Figure 14–42 How chloroplasts and mitochondria collaborate to supply cells with both metabolites and ATP. (a)the inner chloroplast membrane is impermeable to the atp and NaDph that are produced in the stroma during the light reactions of photosynthesis. these molecules are therefore funneled into the carbon-fixation cycle, where they are used to make sugars. the resulting sugars and their metabolites are either stored within the chloroplast—in the form of starch or fat—or exported to the rest of the plant cell. there, they can enter the energy-generating pathway that ends in atp synthesis linked to oxidative phosphorylation inside the mitochondrion. Unlike the chloroplast, mitochondrial membranes contain a specific transporter that makes them permeable to atp (see Figure 14–34). Note that the O2 released to the atmosphere by photosynthesis in chloroplasts is used for oxidative phosphorylation in mitochondria; similarly, the CO2 released by the citric acid cycle in mitochondria is used for carbon fixation in chloroplasts. (B) In a leaf, mitochondria (red) tend to cluster close to the chloroplasts (green), as seen in this light micrograph. (B, courtesy of Olivier Grandjean.) b6-f complex, which both functionally and structurally resembles cytochrome CH2 c reductase in the respiratory chain; and the chloroplast ATP synthase, which closely resembles the mitochondrial ATP synthase and works in the same way.
The photosystems in the thylakoid membrane are multiprotein assemblies of a complexity comparable to that of the protein complexes in the mitochondrial electron-transport chain. They contain large numbers of specifically bound chlorophyll molecules, in addition to cofactors that will be familiar from our discus-CH2 sion of mitochondria (heme, iron–sulfur clusters, and quinones). Chlorophyll, the green pigment of photosynthetic organisms, has a long hydrophobic tail that makes it behave like a lipid, plus a porphyrin ring that has a central Mg atom and an extensive system of delocalized electrons in conjugated double bonds (Figure 14–43). When a chlorophyll molecule absorbs a quantum of sunlight (a photon), the energy of the photon causes one of these electrons to move from a low-energy molecular orbital to another orbital of higher energy.
The excited electron in a chlorophyll molecule tends to return quickly to its ground state, which can occur in one of three ways: 1. By converting the extra energy into heat (molecular motion) or to some combination of heat and light of a longer wavelength (fluorescence); this is what usually happens when light is absorbed by an isolated chlorophyll molecule in solution.
2.
By transferring the energy—but not the electron—directly to a neighboring chlorophyll molecule by a process called resonance energy transfer.
3.
By transferring the excited electron with its negative charge to another nearby molecule, an electron acceptor, after which the positively charged chlorophyll returns to its original state by taking up an electron from some other molecule, an electron donor.
Figure 14–43 The structure of chlorophyll. a magnesium atom is held in a porphyrin ring, which is related to the porphyrin ring that binds iron in heme (see Figure 14–15). Electrons are delocalized over the bonds shaded in blue.
The latter two mechanisms occur when chlorophylls are attached to proteins in a chlorophyll–protein complex. The protein coordinates the central Mg atom in the chlorophyll porphyrin, most often through a histidine side chain located in the hydrophobic interior of a membrane, causing each of the chlorophylls in a protein complex to be held at exactly defined distances and orientations. The flow of excitation energy or electrons then depends on both the precise spatial arrangement and the local protein environment of the protein-bound chlorophylls.