Light behaves as both a particle and a wave

Light is a form of electromagnetic radiation. It comes in discrete packets called photons. Light also behaves as if it were propagated in waves. The amount of energy contained in a single photon is inversely proportional to its wavelength: the shorter the wavelength, the greater the energy of the photons. For example, a photon of red light of wavelength 660 nm has less energy than a photon of blue light at 430 nm. Two things are required for photons to be active in a biological process:

Photons must be absorbed by a receptive molecule.

Photons must have sufficient energy to perform the chemical work required.

Absorbing a photon puts a pigment in an excited state

When a photon meets a molecule, one of three things happens:

The photon may bounce off the molecule—it may be scattered.

The photon may pass through the molecule—it may be transmitted.

The photon may be absorbed by the molecule.

Neither of the first two outcomes causes any change in the molecule. In the third case, the photon disappears. Its energy, however, cannot disappear, because energy is neither created nor destroyed.

When a molecule absorbs a photon, that molecule acquires the energy of the photon. It is thereby raised from a ground state(lower energy) to an excited state(higher energy). The difference in energy between the molecule’s excited state and its ground state is exactly equal to the energy of the absorbed photon. The increase in energy boosts one of the electrons within the molecule into an orbital farther from its nucleus; this electron is now held less firmly, making the molecule more chemically reactive, as we will see later in the chapter. The electromagnetic spectrum shows the wide range of wavelengths (and hence, energy levels) that photons can have. The specific wavelengths absorbed by a particular molecule are characteristic of that type of molecule. Molecules that absorb wavelengths in the visible spectrum — that region of the spectrum that is visible to humans—are called pigments. When a beam of white light (light containing visible light of all wavelengths) falls on a pigment, certain wavelengths of the light are absorbed. The remaining wavelengths, which are scattered or transmitted, make the pigment appear to us to be colored. For example, if a pigment absorbs both blue and red light — as chlorophyll does —what we see is the remaining light which is primarily green.

Absorbed wavelengths correlate with biological activity

If we plot the wavelengths of the light absorbed by a purified molecule, the result is an absorption spectrumfor that molecule. If we plot the biological activity of a photosynthetic organism as a function of the wavelengths of light to which the organism is exposed, the result is an action spectrum. Figure shows the absorption spectrum for a pigment, chlorophyll a, isolated from the leaves of a plant and the action spectrum for photosynthetic activity for the same plant. Acomparison of the two spectra shows that the wavelengths at which photosynthesis is maximal are the same wavelengths at which chlorophyll a absorbs light.

Photosynthesis uses energy absorbed by several pigments

The light energy used for photosynthesis is not absorbed by just a single type of pigment. Instead, several different pigments with different absorption spectra absorb the energy that is eventually used for photosynthesis. In photosynthetic organisms of all kinds (plants, protists, and bacteria), these pigments include chlorophylls, carotenoids, and phycobilins. In plants, two chlorophyllspredominate: chlorophyll a and chlorophyll b. These two molecules differ only slightly in their molecular structure. Both have a complex ring structure similar to that of the heme group of hemoglobin. In the center of each chlorophyll ring is a magnesium atom, and attached at a peripheral location on the ring is a long hydrocarbon “tail,” which can adhere the chlorophyll molecule to proteins in the hydrophobic portion of the thylakoid membrane. The chlorophylls absorb blue and red wavelengths, which are near the two ends of the visible spectrum. Thus, if only chlorophyll pigments were active in photosynthesis, much of the visible spectrum would go unused. However, all photosynthetic organisms possess accessory pigments, which absorb photons intermediate in energy between the red and the blue wavelengths (for instance, yellow light) and then transfer a portion of that energy to the chlorophylls. Among these accessory pigments are carotenoids, such as β-carotene, which absorb photons in the blue and blue-green wavelengths and appear deep yellow. The phycobilins, which are found in red algae and in cyanobacteria, absorb various yellow-green, yellow, and orange wavelengths. Such accessory pigments, in collaboration with the chlorophylls, constitute an energy-absorbing system covering much of the visible spectrum.

Light absorption results in photochemical change

After a pigment molecule absorbs a photon and enters an excited state, that molecule may return to the ground state. When this happens, some of the absorbed en- ergy is given off as heat and the rest is given off as light energy or fluorescence. Because some of the absorbed light energy is lost as heat, the fluorescence has less energy and longer wavelengths than the absorbed light. When there is fluorescence, there are no permanent chemical changes or biological functions—no chemical work is done. Any pigment molecule can become excited when its absorption spectrum matches the energies of incoming photons. If fluorescence does not occur, that pigment molecule may pass the absorbed energy along to another molecule, provided that the target molecule is very near, has the right orientation, and has the appropriate structure to receive the energy. The pigments in photosynthetic organisms are arranged into energy-absorbing antenna systems. In these systems, the pigments are packed together and attached to thylakoid membrane proteins in such a way that the excitation energy from an absorbed photon can be passed along from one pigment molecule in the system to another. Excitation energy moves from pigments that absorb shorter wavelengths (higher energy) to pigments that absorb longer wavelengths (lower energy). Thus the excitation ends up in the one pigment molecule in the antenna system that absorbs the longest wavelengths; this molecule is in the reaction centerof the antenna system. It is the reaction center that converts the light absorbed into chemical energy. It is in the reaction center that a molecule absorbs sufficient energy that it actually gives up its excited electron (is chemically oxidized) and becomes positively charged. In plants, the pigment molecule in the reaction center is always a molecule of chlorophyll a. There are many other chlorophyll a molecules in the antenna system, but all of them absorb light at shorter wavelengths than does the molecule in the reaction center.

Excited chlorophyll in the reaction center acts as a reducing agent for electron transport

Ultimately, photosynthesis stores chemical energy by using the excited chlorophyll molecule in the reaction center as a reducing agent to reduce a stable electron acceptor. Ground-state chlorophyll (symbolized Chl) is not much of a reducing agent, but excited chlorophyll (Chl*) is a good one. To understand the reducing capability of Chl*, recall that in an excited molecule, one of the electrons is zipping around in an orbital farther away from its nucleus. Less tightly held, this electron can be passed on in a redox reaction to an oxidizing agent. Thus Chl* (but not Chl) can react with an oxidizing agent Ain a reaction like this:

Chl* + A®Chl+ + A–

This, then, is the first consequence of light absorption by chlorophyll. The chlorophyll becomes a reducing agent and participates in a redox reaction. As we are about to see, the further adventures of the electrons from chlorophyll reduce the electron carrier NADP+ and generate a proton-motive force that is eventually used to synthesize ATP.