FEATURE ARTICLE
How Plants Produce Dioxygen
At its core, oxygen production comes down to the chemistry of a poorly understood manganese-containing complex in the membranes of plant chloroplasts
Veronika Szalai, Gary Brudvig
Light Harvesting
In order to convert solar energy into chemical energy, photosynthetic organisms must be able to harness light energy. Typically, plants "harvest" light by using molecules that absorb light energy from different regions of the solar spectrum. Because these light-harvesting compounds absorb only some energies of visible light and reflect others, they are brightly colored and are referred to as chromophores or pigment molecules.
The primary chromophore in plants is chlorophyll. It is also what makes plants green (Figure 6, top). Chlorophylls are found inside the membranes of organelles called chloroplasts. Probably descendants of cyanobacteria, plant chloroplasts contain all of the proteins and pigments required for the synthesis of carbohydrates. They vary in size, but are typically ellipsoids about 5 micrometers long. In addition to chlorophyll, the chloroplasts contain all of the proteins and auxiliary molecules of the chemical reactions of photosynthesis.
There are several different types of chlorophyll, all of which possess two general structural features: a nearly planar core that contains a magnesium ion in the center and a long tail to anchor the chlorophyll molecule in its host environment. Depending on other structural elements, plant chlorophylls are designated "a" or "b." Chlorophylls in bacteria (or bacteriochlorophylls) differ from those found in plants, although they also exist in a and b forms. Other chromophores involved in photosynthesis include the linear carotenoids, a familiar example of which is β-carotene, the pigment that makes carrots orange.
The pigment molecules in photosynthetic organisms vary, but all species exploit the ability of the various chromophores to absorb sunlight of particular energies and wavelengths. Therefore, when a plant uses carotenoids in combination with chlorophylls a and b, almost the entire spectrum of light coming from the sun can be absorbed for use in photosynthesis (Figure 6, bottom).
Even though chlorophyll absorbs light in two different regions of the solar spectrum (wavelengths of 400–500 nanometers and 600–700 nanometers), all of the absorbed light is equally useful in photosynthesis because of the unique electronic properties of chlorophyll. Another advantage of using chlorophylls to harvest light is that chlorophyll molecules can transfer energy from one to another with very little energy loss. Therefore, when a single chlorophyll molecule absorbs light energy, it can transfer that energy to a neighboring chlorophyll molecule. The second chlorophyll can then pass the absorbed energy on to a third chlorophyll molecule. In this way, light energy can be relayed to a particular location in the photosynthetic organism (Figure 7).
Once harvested, the light energy is ultimately relayed to the reaction center, where the chemical reactions of photosynthesis take place. Photosynthetic reaction centers are multiple-protein assemblies that bind other small molecules, called cofactors, required for photosynthesis. They are further characterized by their location in membranous regions of the chloroplasts.
Two types of membranes make up a chloroplast: an outer encapsulating membrane and an inner membrane called the thylakoid membrane (Figure 4). The thylakoid membrane is a long, continuous membrane that folds on itself to produce two types of secondary structures called grana and stroma lamellae. The grana are regions of thylakoid membrane that are folded into stacks, and the stroma lamellae are unfolded lengths of thylakoid membrane that connect the grana.
Chlorophyll and all of the protein complexes required for the light reaction of plant photosynthesis are found in the thylakoid membranes. This includes the photosynthetic reaction centers, Photosystem I (PSI) and Photosystem II (PSII), in addition to a protein complex that synthesizes ATP (a molecule that stores chemical energy) and the cytochrome b6/f complex, which is chiefly involved in shuttling protons and electrons from one side of the thylakoid membrane to the other (Figure 5). Because PSI and PSII are intimately involved in the light reaction in plants, they are major targets of photosynthesis research. PSI uses the electrons received from PSII to generate the strong reductant, NADPH, which is important in fixing carbon dioxide.
PSII is of particular interest because it is the reaction center directly responsible for the oxidation of H2O to O2. Experiments involving PSI and PSII are simplified when discrete PSI and PSII units can be selectively isolated. This is accomplished by taking advantage of the fact that stroma lamellae contain mostly PSI, whereas the grana contain predominantly PSII.
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