Chlorophyll is the major entry point for light energy in photosynthesis.

To understand how light energy is captured and stored by photosynthesis, we need to know a little about light. The sun, like all stars, produces a broad spectrum of electromagnetic radiation ranging from gamma rays to radio waves. Each point along the electromagnetic spectrum has a different energy level and a corresponding wavelength. Visible light is the portion of the electromagnetic spectrum apparent to our eyes, and it includes the range of wavelengths used in photosynthesis. The wavelengths of visible light range from 400 nm to 700 nm. Approximately 40% of the sun’s energy that reaches Earth’s surface is in this range.

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FIG. 8.8 Light absorbed by leaves. The graph shows the extent to which wavelengths of visible light are absorbed by pigments in an intact leaf.
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FIG. 8.9 Chemical structure of chlorophyll. Shown is chlorophyll a, found in all photosynthetic eukaryotes and cyanobacteria.

Pigments are molecules that absorb some wavelengths of visible light (Fig. 8.8). Pigments look colored because they reflect light enriched in the wavelengths that they do not absorb. Chlorophyll is the major photosynthetic pigment; it appears green because it is poor at absorbing green wavelengths. The chlorophyll molecule consists of a large, light-absorbing “head” containing a magnesium atom at its center and a long hydrocarbon “tail” (Fig. 8.9). The large number of alternating single and double bonds in the head region explains why chlorophyll is so efficient at absorbing visible light.

Chlorophyll molecules are bound by their tail region to integral membrane proteins in the thylakoid membrane. These protein–pigment complexes, referred to as photosystems, are the functional and structural units that absorb light energy and use it to drive electron transport.

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Photosystems contain pigments other than chlorophyll, called accessory pigments. The most notable are the orange-yellow carotenoids, which can absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll. Thus, the presence of these accessory pigments allows photosynthetic cells to absorb a broader range of visible light than would be possible with just chlorophyll alone. As we will see in section 8.4, carotenoids play an important role in protecting the photosynthetic electron transport chain from damage.