The pigments plants use

A leaf doesn’t rely on a single pigment. Plants and green algae combine several kinds of light-absorbing molecules, and each one soaks up a different part of the visible spectrum.

Chlorophylls a and b

Green plants and green algae contain two kinds of chlorophyll, chlorophyll a and chlorophyll b. Inside the chloroplast, both are bound to proteins that are embedded in the thylakoid membrane.

A chlorophyll molecule has two main parts:

• A porphyrin ring, a flat ring of atoms with a magnesium ion (Mg²⁺) held at its center. One electron in the ring isn’t tied to any single atom; it can move from atom to atom around the ring. This delocalized electron is the one that absorbs light.
• A long phytol tail, a chain made of carbon and hydrogen. Like oil, the tail is hydrophobic (it avoids water), so it anchors the chlorophyll to water-avoiding regions of proteins inside the thylakoid membrane.

Chlorophylls a and b differ at just one spot. Where chlorophyll a carries a CH₃ group, chlorophyll b carries a CHO group. That small change makes their absorption of light slightly different.

Many chlorophyll molecules cluster together with proteins in a light-harvesting complex. Its pigments and proteins are arranged so that light energy absorbed anywhere in the cluster can be collected and passed along to one spot.

Carotenoids

Carotenoids are pigments in shades of yellow, orange, and red. A familiar one is β-carotene, the pigment that makes carrots orange. Like chlorophylls, carotenoids absorb light that powers photosynthesis in the chloroplast.

Carotenoids also give many flowers and fruits their color. In a green leaf you rarely notice them, because chlorophyll far outnumbers them and its green covers them up. Where leaves change color in autumn, the leaf is losing its chlorophyll. The carotenoids were there all along, and now their yellows, oranges, and reds show.

Absorption and action spectra

Two kinds of graphs show which wavelengths matter to a plant:

• An absorption spectrum plots how much light a pigment absorbs at each wavelength.
• An action spectrum plots how fast photosynthesis runs at each wavelength.

Chlorophylls a and b have slightly different absorption spectra, but both absorb red and violet light most strongly and absorb green light poorly. The green light they reflect is why leaves look green during the growing season. Carotenoids absorb mainly blue and blue-green light and reflect yellow and red.

A green plant’s action spectrum peaks at the wavelengths that chlorophylls and carotenoids absorb strongly, and it dips in the green part of the spectrum, where those pigments absorb little. The match shows that the light these pigments absorb is the light that drives photosynthesis.

Why have several pigments?

A mix of pigments lets a plant use more of the sunlight that reaches it.

Each pigment absorbs its own range of wavelengths, so together they cover more of the visible spectrum than any one could alone. The wider that range, the more of the sun’s energy the plant captures.

Check yourselfPhotosynthesis runs slowly in green light. Why, and how does having several pigments help a plant?

Chlorophylls and carotenoids absorb green light poorly, so most of it is reflected rather than used. Because each pigment absorbs a different range of wavelengths, having several lets the plant absorb a wider range of light and capture more of its energy.