This NBC Learn video explains the basic role of pigment molecules -- carotenoids and anthocyanins -- in producing what humans see as color in flowers, as the pigment molecules absorb visible light of various color wavelengths.
The Chemistry of Flowers
BETH NISSEN, reporting:
Hydrangea blue, marigold yellow, lily white, carnation pink, rose red: Why are roses red and violets…well, violet?
Dr. NANCY GOROFF (Stony Brook University): The color is really to make the flower stand out from the rest of the plant so that they will attract insects to spread their pollen.
NISSEN: What determines a flower’s coloring is the same thing that decides our own hair and eye color: genetics. Genes determine which colors we’ll see in a plant – a “blonde” daffodil or a “redhead” rose, for instance. Genes in plant cells direct the production of specific ‘color’ molecules, called pigments.
Dr. GOROFF: The pigment is just something that is colored. That's all it means.
NISSEN: Pigments are stored in parts of plant cells called plastids that are like tidy little storage bins. Some store starch or food for the plant; others store pigments. Most of the pigments stored in those plastid ‘bins’ are called chloroplasts – they contain the green pigment chlorophyll, essential for photosynthesis.
Other plastids contain chromoplasts. “Chromo” comes from the Greek word meaning “color.” So chromoplasts are anything with color (other than green) in plants, fruits and flowers.
So, what is color? Well, when we think of color, we’re really thinking of light. Here’s a quick review of light: Light is on a spectrum – the Electromagnetic Spectrum, running from short wavelengths (gamma rays and x-rays) to long wavelengths (microwaves and AM/FM radio waves). The only part humans can see (without night vision goggles or other aids) is this tiny “rainbow” sliver in the middle: the aptly-named “visible light spectrum”…blue and violet…to red and orange.
Dr. GOROFF: And so if you are looking at, say, chlorophyll, chlorophyll absorbs in the blue part of the spectrum and at the other end-- at other side of the energy scale, it absorbs in the red. We see the color of the light that’s reflected, that’s not absorbed.
NISSEN: So we see violets as violet because these flowers have pigments that absorb all the visible wavelengths of light except violet.
Now, there are two types of pigments that absorb and reflect light to make flower colors – carotenoids and flavonoids – especially a type of flavonoid called anthocyanin.
Let’s take carotenoids first. Carotenoids are molecules that are similar (that’s what the “oid” means – “similar”) to carotene. Are you thinking “carrots?” In fact, carotenoids do produce the orange in carrots, as well as in apricots and pumpkins, and in orange flowers, like tiger lilies, yellow flowers, like daffodils, and red flowers and fruits, like tomatoes (yes, tomatoes are, botanically, fruits).
Then there are the anthocyanin pigments. “Antho-” comes from the Greek word meaning “flower” – and “-cyanin” from the Greek word meaning “blue.” (So when doctors and nurses say that someone is “cyanotic,” that means lack of oxygen is causing some part of the body to turn blue.) So anthocyanins are responsible for blue colors, in blueberries and irises, as well as purples in eggplant and pansies, and reds in strawberries and roses.
Why do these different carotenoid and anthocyanin pigments absorb the lightwaves they do? Like everything in chemistry, the answer is in which atoms make up their molecules and how they are bonded together.
Dr. GOROFF: To a chemist, when I think of color with organic molecules, molecules of carbon and hydrogen and oxygen, I think of bonds.
NISSEN: Molecules, as you know, are formed by atoms that ‘bond’ to each other – most often with a single bond or double bonds. If atoms share a single pair of electrons from their outer shells, it’s a single bond. If they share two pairs of electrons – that’s four electrons – it’s a double bond.
If you look at drawings of the molecular structures of carotenoids and anthocyanins, you can see these bonds. (A single line indicates a single bond; a double line indicates a double bond.) You’ll also see something else: the places where they alternate – a single bond, then a double bond, then a single bond and so on. Chemists say molecules joined together like these are “conjugated.”
We know – we’re edging here into quantum mechanics, a field of physics that (brace yourselves) uses mathematics to describe the behavior and interaction of matter and energy – including what’s called the “wave function.”
But relax. For now, here’s what you need to know. You remember all the electrons in our pigment molecules? When sunlight hits a flower, certain wavelengths of light – unique to each flower’s pigment molecules – cause their electrons to ‘jump’ from a low-energy state to a high-energy state (in chemistry, called an “excited state”). This ‘high’ energy – and those wavelengths of light – can then be absorbed.
And remember the alternating “conjugated” single and double bonds? Molecules like carotenoids and anthocyanins – with long sections of “conjugation” – absorb long wavelength light, light in the visible range. In contrast, molecules with shorter “strings” – take ethylene, as an example – absorb short wavelength light, light we can not see – which is what makes ethylene a colorless gas.
Plant genetics determine the general color of the flower – but many factors can alter the shade or brightness of that color. Air temperature, amounts of sunlight, and the acidity, or pH of the soil – all can affect the amount and type of pigment molecules made by the plant.
There’s much more to know about light, waves, wavelengths, and energy, for those with a ‘budding’ interest in chemistry. Think of this as just a “pop” of color.
Flowers, Color, Pigment, Genes, Genetics, Plastid, Chloroplast, Chlorophyll, Chromoplast, Light, Spectrum, Electromagnetic, Wavelength, Gamma Ray, X-Ray, Microwave, Radio Wave, Visible Light, Carotenoid, Flavonoid, Anthocyanin, Cyanotic, Molecules, Single Bond, Double Bond, Atoms, Electrons, Conjugated Bonds, High Energy, Ground State, Excited State, Ethylene, Reflect, Reflection, pH, Acidity, Aluminum, "Chemistry Now"