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Thylakoid

Granum

Inner membrane

Light and Pigments

To understand how chloroplasts absorb light in photosynthesis, it is important to understand some of the properties of light. Light from the sun appears white, but it is actually made of a variety of colors. As shown in Figure 6-4, white light can be separated into its component colors by passing the light through a prism. The resulting array of colors, ranging from red at one end to violet at the other, is called the visible spectrum.

When white light strikes an object, its component colors can be reflected, transmitted, or absorbed by the object. Many objects contain pigments, compounds that absorb light. Most pigments absorb certain colors more strongly than others. By absorbing certain colors, a pigment subtracts those colors from the visible spectrum. Therefore, the light that is reflected or transmitted by the pigment no longer appears white. For example, the lenses in green-tinted sunglasses contain a pigment that reflects and transmits green light and absorbs the other colors. As a result, the lenses look green.

Chloroplast Pigments

Located in the membrane of the thylakoids are several pigments, the most important of which are called chlorophylls (KLAWR-uh-FILZ). There are several different types of chlorophylls. The two most common types are known as chlorophyll a and chlorophyll b. As Figure 6-5 shows, chlorophyll a absorbs less blue light but more red light than chlorophyll b absorbs. Neither chlorophyll a nor chlorophyll b absorbs much green light. Instead, they allow green light to be reflected or transmitted. For this reason, leaves and other plant structures that contain large amounts of chlorophyll look green.

Only chlorophyll a is directly involved in the light reactions of photosynthesis. Chlorophyll b assists chlorophyll a in capturing light energy, and therefore chlorophyll b is called an accessory pigment. Other compounds found in the thylakoid membrane, including the yellow, orange, and brown carotenoids (kuh-RAHT'n-OYDZ), also function as accessory pigments. Looking again at Figure 6-5, notice that the pattern of light absorption of one of the carotenoids differs from the pattern of either type of chlorophyll. By absorbing colors that chlorophyll a cannot absorb, the accessory pigments enable plants to capture more of the energy in light.

In the leaves of a plant, the chlorophylls are generally much more abundant and therefore mask the colors of the other pigments. But in the nonphotosynthetic parts of a plant, such as fruits and flowers, the colors of the other pigments may be quite visible. During the fall, many plants lose their chlorophylls, and their leaves take on the rich hues of the carotenoids.

figure 6-4

White light contains a variety of colors called the visible spectrum. Each color has a different wavelength, measured in nanometers.

figure 6-4

White light contains a variety of colors called the visible spectrum. Each color has a different wavelength, measured in nanometers.

figure 6-5

The three curves on this graph show how three pigments involved in photosynthesis differ in the colors of light they absorb. Where a curve has a peak, much of the light at that wavelength is absorbed. Where a curve has a trough, much of the light at that wavelength is reflected or transmitted.

Absorption Spectra of Photosynthetic Pigments

Chlorophyll b

Carotenoid

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