Light and how pigments absorb it

The light reactions begin the moment light strikes a leaf. To understand what it means for a plant to capture light, you need two ideas: what light is, and what happens inside a molecule when it absorbs light.

Light is energy that travels in waves

Light is one form of electromagnetic radiation: energy carried by electric and magnetic fields that ripple through space as waves. The distance from one wave peak to the next is the wavelength.

All the possible wavelengths together make up the electromagnetic spectrum. Gamma rays have the shortest wavelengths, followed by X-rays and ultraviolet (UV) radiation. Radio waves have the longest.

Partway between sits visible light, the narrow band your eyes can detect, from about 380 to 740 nm (a nanometer, nm, is a billionth of a meter). Visible light is what powers photosynthesis.

Light also comes in packets

Besides acting like a wave, light behaves like a stream of particles called photons. A photon has no mass and moves at the speed of light, and each one carries a fixed amount of energy.

How much energy depends on the wavelength: the shorter the wavelength, the more energy each photon carries. A gamma-ray photon packs far more energy than a radio-wave photon.

That difference decides what radiation does to living things. Photons of gamma rays, X-rays, and UV carry so much energy that molecules absorbing them can be damaged; this radiation can cause mutations (changes) in DNA and even lead to cancer. The ozone layer, high in the atmosphere, blocks much of the Sun’s UV. Photons of visible light carry far less energy, so molecules can absorb them without harm.

Pigments absorb some colors and reflect others

When light hits an object, it can pass through, bounce off in a new direction, or be absorbed. A pigment is a molecule that absorbs light energy. Each pigment absorbs certain wavelengths and reflects the rest, and the reflected wavelengths are the color you see.

Leaves look green because their pigments reflect green light and absorb other colors. At the extremes, a white object reflects nearly all visible light, and a black object absorbs nearly all of it. That’s why a black T-shirt feels hotter in the sun than a white one.

Absorbing light boosts an electron

At the level of atoms, absorbing light means moving an electron. Electrons occupy regions around an atom’s nucleus called orbitals, and each orbital has its own energy level. When a photon with the right amount of energy strikes an electron, the electron jumps to a higher-energy orbital, farther from the nucleus.

The photon’s energy has to match the gap between the two levels exactly. Different pigments have electrons with different gaps, so each pigment absorbs its own set of wavelengths.

An electron in its normal, low-energy position is in its ground state. After absorbing a photon, it’s in an excited state. Now farther from the nucleus and held less firmly, the excited electron is unstable and won’t stay that way for long.

Four ways an excited electron settles down

An excited electron becomes stable again in one of four ways:

Photosynthesis depends on the last two. Pigments pass energy among themselves by resonance energy transfer, and a few special pigments give away the excited electron itself. Handing off the electron is how light energy gets captured rather than lost as heat or light.

Check yourselfWhy can cells absorb visible light safely, while UV and gamma rays can damage DNA?

The shorter the wavelength, the more energy each photon carries. UV and gamma-ray photons carry enough energy to damage molecules such as DNA, causing mutations. Visible-light photons carry much less, so pigments can absorb them without harm.

Check yourselfWhat four things can happen to an excited electron as it becomes stable again?

It can drop back down and release its energy as heat, or drop back down and release it as light (fluorescence). It can pass the energy to an electron in a nearby molecule (resonance energy transfer). Or the electron itself can move to another molecule where it’s stable, which captures the energy.