light light

Figure 10.5 A simplified summary of photosynthetic reactions.

dioxide from the air and RuBP are combined and then converted during the light-independent reactions (Fig. 10.5). Some grasses and also many plants of arid regions fix carbon differently. They produce a 4-carbon acid as the first product, followed by the Calvin cycle. This 4-carbon pathway is discussed, along with another variation found mostly in desert plants, in the next section, "A Closer Look at Photosynthesis."

3. A Closer Look at Photosynthesis

A great deal has been learned about photosynthesis since 1772 when Joseph Priestly (1733-1804), a naturalist in England, received a medal for demonstrating that a sprig of mint "restored" oxygen so that a mouse could live in air that had been used up by a burning candle. Seven years later, Jan Ingen-Housz (1730-1799) of Holland, who visited England to treat the royal family for smallpox, carefully repeated Priestly's demonstrations. He showed that the air was restored only when green parts of plants were receiving sunlight.

In 1782, Jean Senebier (1742-1809), a Swiss pastor, discovered that the photosynthetic process required carbon dioxide, and in 1796, Ingen-Housz showed that carbon went into the nutrition of the plant. The final component of the overall photosynthetic reaction was explained in 1804 by another Swiss, Nicholas Theodore de Saussure (1767-1845), who showed that water was involved in the process.

A little current information about the details of photosynthesis is given in the following modest amplification of the preceding section, "Introduction to the Major Steps of Photosynthesis." Those who wish more information are referred to the reading list at the end of the chapter.

The Light-Dependent Reactions Reexamined

As noted earlier, light has characteristics of both waves and particles. Sir Isaac Newton, while experimenting with a prism over 300 years ago, produced a spectrum of colors from visible white light and postulated that light consisted of a series of discrete particles he called "corpuscles."

Newton's theory only partially explained light phenomena, however, and by the middle of the 19th century, James Maxwell and others showed that light and all other parts of the extensive electromagnetic spectrum travel in waves.

By the late 1800s, with the discovery that a photoelectric effect can be produced in all metals, the wave theory also became inadequate to explain certain attributes of light. When a metal is exposed to radiation of a critical wavelength, it becomes positively charged because the radiation forces electrons out of the metal atoms. The ability of light to force electrons from metal atoms depends on its particular wavelength—its energy content—and not on its intensity or brightness. The shorter the wavelength, the greater the energy, and vice versa.

In 1905, Albert Einstein proposed that the photoelectric effect results from discrete particles of light energy he called photons. In 1921, he received the Nobel Prize in physics for this work. Both waves and particles (photons) are today almost universally recognized as aspects of light. The energy (quantum) of a photon is not the same for all kinds of light; those of longer wavelengths have lower energy, and those of shorter wavelengths have proportionately higher energy.

Chlorophylls, the principal pigments of photosynthetic systems, absorb light primarily in the violet to blue and also in the red wavelengths; they reflect green wavelengths,

Chapter 10

Engelmann Experiment Bacteria
Figure 10.6 Engelmann's experiment. A tiny spectrum of light was focused on a microscope slide with a row of algal cells suspended in water containing bacteria that move toward an oxygen source. The bacteria assembled mostly in the areas exposed to the red and blue portions of the spectrum.

which is why leaves appear green. This was first ingeniously demonstrated in 1882 by T. W. Engelmann. He focused a tiny spectrum of light on a filament (single row of cells) of Spirogyra, a freshwater alga. The alga had been mounted in a drop of water on a microscope slide containing bacteria that move toward an oxygen source. As shown in Fig. 10.6, the bacteria assembled in greatest numbers along the algal filament in the blue and red portions of the spectrum, demonstrating that oxygen production is directly related to the light the chlorophyll absorbs. An analysis using living material is called a bioassay. Information gained from bioas-says often is significant.

Each pigment has its own distinctive pattern of light absorption, which is referred to as the pigment's absorption spectrum (Fig. 10.7). When a pigment absorbs light, the energy levels of some of the pigment's electrons are raised. When this occurs, the energy may be emitted immediately as light (a phenomenon called fluorescence). In chlorophyll, this is characteristically in the red part of the visible light spectrum, so an extract of chlorophyll placed in light (especially ultraviolet or blue light) will appear red. The absorbed energy may also be emitted as light after a delay (a phenomenon called phosphorescence); it may otherwise be converted to heat. The most important use of the absorbed energy, however, is its storage in chemical bonds for photosynthesis.

Photosystems The two types of photosynthetic units present in most chloroplasts make up photosystems known as photosystem I and photosystem II (Fig. 10.8). Photosystem

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