One Flash of Light, One Vision: Carrots, Colour and Chemistry

“White” light is made up of all the colours of the rainbow.

Sometimes you have one of those weeks when the universe seems to be determined to yell at you about a certain thing. That’s happened to me this week, and the shouting has been all about light and vision (earworm, anyone?).

I started the week writing about conjugated molecules and UV spectrometry for one project, was asked a couple of days ago if I’d support a piece of work on indicators for the RSC Twitter Poster Conference that’s happening from 2-3rd March, and then practically fell over a tweet by Dr Adam Rutherford about bacteria that photosynthesise from infrared light in a hydrothermal vent*.

Oh well, who am I to fight the universe?

Light is awesome. The fact that we can detect it is even awesome-er. The fact that we’ve evolved brains clever enough built all sorts of machines to measure other kinds of light that our puny human eyes cannot detect is, frankly, astonishing.

The electromagnetic spectrum covers all the different kinds of light. (Image source)

Let’s start with some basics. You probably met the electromagnetic (EM) spectrum at some point in school. Possibly a particularly enthusiastic physics teacher encouraged you to come up with some sort of mnemonic to help you remember it. Personally I like Rich Men In Vegas Use eXpensive Gadgets, but maybe that’s just me.

The relevant thing here is that the EM spectrum covers all the different wavelengths of light. Visible light, the stuff that’s, well, visible (to our eyes), runs from about 400 to 700 nanometres.

A colour wheel: when light is absorbed, we see the colour opposite the absorbed wavelengths. (Image source)

Now, we need another bit of basic physics (and biology): we see light when it enters our eyes and strikes our retinas. We see colours when only certain wavelengths of light make it into our eyes.

So-called “white” light is made up of all the colours of the rainbow. Take one or more of those colours away, and we see what’s left.

For example, if something looks red, it means that red light made it to our eyes, which in turn means that, somewhere along the way, blue and green were filtered out.

(Before I go any further, there are actually several causes of colour, but I’m about to focus on one in particular. If you really want to know more, there’s this book, although it is a tad expensive…)

Back to chemistry. Certain substances absorb coloured light. We know them as pigments. Carrots are orange, for example, largely because they contain a pigment called beta-carotene (or β-carotene). This stuff appears, to our eyes, as red-orange, and the reason for that is that it absorbs green-blue light, the wavelengths around 400-500 nm.

β-Carotene is a long molecule with lots of C=C double bonds. (Image source.)

Why does it absorb light at all? Well, β-carotene is a really long molecule, with lots of C=C double bonds. These bonds form what’s called a conjugated system. Without getting into the complexities of molecular orbital theory, that means the double bonds alternate along the chain, and they basically overlap and… smoosh into one long thing. (Look, as the saying goes, “all models are wrong, but some are useful,” – it’ll do for now.)

When molecules with conjugated systems are exposed to electromagnetic light, they absorb it. Specifically, they absorb in the ultraviolet region – the wavelengths between about 200 and 400 nanometres. Here’s the thing, though, those wavelengths are right next to the violet end of the visible spectrum – that’s why it’s called ultraviolet after all.

Molecules with really long conjugated systems start to absorb in the coloured light region, as well. And because they’re absorbing violet and blue, possibly a smidge of green, they look… yup! Orangey, drifting into red.

So now you know why carrots are orange. Most brightly coloured fruit, of course, is that way to attract animals and birds to eat it, and thus spread its seeds. As fruit ripens, it usually changes colour, making it stand out better against green foliage and easier to find. This is the link with indicators that I mentioned at the start: many fruits contain anthocyanin pigments, and these often have purple-red colours in neutral-acidic environments, and yellow-green at the more alkaline end. In other words, the colour change is quite literally an indicator of ripeness.

But the bit of the carrot that we usually eat is underground, right? Not particularly easy to spot, and they don’t contain seeds anyway. Why are carrots bright orange?

Modern carrots are mostly orange, but purple and yellow varieties also exist.

Well, they weren’t. The edible roots of wild plants almost certainly started out as white or cream-coloured, as you might expect for something growing underground, but the carrots which were first domesticated and farmed by humans in around 900 CE were, most probably, purple and yellow.

As carrot cultivation became popular, orange roots began to appear in Spain and Germany in the 15th/16th centuries. Very orange carrots, with high levels of β-carotene, appeared from the 16th/17th centuries and were probably first cultivated in the Netherlands. Some have theorised that they were particularly selected for to honour William of Orange, but the evidence for this seems to be a bit slight. Either way, most modern European carrots do descend from a variety that was originally grown in the Dutch town of Hoorn.

In other words, brightly-coloured carrots are a mutation which human plant breeders selected for, probably largely for appearances.

But wait! There was an advantage for humans, too – even if we didn’t realise it straight away. β-carotene (which, by the way, has the E number E160a – many natural substances have E numbers, they’re nothing to be frightened of) is broken up in our intestines to form vitamin A.

Vitamin A is essential for good eye health.

Vitamin A, like most vitamins, is actually a group of compounds, but the important thing is that it’s essential for growth, a healthy immune system and – this is the really clever bit – good vision.

We knew that. Carrots help you see in the dark, right?

Hah. Well. The idea that carrot consumption actually improves eyesight seems to be the result of a World War II propaganda campaign. During the Blitz, the Royal Air Force had (at that time) new, secret radar technology. They didn’t want anyone to know that, of course, so they spread the rumour that British pilots could see exceptionally well in the dark because they ate a lot of carrots, when the truth was that those pilots were actually using radar.

But! It’s not all a lie – there is some truth to it! Our retinas, at the back of our eyes, have two types of light-sensitive cells. Cone cells help us distinguish colours, while rod cells help us detect light in general.

In those rod cells, a molecule called 11-cis-retinal is converted into another molecule called rhodopsin. This is really light-sensitive. When it’s exposed to light it photobleaches (stops being able to fluoresce), but then regenerates. This process takes about thirty minutes, and is a large part of the reason it takes a while for your eyes to “get used to the dark.”

Guess where 11-cis-retinal comes from? Yep! From vitamin A. Which is why one of the symptoms of vitamin A deficiency is night blindness. So although eating loads of carrots won’t give you super-powered night vision, it does help to maintain vision in low light.

Our brain interprets electrical signals as vision.

How do these molecules actually help us to see? Well, when rhodopsin is exposed to light, the molecule changes, which ultimately results in an electrical signal being transmitted along the optic nerve to the brain, which interprets it as vision!

In summary, not only is colour all about molecules, but our whole visual system depends on some clever chemistry. I told you chemistry was cool!

Just gimme fried chicken 😉


*Ah. I sort of ran out of space for the weird hydrothermal bacteria thing. At least one of the relevant molecules seems to be another carotenoid, probably chlorobactene. The really freaking amazing thing is that there seems to be an absorption at 775 nm, which is beyond red visible light and into the infrared region of the EM spectrum. Maybe more on this another day…


If you’re studying chemistry, have you got your Pocket Chemist yet? Why not grab one? It’s a hugely useful tool, and by buying one you’ll be supporting this site – it’s win-win! If you happen to know a chemist, it would make a brilliant stocking-filler! As would a set of chemistry word magnets!

Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. Content is © Kat Day 2021. You may share or link to anything here, but you must reference this site if you do. If you enjoy reading my blog, and especially if you’re using information you’ve found here to write a piece for which you will be paid, please consider buying me a coffee through Ko-fi using the button below.
Buy Me a Coffee at ko-fi.com

Want something non-sciency to distract you from, well, everything? Why not check out my fiction blog: the fiction phial.

 

In the fridge or on the windowsill: where’s the best place to keep tomatoes?

Fresh fruit and vegetables are great, but where’s the best place to store them?

I’ve mentioned before that my Dad is a professional plant-wrangler (if you’ve never read the electric daisies post, do go and have a look – it’s a little-read favourite) and he often brings me home-grown fruit and vegetables.

What follows is an inevitable disagreement about storage, specifically, my habit of putting everything in the fridge.

In my defence, modern houses rarely have pantries (boo) and we don’t even have a garage. We do have a shed, but it’s at the bottom of our poorly-lit, somewhat muddy garden. Do I want to traipse out there on a cold, dark, autumn evening? No, I do not. So the fabled “cool, dark place” is a bit of problem. My fridge is cool and dark, I have argued, but here’s the thing – turns out, it’s too cool. And quite probably too dark.

This I have learned from the botanist James Wong (@botanygeek on Twitter), whose talk I attended on Monday this week at the Mathematical Institute in Oxford. James, it turns out, had a rather similar argument with his Mum, particularly regarding tomatoes.

We should’ve listened to out parents, because they were right. A lot of fruit and vegetables really are better stored outside of the fridge, and for tomatoes in particular “better” actually means “more nutritious”.

Lycopene is a very long molecule with lots of double C=C bonds.

Tomatoes, James explained, contain a lot of a chemical called lycopene. It’s a carotene pigment, and it’s what gives tomatoes their red colour.

Lycopene has lots of double bonds between its carbon atoms which form something chemists call a conjugated system. This has some rather cool properties, one of which is an ability to absorb certain wavelengths of light. Lycopene is especially good at absorbing blue and green wavelengths, leaving our eyes to detect the red light that’s left.

Lycopene absorbs blue and green light, which is why tomatoes appear red.

Tomatoes and lycopene also seem to have a lot of health benefits. There’s some evidence that lycopene might reduce the risk of prostate and other cancers. It also appears to reduce the risk of stroke, and eating tomato concentrate might even help to protect your skin from sun damage (don’t get any ideas, you still need sunblock). Admittedly the evidence is currently a bit shaky – it’s a case of “more research is needed” – but even if it turns out to that the causative relationship isn’t terribly strong, tomatoes are still a really good source of fibre and vitamins A, C and E. Plus, you know, they taste yummy!

But back to the fridge. Surely they will keep longer in the fridge, and the low temperatures will help to preserve the nutrients? Isn’t that how it works?

Well, no. As James explained, once tomatoes are severed from the plant they have exactly one purpose: to get eaten. The reason, from the plant’s point of view, is that the critter which eats them will hopefully wander off and – ahem – eliminate the tomato seeds at a later time, somewhere away from the parent plant. This spreads the seeds far and wide, allowing little baby tomato plants to grow in a nice, open space with lots of water and sun.

For this reason once the tomato fruit falls, or is cut, from the tomato plant it doesn’t just sit there doing nothing. No, it carries on producing lycopene. Or rather, it does if the temperature is above about 10 oC. Below that temperature (as in a fridge), everything more or less stops. But, leave a tomato at room temperature and lycopene levels increase significantly. Plus, the tomato pumps out extra volatile compounds – both as an insect repellant and to attract animals which might usefully eat it – which means… yes: room temperature tomatoes really do smell better. As if that weren’t enough, chilling tomatoes can damage cell membranes, which can actually cause them to spoil more quickly.

In summary, not only will tomatoes last longer out of the fridge, they will actually contain more healthy lycopene!

Anecdotally, once I got over my scepticism and actually started leaving my tomatoes on my windowsill (after years of refrigeration) I discovered that it’s true. My windowsill tomatoes really do seem to last longer than they used to in the fridge, and they almost never go mouldy. Of course, it’s possible that I might not be comparing like for like (who knows what variety of tomato I bought last year compared to this week), but I urge you to try it for yourself.

James mentioned lots of other interesting bits and pieces in his talk. Did you know that sun-dried shiitake mushrooms are much higher in vitamin D? Or that you can double the amount of flavonoid you absorb from your blueberries by cooking them? (Take that, raw food people!) Storing apples on your windowsill is likely to increase the amount of healthy polyphenols in their skin, red peppers are better for you than green ones, adding mustard to cooked broccoli makes it more nutritious, and it would be much better if we bought our butternut squash in the autumn and saved it for Christmas – it becomes sweeter and more flavoursome over time.

In short, fascinating. Who wants to listen to some “clean eater” making it up as they go along when you can listen to a fully-qualified botanist who really knows what he’s talking about? Do check out the book, How to Eat Better, by James Wong – it’s packed full of brilliant tidbits like this and has loads of recipes.

And yes, Dad: you were right.


Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. All content is © Kat Day 2017. You may share or link to anything here, but you must reference this site if you do.


All comments are moderated. Abusive comments will be deleted, as will any comments referring to posts on this site which have had comments disabled.