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Photosynthesis: The Role of Light

The heart of photosynthesis as it occurs in most autotrophs consists of two key processes:

the removal of hydrogen (H) atoms from water molecules (photolysis) : light

reaction

the reduction of carbon dioxide (CO2) by these hydrogen atoms to form

organic molecules. (carbon fixation/Calvin cycle) : dark reaction

Light Reaction

The electrons (e) and protons (H+) that make up hydrogen atoms are stripped away

separately from water molecules.

2H2O - 4e + 4H+ + O2

The electrons serve two functions:

They reduce NADP+ toNADPHfor use in the Calvin Cycle.

They set up an electrochemical charge that provides the energy for pumping

protons from the stroma of the chloroplast into the interior of the thylakoid.

The protons also serve two functions:

They participate in the reduction of NADP+ to NADPH.

As they flow back out from the interior of the thylakoid (by facilitated diffusion),passing down their concentration gradient), the energy they give up is harnessed

to the conversion of ADP to ATP.

Because it is drive by light, this process is called photophosphorylation.

ADP + Pi - ATP

The ATP provides the second essential ingredient for running the Calvin Cycle.

The removal of electrons from water molecules and their transfer to NADP+ requires

energy. The electrons are moving from a redox potential of about +0.82 volt in water to

0.32 volt in NADPH. Thus enough energy must be available to move them against atotal potential of 1.14 volts. Where does the needed energy come from? Light.

The Chloroplast

Chloroplasts contain a system ofthylakoid membranes surrounded by a fluid stroma.
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Chloroplasts

A typical plant cell (e.g., in thepalisade layer of a leaf) might contain as many as 50

chloroplasts.

The chloroplast is made up of3 types of membrane:

1. A smooth outer membrane which is freely permeable to molecules.

2. A smooth inner membrane which contains many transporters: integral

membrane proteins that regulate the passage in an out of the chloroplast of

o small molecules like sugars

o proteins synthesized in the cytoplasm of the cell but used within thechloroplast

3. A system ofthylakoid membranes

Thylakoids

The thylakoid membranes enclose a lumen: a system of vesicles (that may all be

interconnected).

At various places within the chloroplast these are stacked in arrays called grana(resembling a stack of coins).

Four types ofprotein assemblies are embedded in the thylakoid membranes:

1. Photosystem I which includes chlorophyll and carotenoid molecules2. Photosystem II which also contains chlorophyll and carotenoid molecules

3. Cytochromes b and f

4. ATP synthase

These carry out the so-called light reactions of photosynthesis.

The thylakoid membranes are surrounded by a fluid stroma.
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The stroma contains:

o all the enzymes, e.g., RUBISCO, needed to carry out the "dark" reactions

of photosynthesis; that is, the conversion of CO2 into organic molecules like

glucose.o A number of identical molecules ofDNA, each of which carries the

complete chloroplast genome. The genes encode some - but not all - of the

molecules needed for chloroplast function. The others are transcribed from genes in the nucleus of the cell

translated in the cytoplasm and

transported into the chloroplast.

The electron micrograph on the left

(courtesy of Kenneth R. Miller) shows

the inner surface of a thylakoidmembrane. Each particle may represent

one photosystem II complex. In the

functioning chloroplast, these particlesmay not be as highly ordered as seen

here.

Six different complexes ofintegral membrane proteins are embedded in the thylakoid

membrane. The exact structure of these complexes differs from group to group (e.g.,plant vs. alga) and even within a group (e.g., illuminated in air or underwater). But, in

general, one finds:

1. Photosystem I

The structure of photosystem I in a cyanobacterium ("blue-green alga") has been

completely worked out. It probably closely resembles that of plants as well.

It is a homotrimer with each subunit in the trimer containing:

12 different protein molecules bound to 96 molecules ofchlorophyll a

o 2 molecules of the reaction center chlorophyllP700

o 4 accessory molecules closely associated with them

o 90 molecules that serve as antenna pigments

22 carotenoid molecules

4 lipid molecules 3 clusters of Fe4S4 + 2 phylloquinones
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Chlorophylls and Carotenoids

Chlorophylls

Two types of chlorophyll are found in plants and the green algae.

chlorophyll a and

chlorophyll b

In the chloroplast, both types are associated with integral membrane proteins in the

thylakoid membrane.

Note the system of alternating single and double bonds (white bars) that run around theporphyrin ring. Although I am forced to draw the single and double bonds in fixed

positions, actually the "extra" electrons responsible for the double bonds are not fixedbetween any particular pair of carbon atoms but instead are free to migrate around the ring.

This property enables these molecules to absorb light.

Both chlorophylls absorb light most strongly in the red and violet parts of the spectrum.

Green light is absorbed poorly. Thus when white light shines on chlorophyll-containing

structures like leaves, green light is transmitted and reflected and the structures appear

green.

Carotenoids

Chloroplasts also contain carotenoids. These are also pigments with colors ranging from

red to yellow.

Carotenoids absorb light most strongly in the blue portion of the spectrum. They thusenable the chloroplast to trap are larger fraction of the radiant energy falling on it.

Carotenoids are often the major pigments in flowers and fruits. The red of a ripe tomato

and the orange of a carrot are produced by their carotenoids.

In leaves, the carotenoids are usually masked by the chlorophylls. In the autumn, as the

quantity of chlorophyll in the leaf declines, the carotenoids become visible and produce theyellows and reds of autumn foliage.

beta-carotene, one of the most abundant carotenoids. Note again the system of alternating

single and double bonds that in this molecule runs along the hydrocarbon chain that

connects the two benzene rings. As in chlorophyll, the electrons of the double bondsactually migrate though the chain and also make this molecule an efficient absorber of

carotenoid
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light.

2. Photosystem II

Photosystem II is also a complex of

as many as 20 different protein molecules bound to

50 or more chlorophyll a molecules

o 2 molecules of the reaction center chlorophyll P680

o 2 accessory molecules close to them

o 2 molecules of pheophytin (chlorophyll without the Mg++)

o the remaining molecules ofchlorophyll a serve as antenna pigments.

some half dozen carotenoid molecules. These also serve as antenna pigments.

2 molecules ofplastoquinone

3. Light-Harvesting Complexes (LHC)

LHC-I associated with photosystem I

LHC-II associated with photosystem II

These contain several protein molecules associated with scores of chlorophylls both

chlorophyll a and chlorophyll b. These LHCs also act as antenna pigments harvestinglight and passing its energy on to their respective photosystems.

4.Cytochromes b6 and f

5.ATP synthase

How the System Works

Light is absorbed by the antenna pigments ofPhotosystems II and I

The absorbed energy is transferred to the reaction center pigment, P680 in

Photosystem II, P700 in Photosystem I Activation ofP680 removes an electron from it. With its resulting positive charge, P680 is sufficiently electronegative that it can

remove electrons from water.

These electrons are transferred (by way ofplastoquinone PQ in the figure) tothe cytochrome b6/fcomplex where they provide the energy for

chemiosmosis(ATP).
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chemiosmosis

Each CO2 taken up by the Calvin cycle) requires:

o 2 NADPH molecules and

o 3 ATP molecules

Each molecule of oxygen released by the light reactions supplies the 4 electrons

needed to make 2 NADPH molecules.
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The chemiosmosis driven by these 4 electrons as they pass through the PQ +

cytochrome b6/f complex liberates only enough energy to pump 12 protons into

the interior of the thylakoid.

But in order to make 3 molecules of ATP, the ATPase in chloroplasts appears to

have 14 protons (H+) pass through it.

So there appears to be a deficit of 2 protons. How is this deficit to be made up?

One likely answer: cyclic photophosphorylation.

In cyclic photophosphorylation,

the electrons expelled by the energy of light absorbed by photosystem I pass, asnormal, to ferredoxin,

but instead of then going on to reduce NADP+ to NADPH, they pass to

plastoquinone and on back into the cytochrome b6/f complex.

Here the energy each electron liberates pumps 2 protons (H

+

) into the interior ofthe thylakoid enough to make up the deficit left by noncyclic

photophosphorylation.

This process is truly cyclic because no outside source of electrons is required. Like thephotocell in a light meter, photosystem I is simply using light to create a flow of current.

The only difference is that instead of using the current to move the needle on a light

meter, the chloroplast uses the current to help synthesize ATP.

Antenna PigmentsChlorophylls a and b differ slightly in the wavelengths of light that they absorb best

(although both absorb red and blue much better than yellow and green). Carotenoids helpfill in the gap by strongly absorbing green light. The entire complex ensures that most of

the energy of light will be trapped and passed on to the reaction center chlorophylls.

Photosynthesis: Pathway of Carbon Fixation

Photosynthesis is the synthesis of organic molecules using the energy of light. For the

sugar glucose (one of the most abundant products of photosynthesis) the equation is:

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Light provides the energy to:

transfer electrons from water to nicotinamide adenine dinucleotide phosphate(NADP+) forming NADPH;

generate ATP.
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ATP and NADPH provide the energy and electrons to reduce carbon dioxide (CO2) to

organic molecules.

The Steps

CO2 combines with the phosphorylated 5-carbon sugarribulose bisphosphate. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase

oxygenase (RUBISCO)(an enzyme which can fairly claim to be the most

abundant protein on earth).

The resulting 6-carbon compound breaks down into two molecules of3-phosphoglyceric acid (PGA).

The PGA molecules are further phosphorylated (by ATP) and are reduced (by

NADPH) to form phosphoglyceraldehyde (PGAL).

Phosphoglyceraldehyde serves as the starting material for the synthesis ofglucoseandfructose.

Glucose and fructose make the disaccharidesucrose, which travels in solution to

other parts of the plant (e.g., fruit, roots). Glucose is also the monomer used in the synthesis of the polysaccharidesstarch

andcellulose.
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The graphic shows the steps in the fixation of carbon dioxide during photosynthesis.

These steps were worked out by Melvin Calvin and his colleagues at the University ofCalifornia and, for this reason, are named the Calvin cycle.

All the reactions of carbon fixation occur in the stroma of the chloroplast.

Photorespiration and C4 Plants

The first stable compoundare 3 PG, So they named

C3 cycle or Calvin cycle


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All plants carry on photosynthesis by

adding carbon dioxide (CO2) to a phosphorylated 5-carbon sugar called ribulosebisphosphate.

This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase

oxygenase (RUBISCO). The resulting 6-carbon compound breaks down into two molecules of3-

phosphoglyceric acid (PGA). These 3-carbon molecules serve as the starting material for the synthesis of

glucose and other food molecules.

The process is called the Calvin cycle and the pathway is called the C3 pathway.

When temperature rise, photosynthesis increase while the stomata close, which

then lead to a decrease of CO2 and increase O2, as a result of photosynthesis.

High O2 concentration lead to photorespiration.

Photorespiration

As its name suggests, RUBISCO catalyzes two different reactions:

adding CO2 to ribulose bisphosphate the carboxylase activity

adding O2 to ribulose bisphosphate the oxygenase activity.

Which one predominates depends on the relative concentrations of O2 and CO2 with

(1)high CO2, low O2 favoring the carboxylase action,

(2)high O2, low CO2 favoring the oxygenase action.

The light reactions of photosynthesis liberate oxygen and more oxygen dissolves in thecytosol of the cell at higher temperatures. Therefore,

high light intensities and

high temperatures (above ~ 30C)

favor the second (2) reaction, which is photorespiration

The details of photorespiration

The uptake of O2 by RUBISCO forms:

o the 3-carbon molecule 3-phosphoglyceric acid just as in the Calvin

cycle

o the 2-carbon molecule glycolate.

The glycolate entersperoxisomes where it uses O2 to form intermediates that

entermitochondria where they are broken down to CO2.
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So this process uses O2 and liberates CO2 as cellular respiration does which is why it is

called photorespiration.

2 glycolate ------- CO2 + P-glycerate (Sugar)

It undoes the good anabolic work of photosynthesis, reducing the net productivity of the

plant for 25 %

For this reason, much effort so far largely unsuccessful has gone into attempts to

alter crop plants so that they carry on less photorespiration.

The problem may solve itself. If atmospheric CO2 concentrations continue to rise, due to

increase of organic combustion, perhaps this will enhance the net productivity of theworld's crops by reducing losses to photorespiration.

C4 Plants

Over 8000 species ofangiosperms, scattered among 18 different families, have developed

adaptations which minimize the losses to photorespiration.

They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule

instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called

C4 plants. (Plants that have only the Calvin cycle are thus C3 plants.)

Some C4 plants called CAM plants separate their C3 and C4 cycles by time.

CAM plants are discussed below.

Other C4 plants have structural changes in their leaf anatomy so thato their C4 and C3 pathways are separated in different parts of the leaf with

o RUBISCO sequestered where the CO2 level is high; the O2 level low.
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The details of the C4 cycle

After entering through stomata, CO2 diffuses into a mesophyll cell.

o Being close to the leaf surface, these cells are exposed to high levels of O2,

but

o have no RUBISCO so cannot start photorespiration (nor the dark reactionsof the Calvin cycle).

Instead the CO2 is inserted into a 3-carbon compound (C3) called

phosphoenolpyruvic acid (PEP) forming

the 4-carbon compound oxaloacetic acid (C4).

Oxaloacetic acid is converted into malic acid oraspartic acid (both have 4

carbons), which is transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells

o are deep in the leaf so atmospheric oxygen cannot diffuse easily to them;

o often have thylakoids with reduced photosystem II complexes (the one

that produces O2).

o Both of these features keep oxygen levelslow.

Here the 4-carbon compound is broken down into

o carbon dioxide, which enters the Calvin

cycle to form sugars and starch.o pyruvic acid (C3), which is transported back

to a mesophyll cell where it is converted

back into PEP.

These C4 plants are well adapted to (and likely to be found

in) habitats with high daytime temperatures and intense

sunlight.Some examples:

crabgrass

corn (maize)

sugarcane

sorghum
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C4 cells in C3 plants

The ability to use the C4 pathway has evolved repeatedly in different families of

angiosperms. Perhaps the potential is in them all.

A report in the 24 January 2002 issue ofNature (by Julian M. Hibbard and W. PaulQuick) describes the discovery that tobacco, a C3 plant, has cells capable of fixing carbondioxide by the C4 path. These cells are clustered around the veins (containing xylem and

phloem) of the stems and also in thepetioles of the leaves. In this location, they are far

removed from the stomata that could provide atmospheric CO2. Instead, they get theirCO2 and/or the 4-carbon malic acid in the sap that has been brought up in the xylem from

the roots.

If this turns out to be true of many C3 plants, it would explain why it has been so easy for

C4 plants to evolve from C3 ancestors

CAM Plants

These are also C4 plants but instead of segregating the C4 and C3 pathways in different

parts of the leaf, they separate them in time instead. (CAM stands forcrassulacean acid

metabolism because it was first studied in members of the plant family Crassulaceae.)

At night,

CAM plants take in CO2 through their open stomata (they tend to have reduced

numbers of them). The CO2 joins with PEP to form the 4-carbon oxaloacetic acid.

This is converted to 4-carbon malic acid that accumulates during the night in the

central vacuole of the cells.

In the morning,

the stomata close (thus conserving moisture as well as reducing the inward

diffusion of oxygen).

The accumulated malic acid leaves the vacuole and is broken down to releaseCO2.

The CO2 is taken up into the Calvin (C3) cycle.

These adaptations also enable their owners to thrive in conditions of

high daytime temperatures

intense sunlight
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low soil moisture.

Some examples of CAM plants:

cacti

Bryophyllum the pineapple and all epiphytic bromeliads

sedums

the "ice plant" that grows in sandy parts of the scrub forest biome

C4 Diatoms

On 26 October 2000, Nature reported the discovery of both the C3 and C4 pathways in a

marine diatom. In this unicellular organism, the two paths are kept separate by having

the C4 path in the cytosol, and the C3 path confined to the chloroplast. The presence of aC4 pathway probably reflects the frequent low concentrations of CO2 in ocean waters.

WHY STUDY PHOTOSYNTHESIS?

By Devens Gust, Ph.D.

Professor of Chemistry and Biochemistry

Center for the Study of Early Events in Photosynthesis

What is photosynthesis?

Photosynthesis is arguably the most important biological process on earth. By liberating

oxygen and consuming carbon dioxide, it has transformed the world into the hospitable

environment we know today. Directly or indirectly, photosynthesis fills all of our foodrequirements and many of our needs for fiber and building materials. The energy stored

in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the

energy in firewood, which is a major fuel in many parts of the world. This being the case,scientific research into photosynthesis is vitally important. If we can understand and

control the intricacies of the photosynthetic process, we can learn how to increase crop

yields of food, fiber, wood, and fuel, and how to better use our lands. The energy-harvesting secrets of plants can be adapted to man-made systems which provide new,

efficient ways to collect and use solar energy. These same natural "technologies" can help

point the way to the design of new, faster, and more compact computers, and even to new

medical breakthroughs. Because photosynthesis helps control the makeup of ouratmosphere, understanding photosynthesis is crucial to understanding how carbon dioxide

and other "greenhouse gases" affect the global climate. In this document, we will briefly

explore each of the areas mentioned above, and illustrate how photosynthesis research iscritical to maintaining and improving our quality of life.

Photosynthesis and food. All of our biological energy needs are met by the plant

kingdom, either directly or through herbivorous animals. Plants in turn obtain the energy
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Biomes.html#TropicalRainForesthttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Biomes.html#Chaparralhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Protists.html#Diatoms,_Golden_Algae,_and_Brown_Algae_http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Biomes.html#TropicalRainForesthttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Biomes.html#Chaparralhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Protists.html#Diatoms,_Golden_Algae,_and_Brown_Algae_


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to synthesize foodstuffs via photosynthesis. Although plants draw necessary materials

from the soil and water and carbon dioxide from the air, the energy needs of the plant are

filled by sunlight. Sunlight is pure energy. However, sunlight itself is not a very usefulform of energy; it cannot be eaten, it cannot turn dynamos, and it cannot be stored. To be

beneficial, the energy in sunlight must be converted to other forms. This is what

photosynthesis is all about. It is the process by which plants change the energy in sunlightto kinds of energy that can be stored for later use. Plants carry out this process in

photosynthetic reaction centers. These tiny units are found in leaves, and convert light

energy to chemical energy, which is the form used by all living organisms. One of themajor energy-harvesting processes in plants involves using the energy of sunlight to

convert carbon dioxide from the air into sugars, starches, and other high-energy

carbohydrates. Oxygen is released in the process. Later, when the plant needs food, it

draws upon the energy stored in these carbohydrates. We do the same. When we eat aplate of spaghetti, our bodies oxidize or "burn" the starch by allowing it to combine with

oxygen from the air. This produces carbon dioxide, which we exhale, and the energy we

need to survive. Thus, if there is no photosynthesis, there is no food. Indeed, one widely

accepted theory explaining the extinction of the dinosaurs suggests that a comet, meteor,or volcano ejected so much material into the atmosphere that the amount of sunlight

reaching the earth was severely reduced. This in turn caused the death of many plants andthe creatures that depended upon them for energy.

Photosynthesis and energy. One of the carbohydrates resulting from photosynthesis is

cellulose, which makes up the bulk of dry wood and other plant material. When we burn

wood, we convert the cellulose back to carbon dioxide and release the stored energy asheat. Burning fuel is basically the same oxidation process that occurs in our bodies; it

liberates the energy of "stored sunlight" in a useful form, and returns carbon dioxide to

the atmosphere. Energy from burning "biomass" is important in many parts of the world.

In developing countries, firewood continues to be critical to survival. Ethanol (grainalcohol) produced from sugars and starches by fermentation is a major automobile fuel in

Brazil, and is added to gasoline in some parts of the United States to help reduce

emissions of harmful pollutants. Ethanol is also readily converted to ethylene, whichserves as a feedstock to a large part of the petrochemical industry. It is possible to convert

cellulose to sugar, and then into ethanol; various microorganisms carry out this process. It

could be commercially important one day.

Our major sources of energy, of course, are coal, oil and natural gas. These materials areall derived from ancient plants and animals, and the energy stored within them is

chemical energy that originally came from sunlight through photosynthesis. Thus, most

of the energy we use today was originally solar energy!

Photosynthesis, fiber, and materials. Wood, of course, is not only burned, but is an

important material for building and many other purposes. Paper, for example, is nearly

pure photosynthetically produced cellulose, as is cotton and many other natural fibers.

Even wool production depends on photosynthetically-derived energy. In fact, all plantand animal products including many medicines and drugs require energy to produce, and

that energy comes ultimately from sunlight via photosynthesis. Many of our other


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materials needs are filled by plastics and synthetic fibers which are produced from

petroleum, and are thus also photosynthetic in origin. Even much of our metal refining

depends ultimately on coal or other photosynthetic products. Indeed, it is difficult toname an economically important material or substance whose existence and usefulness is

not in some way tied to photosynthesis.

Photosynthesis and the environment. Currently, there is a lot of discussion concerning

the possible effects of carbon dioxide and other "greenhouse gases" on the environment.As mentioned above, photosynthesis converts carbon dioxide from the air to

carbohydrates and other kinds of "fixed" carbon and releases oxygen to the atmosphere.

When we burn firewood, ethanol, or coal, oil and other fossil fuels, oxygen is consumed,and carbon dioxide is released back to the atmosphere. Thus, carbon dioxide which was

removed from the atmosphere over millions of years is being replaced very quickly

through our consumption of these fuels. The increase in carbon dioxide and related gasesis bound to affect our atmosphere. Will this change be large or small, and will it be

harmful or beneficial? These questions are being actively studied by many scientists

today. The answers will depend strongly on the effect of photosynthesis carried out byland and sea organisms. As photosynthesis consumes carbon dioxide and releasesoxygen, it helps counteract the effect of combustion of fossil fuels. The burning of fossil

fuels releases not only carbon dioxide, but also hydrocarbons, nitrogen oxides, and other

trace materials that pollute the atmosphere and contribute to long-term health andenvironmental problems. These problems are a consequence of the fact that nature has

chosen to implement photosynthesis through conversion of carbon dioxide to energy-rich

materials such as carbohydrates. Can the principles of photosynthetic solar energyharvesting be used in some way to produce non-polluting fuels or energy sources? The

answer, as we shall see, is yes.

Why study photosynthesis?

Because our quality of life, and indeed our very existence, depends on photosynthesis, it

is essential that we understand it. Through understanding, we can avoid adverselyaffecting the process and precipitating environmental or ecological disasters. Through

understanding, we can also learn to control photosynthesis, and thus enhance production

of food, fiber and energy. Understanding the natural process, which has been developedby plants over several billion years, will also allow us to use the basic chemistry and

physics of photosynthesis for other purposes, such as solar energy conversion, the design

of electronic circuits, and the development of medicines and drugs. Some examplesfollow.

Photosynthesis and agriculture. Although photosynthesis has interested mankind for

eons, rapid progress in understanding the process has come in the last few years. One of

the things we have learned is that overall, photosynthesis is relatively inefficient. Forexample, based on the amount of carbon fixed by a field of corn during a typical growing

season, only about 1 - 2% of the solar energy falling on the field is recovered as new

photosynthetic products. The efficiency of uncultivated plant life is only about 0.2%. Insugar cane, which is one of the most efficient plants, about 8% of the light absorbed by


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the plant is preserved as chemical energy. Many plants, especially those that originate in

the temperate zones such as most of the United States, undergo a process called

photorespiration. This is a kind of "short circuit" of photosynthesis that wastes much ofthe plants' photosynthetic energy. The phenomenon of photorespiration including its

function, if any, is only one of many riddles facing the photosynthesis researcher.

If we can fully understand processes like photorespiration, we will have the ability to

alter them. Thus, more efficient plants can be designed. Although new varieties of plantshave been developed for centuries through selective breeding, the techniques of modern

molecular biology have speeded up the process tremendously. Photosynthesis research

can show us how to produce new crop strains that will make much better use of thesunlight they absorb. Research along these lines is critical, as recent studies show that

agricultural production is leveling off at a time when demand for food and other

agricultural products is increasing rapidly.

Because plants depend upon photosynthesis for their survival, interfering with

photosynthesis can kill the plant. This is the basis of several important herbicides, whichact by preventing certain important steps of photosynthesis. Understanding the details of

photosynthesis can lead to the design of new, extremely selective herbicides and plantgrowth regulators that have the potential of being environmentally safe (especially to

animal life, which does not carry out photosynthesis). Indeed, it is possible to develop

new crop plants that are immune to specific herbicides, and to thus achieve weed controlspecific to one crop species.

Photosynthesis and energy production. As described above, most of our current energy

needs are met by photosynthesis, ancient or modern. Increasing the efficiency of natural

photosynthesis can also increase production of ethanol and other fuels derived from

agriculture. However, knowledge gained from photosynthesis research can also be usedto enhance energy production in a much more direct way. Although the overall

photosynthesis process is relatively wasteful, the early steps in the conversion of sunlight

to chemical energy are quite efficient. Why not learn to understand the basic chemistryand physics of photosynthesis, and use these same principles to build man-made solar

energy harvesting devices? This has been a dream of chemists for years, but is now close

to becoming a reality. In the laboratory, scientists can now synthesize artificialphotosynthetic reaction centers which rival the natural ones in terms of the amount of

sunlight stored as chemical or electrical energy. More research will lead to the

development of new, efficient solar energy harvesting technologies based on the naturalprocess.

The role of photosynthesis in control of the environment. How does photosynthesis in

temperate and tropical forests and in the sea affect the quantity of greenhouse gases in the

atmosphere? This is an important and controversial issue today. As mentioned above,photosynthesis by plants removes carbon dioxide from the atmosphere and replaces it

with oxygen. Thus, it would tend to ameliorate the effects of carbon dioxide released by

the burning of fossil fuels. However, the question is complicated by the fact that plantsthemselves react to the amount of carbon dioxide in the atmosphere. Some plants, appear


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to grow more rapidly in an atmosphere rich in carbon dioxide, but this may not be true of

all species. Understanding the effect of greenhouse gases requires a much better

knowledge of the interaction of the plant kingdom with carbon dioxide than we havetoday. Burning plants and plant products such as petroleum releases carbon dioxide and

other byproducts such as hydrocarbons and nitrogen oxides. However, the pollution

caused by such materials is not a necessary product of solar energy utilization. Theartificial photosynthetic reaction centers discussed above produce energy without

releasing any byproducts other than heat. They hold the promise of producing clean

energy in the form of electricity or hydrogen fuel without pollution. Implementation ofsuch solar energy harvesting devices would prevent pollution at the source, which is

certainly the most efficient approach to control.

Photosynthesis and electronics. At first glance, photosynthesis would seem to have no

association with the design of computers and other electronic devices. However, there ispotentially a very strong connection. A goal of modern electronics research is to make

transistors and other circuit components as small as possible. Small devices and short

connections between them make computers faster and more compact. The smallestpossible unit of a material is a molecule (made up of atoms of various types). Thus, thesmallest conceivable transistor is a single molecule (or atom). Many researchers today are

investigating the intriguing possibility of making electronic components from single

molecules or small groups of molecules. Another very active area of research iscomputers that use light, rather than electrons, as the medium for carrying information. In

principle, light-based computers have several advantages over traditional designs, and

indeed many of our telephone transmission and switching networks already operatethrough fiber optics. What does this have to do with photosynthesis? It turns out that

photosynthetic reaction centers are natural photochemical switches of molecular

dimensions. Learning how plants absorb light, control the movement of the resulting

energy to reaction centers, and convert the light energy to electrical, and finally chemicalenergy can help us understand how to make molecular-scale computers. In fact, several

molecular electronic logic elements based on artificial photosynthetic reaction centers

have already been reported in the scientific literature.

Photosynthesis and medicine. Light has a very high energy content, and when it is

absorbed by a substance this energy is converted to other forms. When the energy ends

up in the wrong place, it can cause serious damage to living organisms. Aging of the skin

and skin cancer are only two of many deleterious effects of light on humans and animals.Because plants and other photosynthetic species have been dealing with light for eons,

they have had to develop photoprotective mechanisms to limit light damage. Learning

about the causes of light- induced tissue damage and the details of the naturalphotoprotective mechanisms can help us can find ways to adapt these processes for the

benefit of humanity in areas far removed from photosynthesis itself. For example, the

mechanism by which sunlight absorbed by photosynthetic chlorophyll causes tissuedamage in plants has been harnessed for medical purposes. Substances related to

chlorophyll localize naturally in cancerous tumor tissue. Illumination of the tumors with

light then leads to photochemical damage which can kill the tumor while leavingsurrounding tissue unharmed. Another medical application involves using similar


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chlorophyll relatives to localize in tumor tissue, and thus act as dyes which clearly

delineate the boundary between cancerous and healthy tissue. This diagnostic aid does

not cause photochemical damage to normal tissue because the principles ofphotosynthesis have been used to endow it with protective agents that harmlessly convert

the absorbed light to heat.

Conclusions

The above examples illustrate the importance of photosynthesis as a natural process and

the impact that it has on all of our lives. Research into the nature of photosynthesis is

crucial because only by understanding photosynthesis can we control it, and harness itsprinciples for the betterment of mankind. Science has only recently developed the basic

tools and techniques needed to investigate the intricate details of photosynthesis. It is now

time to apply these tools and techniques to the problem, and to begin to reap the benefitsof this research.

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