Photosynthetic Stages and Light-Absorbing Pigments - Molecular Cell Biology - NCBI Bookshelf
Figure 16-34
The structure of a leaf and chloroplast. The chloroplast is bounded by a double membrane: the outer membrane contains proteins that render it permeable to small molecules (MW < 6000); the inner membrane forms the permeability barrier (more...)
Unlike mitochondria , chloroplasts contain a third membrane — the thylakoid membrane — that is the site of photosynthesis . In each chloroplast , the thylakoid membrane is believed to constitute a single, interconnected sheet that forms numerous small flattened vesicles, the thylakoids , which commonly are arranged in stacks termed grana (see Figure 16-34 ). The spaces within all the thylakoids constitute a single continuous compartment, the thylakoid lumen. The thylakoid membrane contains a number of integral membrane proteins to which are bound several important prosthetic groups and light-absorbing pigments, most notably chlorophyll. Carbohydrate synthesis occurs in the stroma, the soluble phase between the thylakoid membrane and the inner membrane. In photosynthetic bacteria extensive invaginations of the plasma membrane form a set of internal membranes, also termed thylakoid membranes, or simply thylakoids, where photosynthesis occurs.
Three of the Four Stages in Photosynthesis Occur Only during Illumination
It is convenient to divide the photosynthetic process in plants into four stages, each occurring in a defined area of the chloroplast : (1) absorption of light, (2) electron transport leading to the reduction of NADP+ to NADPH, (3) generation of ATP, and (4) conversion of CO2 into carbohydrates ( carbon fixation ). All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1 – 3 are catalyzed by proteins in the thylakoid membrane . The enzymes that incorporate CO2 into chemical intermediates and then convert it to starch are soluble constituents of the chloroplast stroma (see Figure 16-34 ). The enzymes that form sucrose from three-carbon intermediates are in the cytosol .
Absorption of Light
The initial step in photosynthesis is the absorption of light by chlorophylls attached to proteins in the thylakoid membranes. Like cytochromes , chlorophylls consist of a porphyrin ring attached to a long hydrocarbon side chain ( Figure 16-35 ). They differ from cytochromes (and heme) in containing a central Mg2+ ion (rather than Fe atom) and having an additional five-membered ring. The energy of the absorbed light is used to remove electrons from an unwilling donor (water, in green plants), forming oxygen,
and then to transfer the electrons to a primary electron acceptor, a quinone designated Q, which is similar to CoQ.
Figure 16-36
The structure of β-carotene, a pigment that assists in light absorption by chloroplasts. β-Carotene, which is related to the visual pigment retinal (see Figure 21-47), is one of a family of carotenoids containing long hydrocarbon chains (more...)
One of the strongest pieces of evidence for the involvement of chlorophylls and β-carotene in photosynthesis is that the absorption spectrum of these pigments is similar to the action spectrum of photosynthesis ( Figure 16-37 ). The latter is a measure of the relative ability of light of different wavelengths to support photosynthesis.
Figure 16-37
Photosynthesis at different wavelengths. (a) The action spectrum of photosynthesis in plants; that is, the ability of light of different wavelengths to support photosynthesis. (b) The absorption spectra for three photosynthetic pigments: chlorophyll (more...)
When chlorophyll a (or any other molecule) absorbs visible light, the absorbed light energy raises the chlorophyll a to a higher energy state, termed an excited state. This differs from the ground (unexcited) state largely in the distribution of electrons around the C and N atoms of the porphyrin ring (see Figure 16-35 ). Excited states are unstable, and will return to the ground state by one of several competing processes. For chlorophyll a molecules dissolved in organic solvents, such as ethanol, the principal reactions that dissipate the excited-state energy are the emission of light (fluorescence and phosphorescence) and thermal emission (heat). The situation is quite different when the same chlorophyll a is bound to the unique protein environment of the reaction center.
Light Absorption by Reaction-Center Chlorophylls Causes a Charge Separation across the Thylakoid Membrane
The absorption of a quantum of light of wavelength ≈680 nm causes a chlorophyll a molecule to enter the first excited state. The energy of such photons increases the energy of chlorophyll a by 42 kcal/mol. In the reaction center, this excited-state energy is used to promote a charge separation across the thylakoid membrane : an electron is transported from a chlorophyll molecule to the primary electron acceptor, the quinone Q, on the stromal surface of the membrane, leaving a positive charge on the chlorophyll close to the luminal surface ( Figure 16-38 ). The reduced primary electron acceptor becomes a powerful reducing agent, with a strong tendency to transfer the electron to another molecule. The positively charged chlorophyll, a strong oxidizing agent, will attract an electron from an electron donor on the luminal surface. These potent biological reductants and oxidants provide all the energy needed to drive all subsequent reactions of photosynthesis : electron transport , ATP synthesis, and CO2 fixation.
Figure 16-38
The primary event in photosynthesis. After a photon of light of wavelength ≈680 nm is absorbed by one of the many chlorophyll molecules in one of the light-harvesting complexes (LHCs) of an antenna (only one is shown), some of the absorbed energy (more...)
The significant features of the primary reactions of photosynthesis are summarized in the following model, in which P represents the chlorophyll a in the reaction center, and Q represents the primary electron acceptor:
According to this model, the ground state of the reaction-center chlorophyll, P, is not a strong enough reductant to reduce Q; that is, an electron will not move spontaneously from P to Q. However, the excited state of the reactioncenter chlorophyll, P*, is an excellent reductant and rapidly (in about 10−10 seconds) donates an electron to Q, generating P+ and Q−. This photochemical electron movement, which depends on the unique environment of both the chlorophylls and the acceptor within the reaction center, occurs nearly every time a photon is absorbed. The acceptor, Q−, is a powerful reducing agent capable of transferring the electron to still other molecules, ultimately to NADP+. The powerful oxidant P+ can remove electrons from other molecules to regenerate the original P. In plants, the oxidizing power of four molecules of P+ is used, by way of intermediates, to remove four electrons from H2O to form O2:
Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 16-37b ). Such absorption raises the molecule into one of several higher excited states, which decay within 10−12 seconds (1 picosecond, ps) to the first excited state P*, with loss of the extra energy as heat. Photochemical charge separation occurs only from the first excited state of the reaction-center chlorophyll a, P*. This means that the quantum yield — the amount of photosynthesis per absorbed photon — is the same for all wavelengths of visible light shorter than 680 nm.
The chlorophyll a molecules within reaction centers are capable of directly absorbing light and initiating photosynthesis . However, even at the maximum light intensity encountered by photosynthetic organisms (tropical noontime sun, ≈1.2 × 1020 photons/m2/s), each reaction-center chlorophyll a absorbs about one photon per second, which is not enough to support photosynthesis sufficient for the needs of the plant. To increase the efficiency of photosynthesis, especially at more typical light intensities, organisms utilize additional light-absorbing pigments.
Light-Harvesting Complexes Increase the Efficiency of Photosynthesis
As noted earlier, each reaction center is associated with an antenna, which contains several light-harvesting complexes (LHCs), packed with chlorophyll a and, depending on the species, chlorophyll b and other pigments. LHCs promote photosynthesis by increasing absorption of 680-nm light and by extending the range of wavelengths of light that can be absorbed (see Figure 16-37 ).
Photons can be absorbed by any of the pigment molecules in each LHC. The absorbed energy is then rapidly transferred (in <10−9 seconds) to one of the two chlorophyll a molecules in the associated reaction center, where it promotes the primary photosynthetic charge separation (see Figure 16-38 ). Within an LHC are several transmembrane proteins whose role is to maintain the pigment molecules in the precise orientation and position that are optimal for light absorption and energy transfer, thereby maximizing the very rapid and efficient process known as resonance transfer of energy from antenna pigments to reaction-center chlorophylls . As depicted in Figure 16-39a , some photosynthetic bacteria contain two types of LHCs: the larger type (LH1) is intimately associated with a reaction center; the smaller type (LH2) can transfer absorbed light energy to an LH1. Figure 16-39b shows the structure of the subunits that make up the LH2 complex in Rhodopseudomonas acidophila. Surprisingly, the molecular structures of plant light-harvesting complexes are completely different from those in bacteria, even though both types contain carotenoids and chlorophylls in a clustered geometric arrangement within the membrane .