Leaves reflect green light, which is why plants look green. Plants also contain red and orange pigments such as carotene that absorb light in other regions of the spectrum. Most of the time, the intense green color of chlorophyll overwhelms the accessory pigments, so we don't notice them. As temperatures drop late in the year, however, chlorophyll molecules break down first, leaving the reds and oranges of the accessory pigments for all to see. The beautiful colors of fall in some parts of the country are the result of this process.

Chloroplasts Recall from Chapter 7 that in plants and other photosynthetic eukaryotes, photosynthesis takes place inside organelles called chloroplasts. Chloroplasts contain an abundance of saclike photosynthetic membranes called thylakoids (THY luh koydz). Thylakoids are interconnected and arranged in stacks known as grana (singular: granum). Pigments such as chlorophyll are located in the thylakoid membranes. The fluid portion of the chloroplast, outside of the thylakoids, is known as the stroma. The structure of a typical chloroplast is shown in Figure 8–5.

Energy Collection What's so special about chlorophyll that makes it important for photosynthesis? Because light is a form of energy, any compound that absorbs light absorbs energy. Chlorophyll absorbs visible light especially well. In addition, when chlorophyll absorbs light, a large fraction of that light energy is transferred directly to electrons in the chlorophyll molecule itself. By raising the energy levels of these electrons, light energy can produce a steady supply of high-energy electrons, which is what makes photosynthesis work.

In Your Notebook In your own words, explain why most plants will not grow well if kept under green light.

ZOOMING IN

THE CHLOROPLAST

FIGURE 8–5 In plants, photosynthesis takes place inside chloroplasts. Observe How are thylakoids arranged in the chloroplast?

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