Photosynthesis

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Virtually all the organic carbon in our environment, from the carbon in a person's fingernails to the carbon in a plastic pen, was formed by plants from CO2 in the air. The energy for this conversion comes entirely from sunlight. Among the few known exceptions in nature are ecosystems found on the ocean floor and hot springs that obtain their energy from oxidation of reduced inorganic compounds issuing from deep below the ocean floor in hot-water vents. Most human-made sources of energy, such as fossil fuels and weather-driven electric plants (wind and hydroelectric), ultimately come from the sun. Only nuclear, geothermal, and tidal electrical facilities, plus a portion of wind energy driven by Earth's rotation, do not derive their energy from the sun.

Use of the sun's energy to synthesize carbohydrates is called photosynthesis. Only certain bacteria, algal protists, and green plants are capable of photosynthesis. Organisms that can synthesize their own carbohydrates from inorganic precursors are called autotrophs. Those that use sunlight to provide the energy for this are called photoautotrophs. Some bacteria can use inorganic energy sources, such as H2, H2S, NH3, or reduced metallic salts such as manganese or ferrous iron, to form carbohydrates from CO2. These are called chemoautotrophs or lithoautotrophs. All other organisms, including all animals and fungi, and most bacteria, depend ultimately on the autotrophs for organic carbon and energy. Organisms such as animals, fungi, and many bacteria that must obtain their organic carbon ultimately from autotrophs are called heterotrophs.

Photosynthesis is somewhat more complicated to describe than respiration. However, once we have understood respiration, there are enough similarities to respiration in reverse to describe it in those terms. Recall that in respiration, there is a separation between the oxidation of organic carbon to CO2 (glycolysis and the Krebs cycle), which produces reducing power as NADH2, and the electron transport system, which consumes the reducing power and reduces oxygen to water.

In photosynthesis, the CO2 gets reduced, producing glucose. The reducing power comes from photons of light. The overall net reaction of photosynthesis is

The two major parts of photosynthesis are the light reactions, which are analogous to electron transport in respiration, and the dark reactions, which can be compared to the reverse of the Krebs cycle and glycolysis. Several basic experimental facts support the division of photosynthesis into light and dark reactions: (1) Plants give off oxygen only in the light; and (2) if a suspension of algae is illuminated for some time in the absence of CO2, then placed in the dark with CO2, the CO2 is incorporated into carbohydrate for a brief time. Additional work with tracer elements further supports the idea.

In the light reactions, energy from light is used to oxidize H2O to O2 and produce ATP and/or NADPH2. As in respiration, electron transport is involved and even uses cytochromes. In fact, one type of organism, the purple nonsulfur bacteria, use the same electron transport chain for both respiration and photosynthesis. In algae and plants there are actually two electron transport systems just for photosynthesis, which act in concert. The light reactions require a membrane structure, the chloroplast (except in cyanobacter), which is similar to the mitochondrion. Furthermore, ATPs are formed as a result of the generation of a chemiosmotic potential by the electron transport system. In the dark reactions, the ATP and NADPH2 are used in a cyclic reaction to form sugars from CO2. Some of the reactions involved are the reverse of parts of glycolysis.

Chloroplasts are the cellular organelles in plants and algae where all of the light reactions and some of the dark reactions are located. Bacteria perform the function of chlor-oplasts using their cytoplasmic membrane, as they do for the respiratory electron transport system. Like mitochondria, they contain an internal membrane structure that divides the interior into stacks of flattened hollow disks, thylakoids (Figure 5.8). The inner compartment of the thylakoid is called the lumen, the outer compartment is called the stroma.

The membranes of the thylakoids are studded with three groups of proteins and other compounds (Figure 5.9). Two of these groups are photosystem I and photosystem II, which perform the principal task of capturing the light energy and transporting the electrons. Each photosystem consists of pigments, proteins, and electron transport compounds, such as cytochromes. The third group is actually a single complex, called the CF1 particle. Like the ATP synthase particle in the mitochondrion, The CF1 particle uses the energy stored in the chemiosmotic potential created by the electron transport systems (in this case of the photosystems) to generate ATP.

The photosystems consist of an association of membrane-bound particles, one of which is called the antenna. The antenna is a complex of numerous chlorophyll molecules, most bound to proteins. Chlorophyll is the green pigment of plants, which absorbs much of the light energy for photosynthesis. The antenna then transfers the energy to a "special pair''

Figure 5.8 Chloroplast structure. (From Fried, 1990. © The McGraw-Hill Companies, Inc. Used with permission.)

stroma outer membrane stroma outer membrane

Figure 5.8 Chloroplast structure. (From Fried, 1990. © The McGraw-Hill Companies, Inc. Used with permission.)

intrathylakoid space thylakoid intrathylakoid space thylakoid

Figure 5.9 Photosynthetic apparatus. (From Fried, 1990. © The McGraw-Hill Companies, Inc. Used with permission.)

of chlorophyll molecules, called the reaction center, which uses the energy to promote an electron to a higher energy level and transfer it to the electron transport system. The reactions centers of photosystem I and photosystem II are known as P700 and P680, respectively, for the optimum wavelengths at which they operate.

The chlorophyll molecule consists of a hydrocarbon tail connected to a ring system with a nonionic magnesium atom at its center (Figure 5.10). The ring system has the alternating single and double bonds that often characterize compounds that absorb visible light. There are two main types of chlorophyll: a and b, but chlorophyll a is most common in plants. Chlorophyll a absorbs most strongly in the blue (400 to 450 nm) and red (640 to 680 nm) regions of the spectrum, leaving green to please our eyes (Figure 5.11). However, photosystem antennas contain other pigments, such as carotenoids, which capture green and yellow light energy and transfer it to the chlorophyll. This is particularly important for plants that live in deep waters, where the longer wavelengths do not penetrate. Diatoms, the brown algae, and dinoflagellates (including the red tide organism) contain pigments that absorb strongly in the range 400 to 550 nm. The carotenoids become visible when the

Figure 5.11 Absorption spectrum of chlorophyll and of a green leaf, and the action spectrum for the rate of photosynthesis vs. wavelength. (From Fried, 1990. © The McGraw-Hill Companies, Inc. Used with permission.)

chlorophyll degrades in deciduous tree leaves in certain areas in the fall, creating the spectacular fall color displays.

Before examining photosynthesis in plants, it may be useful to consider bacterial photosynthesis first, because it is simpler and because its mechanism is retained in algae and plants. Bacteria have only one photosystem. Absorption of a photon by the photosystem starts the process of cyclic photophosphorylation, which results in the production of a single ATP. The photon excites an electron in the chlorophyll, which transfers to the electron transport chain. As the electron is passed down the chain, two protons are secreted outside the cell using energy from the electron. At the end of the chain, the low-energy electron returns to the chlorophyll, ready to start the cycle again. The proton secretion creates a chemiosmotic potential across the membrane, which produces ATP as it passes back into the cell through an enzyme complex. To form carbohydrates, the bacteria need ATP and NADPH2. Bacteria require an external chemical reducing agent to form NADPH2, such as H2, H2S, or organic matter. The ATP and NADPH2 then feed the dark reactions to produce carbohydrates as described below. No oxygen is produced as it is by plants. Indeed, oxygen is toxic to these bacteria. An exception is the prokaryotic nitrogen-fixing cyanobacter, which does produce oxygen.

Light Reactions Plants and algae retain the cyclic photophosphorylation pathway but have developed noncyclic photophosphorylation, which has the advantage of not requiring an external reducing agent to replace the electron given up by the chlorophyll (Figure 5.12). Instead, in photosystem II, water is hydrolyzed to hydrogen and oxygen; the hydrogens provide the needed electrons, and the remaining protons enter the thylakoid lumen to augment the chemiosmotic gradient (ultimately for ATP production). The oxygen formed by hydrolysis of the water is released. After passing through the electron transport chain of photosystem II, using their energy to pump more protons, the low-energy electrons can then replace electrons promoted by another photon absorbed by photosystem I. The reenergized electrons, now in photosystem I, pass through another electron transport chain and ultimately are used to reduce NADP to NADPH2. (Note

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