Why are plants green?

My answer melds the physics and chemistry of light-driven chemical reactions – photosynthesis – with evolutionary biology and some properties of a star system.  In brief, plants and other photosynthetic organisms need a source of stellar energy (good old Sol, for our Earth) with a good distribution of radiation (a good star temperature).  The spectrum  of visible light is highly appropriate, with energies of its photons or light particles adequate for photochemical reactions but not so high as to be prone to breaking up molecules directly.  A chemical that can capture and  hold the energy from absorbing the photons of that energy level is a great rarity – that’s chlorophyll, and by chance it absorbs red and blue, leaving much of the green.  Now we can delve into intriguing details:

Well, this is not really at the popular science level, as it requires some grasp of how molecules interact with light.  If you’re game for a bit of science beyond that of a fair fraction of Web users, here goes:

First, energy availability: most of the energy in the spectrum of the Sun is in visible light, of wavelengths between 400 and 700 nanometers (nm, billionths of a meter).  (Compare a human hair’s diameter, 80,000 to 100,000 nm.) Extreme blue  light, the shortest wavelength most people can see, is 400 nm; deep red is 700 nm. This range of wavelengths happens to be close to the same at that absorbed by green plants.  It makes evolutionary sense for plants to use this abundant energy source.  We haven’t yet argued why the green isn’t used well, thereby being sent off substantially unused from leaves, to reach our eyes as pretty green light.

Second, energy utility: to  make new, stable, energy-packed molecules – that is, sugars – from other molecules in the environment – water and CO2 – takes the amount of energy that’s carried by the particles of light, the photons, in this range of wavelengths.  That photon energy is E = hν, with ν, or “nu” being the frequency of the light; h is Planck’s constant.  We can quote that in various units in science, as 1.77 to 3.09 electron-volts (ah, battery voltage, which we know can cause chemical reactions by breaking or remaking bonds) .  If you like old English units (alas, not used in science) and you consider a mole of photons – a mole is Avogadro’s number of anything – then that’s 171 to 299 kilojoules or 41 to 71 kilocalories (food calories).  Even with that much energy per photon or mole of photons, it takes a pooling of energy from a couple of photons to move an electron from water onto CO2 when making sugars.

Two sidelights on energy utility: First, there are bacteria that use light of even longer wavelengths, to about 850 nm.  Their photochemical reactions differ from those used by green plants.  That’s about the lowest energy that’s usable.  Second, higher energy photons in the ultraviolet can do photochemistry even more vigorously… too vigorously, as we know.  We get sunburn, all organisms get DNA breakage and other damage.  One great feature of the Earth is the ozone shield – among many special  conditions (see my other essays, about habitable planets and about Proxima Centauri b, in particular).  That layer keeps out most UV light.

Third, energy retention: Chlorophyll has unique photophysical properties.  Consider what happens when a molecule of any type is able to absorb light.  There has to be a jump in molecular motion – rotation, vibration, or the “orbital” motion of its electrons – with a difference in energy from the starting point that exactly matches the energy of the light particle or photon.  OK, visible light is fairly energetic, able to excite changes in electronic motion and even to break bonds in some cases.  We’re considering a Chl (chlorophyll) molecule starting out in what’s called the ground electronic state.  When it absorbs light it jumps into a new electronic state.  There are two states readily available when visible light is absorbed.  One, the first excited singlet state (S1) lies at the difference in energy from the ground state that matches the energy embodied by red light.  There is also a second excited singlet state (S2) at the energy embodied by blue light.  Most organic molecules that absorb electromagnetic radiation that is as energetic as red or blue light don’t hold onto the energy of excitation long.  Particularly if the molecule is complex (and Chl is, having 137 atoms), there are many ways of converting the energy of electronic excitation into other motions of the molecule – and of neighboring molecules – such as vibration and rotation, thus, eventually to heat.  Chlorophyll is a rarity, in that it holds onto the “high-quality” energy of electronic excitation a relatively long time, on the order of billionths of a second.  That’s long enough for the excitation to  be used in a photochemical reaction, the start of photosynthesis.  The excitation can be transferred from one Chl to another, in what’s called nonradiative transfer or Förster transfer – the receiving molecule has an energy level to match that of the donor.

One paragraph with some quantum mechanics:That is, Chl has low rates of loss by internal conversion (to heat) or intersystem crossing (to the first excited triplet state, to which direct absorption is first-order forbidden by quantum symmetry rules, and with which there is a danger of creating damaging singlet oxygen from ground-state triplet O2). G. Wilse Robinson at Caltech had a great course on this topic, using a sophisticated analysis of the density of states (rovibronic states, or quantum energy levels). I wrote about this years ago – see V. P. Gutschick. 1978. Concentration quenching in chlorophyll-a and relation to functional charge transfer in vivo. J.Bioenerg. Biomembr.10: 153-170. Note that absorption spectra are broadened by local environmental interactions of the Chls to cover a good fraction of the solar spectrum, and also there are auxiliary pigments (carotenoids) to fill in even more of the spectrum. So, leaves absorbing 85% of the solar spectrum is a good deal! So, chlorophyll makes excellent use of the spectrum of light from the Sun.

Chlorophyll is then a unique molecule, with a few variant forms (Chl a, Chl b, bacteriochlorophyll).  You can’t fault it for not absorbing light at all the visible wavelengths.  Absorb red and absorb blue and you leave green to be passed on to the eye by reflection or transmission.  So, the whole package of properties comes down to the requirement for a molecule that leaves us the green (did I intend that pun?).

There’s more to say, much more, but here is a short discussion of how and why plants seem to waste a lot of the energy in sunlight.  Chlorophyll and the auxiliary pigments absorb more light in bright sunlight than they can use.  This is an interesting situation in evolution.  Here are considerations:

First, leaves can’t pack in enough enzymes, the proteins that carry out the sugar-making reactions by using the energy of light that’s been captured in the photochemistry, which makes molecules that store energy but are unstable.  The products of photochemistry have to be used in a complex series of nonphotochemical or “dark” reactions.  These reactions use proteins, enzymes, that catalyze the combination of various chemical species along a long and tangled path.  It happens that the enzyme that does the first, critical reaction, combining CO2 with a receptor molecule, ribulose bisphosphate, has a low reaction rate or turnover number.  One molecule of ribulose bisphosphate carboxylase/oxygenase (Rubisco, for short, or a great name for a plant physiologist’s dog) can only process a dozen or so CO2 molecules per second.  There are other enzymes blazingly faster: carbonic anhydrase turns over about 500,000 molecules  per second of CO2 from carbonic acid (good for us air breathers so that we can remove CO2 as a waste product of our metabolism). There’s a good reason for Rubisco being so slow – its reaction is the second most difficult reaction in the biological world, after the reaction catalyzed by nitrogenase enzyme in nitrogen-fixing microbes.  The chemical bonds in CO2 are very tough, almost as tough as those in N≡N or N2 (ordinary nitrogen gas).  It takes real energy to change these bonds. A leaf that could use all the energy in full sunlight would be very thick, with concurrent problems in moving reactants around. Thus, leaves hit a maximal rate of photosynthesis at a modest fraction of full sunlight, maybe 1/20 for some trees to 1/2 for some very robust photosynthesizers  such as the “weed” Camissonia claviformis.  You might look at a couple of my publications for more ideas: V. P. Gutschick. 1984. Photosynthesis model for C3 leaves incorporating CO2 transport, radiation propagation, and biochemistry. 1. Kinetics and their parametrization. Photosynthetica. 18: 549-568, and V. P. Gutschick – and  ____. 2. Ecological and agricultural utility. Photosynthetica 18: 569-595.

Second, and related, is a sort of CO2 starvation of plants, another great story in evolution.  In a somewhat simplified view, plants have been too successful in both photosynthesis and self-protection.  In the latter aspect, they make structural compounds, cellulose and lignins, that are hard for bacteria and fungi to break down, even as versatile metabolically as these organisms are.  As a result, a rather tiny fraction of carbon compounds derived from photosynthetic organisms end up not being fully decomposed before the site of their deposition gets buried geologically.  That’s how we get coal, oil, and natural gas.  Even though we’re now down our damnedest (an appropriate adjective, I offer) to burn these all back into CO2 in the air, the level of CO2 in the air has been dropping for eons, on average.  There are a lot of tales tied to this, such as climate change and Snowball Earth (check it out), but the key thing is that CO2 is at “only” 405 parts per million in today’s air (2018).  It’s a trick for leaves to take up CO2 with only a small driving force (concentration of CO2 outside the leaf relative to inside the leaf). In fact, it’s why plants need so much water.  With their leaf pores, the stomata (little stomach) open, they take up CO2 but lose about 100 to 500 times more molecules of water.  The situation has been getting worse, then, for plants; their water-use efficiency is getting lower.  About twenty times, some plant lineages have evolved a new first step for capturing CO2, the so-called C4 plants (and Crassulacean acid metabolism or CAM plants, similarly). I’ll skip the details here.

Third, the inability to use all the energy flow in full sunlight leads to interesting competitive relations among plants, as well as interesting light signalling in plants.  Plants exposed to full sun, the overstory plants (or their top leaves, at least), spend a lot of time light-saturated.  They use part of the solar energy and dump the rest as heat.  They have to really protect themselves from absorbing too much light and thus getting too much photodamage,.  The leaves have the help of xanthophyll pigments that can absorb excess light and convert it to heat effectively (high rates of internal conversion of the energy of electronic excitation).   Still, thicker leaves with more Rubisco could help, as could having leaves presented at a steeper angle to the sun, thus spreading light out over a bigger area at a lower intensity.  The latter strategy has been used in crop breeding, in what’s called the erect-leaf hypothesis.  Modern varieties of maize (corn, in US palance; Zea mays) have rather erect leaves and are more efficient in using light.  Another  strategy is making  leaves with less chlorophyll.  They’d have lower rates of photosynthesis in their top leaves but could share light with lower leaves.  Using results from the papers I just cited, in a follow-on article (oops; kinda big, at 5 MB, a scan of pages), I proposed that “pale mutants” would have 8% higher yields in dense stands of a monoculture.  This idea was picked up by colleague John  Hesketh at the University of Illinois- and it worked!  (W. T. Pettigrew et al. 1989. Crop Science 29: 1024-1029).)  Crop breeders didn’t go for it – what farmer likes pale green crops?  There is also a very good ecological / evolutionary reason that wild plants don’t embody this – why share a resource, light, with competitors!

Signaling with light: The color quality of light changes as one moves deeper into a plant stand.  There’s more green, less red.  Plants have elaborate photosensors to control where they invest in making leaves and in growing stems.  They don’t respond to green, but to the ratio of red light to far-red light.  Deeper in the canopy, the red light is much reduced, having been absorbed by leaves above.  There is less reduction in the intensity of far-red light, at wavelengths just longer than the absorption edge of chlorophyll.  The red:far-red ratio detected by a plant changes the plant responses.  For a plant evolved to be an overstory plant, intolerant of shade, a low red:far-red ratio triggers a response to elongate its stem to help rise to the top, and at the same time to forgo developing leaves at depth.  There is a vasts literature on such signaling responses in plants.

Those 137 atoms in chlorophyll: about 52 aren’t key to handling the electronic excitation arising from absorbing light.  They are in the “phytl tail,” a long chain of hydrocarbon that helps the chlorophyll molecule imbed in a fatty or lipid membrane.  Photosynthesis has to be done across a biological membrane for several reasons.  Among these are safe separation of electrical charges, ability to recoup some energy by letting charges come back through what’s effectively a turbine to make ATP, the cell’s energy currency.  That’s a whole ‘nother story, not to be pursued here.

One final note here: Since there’s less light deeper in a stand of plants (a “canopy”), should lower leaves have progressively less investment in photosynthetic enzymes and overall mass?  Frits Wiegel and I modeled that in 1984, publishing it later (V.P. Gutschick and F. W. Wiegel. 1988. American Naturalist 132: 67-86).  We came up with a profile of leaf mass per area vs. optical depth in the canopy that looks like that of real plants.  Making hypotheses from some deep physics and chemistry to see if the concepts work out in nature is so much fun, and it’s also of potential use.  I have lots of other plant models and tests published.  You can take a look at https://gcconsortium.com/about_us_founders_qualifications.html.

Addendum: A chemist’s joke: What is Avogadro’s number of avocados?  A guacamole.  Note that this number would fill the oceans about 100 times over.  Have fun working out the math.

Another one: Chlorophyll has 137 atoms.  That’s essentially the reciprocal of the fine-structure constant, which is a fundamental constant in the theory of electromagnetic interactions – which includes, of course, light with electrons in molecules.  I mentioned this to Nobel laureate Murray Gell-Mann once in the cafeteria at Los Alamos, to his amusement.

You make more heat than the Sun

Say what?  The sun’s temperature is surely very much higher than our temperature – in fact, in absolute units, it is nearly 20 times hotter (nearly 5800 Kelvin, in scientific units, while we’re less than 310 Kelvin, or K).  The sun is also considerably bigger than any one of us, or all of us. Its mass is about 330,000 times the mass of our own earth, and the earth’s mass is about 86 sextillion times (86 x 1021) more than our tiny little 70 kg or 154 pounds apiece.  Yet the statement in the title is true in one important sense: the average kilogram of our body generates more metabolic heat than the average kilogram of the sun.

 

One way to get at this figure is that, at rest, we put out about 70 Watts, less than the (fortunately less and less common) 100 W incandescent light bulb.  Still, we’re at 1 watt (W) per kilogram.  We eat to maintain that rate.  Over an hour, that 70 W amounts to an energy use of 70 x 3600 Joules.  Now, a Joule is 0.242 calories and a food calorie (Cal) is 1000 of the standard calories, so in that hour we metabolize about  65 Cal. Over a 24-hour day, we then need 24 x 65 = 1560 Cal…well, more like 2,000 if we’re minimally active or 10,000 if we’re top-notch mountain climbers or skiing across Antarctica.

 

How much power, which is energy per unit time, does the sun put out?  By the time sunlight reaches the surface of the earth, its peak at noon is as high as 1000 Watts on a square meter; that’s a hair-dryer’s worth, for every square meter (a bit bigger than a square yard).  Above our atmosphere, it’s higher, about 1360 W per square meter.  It gets more intense as we get closer to the sun.   You can search a book or the Internet for the inverse-square law to find out how much more intense.  Using the fact that we are, on average, 150 million km (93 million miles) from the sun and the sun’s diameter is close to 1,390,000 km (870,000 miles), we can calculate the intensity at its “surface” (remember, no hard surface – it’s a gas bag).  It’s 46,000 times greater than at our distance, or about 63 million watts per square meter, compared to our 70 Watts per 2 square meters of skin surface.  Let’s go on.  Over its whole surface, the sun radiates energy to space.  That surface area is 4 π times the sun’s radius squared, or 6.07 quintillion square meters (6.07 x 1018).  This is 2.36 trillion square miles, if you’d like old English units, and that’s close to a million times the area of the US.  Multiply this by the output per square meter, to get 382 septillion W (3.82 x 1026).  Divide that by the mass of the sun, which is nearly 2,000,000,000,000,000,000,000,000,000,000 kg (or 2 x 1030 in easier notation).  The sun puts out a measly 0.00019 Watts per kg.  You expend 5,000 times more, on a mass or “weight” basis.

 

This low output from the sun is a good thing, so that the sun does not run out of its nuclear fuel in less than its long life of about 10 billion years.  Other stars are much brighter and burn up much faster, in as little as a few million years.  Compared to the sun, we are profligate in using energy…which we capture from the sun through our crops and pastures.  The plants on earth use only 0.3% of the solar energy reaching us, and we capture as food only several percent of what they capture.  This low capture rate can only be helped a little, and it’s why there can’t be too many more of us.

Are you a big corn chip?

 

How much corn, or maize, do you eat?  You can probably count up the number of ears of corn on the cob, and maybe estimate the number of corn chips or corn tortillas you eat.  You’d likely say that you eat much more as red meat, cheese, chicken, eggs, and, I hope, vegetables, not to forget a good batch of calories in sodas and other drinks that clearly aren’t corn.  However, you need to consider where that meat, that cheese, those eggs, and those drink calories came from.  The meat and the milk came from cattle that were fed corn for most of their weight gain or much of their milk production.  Laying hens are fed mostly corn for the time that they produce eggs.  Soft drinks, the full-calorie type, are sweetened with high-fructose corn syrup.  (Ever tasted a kola nut by itself?  I did, in western Cameroon, and I unpuckered  after a few minutes.)  That corn syrup likely appears in more than half of every processed food you eat – check those labels – and in the US our diet is almost entirely processed food, short of some fresh fruits and vegetables.

 

There’s an interesting technique to detect the broad sources of our food, by analyzing bits of our food (or even of us, say, a fingernail clipping).  Remember, all our food comes from plants, directly or from the animals that eat it or the fungi (mushrooms) that grow on animal manure (very well sterilized).  Plants can be distinguished into two types, called C3 and C4, based on the first steps in photosynthesis.  The C4 plants are less common globally but corn, sugar cane, and sorghum are major examples.  They take up carbon dioxide from the air with a “carrier” molecule than changes into a 4-carbon acid.  This molecule gives up CO2 inside a special section of the leaf, effectively pumping up the level of available CO2 and allowing the plant to be more efficient in using water, light, and nitrogen.  Why they don’t yet dominate the globe is another story.  Now, the carbon in CO2 is overwhelmingly carbon-12, with six protons and six neutrons in its nucleus.  A small portion, about 0.37%, is as carbon-13, with 7 neutrons.  It’s stable, not radioactive.  Its compounds react a tiny bit more slowly than those of carbon-12.  So, compared to the carbon found in CO2 in the air, there’s even less carbon-13 in the sugars and the tissues made in the plant.

 

The C4 plants such as corn discriminate less against carbon-13 than do the C3 plants.  We can detect the amount of discrimination with extremely precise (and pretty pricey) instruments called isotope-ratio mass spectrometers.  We have one here at NMSU.  The ratio of carbon-13 to carbon-12 stays nearly unchanged when an animal eats plant matter and then uses the carbon in making its own tissues.  The same near-preservation of the ratio occurs when another animal (such as one of us) eats that animal.  When animals eat a blend of foods, this isotope ratio is a corresponding blend of the ratios in the different foods.  In short, if we measure this ratio in one of our foods, we can estimate what fraction of the food came ultimately from C4 plants and what fraction came from C3 plants.  Here in the US, we can state that the C4 food was corn; we eat little sugar cane or sorghum; Hawaii grows cane but even more marijuana, in dollar value.  To continue, Todd Dawson at the University of California, Berkeley, made such measurements of the isotope ratio.  A soft drink came out as 100% derived from corn.  A milk shake at 78%.  Chicken nuggets at 56%. Even French fries were 23 percent corn; they’re sugar-enriched.  So, if you’re a typical American, the quip is that you’re close to being a walking corn chip.

 

How did we get all this corn in our food chain?  How does it relate to our nutrition and our health? Those, too, are other big stories.  If you find insights on your own, please email me at vince.gutschick@gmail.com and we’ll share them in another column.

Vince Gutschick    Las Cruces Academy       2009

 

How do we know how far away the Sun is?

We need a big distance scale to do this, and a good one is the diameter of the earth.  How have we come to know this distance, the diameter?  The first accurate estimate may be said to be one developed by the Greek mathematician Eratosthenes of Cyrene over 2,200 years ago.  He was sure that the earth was round.  He was also sure that the sun was very far away, so that its rays came in almost exactly the same direction to every spot on earth.  He then considered how the shadow of a vertical stick would have different angles at different locations on earth.  He could calculate the angle from the height of the stick and the length of the shadow.  His next, critical insight was knowing that this shadow angle was zero (the sun was directly overhead) at modern Aswan in Egypt on the date of the summer solstice.  On the same day he measured to angle at his hometown of Alexandria, 950 km (about 500 miles) farther north.  Now he knew the angle between the vertical at those two locations, which turned out to be almost exactly 1/50 of the 360 degrees in a circle.  So, the circumference of the earth would be 50 times the distance between the cities, or very nearly 40,000 km (25,000 miles).   That makes the diameter of the earth 1/π times this, or about 12,800 km (8,000 miles).  This translation into units we use today assumes that he used as his length unit the Egyptian stadion, not the Greek stadion.

 

In any event, this surprisingly accurate measurement stood for a long time.   The report of his method was then corrupted in copying and the distance unit was misinterpreted by Columbus, who thought the circumference of the earth was less than 2/3 the true value.  His estimates would have put Asia close to where America really is.  Thus, Columbus stumbled on the outlying islands of America by luck, and he never really admitted that he hadn’t reached some novel part of Asia.

 

Back to the distance to the sun.  If we look at a celestial object – say, a planet – from sites far apart on the earth, we see it appear against different elements of the background formed by the distant stars.  That is, the planet appears to be at slightly different angles relative to the stars at the two locations. To appreciate this phenomenon of parallax, we can hold up a finger at arms length and view it with one eye closed and then the other.  We see the finger against different elements on a distant wall.  Our head plays the role of the earth, with our two eyes at different locations on it.  Back to the planet: we know the distance between our observing locations on earth, because we know the diameter of the earth and how many degrees apart these locations are on earth.  The distance to the planet is this separation on earth, divided by the amount of parallax, if we express this parallax angle in radians (360 degrees divided by π , or about 57.3 degrees).  As an example, consider a point in time when Mars is 100 million km from us.  If we view it from two places on earth that are 10,000 km apart (along the line perpendicular to the direction of Mars), the parallax angle will be 10,000/100,000,000 radians.  This is 0.0058 degrees – about 1/100th of the angle subtended by the moon from earth.  This is a small angle, but readily measured with good telescopes.

 

We can’t readily view the sun against the distant stars, since it blocks out the stars with its brightness, to some wide angle around its own disk.  Instead, we first measure the distance from earth to a planet.  The astronomer Cassini measured the distance to Mars using parallax in 1672.  The next step is to calculate the ratio (distance to sun) / (distance to planet).  We can do this by using geometry.  We measure the angle between the sun and a planet (an inner one, say, Venus) when the planet has reached its farthest angle away from the sun.  For a planet is a very nearly circular orbit, we then have the earth, Venus, and the sun in a right triangle.  The side of the triangle from earth to sun is the hypotenuse, and the other two sides (earth-Venus and Venus-sun) meet at a right angle.  The earth-sun distance is then the distance to Venus, divided by the cosine of the angle between Venus and the sun.

 

Modern methods of measuring the distance to planets use radar beams reflected from the planet.  We measure the time for the signal to bounce back.  This time is twice the distance to the planet, divided by the speed of light. Radar waves move at the same speed, and we know the speed of light with exquisite accuracy; in fact, we now define the speed of light exactly in meters, and the uncertainty is in the meter, at about 25 parts per trillion.  Over 100 million km, this uncertainty corresponds to only 2.5 m.  The total uncertainty is somewhat larger than this, but still remarkably small.

Vince Gutschick    Las Cruces Academy    2009