And to think that I saw it on mulberry trees
Marco is one of the great, unsung heroes in literature.
The protagonist of Dr. Seuss’s “And To Think That I Saw It On Mulberry Street” has an imagination of enviable proportions. He is a masterful inventor of yarns – a storyteller fit to stand amongst the pantheon of great storytellers. He is able to transform a ramshackle horse and cart meandering up the street into the grandest, full-blown, state-sponsored, musical parade conceivable.
To his credit, Marco is also a rather self-aware young person. He recognises his embellishments as such. He is happy to share his thoughts with us as he constructs his outlandish tale, but he is prepared to pare the story back to reality for his unwitting father.
It is this latter quality that establishes Marco as a true hero. He sacrifices his greatest gift, his storytelling, to share the truth. He is a person of great integrity - a compromiser of the highest order.
Science would have saved Marco from such a compromise. Had Marco had scientific knowledge amongst his arsenal of wonderful character traits, he might have been able to reconcile the fantastical with reality. Science would have provided him with truths fitting for his grand imagination. Science would have equipped Marco with a reality that is every bit as inspired as that envisioned in his mind’s eye.
He could have shared an ingenious story with absolutely no need for embellishment.
In fact, Marco need not have looked any further than the eponymous mulberry trees of Mulberry Street to tell a story of wonder.
Mulberry trees are widely distributed in temperate climates. In North America, where Marco lived, both native and introduced mulberry trees line urban streets, giving some of them their name – Marco had Mulberry Street, located in Springfield, Massachusetts; in my neighbourhood we have Mulberry Crescent. The trees produce the fruit from which they derive their name, the mulberry. The mulberry is a multifaceted fruit superficially resembling a raspberry or blackberry. It can be white, red, or deep purple and very sweet when ripe. The mulberry tree would have been, both figuratively and literally, a fruitful source for Marco’s storytelling.
Phenylalanine is as good a starting point for a new story for Marco as the horse and cart were on Mulberry Street.
Phenylalanine is an amino acid – one of the 20 naturally occurring amino acids that organisms use to make proteins. For humans, phenylalanine is one of the nine essential amino acids. Human diets must contain essential amino acids like phenylalanine because we are unable to make them from simpler molecular building blocks. Plants, like the mulberry tree, have no such requirement. Like other plants, the mulberry trees on Mulberry Street can make phenylalanine from scratch.
Plants are able to make phenylalanine from carbon skeletons derived from the sugars made via photosynthesis. Phenylalanine is made from simple, linear molecules – two containing just 3 carbons, and one containing 4 carbons. With these starting materials, via a series of enzyme-catalysed reactions, plants construct a new molecule that contains a 6-carbon ring attached to a chain of three carbons. This molecule is converted to phenylalanine by adding a nitrogen group – the amino group that gives amino acids their name – to the second carbon in the chain. The nitrogen-based amino group is what enables amino acids like phenylalanine to link up in long chains – through peptide bonds – giving rise to polypeptides, also known as proteins.
The nitrogen in the amino group of amino acids is a fairly costly commodity for plants.
For most plants, like mulberry trees, nitrogen must be extracted from the soil. While most of our atmosphere is made of nitrogen gas, this is largely inaccessible to most plants. Instead, plants must access much less abundant mineral nitrogen – nitrate. Roots take up nitrate where it is added to carbon skeletons for transport throughout the plant body in a fairly energy-intensive process. The amino acids glutamate and glutamine function as the major means by which nitrogen is moved around plants. Glutamate passes nitrogen onto other carbon skeletons so as to complete the synthesis of other amino acids. Such is the origin of the amino group of phenylalanine.
Now here is the truly amazing thing about Marco’s mulberry plant. Despite the cost in obtaining nitrogen, and moving it around the plant, and then using it to create phenylalanine, mulberry trees, like all land plants, turn around and deliberately chop the amino group off of phenylalanine. That’s right, they remove the very same amino group from phenylalanine that they have so carefully added to the carbon skeleton. How and why would they engage in this futile act?
The removal of the amino group from phenylalanine is accomplished by an enzyme, a protein catalyst, known as phenylalanine ammonia-lyase, or PAL.
The PAL enzyme has a pocket that perfectly fits a single molecule of phenylalanine. In this pocket, the enzyme active site, PAL catalyses the specific removal of amino group from phenylalanine. When the amino group is removed, it leaves the second carbon of former phenylalanine molecule without a bonding partner. In order to compensate for this, PAL creates a double bond between the second and third carbons in the 3-carbon chain that is attached to the 6-carbon ring. This new molecule, comprising the 6-carbon ring and a 3-carbon tail with a double bond between the second and third carbon, is called cinnamic acid, or cinnamate. So, PAL takes a molecule of phenylalanine and catalyses the formation of cinnamate and ammonia.
PAL proteins function throughout the plant body, liberating so much nitrogen in the form of ammonia that the plant would suffer from a nitrogen deficit if it weren’t recovered. Not surprisingly, evolution has equipped plants with a mechanisms that functions in coordination with PAL to recover the liberated nitrogen and recycle to make other amino acids, including even phenylalanine.
So PAL tells us how plants engage in the futile act of removing the amino group from phenylalanine, but it doesn’t tell us why. The why is what turns Marco’s tale of a horse and cart into a full-fledged parade. The parade emanates from cinnamate.
Cinnamate is one of the most remarkable molecular building blocks ever devised by nature. It is used in the manufacture of a veritable cornucopia of chemicals.
Cinnamate is a very flat, or planar, molecule with a large number of double bonds given its small size. This bestows a number of other properties on cinnamate. It is relatively hydrophobic, or water repellent. It also has the capacity to absorb light over a fairly broad range of wavelengths, notably those in the UV range. Plants have been able to make great use of these properties.
In the mulberry trees, a polymer ultimately derived from cinnamate units imparts rigidity to the trunk, the stems, and the leaves. Modified cinnamate units are polymerised together to make large three-dimensional molecules, lignins. Lignins are both highly water repellent and hard to digest by insects and microbes alike. Lignins are integrated with other large molecules, particularly cellulose, on the exterior of plant cells to make a matrix that helps plants, particularly trees, to be rigid, to resist pests and pathogens, and to transport water throughout the plant body. The innovation of lignins was thought to be a key evolutionary innovation in the colonisation of terrestrial environments by plants, and in the establishment of forest ecosystems.
Lignins are merely the tip of the proverbial iceberg in terms of the diversity of cinnamate-derived compounds, even just considering mulberry trees.
In mulberry trees, cinnamate and its derivatives, particularly flavonoids, absorb UV light striking the aerial tissues. Flavonoids are made by joining cinnamate with other carbon skeletons, giving rise to a highly planar, multi-ring, double-bonded structure that is particularly suited to absorb UV light. Together they protect the leaves and flowers from the damaging effects of UV light, including the destruction of proteins and cell membranes, and the mutation of DNA. Beneath the soil, a different class of flavonoids functions to attract beneficial microbes to support root growth and protection, while yet other cinnamate derivatives ward off harmful microbes and pests.
A special class of flavonoids, known as anthocyanins, are prominent in the mulberry fruit. Anthocyanins absorb light of visible wavelengths. In doing so, they impart colour on those tissues in which they are synthesised. In mulberries, anthocyanins provide the distinctive pink, red, purple and black hues to the fruit. They function to attract fruit eaters – an attraction that is rewarded by the luscious sweetness of the sugar-rich mulberries. The seed embedded within each facet of the fruit is protected by a hard outer coat, which also contains lignin. When the fruit is consumed, some of these berries pass through the digestive system of the fruit eater, who then distributes the seeds in their nutrient-rich manure as they move about the countryside. In this way, the mulberry is able to spread its offspring beyond the competitive shade of the seed parent.
If Marco had consumed the mulberry, he would have detected an astringency that is attributable to anthocyanins within the fruits. These condensed anthocyanins, known as tannins, function to inactivate proteins by denaturing them, making the fruit somewhat less digestible and protecting the precious seed cargo they carry. So abundant are anthocyanins in mulberries that they literally colour the ground on which they drop – turning grassy greens to lovely purples. They are so abundant that an industry has emerged using them as food and fabric colourants.
Marco might not have detected it, but be would have been unknowingly affected by another cinnamate derivative in the mulberries, resveratrol. Mulberries also contain significant quantities of resveratrol, a member of the stilbene class of compounds. Resveratrol protects the mulberry from some herbivores and pathogens. On the other hand, resveratrol also has a remarkably positive effect on the lives of other organisms. Notably, anti-aging effects have been attributed to resveratrol consumption in a number of species, including yeast, bees, mice and humans. Marco could enhance his mulberry story with the anti-ageing effects of resveratrol, but this would be a return to embellishment, as the science is still not firm on this part of the mulberry chemical parade.
Now the best bit of Marco’s story comes at the end, of course. And that’s the bit about Marco’s relationship with the mulberry tree. With their shared evolutionary history, Marco has arrived at a point where the rustling of the leaves imparted by lignin’s rigidity attracts his attention. He turns, and he eyes the mulberries, their hue perfected over time to excite the colour-sensing cells of his retina. With a forefinger and opposable thumb perfectly crafted for fruit picking, he plucks a plump, ripened mulberry from a branch and pops it in his mouth. As the juice squirts from the fruit, the flavour-sensing cells of his tongue sense the sweetness of fructose and the astringency of tannins. Gritty, indigestible seeds distribute themselves between his teeth. As he walks away he periodically spits those seeds he can loosen from his gumline, spreading the next generation of mulberries as he does so. He walks on, potentially benefitting from age-repelling effects of the stilbene he has consumed. He smiles – the ear-to-ear, purple-toothed grin of the satisfied.
Such might have been the tale that Marco might have told. It is equally a tale of the imagination, but one of an imagination informed by science.
Recently, it has been suggested that tales of science need to step away from the imaginative. Tales of science, it is argued, need to focus more on human endeavours, like politics, economics, military actions, and the like. The case is made that stories about mulberry trees, birdsong, mushrooms, cheetahs, or sunlight are all fine and well, but science really needs to speak to the latest financial crises, political upheavals, or, presumably, celebrity goings on.
It is certain that science has a place in helping us understand human endeavours, but restricting ourselves to the workings of one species, in one miniscule patch of the universe, at only one scale, seems rather an insult to the wonderful capacity of the human intellect. It’s the equivalent of telling Marco to give up on his imaginings.
Rather than constraining the Marcos of this world, we need to let them use science to give voice to the vast wonders this universe holds. Perhaps in helping people revel in the same things that the Marcos see, we can better contextualise our place in the universe, and identify the substantive things that bind humanity together, rather than the trivial that drive us apart.
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Images: All photographs by Malcolm M. Campbell.