Reduced form (colorless)


SUMMARY 19.6 General Features of Photophosphorylation

■ The light reactions of photosynthesis are those directly dependent on the absorption of light; the resulting photochemistry takes electrons from H2O and drives them through a series of membrane-bound carriers, producing NADPH and ATP.

■ The carbon-assimilation reactions of photosynthesis reduce CO2 with electrons from NADPH and energy from ATP.

19.7 Light Absorption

Visible light is electromagnetic radiation of wavelengths 400 to 700 nm, a small part of the electromagnetic spectrum (Fig. 19-39), ranging from violet to red. The energy of a single photon (a quantum of light) is greater at the violet end of the spectrum than at the red end; shorter wavelength (and higher frequency) corresponds to higher energy. The energy, E, in a "mole" of photons (1 einstein, or 6 X 1023 photons) of visible light is 170 to 300 kJ, as given by the Planck equation:

E = hv where h is Planck's constant (6.626 X 10~34 J • s) and v is the wavelength. These amounts of energy are almost an order of magnitude greater than the 30 to 50 kJ required to synthesize a mole of ATP from ADP and Pj.

When a photon is absorbed, an electron in the absorbing molecule (chromophore) is lifted to a higher energy level. This is an all-or-nothing event; to be absorbed, the photon must contain a quantity of energy (a quantum) that exactly matches the energy of the electronic transition. A molecule that has absorbed a photon is in an excited state, which is generally unstable. An electron lifted into a higher-energy orbital usually returns rapidly to its normal lower-energy orbital; the excited molecule decays to the stable ground state, giving up the absorbed quantum as light or heat or using it to do chemical work. Light emission accompanying decay of excited molecules (called fluorescence) is always at a longer wavelength (lower energy) than that of the absorbed light (see Box 12-2). An alternative mode of decay important in photosynthesis involves direct transfer of excitation energy from an excited molecule to a neighboring molecule. Just as the photon is a quantum of light energy, so the exciton is a quantum of energy passed from an excited molecule to another molecule in a process called exciton transfer.

Chlorophylls Absorb Light Energy for Photosynthesis

The most important light-absorbing pigments in the thy-lakoid membranes are the chlorophylls, green pigments with polycyclic, planar structures resembling the protoporphyrin of hemoglobin (see Fig. 5-1), except that Mg2+, not Fe2+, occupies the central position (Fig. 19-40). The four inward-oriented nitrogen atoms of chlorophyll are coordinated with the Mg2+. All chlorophylls have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV, and chlorophylls also have a fifth five-membered ring not present in heme.

The heterocyclic five-ring system that surrounds the Mg2+ has an extended polyene structure, with alternating single and double bonds. Such polyenes characteristically show strong absorption in the visible region of the spectrum (Fig. 19-41); the chlorophylls have unusually high molar extinction coefficients (see Box 3-1) and are therefore particularly well-suited for absorbing visible light during photosynthesis.

Chloroplasts always contain both chlorophyll a and chlorophyll b (Fig. 19-40a). Although both are green, their absorption spectra are sufficiently different (Fig. 19-41) that they complement each other's range of light absorption in the visible region. Most plants contain about twice as much chlorophyll a as chlorophyll b. The pigments in algae and photosynthetic bacteria include chlorophylls that differ only slightly from the plant pigments.

Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in which chlorophyll molecules are fixed in relation to each other, to other protein complexes, and to the membrane. The detailed structure of one light-harvesting complex is known from x-ray crystallography (Fig. 19-42). It contains seven molecules of chlorophyll a, five of chlorophyll b, and two of the accessory pigment lutein (see below).

FIGURE 19-39 Electromagnetic radiation. The spectrum of electromagnetic radiation, and the energy of photons in the visible range of the spectrum. One einstein is 6 X 1 023 photons.
Chlorophyll a







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