Hydrogen, Phosphorus, Potassium, Nitrogen, Sulphur, Calcium, Iron, Magnesium, Boron, Manganese, Copper, Zinc, Molybdenum, and Chlorine. We may someday have to come up with a modified version to account for the concern that Sodium (Na), Silicon (Si), and Nickel (Ni) are potential additions to the list. In the meanwhile there is plenty to do just discovering the roles of the known nutrients in the life of a plant. Three of the most important elements, carbon, hydrogen, and oxygen, are directly involved in photosynthesis, which is usually summarized by the following convenient, but greatly simplified and problematic statement:

Carbon dioxide + water + sunlight Þ sugars and oxygen

DIAMONDS AND RUST (Carbon & Oxygen); YET MORE INTRODUCTION THAT YOU SHOULD READ ONLY IF USE OF THE POSTER WILL BE ORGANIZED AROUND CHEMISTRY:

The properties of carbon, oxygen, and hydrogen are basic to the photosynthetic reaction, and to life itself. Carbon is a nonmetal. In pure state it can appear black or crystalline clear, existing naturally as graphite or diamond, as well as a recently discovered configuration called buckminsterfullerene (due to structural resemblance to the plan of a geodesic dome). Soot and carbon black are forms of graphite, carbon atoms bonded to each other in sheetlike crystalline configurations. Graphite is used for air and water purification (through surface adsorption), lubrication, pencil lead, and electrical conductivity. Graphite can be burned, that is oxidized, to form carbon dioxide. When present as diamond, carbon has completely different characteristics. Formed under considerable heat and pressure, the carbon-carbon bonds of diamond are built into a three-dimensional crystalline framework. Unlike graphite, "diamond is a rigid, transparent, electrically insulating solid. It is the hardest substance known and the best conductor of heat." (General Chemistry, Atkins & Beran, Sc. Am. Books)

Important to the formation of compounds necessary for life, "carbon atoms possess... the capacity to bond with each other. Since a carbon atom may either accept or donate four electrons to complete an outer octet, it can form covalent bonds with four other carbon atoms. In this way covalently linked carbon atoms can form linear or branched or cyclic backbones for an immense variety of different organic molecules. Moreover, since carbon atoms also form covalent bonds with oxygen, hydrogen, nitrogen, and sulfur, many different kinds of functional groups can be introduced into the structure of organic molecules.

Organic compounds of carbon have yet another distinctive feature. Because of the tetrahedral configuration of electron pairs... many different three-dimensional structures can be achieved... No other chemical element can form molecules of such widely different sizes and shapes or with such a variety of functional groups." (Biochemistry, Leninger, Worth Publ.)

But carbon, so critical to life, is not the most common element on Earth. "Oxygen is the most abundant element in the Earth's crust and accounts for 23% by mass of the atmosphere" (Atkins & Beran, General Chemistry, 1990). In league with water, oxygen is a major force in corrosion, such as in the formation of rust (Fe₂O₃ H₂0). Pure oxygen is a clear, odorless, tasteless gas. In the trophosphere (the lower zone of the atmosphere in which weather is produced and living forms exist) free oxygen is normally in the molecular form O₂. In the upper atmosphere, in the presence of sunlight, O₂ actually decomposes to single atoms of oxygen. Those single atoms may then react with oxygen molecules to form ozone, O₃.

Carbon and oxygen exist together in multitudinous and vastly different circumstances. They can form the colorless, odorless gases carbon monoxide and carbon dioxide. Together they form mineral carbonates, such as CaCO₃. Of most immediate interest to us as living beings, however, are the roles of these two elements in what are normally termed organic compounds. Photosynthesis is the process that takes inorganic compounds (carbon dioxide and water) and fixes the carbon to small chains of other carbons. But pure carbon linked to carbon makes graphite or diamond. The compounds that result from photosynthesis (basically sugars) contain hydrogen and oxygen as well, and have amazingly different properties. These sugars, classified as carbohydrates, are energy storage compounds and the building blocks for more complicated organic molecules.

ACTIVITIES:

The PHOTOSYNTHESIS poster can be used to talk with students of many different age groups about photosynthesis and plant nutrition in various ways. According to your students' level of comprehension in the sciences, here are a few very short activities that may prove useful and engaging.

THE REAL SCOOP (Suggested grade levels: 8 - 12; for students who understand that matter is made of atoms, that names are assigned to atoms of each different type, and that the various elements are charted on the periodic table.)

Give students the listings of elements on page 6. Have them compare the required nutrients for plants to those required for humans and to the relative abundance of elements in the Earth's crust and soil samples. A good activity for high school students would be to construct a graph (a pie chart or histogram) for each situation showing the abundance of each element. Examine the formula for photosynthesis at the top of the poster. Solicit observations/ask questions along the following lines:

- How does the listing of essential elements for plants compare to the relative abundance of elements in the soil? Which elements that are very abundant in a normal soil are not part of a plant's chemistry? Silicon and aluminum are abundant in the Earth's crust and in soils. Sodium is also fairly common. It has been demonstrated that "some plants may require sodium or silicon. In plant species adapted to saline environments, sodium is taken up in high amounts and is required for growth. Silicon is essential for rice and may also be essential for...scouring rush." (Taiz and Zeiger) In general, however, silicon and aluminum, though abundant, are not necessary for plant growth. Many metals are toxic to plants and other living creatures.

- Examine a periodic table. Which elements that plants require are nonmetals, which are considered metals? Which are most abundant in plants? What essential element for plants has the highest atomic number and mass? Boron, Carbon, Nitrogen, Oxygen, Phosphorous, Sulfur, and Chlorine are considered non-metals and the rest are considered metals. "A metal is a substance that conducts electricity, has a metallic luster, and is malleable and ductile. A nonmetal is a substance that does not conduct electricity and is neither malleable nor ductile." (Atkins and Beran) Molybdenum is the essential element with the highest atomic number (42) and molar mass (95.94 gms/mole).

- What are the sources of Carbon, Hydrogen, and Oxygen that enter the photosynthetic reaction? The Carbon comes from carbon dioxide in the atmosphere. The Oxygen that enters sugar synthesis also comes from atmospheric carbon dioxide. Free oxygen that is released to the atmosphere is produced through the splitting of water (H₂O), and thus comes from the soil water (in most plants).

- Where do the next three elements (P, K, N) come from? The soil. Have students ever heard of N:P:K ratio? Ask if any student has ever bought a bag of fertilizer. Did the shopkeeper ask them to specify three numbers (i.e. the N:P:K ratio)? What does the N:P:K ratio represent? The relative percentages of usable Nitrogen, Phosphorous, and Potassium in the formulation. Which is highest in Nitrogen, a 10:10:10 or a 15:10:5 fertilizer? The 15:10:5 has fifteen pounds of available nitrogen but only ten pounds of phosphorous and five pounds of potassium for every 100 pounds of weight.

- Compare the relative abundance of the macronutrient elements for humans (Magnesium and above) to the macronutrient elements for plants (Sulfur and above). There are a few real differences. Can students offer explanations? Calcium is much more abundant in humans, due probably to formation of bones as well as use for chemical transmitter release, muscle contraction, etc. The relative positions of H, C, and O are different because the plant sample was oven-dried material and does not reflect the large amount of water normally present in this tissue.

Relative abundance of elements in a Soil Sample (not including water or water of hydration), as a percent of total number of atoms:

O 62.4

Si 22.5

Al 5.4

C 4.2

Ca 1.2

Na 1.1

Fe 0.9

K 0.8

Mg 0.7

N 0.3

S 0.1

Ti 0.1

F 0.1

P 0.03

Mn 0.02

Ba <0.01

B <0.01

Cl <0.01

Also present at levels lower than 0.01%: Sr, Zr, V, Rb, Zn, Cr, Nd, La, Cu, Y, Li, Ni, Pb, Ga, Nb, Th, Co, Sc, As, Ce, U, Sn, Ge, I, Mo, Be, Br, Sb, Se, Cd, Hg, and Ag.

Elements found in dried Plant Material, percentage based on number of atoms:

H 45.6

C 30.4

O 22.8

N 0.76

K 0.19

Ca 0.09

Mg 0.06

P 0.05

S 0.02

Cl 0.002

B 0.001

Fe 0.001

Mn 0.0008

Zn 0.0002

Cu 0.00008

Mo 0.0000007

Relative abundance of the major chemical elements in the Earth's crust, as a percent of total number of atoms:

O 47

Si 28

Al 7.9

Fe 4.5

Ca 3.5

Na 2.5

K 2.5

Mg 2.2

Ti 0.46

H 0.22

C 0.19

Major Elements in the Human Body as a percentage of total number of atoms:

H 63

O 25.5

C 9.5

N 1.4

Ca 0.31

P 0.22

Cl 0.08

K 0.06

S 0.05

Na 0.03

Mg 0.01

Fe <0.001

Zn <0.001

Se <0.001

Mn <0.001

Cu <0.001

I <0.001

Mo <0.001

Co <0.001

Cr <0.001

F <0.001

MISSING ELEMENTS I (Suggested grade levels: 5 - 7; for students just learning the names and symbols for atoms.)

- Have students learn the names and symbols for essential elements by comparing the mnemonic phrase to the names of the elements that have been written into the soil on the poster (the most important essential elements are written in rust color, the trace elements are written in tan). Do the students discover there is a missing element? Yes, it is magnesium (Mg). What is the term that describes this condition for a plant, when an element necessary for normal growth is missing? This is called a deficiency.

MISSING ELEMENTS II (Suggested grade levels: 7 - 9; For students who understand the basics of chemical structure and reactions.)

- Ask students to estimate roles that some of the essential elements have in the structure or physiology of plants. They should be able to guess one basic importance of carbon, hydrogen, and oxygen from study of the poster. Scientists have explained roles for most other essential elements; some are listed below:

Nitrogen - a constituent of amino acids, amides, proteins, nucleic acids, nucleotides, porphyrin-like structures (such as chlorophyll)

Phosphorous - component of sugar phosphates (therefore necessary to form the sugars that are exported from chloroplasts), nucleic acids, phospholipids, coenzymes; important role in energy transfer compounds

Potassium - a required cofactor for over 40 enzymes; functions in stomatal control

Sulfur - component of three amino acids, thus of proteins; constituent of many important compounds

Calcium - constituent of middle lamella that binds the walls of separate cells together, cofactor in various enzymes

Magnesium - constituent of chlorophyll

Iron - Constituent of cytochromes and proteins involved in photosynthesis, nitrogen fixation, and respiration

Manganese - required for production of O₂ during photosynthesis, necessary for various enzymes

Boron - required for plant gravitropic response (note that humans are not known to require boron); other unclear activities

Copper - essential component of various oxidases (enzymes)

Zinc - essential component of various dehydrogenases and other enzymes

Molybdenum - essential to Nitrogen fixation

Chlorine - required for photosynthetic reactions involved in oxygen production

Carbon Calcium

Hydrogen Iron

Oxygen Magnesium

Phosphorus Boron

Potassium Manganese

Nitrogen Copper

Sulfur Zinc

Sulphur Molybdenum

Chlorine

A FUNGUS AMONG US (Suggested grade levels: 6 - 8; for students with grounding in physical and earth sciences.)

We have illustrated roots in the soil, and even given an enlargement of a growing root tip. What is not obvious is the means by which nutrients enter a plant. It is not even obvious which nutrients must come from the soil. Examining the mnemonic device for essential elements, hold a brief discussion with students as to how plants take in elements. Here are some starter questions:

- There is carbon in the soil and carbon in the air. What is the source of carbon involved in photosynthesis? Carbon dioxide in the atmosphere.

- Which elements are taken from the atmosphere, which from the soil? The carbon and oxygen involved in building the sugars come from CO₂ in the atmosphere. Oxygen also enters the plant in other forms, from water to various oxides. All of the other required elements are taken in from the soil (the growth medium).

- How are elements taken in by the roots? Points of entry for nutrients can be along the root surface, especially near the growing tip where the young root hairs are still existent. Shown in rusty orange color is another mode of entry, through mediation by a fungus. Scientists have discovered in the last two decades that many more plants form mycorrhizal (fungus and root) relationships than previously thought. The relationship between the infecting fungus and the plant root is considered to be mutualistic, with the fungus receiving sugars from the host plant, while the plant receives nutrients taken-up from the soil. Much study is underway in this area of inquiry.

Typically, when nutrients are taken up, it will be in the form of a charged particle dissolved in water, for example chlorine, potassium, or sodium ions, or oxygen-containing ions such as phosphates or borates, or even in chelated forms, as with iron or copper. This explains why commercial fertilizers often consist of compounds, such as potassium nitrate or ammonium phosphate.

- Can roots affect the nature of the soil that surrounds them? Yes, indeed. Recent studies indicate that roots exude considerable quantities of organic compounds into the soil. This might include citric acid, amino acids, or many other small molecules. The presence of these compounds can significantly change the acidity of the soil in direct contact with growing roots.

O TWO, BRUTE (Suggested for any grade level; for students just introduced to the physical sciences.)

- Have the students examine claims at the bottom of the poster. "Photosynthesis: Without it we would have no food, no fuel, and no oxygen." Concentrate on the "no oxygen" part. Is this statement valid? Certainly there would be oxygen without photosynthesis, but from all accounts, there would be little free oxygen, O₂, for animals to breathe. Most of the Earth's supply of oxygen is bound up in various kinds of molecules, carbon dioxide being one of them, water being another. Through the splitting of water (which is set in motion by the energy of light) into protons (hydrogen), electrons, and free oxygen, photosynthesizing plants indeed drive the natural replenishment of oxygen in the atmosphere.

Examine the other claims:

- Without Photosynthesis we would have no food. True or False? This is absolutely true. "Life on Earth ultimately depends on energy derived from the sun. Photosynthesis is the only process of biological importance that can harvest this energy." (Plant Physiology, Taiz & Zeiger, 1991) This chemical energy, bound in organic (read carbon-based) compounds and packaged in some acceptable form is what we call food.

- Without Photosynthesis we would have no fuel. True or False? This one is semantic. All fossil fuels (coal, oil, natural gas) are based on deposits of ancient plant and animal life and are therefore based on photosynthesis. Nuclear fuel, solar power, wind and water power are certainly not dependent on living plants, though continental weather patterns (wind and rainfall) are affected by the presence (or absence) of huge forested areas.

NAME YOUR FIRST CHILD PHLOEM (Suggested grade levels: 6 - 12; for students who understand that plants are made of cells and that cells differentiate to take on specialized functions.)

There are so many ways to understand the structure of a plant, and how that structure relates to plant physiology and ecology. The internal arrangements for the flow of water and nutrients, as well as the movement of sugars, are critical to the process of photosynthesis. Without launching a lesson in anatomy, it is useful for students to understand that in a manner somewhat analogous to animals, plants have systems for the movement of liquids. But the tissues involved, phloem and xylem, are not at all equivalent to the venal and arterial circulatory systems of animals for they do not have a main role in carrying oxygen for respiration or carbon dioxide for release, they are not actively pumped, and they do not have any filtration or breathing organs (liver, kidney, lungs) involved. The similarity is that phloem and xylem conduct liquids carrying nutrients and break out into recognizable veins (or vascular bundles) that branch and branch to make close connection with cells throughout organs such as leaves and flowers. You could just as well compare the vascular system of a plant to the plumbing in a city. Water, pumped from the ground, arrives at homes and businesses through one set of pipes. Organic material, from toilets and garbage disposals leaves homes and businesses through another set of pipes. The water piped into homes may carry fluorine and trace amounts of iron or other elements. The organic efflux from homes (sewage) is piped to lagoons or pools where it becomes an energy source for hoards of microorganisms.

Instead of animal arteries and veins, or city water and septic, plants have xylem and phloem. Essentially, a plant vein will encompass both systems, which run in parallel. The xylem tissue constitutes a tubular network of cells that connects to the roots and transports water and dissolved nutrients from the soil (or growth medium) to all living parts of the plant. Sugars and other products produced by the plant are not typically transported in the xylem, because xylem is basically a one way street from the soil through the plant to the atmosphere (water vapor escapes through various places, most importantly the stomata in the leaves.) Moreover, xylem tube cells develop and die - the functioning water-conducting element is the non-living shell of a cell. To move molecules around in the entire plant, including the translocation of sugars from the point of production (normally the leaf) to places where sugars are used for growth (the roots, new shoots, flowers and fruit, etc.) requires more work and thus the unique qualities of phloem tissue. Phloem sieve cells are alive at functional maturity, which is necessary to the transport and regulation of plant sugars. There are many lessons from these observations, perhaps the least of which is that if you are searching around for a clever plant name to give to your first offspring, unless you are looking to raise dead wood, Phloem is probably a better choice than Xylem.

- From examining the diagram at the top of the poster, can you determine the relative orientations of xylem and phloem in the stem? In the leaf? Xylem tissue is normally formed to the inside of phloem in a stem, and thus as a vein essentially peels off a leaf connection with both systems, the phloem becomes positioned below xylem in leaves.

- What color is xylem and what color is phloem in a living plant? They are basically colorless. Students need to understand that most demonstration slides are stained so you can easily distinguish the different cell types, either with the red and green of safranin and fast green, or the varying colors of toluidine blue. In the most common preparations, xylem tissue stains red and phloem a blue-green.

- What is the wood of a tree? Wood is accumulated layers of xylem tissue. Phloem tissue is formed toward the outside of a tree as rings are grown. Thus, each year's new phloem replaces the last year's phloem, which is eventually pushed toward the outside of the tree, into the bark where it finally is shed.

IT'S NOT THAT EASY BEING GREEN (Suggested grade levels: 7 - 12; for students who understand the nature of cells.)

Scientists have discovered many facts about chlorophyll, the green matter of plants. We know that it does not float around free in a cell, it occurs in membrane-bound bodies called plastids. Plastids, in this case chloroplasts, are not fixed in one position, but move in the general cytoplasmic flow that can be observed in a living cell. Cases have been noted to indicate that the positioning of chloroplasts within a cell, indeed the hourly positioning of leaves for some types of plants, relates to the direction and intensity of sunlight. Inside the plastids chlorophyll is not loose, rather it is layered into structures called thylakoids. Thylakoids are arranged into arrays analogous to those of solar energy panels.

In each layer of a thylakoid membrane molecules of chlorophyll are organized, along with other pigments, as well as enzymes, very complex proteins, and other compounds, into reaction centers for the capture of light energy and also sites for the conversion of light energy to a usable form of chemical energy.

- How long can a photosynthetic plant cell live? How long can chlorophyll function? Some photosynthetic cells have quite long lives. Cacti can maintain green stems for decades, indicating that the same cells remain photosynthetic over that long period of time. Chlorophyll molecules in those cells, on the other hand, are constantly being damaged and restructured. The capability to repair the photosynthetic apparatus is critical for chloroplasts and chlorophyllous cells to retain long-term utility.

- Have you ever heard of a plant getting sunburned? How might that happen? Sunburn on plants is usually the irreversible destruction of cells from too much light and heat. This normally happens when plants have been grown under shaded conditions and are exposed suddenly to full sunlight, without a period of gradual increase in light. The compounds that typically help mask photosynthetic pigments from too much light may not have developed in the shade. The leaf may be larger and thinner than it would have been had it grown to maturity in full sunlight. There may not be as much protective hair or wax on the surfaces of leaves that developed in the shade as compared to those that would normally have matured in full sunlight. Some types of plants are simply constitutionally incapable of growing in full sunlight; others may grow in either shade or full sun, but require a gradual change to make the necessary preparations.

- Why are plants green? The capture of light energy begins when chlorophyll is excited to the point that it can give up an energized electron. The wavelengths, therefore colors of light, that can excite chlorophyll are mainly in the red and blue portions of the spectrum. Therefore the light that a chlorophyll-bearing leaf will reflect or transmit (but not absorb) is conspicuously green.

THE DARK SIDE (Suggested grade levels; 8 - 12.)

Photosynthesis is a simple enough word that embraces a process, a set of complex reactions having the significant outcome that carbon dioxide and water are converted to fixed carbon compounds and oxygen using the energy from sunlight (or any light of the appropriate wavelengths). The harvesting of light energy occurs in two distinct operations, photosystems I and II, that are termed the light reactions. In photosystem II, water is "split" (oxidized) as electrons are removed to yield hydrogen ions and free oxygen.

The rest of the simplified photosynthesis formula is the conversion of carbon dioxide to a fixed (reduced), organic form, that is sugars. This occurs independent of whether or not it is daylight, though it is totally dependent on a supply of carbon dioxide as well as the correct forms of chemical energy that were created during the light reactions. Because this phase of photosynthesis that can occur in darkness, we have come to call the carbon metabolism phase the "dark reactions." The heart of the dark reactions is the C3 Photosynthetic Carbon Reduction Cycle, also termed the Calvin cycle, honoring the person who discovered it. Students may have heard of this cycle, as well as ecological modifications known as C4 and CAM systems.

- Is chlorophyll involved in the dark reactions? Not directly. Chlorophyll is only involved in the light reactions, when light energy is captured.

- How does it do that? How is carbon dioxide captured and made part of an organic compound? The carbon is attached to a pre-existing sugar through the activity of an enzyme nicknamed Rubisco (its real name is ribulose bisphosphate carboxylase/oxygenase.) Scientists estimate that about 200 billion tons of carbon dioxide are converted into biomass each year. That means that about 10,000,000 tons of Rubisco are needed to accomplish the task. Plant physiologists state that without a doubt, Rubisco is the world's most abundant enzyme. That is quite plausible when one considers that about 40% of the soluble protein of most leaves is Rubisco.

Rubisco can function with very low concentrations of carbon dioxide in the atmosphere, but because it also catalyzes the reverse reaction (oxygenation to remove a carbon and produce carbon dioxide), the right environmental conditions (relative atmospheric concentrations of oxygen and carbon dioxide, temperature, water, light) must exist for photosynthesis to proceed at the expected level.

THE CHARGE OF THE LIGHT BRIGADE (Grades 6-8)

Sometimes even college students are lost when challenged to understand the complex sequence of events involved in transforming the energy of light into chemical energy. Chemical energy, of course, is necessary to drive the reactions that bind the carbon of CO₂ into organic compounds (that is, make sugars out of carbon dioxide).

This simple floor game of scientific hot potatoes can yield an outline view of the types of interactions that occur, providing a very early appreciation for the basic process of photosynthesis. [You will need several similar soft objects to represent electrons to be tossed from student to student (bean bags, plastic balls, potatoes, fruit, etc.) and a working flashlight.]

The most useful experience comes from simply constructing the game, which is truly a brigade (like a fire brigade) of students who will pass along electrons and create chemical energy as part of the process. Start with one student, who will be seated on the floor as the energy Receptor for one of the two systems that must be built. This student is given an "electron" that must be energized in some manner. A second student is given the flashlight and the role as the sun. When Sun flashes Receptor with light, the student tosses the electron up in the air.

If no other compounds were involved, the electron would settle back down and little would be achieved except perhaps the release of energy in the form of light. Now have a third student stand above and behind the Receptor. For our simplified game we can call this student (who should be one of the taller members of the class) Q. Now try the system again. When Receptor is flashed and the electron is tossed upward, Q catches the electron.

At this point talk with the students about what has happened. So far, not much. The electron is elevated energetically due to the power in the flash of light. It has been captured at this new state, but is something like a hot potato. The electron must be passed along a chain of compounds to a lower energy state while energy that had been associated with the electron powers a process that creates chemically potent ATP.

So you need more students - to form the Chain gang. Line up two or three (or as many as you wish) students who are shorter than Q, in a declining sequence. Q hands the electron off to the first student, who then passes it down through the other members of the Chain gang. In the process energy is moved into the chemically legal tender called ATP (adenosine triphosphate is formed from adenosine diphosphate through addition of an inorganic phosphate and the use of the energy from the electron).

What happens to the electron now? Probably many things, but in our photosynthetic thoughts, the electron is taken up by another kind of receptor, who we will call Receptor I. Like the student we seated at the beginning of the Chain gang, Receptor I sits on the floor, but of course in a position to receive the electron from the Chain gang.

Once this system is put together, it's time to get back to the powerpak that fuels the entire operation, the Sun. Have Sun flash Receptor again. Receptor doesn't have another electron to toss. That need should have been satisfied earlier. Where does Receptor get another electron? Someone must sit next to Receptor and hold the supply of extra electrons, this someone is Watermaster. It is water that gives up the electrons (manganese is the major element of the Watermaster and must be present for this to occur). When the H₂O has been split, oxygen is released. Watermaster can make this happen, but doesn't just hand over the electron. Receptor has to be fairly aggressive about reaching out and grabbing it. In future run-throughs, as soon as Receptor is flashed (and tosses its electron to Q), he/she should immediately reach out and take an electron from Watermaster.

Now have Sun flash Receptor I. We encounter the same problem as before. Receptor I tosses the electron into the air, but there must be someone to take it. Just as in the first situation a tall student, this time called A, will have to stand behind Receptor I (playing the identical role here as Q does at the head of the first chain gang.) Then we will of course need a new group of students, Son of Chain gang, to pass along the electron. The end of this process is different, however, from the earlier system. The last student in Son of Chain gang actually has two choices:

1. to lose the electron to an energy hoarding compound called Nadp, or

2. to pass it back to the first student in the original Chain gang. In that case it cycles back to Receptor I, yielding more ATP in the process.

Now let Sun make a few trials. After each flash of light take a moment to discuss with the students what has happened. Several developments will take place:

Watermaster will eventually run out electrons, i.e. there will be no more water to split. It becomes clear that photosynthesis cannot proceed unless water is available.

Receptor cannot function unless electrons are replenished; indeed continued reception of light without the necessary water can result in damage to the photosynthetic apparatus. There must also be someone to play the roles of Q and the rest of the intact chain gang. Moreover, Q has to have the correct physical placement to readily accept the electron.

Receptor I may reuse electrons that were cycled through Son of Chain gang and then back through the original Chain gang to Receptor I again. If, however, the last member of Son of Chain gang opts for the alternative outcome, which is the conversion of energy by passing the electron on to Nadp, then electrons are consumed as NADP is reduced to its energy-rich form called NADPH_2. In this case, Receptor I is still dependent on receiving electrons that came from the splitting of water and were passed from the original Receptor through the original Chain gang. Scientists have learned in more recent years that the connection between Chain gang and Receptor I is not especially as direct as our game implies.

We did not want to complicate the game further with yet more compounds, but to form ATP and NADPH₂ requires a supply of ADP and NADP. These lower energy compounds are regenerated when ATP and NADPH₂ release their energy during many processes that occur in cells. In the current example ADP and NADP are produced during the dark reactions of photosynthesis when their energy rich forms (ATP and NADPH₂) provide the power to fix carbon and build sugars. Advanced students should be able to improve the game and actually calculate the exact relationship between numbers of ATP and/or NADPH₂ that can be produced and the pulses of light given off by the sun.

Sun is absolutely necessary to this game. That is why the reactions demonstrated here are called the light reactions of photosynthesis. Light is necessary as the driving force.

Receptor and Receptor I are really forms of chlorophyll. Each is the focus of an entire array of other types of chlorophylls that collect light and pass the energy along to the receptors.

All of this process happens in the sunlight. Water is split, energy is converted to a usable chemical form. To get sugars is yet another matter. By now it should be very clear to students that the simple formula for photosynthesis:

Carbon dioxide + water + sunlight Þ sugars and oxygen

is truly too simplistic. The formula suggests that somehow water and carbon dioxide react in the presence of light to yield sugars and oxygen. This is clearly far from the truth. The splitting of water is part of the light reactions and is quite a separate event from the production of sugars using carbon dioxide (the dark reactions).

If you compare this game to a standard text on photosynthesis, the first portion we constructed (Receptor, Q, and Chain Gang) constitutes Photosystem II. The remaining players (Receptor I, A, and Son of Chain Gang) represent Photosystem I. Look at the grouping of students - it may appear like the shape of the letter "Z" laid on its side, hence the reason we label this model the "Z Scheme."

Technical References:

Walker, David. Energy, Plants & Man. Mill Valley, CA, Otygraphics Ltd., 1993. Revised edition.

Taiz, Lincoln and Eduardo Zeiger. Plant Physiology. Benjamin/Cummings Publ. Co., 1991.

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© 1997, Huntington Library. All rights reserved.
Last revised: October, 1997

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