Why do pumpkins look orange? BETA-CAROTENE! Orange-yah glad you asked? Hopefully readers will delight if I discuss how these pumpkins steal sunlight! (but only specific wavelengths of it!)

Light is “just” little packets of energy called photons traveling in waves. You can think of them kinda like baseballs wiggling through the air. The balls are all thrown at the same speed (the speed of light) so, when a group of balls is thrown by the pitcher (light source), all the balls will reach the catchers at the same time. But some of the balls have more energy, so they take a wavier route, oscillating up and down more as they travel giving you closer together peaks (higher frequency (f), shorter wavelength (λ)).If you think of slalom racing, it’s like you have a bunch of skiers that all get to the finish line at the same time, but the skiers with higher energy make more S’s during that time.

White light has a combination of “all the colors” – so it’s like the sun is a pitcher throwing lots of baseballs at us, but molecules can “catch” some of the balls and “hide them” so that things look colored. (ROYGBIV is white, but OYGBIV or ROYBIV, etc. isn’t).

Molecules can only catch those photon “baseballs” if they have the right “catcher’s mitt.” These “mitts” – the photon-absorbing molecules (or parts of bigger molecules) – are called chromophores, and different ones absorb different photons (catch different balls). Why? 

They can only “catch a ball” if the photon’s energy is just right for exciting an electron in a molecule. An electron is a type of negatively-charged subatomic particle that molecules use to interact & bond with one another. They can live in different “orbitals” and the highest energy electrons live in the furthest orbitals from the nucleus (the central hub of the atom where the positively-charged protons and the neutral neutrons live). More here: http://bit.ly/33RznDA

It’s kinda like the “electron housing” landlords charge more in rent (in the form of energy) to live further from the central nucleus – out in the outer atomic suburbs. If they get some extra energy income, electrons can afford to move out from a “ground state” to a higher orbital – a so-called “excited state.” But they can only do this if they pay “exact” change, so they can only absorb photons that have an energy amount equal to the difference between the higher orbital and the current one. 

So who determines the rent? The atoms making up the molecules themselves (with some influence from the local environment). Different molecules have different housing arrangements and different differences in rent between the different housing levels, so “moving up” costs different amounts and, as a result, different molecules absorb photons with different amounts of energy.

And if we go back to that energy-wavelength-frequency-color relationship, this means that different molecules are absorbing different colors of light. And since they’re stealing different slices of the rainbow, the leftovers they leave us with look different. 

For example, if the orbitals are really far apart, it takes a lot of energy for an electron to move to those outer suburbs, so they’ll absorb high-energy (and thus high-frequency, short wavelength) light like blue light. And since they’re stealing that blue light, the light they leave us with (either going through it (transmission) or bouncing off (reflection)) looks yellow-orange-y. (To see what color something will look if it absorbs a color, look across from the absorbed color in a color wheel).

If a chromophore’s orbitals are closer together, the difference in rent is smaller, so they’ll absorb lower-energy photons (lower-frequency, longer-wavelength) – for example, they might absorb red red light & look green. 

I put “exact” energy quotes because there’s a little wiggle room because electrons can have different “vibrational levels” within orbitals and stuff. So if you look at an absorption spectrum for a molecule (which shows you what wavelengths the molecule “steals”) – instead of sharp peaks you see more of bell curves – peaks at the maximum absorption wavelength and some absorption on either side, petering out the further you get from that “optimal wavelength.” And you’ll likely also see multiple peaks because, for example, molecules can have multiple chromophores in the same molecule,

Most of the molecules I study are invisible to us because their orbitals are arranged such that the photons they absorb are outside of the visible range (either too low in energy or too high in energy for us to see) – there’s only a segment of the electromagnetic radiation (EMR) spectrum that we can actually see. We call this the visible spectrum – it spans from wavelengths of about 380-740 nanometers (nm)(a nanometer is 1 billionth of a meter so even the longest of these are pretty dang short).

Speaking of longest – that’d be the red light. It’s at the 740 end and then violet is at the 380 end with the OYGBI of ROYGBIV in between them in that order. Remember longer wavelength is lower energy & lower frequency. There’s a lot of other EMR that we can’t see – below red we have things like infrared & microwaves while above violet we have things like ultraviolet (UV) and x-rays.

But it’s easy to see the color of beta-carotene in this pumpkin! If you look at the chemical structure of beta-carotene – or the structure of many chromophores for that matter – you see a bunch of alternating single and double bonds. Atoms link up to form molecules by sharing electrons with their next-door neighbors in strong bonds called covalent bonds – pairs of electrons “moving in together” – one pair shared is a single bond & 2 pairs makes a double bond. 

Those “normal” covalent bonds normally just involve next door neighbors (adjacent atoms), but in the case of resonance – aka electron delocalization – which can happen when you have alternating single and double bonds like in beta-carotene- atoms can share electrons with others in the neighborhood – basically the atoms do some “redistricting” so that they can all evenly share some of their electrons (a sort of electron commune). And this often lowers the rent differences between orbitals, so these molecules often absorb visible light. 

A lot of plants (or at least parts of a lot of plants) look green – and pumpkins do initially too – this is because of a different chromophore, chlorophyll. Chlorophyll absorbs photons to “cash in” for sugar in the process of photosynthesis (plant food-making). Chlorophyll absorbs lower-energy light than beta-carotene – it absorbs reddish light so looks green. And since it’s absorbing reddish light, it “hides” the reddish light that beta-carotene leaves behind. So the plant looks green. So some vegetables like kale might look just green, but they’re also high in beta-carotene!

Beta-carotene is a “provitamin” – a vitamin precursor – in this case for Vitamin A. You can shell out big bucks for brand-name vitamin A supplements, but “vitamin A” itself’s “generic” – the term’s often used to describe a whole family of interrelated compounds including active RETINOIDS (which are found in animal sources (e.g. liver, kidney, eggs, & milk) & their CAROTENOID precursors (provitamins (vitamin precursors)) which are found in plants (e.g. dark or yellow vegetables)

The retinoids are important for things like vision –  helping us see – and acting as antioxidants (grabbing extra electrons from overly-energetic molecules before they can do damage). More here: http://bit.ly/2Rgr0xQ

Retinoids can be made from carotenoids (some of them) – in animals – so you can only get retinoids “premade” in animal-based food sources – but your body can easily make retinoids from the precursor carotenoids you can get from plants (so us vegetarians can breathe a sigh of relief 🙂 ).

There are several types of pro-A carotenoids (vitamin A precursors). The “best” carotenoid in terms of retinoid-making is β-CAROTENE because it gives you the most bang for your buck – each β-CAROTENE can be split into 2 retinoids, while the other carotenoids (α-carotene & β-cryptoxanthin) only give you 1.  Beta-carotene has highest absorption at 450nm. 

note: There are also non-pro-A carotenoids including lycopene (makes tomatoes red) & zeaxanthin (corn-yellower). Other common carotenoids are of the “xanthophyll” subclass – with a major one being lutein which makes leaves (and egg yolks) look yellow.

You might be wondering: “what’s in it for the plants?” Carotenoids absorb violet & blue-green light. Those are a couple of the more energetic wave lengths and more energy isn’t always a good thing (especially if it isn’t controlled!). You can imagine carotenoids kinda like plant sunscreen – by absorbing high-energy light, they act as molecular shields to keep the plants from getting fried. They do this by helping capture extra energy and dissipate it as heat before it can do damage. And their pretty colors attract seed-spreaders. (not to mention they can lead one lucky pumpkin to get nerdily carved!)

Carotenoids help make light less harmful, but they don’t really make the light “useful” – instead they’re just making it kinda fizzle out as heat. But what if you could put the energy of light to use for things like photosynthesis (sugar-making from sunlight)? Could you capture energy during long summer days and save it for winter nights? In order to do this, you’d need to capture that energy and convert it to a storable, more controllable, form. A sort of molecular energy money if you will… enter the chlorophyll!

There are actually multiple types of chlorophyll, with chlorophyll a & B being the main photosynthetic pigments. Chlorophylls absorb blue & red wavelengths, but not green – so green is the color that is seen! Chlorophyll a is the most abundant plant pigment & it absorbs at maxima of 430nm (blue) & 662nm (red). Chlorophyll b has a slightly shifted absorption spectrum (453 & 642 maxima) so it helps expand the range of light that can be captured & used to make sugar. 

Chlorophyll’s photon-catching is done by a “tetrapyrrolic ring” that holds a metal that helps it with electron transferring. It also has another, not-visible-light-abasorbing part – a long hydrophobic (water-avoiding) tail that lets it embed in membranes. 

The tetrapyrrolic (4 pyrorole-like) ring forms a “porphyrin” structure that is like a big electron orgy – 18 atoms participating in one of those conjugated systems where electrons are shared among them. This porphyrin is similar to that of hemoglobin – the oxygen-carrying molecule in our blood – even though these “mitts” may look similar at the chemical level, hemoglobin makes our blood look red while chlorophyll makes plants look green – clearly little differences in chemical makeup can cause big differences in color! If you compare hemoglobin & chlorophyll you can see that hemoglobin holds iron, while chlorophyll has magnesium, and there are also some other differences that affect electronic “rent costs” and thus the absorbed colors.

To take full advantage of the summer sun, plants produce lots of chlorophyll “catcher’s mitts” and spread them out over a large surface area so that mitt and light have a chance to meet! They grow big flat leaves with compartments called chloroplasts that contain lots and lots of a chromophore called chlorophyll that can absorb specific photons that the plant can “cash in” for sugar through the process of photosynthesis. 

As the days start getting shorter and there’s less available sunlight, it gets to the point where, if the plants kept up stocking chlorophyll at the current levels, they’d spend more energy preparing to catch sunlight than they’d get back in usable energy captured. Unfavorable energy economics… So the plants stop making chlorophyll. What’s left gets degraded (and parts recycled), and the red and blue light stop getting stolen.

But instead of looking white, the leaves usually look orange, yellow, or red. Because, even though the leaves had “only looked green” before, when there was sooo much chlorophyll, there were actually other chromophores present with “different mitts”- they were just hidden. Pigments including the carotenoids, as well as other pigments like flavonoids. http://bit.ly/lightleafcolor  

I had a really nice time carving pumpkins with my mom (and I love her pumpkin – and HER!) She’s back home now and I miss her a lot already… I’m so grateful she came to visit me and I have a newfound extra motivation to try to finish up my PhD and move closer to family. Planning to lose myself into work for a while to distract from things…

note: video’s of last year’s pumpkin 

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0

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