4.7: Light energy travels in waves: plant pigments absorb specific wavelengths.

You can’t eat sunlight. That’s because sunlight is light energy rather than the chemical energy found in the bonds of food molecules. Photosynthesis is powered by light energy, a type of kinetic energy made up of little energy packets called photons, which are organized into waves. Photons can do work as they bombard surfaces such as your face (heating it) or a leaf (enabling it to build sugar from carbon dioxide and water).

Photons have various amounts of energy, and the length of the wave in which they travel corresponds to the amount of energy carried by the photon. The shorter the wavelength, the more energy the photon carries. Within a ray of light, there are super-high-energy photons (those with short wavelengths), relatively low-energy photons (those with longer wavelengths), and everything in between. This range, which is called the electromagnetic spectrum, extends from extremely short, high-energy gamma rays and X rays, with wavelengths as short as 1 nanometer (nm; a human hair is about 50,000 nm in diameter), to very long, low-energy radio waves, with wavelengths as long as a mile (FIGURE 4-14).

Just as we can’t hear some super-high-pitched frequencies of sound (even though many dogs can), there are some wavelengths of light that are too short or too long for us to see. The light that we can see, visible light, spans all the colors of the rainbow. Humans (and some other animals) can see colors because our eyes contain light-absorbing molecules called pigments. These pigments absorb wavelengths of light within the visible range. The energy in these light waves excites electrons in the pigments, stimulating nerves in our eyes, which then transmit electrical signals to our brains. We perceive different wavelengths within the visible spectrum as different colors. The pigments in the human eye absorb many different wavelengths pretty well: that’s why we can see so many colors. When plants use sunlight’s energy to make sugar during photosynthesis, they also use the visible portion of the electromagnetic spectrum. Unlike the pigments in our eyes, however, plant pigments (the energy-capturing parts of a plant) absorb and use only a portion of visible light wavelengths.

Chlorophyll is the main pigment molecule in plants that absorbs light energy from the sun. Chlorophyll molecules are embedded in the thylakoid membranes of chloroplasts, which are found primarily in plants’ leaves. Just as light energy excites electrons in the pigments responsible for color vision in humans, electrons in a plant’s chlorophyll can become excited by certain wavelengths of light and can capture a bit of this light energy.

Plants produce several different light-absorbing pigments (FIGURE 4-15). The primary photosynthetic pigment, called chlorophyll a, absorbs red and blue-violet wavelengths of light. Every other wavelength generally travels through or bounces off this pigment. Chlorophyll a cannot efficiently absorb green light and instead reflects those wavelengths. We perceive the reflected light waves as green, and so the pigment and the leaves that contain it appear green. Another pigment, chlorophyll b, is similar in structure but absorbs blue and red-orange wavelengths and reflects yellow-green wavelengths. Some related pigments called carotenoids absorb blue-violet and blue-green wavelengths and reflect yellow, orange, and red wavelengths.

In the late summer, cooler temperatures cause some trees to prepare for the winter by shutting down chlorophyll production and reducing photosynthesis rates, much like an animal’s hibernation. Gradually, the chlorophyll a and b molecules in the leaves are broken down and their chemical components are stored in the branches. As the amounts of chlorophyll a and b in the leaves decrease relative to the remaining carotenoids, the striking colors of the fall foliage are revealed (see Figure 4-15). During the rest of the year, chlorophylls a and b are so abundant in leaves that green masks the colors of the other pigments.