5.4.1 Conversion of light energy to chemical energy
The core of photosynthesis is conversion of light energy to chemical energy or energy transduction. Conversion occurs when the energy in a photon of light is transferred to an electron in a light-absorbing molecule (e.g., chlorophyll). Electrons excited in this first energy-transduction step (photosystem II) are then passed through multiple carriers that undergo reversible oxidation-reduction reactions. Multiple reactions allow for a stepwise release of energy from the electrons, and as in electron transport, the released energy is coupled to synthesis of ATP. The electrons are then reexcited by absorption of light energy in a second energy-transduction step (photosystem I). The reexcited electrons can be transferred to the coenzyme nicotinamide adenine dinucleotide phosphate (NADP+) or used for other purposes. Eventually, much of the energy stored as ATP and high-energy electrons stored in NADPH will be used to convert carbon dioxide to carbohydrates (i.e., sugars) and various other organic compounds.
The general reaction for photosynthesis, where water is the electron donor, can be written as in Eq. (5.7). Extraction of electrons from water yields molecular oxygen. Therefore, the oxygen that is critical for aerobic respiration is simply a photosynthetic by-product. The low-energy electrons obtained from water are excited by energy from light and stored as high-energy electrons in NADPH. The stored electrons are then used to reduce carbon dioxide, producing a unit of carbohydrate (CH2O). For a couple of billion years, hydrogen sulfide (H2S) was the primary source of electrons. Today, H2S serves as an electron source for only some bacteria.
Carbohydrate synthesis is an endergonic process that requires a substantial input of energy. The simplified reaction above shows that synthesis of glucose (i.e., the reverse of aerobic respiration) requires 686 kcal/mol. The necessary energy (ATP and NADPH) is generated by energy trans-duction and energy transfer in the light-dependent reactions of photosynthesis. Carbohydrates are then synthesized in the light-independent or "dark" reactions of photosynthesis. Carbohydrate synthesis is not truly light independent, because stored energy (ATP and NADPH) is exhausted quickly in the absence of light.
The primary photosynthetic organelles in plants and algae are chloroplasts. Like the mitochondrion, the chloroplast is believed to have evolved from a bacterium. Consistent with a bacterial lineage, the chloroplast has its own DNA and RNA, and the organelle synthesizes some of its own proteins. Like some bacteria, the chloroplast has an outer and inner membrane (Fig. 5.12). The outer membrane is relatively permeable,
Figure 5.12 Chloroplast. The plant organelle consists of two membranes (inner and outer) that surround an inner stroma. The chloroplast also contains a third membrane system consisting of flattened membranous disks called thylakoids that are arranged in interconnected stacks called grana (one stack is a granum).
because it contains large membrane channels called porins. The inner membrane is much less permeable; therefore, crossing this membrane generally requires specific transporters. Chloroplasts have a third membrane system consisting of membranous disks called thylakoids, which are arranged in stacks called grana. The space enclosed by the thy-lakoid membrane is the lumen. For the sake of discussion, the lumen will be treated as an isolated compartment but in truth, the thylakoids are interconnected. The thylakoid membrane serves as a scaffold for the components (i.e., pigments, photosystem I, photosystem II, and ATP synthase) required for the light-dependent reactions of photosynthesis. Surrounding the thylakoids is the stroma, which contains the components required for carbohydrate synthesis.
Chlorophyll is the primary pigment responsible for transducing light energy into chemical energy. The chlorophyll molecule is composed of a porphyrin ring and a hydrophobic phytol tail that can embed in the lipid bilayer of the thylakoid membrane (Fig. 5.13). Recent findings suggest that the properties of the phytol tail make it suitable for use as an adjuvant for vaccines (see Clinical Box 5.2). The alternating single and double bonds in the porphyrin ring provide a cloud of delocalized electrons with a magnesium ion serving as an electron holder/donor. This bond system provides a variety of electron orbitals that can absorb different amounts of energy. Thus, electrons in chlorophyll can effectively absorb energy from photons of a broad absorption spectrum (i.e., a broad range of wavelengths of light). The absorption spectrum of chlorophyll is further broadened by interaction of the molecule with different binding proteins and by the presence of "a" and "b" forms of the molecule. Accessory pigments such as carotinoids and phycobilins absorb energy at wavelengths that are not effectively utilized by chlorophyll. These accessory pigments expand photosynthesis' action spectrum, which indicates the relative efficiency of photosynthesis at different wavelengths of light. The action spectrum for photosynthesis in green plants (primarily chlorophylls a and b) is maximal in the blue-violet region (wavelengths ~400 to 600 nm) and high in the orange-red region (wavelengths ~600 to 700 nm) (Fig. 5.14).
Chlorophyll and other pigments are associated with large pigmentprotein complexes called photosystems or with light-harvesting complexes (LHCs). Most of the pigments simply serve as antenna pigments or light-gathering molecules that transfer photon energy to nearby pigments.
Eventually, the photon energy is passed to a reaction center chlorophyll in a photosystem. Reaction centers receive energy collected by approximately 250 antenna pigments. Thus, the antenna pigments (like a satellite dish) increase the efficiency of energy reception by the reaction center
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