This NBC Learn video explains the basic role of the pigment molecule chlorophyll in photosynthesis, and explains why plants are -- or appear to be -- green: because chlorophyll molecules absorb visible light in all color wavelengths except green, which is reflected back, into our eyes.
The Chemistry of Green
BETH NISSEN, reporting:
Here are some green things: Grass, limes, emeralds, the Emerald Isle, the Chicago River on St. Patrick’s Day. And, while we’re on this theme, shamrocks. In fact, almost all leafy green plants. Why are plants green? That’s actually a trick question. We’ll reveal the trick later; for now, the answer is chlorophyll, a very well-named molecule: The first half of its name comes from the Greek word khloros, meaning green. The other half from the Greek phyllon, meaning leaf. And the plant leaf is where chlorophyll works at one of the most vital jobs on the planet: photosynthesis.
DR. NATE LEWIS (California Institute of Technology): Photosynthesis is essential to everyday life. Without taking the energy from the sun and storing it in chemical fuel we wouldn’t have plants. We wouldn’t have bacteria; we wouldn’t have food sources to support life on earth. We wouldn’t have oxygen in our atmosphere.
NISSEN: You probably know the basics of photosynthesis: Plants, grasses, ferns, trees, take in water from their root and sunlight, and carbon dioxide, through their leaves; materials they use to produce sugars they eat, and starches they store as food and to produce food we eat and store. As a by-product of all this, plants exhale oxygen into the air that other living things need, to keep living. Let’s rewind to that part about taking in sunlight. All those photosynthesis processes we just reviewed need to be fueled, need a source of energy. Earth’s primary source of energy? The sun, by far the largest object in our solar system.
There’s a lot to learn about energy, but here’s the most important thing: it’s always conserved. Conservation of energy is a fundamental principle of chemistry and physics. It’s actually a natural law, the First Law of Thermodynamics, which almost sounds like a riddle: energy can’t be created, can’t be destroyed, but energy can be captured and can change forms.
LEWIS: Photosynthesis takes sunlight and chemicals that are abundant all around us and converts them into fuel.
NISSEN: How? Think of a green plant as, well, a plant, a power plant, a solar power plant. Our solar panels and arrays capture sunlight and turn it into electricity. Plants do the same thing and chlorophyll is key. Chlorophyll is a pigment, a chemical compound that absorbs, or takes in, light. Inside the plant, chlorophyll acts like a funnel: taking in sunlight; redirecting it.
LEWIS: Chlorophyll takes the energy in the sunlight and channels it into the parts of the biological system that let it make chemical fuel and not just go right through, like through a piece of glass.
NISSEN: Once captured and channeled, the sunlight is transformed. It all works a little like that carnival strong-man game: when sun hits a leaf, electrons in the chlorophyll molecules jump from a ground state to a high-energy state in chemistry called an excited state. This sets off a series of chemical reactions that eventually converts carbon dioxide and water into a carbohydrate called glucose, a plant sugar that plants eat like we eat sugars for energy, to keep all systems running. The by-product of all this is the oxygen plants release into the air we breathe.
Probably time to return to that trick question: why are plants green? The trick is in the wording of the question: it should be why do plants appear green? You know that phrase ‘see the light’? Well, we can see only part of it. 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 violet: and blue to orange and red.
LEWIS: We'd like to absorb as much of the colors of the rainbow of spectrum of the sun as we can, so that we can efficiently utilize all that. Chlorophyll starts to absorb light at a specific wavelength: 670 nanometers.
NISSEN: That red-orange light is strongly absorbed by chlorophyll molecules and so is the violet-blue light at around the 450-nanometer mark. Chlorophyll absorbs the color wavelengths in between, but the closer the wavelengths get to the middle, around 500 nanometers, the less chlorophyll absorbs until it quits.
LEWIS: What’s left behind is the green.
NISSEN: The primarily green wavelength light that the chlorophyll doesn’t absorb is reflected back, where it hits, among other things, our eyes. And because green is the color of the light reflected, we see plants as green. Photosynthesis is wonderfully complex. This is just a basic explanation of the way it all works on Earth. Scientists think that plants on other planets, in different atmospheres, could be red, blue, even black. Although, that might be just a lucky guess.
