It is helpful here to discuss light in terms of its photochemistry and photobiology.

PHOTOCHEMISTRY Light is a form of electromagnetic radiation. Electromagnetic radiation is said to have a dual nature. Although it is propagated in waves, it also has particle-like behaviors. Particles of light can be described as packets of energy called photons, which have no mass. The amount of energy in electromagnetic radiation is inversely proportional to its wavelength—the shorter the wavelength, the greater the energy. The visible portion of the electromagnetic spectrum (Figure 10.3) encompasses a wide range of wavelengths and energy levels. In plants and other photosynthetic organisms, receptive molecules absorb photons and thereby harvest their energy for biological processes. These receptive molecules absorb only specific wavelengths of light—photons with specific amounts of energy.

When a photon meets a molecule, one of three things can happen:

Neither of the first two outcomes causes any change in the molecule. However, in the case of absorption, the photon disappears and its energy is absorbed by the molecule. The photon’s energy cannot disappear, because according to the first law of thermodynamics, energy is neither created nor destroyed. When the molecule acquires the energy of the photon, it is raised from a ground state (with lower energy) to an excited state (with higher energy):

The difference in free energy between the molecule’s excited state and its ground state is approximately equal to the free energy of the absorbed photon (a small amount of energy is lost to entropy, according to the second law of thermodynamics). The increase in energy boosts one of the electrons in the molecule into a *shell farther from its nucleus; this electron is now held less firmly, making the molecule unstable and more chemically reactive.

PHOTOBIOLOGY The specific wavelengths absorbed by a particular molecule are characteristic of that type of molecule. Molecules that absorb wavelengths in the visible spectrum are called pigments.

When a beam of white light (containing all the wavelengths of visible light) falls on a pigment, certain wavelengths are absorbed. The remaining wavelengths are scattered or transmitted and make the pigment appear to us as colored. For example, the pigment chlorophyll absorbs both blue and red light, and we see the remaining light, which is primarily green. If we plot light absorbed by a purified pigment against wavelength, the result is an absorption spectrum for that pigment.

In contrast to the absorption spectrum, an action spectrum is a plot of the rate of photosynthesis carried out by an organism against the wavelengths of light to which it is exposed. An action spectrum for photosynthesis can be determined as follows:

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Figure 10.4 shows the absorption spectra of pigments in Anacharis, a common aquarium plant, and the action spectrum for photosynthesis by that plant. The two spectra can be compared to see which pigments in Anacharis contribute most to light harvesting for photosynthesis.

Question

In phycobilins, yellow light absorbs at a shorter wavelength (540 nm) that is more energetic than the longer wavelength (660 nm) at which chlorophyll absorbs. This means that the energy transfer from phycobilins to chlorophyll is thermodynamically favored (higher to lower energy).

The major pigment used to drive the light reactions of oxygenic photosynthesis is chlorophyll a. (In Figure 10.4 you can see that the wavelengths at which photosynthesis is highest in Anacharis are the same wavelengths at which chlorophyll a absorbs the most light.) Chlorophyll a has a complex ring structure (Figure 10.5), similar to that of the heme group of hemoglobin, with a magnesium ion at the center. A long hydrocarbon “tail” anchors the molecule to proteins within a large multi-protein complex called a photosystem, which spans the thylakoid membrane. Molecules of chlorophyll a and various accessory pigments (such as chlorophylls b and c, carotenoids, and phycobilins; see below) are arranged into light-harvesting complexes, also called antenna systems. Multiple light-harvesting complexes surround a single reaction center within the photosystem.

Light energy is captured by the light-harvesting complexes and transferred to the reaction center, where chlorophyll a molecules participate in redox reactions that convert the light energy to chemical energy.

Chlorophyll absorbs blue and red light, which are near the two ends of the visible spectrum (see Figure 10.3). The various accessory pigments absorb light in other parts of the spectrum and thus function to broaden the range of wavelengths that can be used for photosynthesis. You can see this in the action spectrum in Figure 10.4B: Anacharis is able to photosynthesize at wavelengths of light that chlorophyll a doesn’t absorb but that other pigments absorb (e.g., 500 nanometers [nm]). Different photosynthetic organisms have different combinations of accessory pigments. Higher plants and green algae have chlorophyll b (with a structure and absorption spectrum very similar to that of chlorophyll a) and carotenoids such as β-carotene, which absorb photons in the blue and blue-green wavelengths. Phycobilins, which are found in red algae and cyanobacteria, absorb various yellow-green, yellow, and orange wavelengths.