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Photosynthesis: Energy from the Sun

 

Powered by sunlight, green plants convert CO2 and water into carbohydrates by a process called photosynthesis. The emergence of this metabolic pathway was a key event in the evolution of life. Photosynthesizing organisms, called autotrophs (“self-feeders”), use solar energy to make their own food from simple chemicals in the environment. In this way, they provide an entry point to the biosphere for chemical energy. Heterotrophs (“other-feeders”) cannot photosynthesize, and they depend on autotrophs (or other heterotrophs) for the raw materials of metabolism, such as glucose. The “food chain” from autotrophs to heterotrophs requires a lot of photosynthesis. On the African plain, it takes 6 acres of grassland to convert enough CO2 into plant matter to support the growth of one gazelle that consumes the grass. Globally, more than 10 billion tons of carbon is fixed—converted from being part of a simple gas (CO2) into a more complex molecule (carbohydrate)—by plants every year. This huge amount of photosynthetically-fixed carbon is available for use by all species that need it. Humans consume a huge amount of Earth’s photosynthetic output. Recent calculations of total plant growth in agriculture, pastures, and forests and the products consumed by people indicate that one-third of all the carbon fixed annually is appropriated by humans, leaving two-thirds for the entire remainder of the biosphere. This is by far the greatest proportion of consumption for any single species in known history. Is this situation sustainable? Conferences such as the 2002 United Nations Conference on Sustainability have demonstrated concern for our photosynthetic future. An important first step in examining ecological sustainability is a thorough understanding of photosynthesis. The process of photosynthesis can be neatly broken down into two steps. The first step is the conversion of energy from light to chemical bonds in reduced electron carriers and ATP. In the second step, these two sources of chemical energy are used to drive the synthesis of carbohydrates from carbon dioxide. In this chapter, we will examine these two processes and show how they are related to each other and to plant growth.

 

 

Identifying Photosynthetic Reactants and Products

By the beginning of the nineteenth century, scientists understood the broad outlines of photosynthesis. It was known to use three principal ingredients—water, carbon dioxide (CO2), and light—and to produce not only carbohydrates but also oxygen gas (O2). Scientists had learned several things:

The water for photosynthesis in land plants comes primarily from the soil and must travel from the roots to the leaves.

Carbon dioxide is taken in, and water and O2 are released, through tiny openings in leaves, called stomata (singular, stoma).

Light is absolutely necessary for the production of oxygen and carbohydrates.

Photosynthesis can be illustrated as follows schema: carbon dioxide + water + light energy ®sugar + oxygen



which turns into an equation that is the reverse of the overall equation for cellular respiration:

6 CO2 + 6 H2O ®C6H12O6 + 6 O2

What role does light play in these reactions? How do the carbons become linked to form sugars? And does the oxygen gas come from the CO2 or from the H2O? Almost a century and a half passed before the source of the O2 released during photosynthesis was determined. One of the first uses of an isotopic tracer in biological research resulted in its identification. In these experiments, two groups of green plants were allowed to carry on photosynthesis. Plants in the first group were supplied with water containing the oxygen isotope 18O and with CO2 containing only the common oxygen isotope 16O; plants in the second group were supplied with CO2 labeled with 18O and water containing only 16O. When oxygen gas was collected from each group of plants and analyzed, it was found that O2 containing 18O was produced in abundance by the plants that had been given 18Olabeled water, but not by the plants given 18O-labeled CO2. These results showed that all the oxygen gas produced during photosynthesis comes from water. This discovery is reflected in a revised balanced equation:

6 CO2 + 12 H2O →C6H12O6 + 6 O2 + 6 H2O

 

Water appears on both sides of the equation because water is both used as a reactant (the twelve molecules on the left) and released as a product (the six new ones on the right). In this revised equation, there are now sufficient water molecules to account for all the oxygen gas produced.

 

The Two Pathways of Photosynthesis: An Overview

The equation above summarizes the overall process of photosynthesis. However, like glycolysis and the other metabolic pathways that yield energy in cells, photosynthesis consists not of one single reaction but of many reactions. The reactions of photosynthesis can be divided into two pathways:

The light reactions are driven by light energy. This pathway produces ATP and a reduced electron carrier (NADPH + H+).

The second pathway, called the Calvin–Benson cycle, does not use light directly. It uses ATP, NADPH + H+ (made by the light reactions), and CO2 to produce sugars. The reactions of the Calvin–Benson cycle are sometimes called the dark reactions because they do not directly require light energy. However, both pathways stop in the dark because ATP synthesis and NADP+ reduction require light. The reactions of both pathways proceed within the chloroplast but they reside in different parts of that organelle. The two pathways are linked by the exchange of ATP and ADP, and of NADP+ and NADPH, and the rate of each set of reactions depends on the rate of the other. We will discuss the light reactions at length, to be followed by the details of the Calvin–Benson cycle. However, because these two photosynthetic pathways are powered by the energy of light, we begin with discussing the physical nature of light and the nature of the specific photosynthetic molecules that capture its energy.

 

The Interactions of Light and Pigments

Light is a source of both energy and information. In later chapters, we’ll examine the many roles of light in the transmission of information. In this chapter, our focus is on light as a source of energy.

Light behaves as both a particle and a wave

Light is a form of electromagnetic radiation. It comes in discrete packets called photons. Light also behaves as if it were propagated in waves. The amount of energy contained in a single photon is inversely proportional to its wavelength: the shorter the wavelength, the greater the energy of the photons. For example, a photon of red light of wavelength 660 nm has less energy than a photon of blue light at 430 nm. Two things are required for photons to be active in a biological process:

Photons must be absorbed by a receptive molecule.

Photons must have sufficient energy to perform the chemical work required.

 

Absorbing a photon puts a pigment in an excited state

When a photon meets a molecule, one of three things happens:

The photon may bounce off the molecule—it may be scattered.

The photon may pass through the molecule—it may be transmitted.

The photon may be absorbed by the molecule.

Neither of the first two outcomes causes any change in the molecule. In the third case, the photon disappears. Its energy, however, cannot disappear, because energy is neither created nor destroyed.

When a molecule absorbs a photon, that molecule acquires the energy of the photon. It is thereby raised from a ground state (lower energy) to an excited state (higher energy). The difference in energy between the molecule’s excited state and its ground state is exactly equal to the energy of the absorbed photon. The increase in energy boosts one of the electrons within the molecule into an orbital farther from its nucleus; this electron is now held less firmly, making the molecule more chemically reactive, as we will see later in the chapter. The electromagnetic spectrum shows the wide range of wavelengths (and hence, energy levels) that photons can have. The specific wavelengths absorbed by a particular molecule are characteristic of that type of molecule. Molecules that absorb wavelengths in the visible spectrum — that region of the spectrum that is visible to humans—are called pigments. When a beam of white light (light containing visible light of all wavelengths) falls on a pigment, certain wavelengths of the light are absorbed. The remaining wavelengths, which are scattered or transmitted, make the pigment appear to us to be colored. For example, if a pigment absorbs both blue and red light — as chlorophyll does —what we see is the remaining light which is primarily green.

 

Absorbed wavelengths correlate with biological activity

If we plot the wavelengths of the light absorbed by a purified molecule, the result is an absorption spectrum for that molecule. If we plot the biological activity of a photosynthetic organism as a function of the wavelengths of light to which the organism is exposed, the result is an action spectrum. Figure shows the absorption spectrum for a pigment, chlorophyll a, isolated from the leaves of a plant and the action spectrum for photosynthetic activity for the same plant. Acomparison of the two spectra shows that the wavelengths at which photosynthesis is maximal are the same wavelengths at which chlorophyll a absorbs light.

 

Photosynthesis uses energy absorbed by several pigments

The light energy used for photosynthesis is not absorbed by just a single type of pigment. Instead, several different pigments with different absorption spectra absorb the energy that is eventually used for photosynthesis. In photosynthetic organisms of all kinds (plants, protists, and bacteria), these pigments include chlorophylls, carotenoids, and phycobilins. In plants, two chlorophylls predominate: chlorophyll a and chlorophyll b. These two molecules differ only slightly in their molecular structure. Both have a complex ring structure similar to that of the heme group of hemoglobin. In the center of each chlorophyll ring is a magnesium atom, and attached at a peripheral location on the ring is a long hydrocarbon “tail,” which can adhere the chlorophyll molecule to proteins in the hydrophobic portion of the thylakoid membrane. The chlorophylls absorb blue and red wavelengths, which are near the two ends of the visible spectrum. Thus, if only chlorophyll pigments were active in photosynthesis, much of the visible spectrum would go unused. However, all photosynthetic organisms possess accessory pigments, which absorb photons intermediate in energy between the red and the blue wavelengths (for instance, yellow light) and then transfer a portion of that energy to the chlorophylls. Among these accessory pigments are carotenoids, such as β-carotene, which absorb photons in the blue and blue-green wavelengths and appear deep yellow. The phycobilins, which are found in red algae and in cyanobacteria, absorb various yellow-green, yellow, and orange wavelengths. Such accessory pigments, in collaboration with the chlorophylls, constitute an energy-absorbing system covering much of the visible spectrum.

 

Light absorption results in photochemical change

After a pigment molecule absorbs a photon and enters an excited state, that molecule may return to the ground state. When this happens, some of the absorbed en- ergy is given off as heat and the rest is given off as light energy or fluorescence. Because some of the absorbed light energy is lost as heat, the fluorescence has less energy and longer wavelengths than the absorbed light. When there is fluorescence, there are no permanent chemical changes or biological functions—no chemical work is done. Any pigment molecule can become excited when its absorption spectrum matches the energies of incoming photons. If fluorescence does not occur, that pigment molecule may pass the absorbed energy along to another molecule, provided that the target molecule is very near, has the right orientation, and has the appropriate structure to receive the energy. The pigments in photosynthetic organisms are arranged into energy-absorbing antenna systems. In these systems, the pigments are packed together and attached to thylakoid membrane proteins in such a way that the excitation energy from an absorbed photon can be passed along from one pigment molecule in the system to another. Excitation energy moves from pigments that absorb shorter wavelengths (higher energy) to pigments that absorb longer wavelengths (lower energy). Thus the excitation ends up in the one pigment molecule in the antenna system that absorbs the longest wavelengths; this molecule is in the reaction center of the antenna system. It is the reaction center that converts the light absorbed into chemical energy. It is in the reaction center that a molecule absorbs sufficient energy that it actually gives up its excited electron (is chemically oxidized) and becomes positively charged. In plants, the pigment molecule in the reaction center is always a molecule of chlorophyll a. There are many other chlorophyll a molecules in the antenna system, but all of them absorb light at shorter wavelengths than does the molecule in the reaction center.

 

Excited chlorophyll in the reaction center acts as a reducing agent for electron transport

Ultimately, photosynthesis stores chemical energy by using the excited chlorophyll molecule in the reaction center as a reducing agent to reduce a stable electron acceptor. Ground-state chlorophyll (symbolized Chl) is not much of a reducing agent, but excited chlorophyll (Chl*) is a good one. To understand the reducing capability of Chl*, recall that in an excited molecule, one of the electrons is zipping around in an orbital farther away from its nucleus. Less tightly held, this electron can be passed on in a redox reaction to an oxidizing agent. Thus Chl* (but not Chl) can react with an oxidizing agent Ain a reaction like this:

Chl* + A®Chl+ + A–

This, then, is the first consequence of light absorption by chlorophyll. The chlorophyll becomes a reducing agent and participates in a redox reaction. As we are about to see, the further adventures of the electrons from chlorophyll reduce the electron carrier NADP+ and generate a proton-motive force that is eventually used to synthesize ATP.

 

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation

The energized electron that leaves the activated chlorophyll in the reaction center needs somewhere to go. It immediately participates in a series of oxidation-reduction (redox) reactions. The energy-rich electron is passed through a chain of electron carriers in the thylakoid membrane in a process termed electron transport. Two energy-rich products of the light reactions, NADPH + H+ and ATP, are the result. The energy-rich NADPH + H+ is a stable, reduced coenzyme. Its oxidized form is NADP+ (nicotinamide adenine dinucleotide phosphate). Just as NAD+ couples the metabolic pathways of cellular respiration, NADP+ couples the two photosynthetic pathways. NADP+ is identical to NAD+ except that the former has an additional phosphate group attached to each ribose. Whereas NAD+ participates in catabolism, NADP+ is used in anabolic (synthetic) reactions, such as carbohydrate synthesis from CO2, that require energy from reducing power. Electron transport in the thylakoid membrane sets up a charge separation, just as electron transport in the inner mitochondrial membrane does .This potential energy is captured by the chemiosmotic synthesis of ATP in a process called photophosphorylation. Both NADPH + H+ and ATP are used in the Calvin–Benson cycle as a source of energy for the endergonic synthesis of carbohydrates. There are two different systems of electron transport in photosynthesis:

Noncyclic electron transport produces NADPH + H+ and ATP.

Cyclic electron transport produces only ATP.

 

Noncyclic electron transport produces ATP and NADPH

In noncyclic electron transport, light energy is used to oxidize water, forming O2, H+, and electrons. Electrons from water replenish the electrons that chlorophyll molecules lose when they are excited by light. As the electrons are passed from water to chlorophyll, and ultimately to NADP+, they pass through a chain of electron carriers. These redox reactions are exergonic, and some of the free energy released is used ultimately to form ATP by a chemiosmotic mechanism.

 

Two photosystems are required

Noncyclic electron transport requires the participation of two different photosystems. These photosystems are light-driven molecular units, each of which consists of many chlorophyll molecules and accessory pigments bound to proteins in separate energy-absorbing antenna systems.

Photosystem I uses light energy to reduce NADP+ to NADPH + H+.

Photosystem II uses light energy to oxidize water molecules, producing electrons, protons (H+) and O2.

The reaction center for photosystem I contains a chlorophyll a molecule called P700 because it can best absorb light of wavelength 700 nm. The reaction center for photosystem II contains a chlorophyll a molecule called P680 because it absorbs light maximally at 680 nm. Thus photosystem II requires photons that are somewhat more energetic (i.e., shorter wavelengths) than those required by photosystem I. To keep noncyclic electron transport going, both photosystems I and II must constantly be absorbing light, thereby boosting electrons to higher orbitals from which they may be captured by specific oxidizing agents.

 


 

Detalis of the reactions

Photosystem II absorbs photons, sending electrons from P680 to the primary electron acceptor— the first carrier in the redox chain— and causing P680 to become oxidized to P680 +. Electrons from the oxidation of water are passed to P680 +, reducing it once again to P680, which can then absorb more photons. The electron from photosystem II passes through a series of exergonic reactions in the redox chain that are indirectly coupled across the thylakoid membrane to proton pumping. This pumping creates a proton gradient that produces energy for ATP synthesis. In photosystem I, the reaction center containing P700 becomes excited to P700*, which then leads to the reduction of an oxidizing agent called ferredoxin (Fd) and the production of P700 +. Then P700 + returns to the ground state by accepting electrons passed through the redox chain from photosystem II. With this accounting for the source of the electrons entering photosystem II, we can now consider the fate of the electrons from photosystem I. These electrons are used in the last step of noncyclic electron transport, in which two electrons and two protons are used to reduce a molecule of NADP+ to NADPH + H+.

 

In summary:

Noncyclic electron transport uses a molecule of water, four photons (two each absorbed by photosystems I and II), one molecule each of NADP+ and ADP, and one Pi. Noncyclic electron transport produces NADPH + H+ and ATP and half a molecule of oxygen (1/2 O2).

 

Cyclic electron transport produces ATP but no NADPH

Noncyclic electron transport produces ATP and NADPH + H+. However, as we will see, the Calvin–Benson cycle uses more ATP than NADPH + H+. Cyclic electron transport occurs in some organisms when the ratio of NADPH + H+ to NADP+ in the chloroplast is high. This process, which produces only ATP, is called cyclic because an electron passed from an excited chlorophyll molecule at the outset cycles back to the same chlorophyll molecule at the end of the chain of reactions Before cyclic electron transport begins, P700, the reaction center chlorophyll of photosystem I, is in the ground state. It absorbs a photon and becomes P700*. The P700* then reacts with oxidized ferredoxin (Fdox) to produce reduced ferredoxin (Fdred). The reaction is exergonic, releasing free energy. Reduced ferredoxin (Fdred) passes its added electron to a different oxidizing agent, plastoquinone (PQ, a small organic molecule), which pumps 2 H+ back across the thylakoid membrane. Thus, Fdred reduces PQ and PQred passes the electron to a cytochrome complex (Cyt). The electron continues down the electron transport chain until it completes its cycle by returning to P700 +, resulting in a restoration of its uncharged form, P700. By the time the electron from P700* travels through the redox chain by way of plastocyanin (PC), and comes back to reduce P700 +, all the energy from the original photon has been released. This cycle is a series of redox reactions, each exergonic, and the released energy is stored in the form of a proton gradient that can be used to produce ATP. Having seen how a proton gradient is established across the thylakoid membrane, we’ll now examine in more detail the role of this gradient in ATP synthesis.

 

Chemiosmosis is the source of the ATP produced in photophosphorylation

The chemiosmotic mechanism for ATP formation in the mitochondrion. The chemiosmotic mechanism also operates in photophosphorylation (Figure ). In chloroplasts, as in mitochondria, electron transport through the redox chain is coupled to the transport of protons (H+) across the thylakoid membrane, which results in a proton gradient across the membrane. The electron carriers in the thylakoid membranes are oriented so that protons move from the stroma into the interior of the thylakoid. The interior compartment becomes acidic with respect to the stroma. When there is sufficient light, the ratio of H+ inside versus outside a thylakoid is usually 10,000:1, which is a difference of 4 pH units. This difference leads to the diffusion of H+ back out of the thylakoid interior through specific protein channels in the thylakoid membrane. These channels are enzymes—ATP synthases—that couple the diffusion of protons to the formation of ATP, just as in mitochondria. In the chloroplast, the ATP is generated in the stroma, where it will be available to provide the energy for the fixation of CO2 in the production of carbohydrate by the Calvin-Benson cycle.

 

 

Making Carbohydrate from CO2: The Calvin–Benson Cycle

At the start of this chapter we identified two distinct metabolic pathways operating in photosynthesis. We have now discussed the first pathway: the light reactions, which use light energy to produce ATP and NADPH + H+ in the chloroplasts of green plants. The second pathway, the Calvin–Benson cycle, uses this ATP and NADPH + H+ to incorporate CO2 into carbohydrates. Most of the enzymes that catalyze the reactions of the Calvin–Benson cycle are dissolved in the chloroplast stroma (the “soup” outside the thylakoids) and that is where those reactions take place. However, these enzymes use the energy in ATP and NADPH, produced in the thylakoids by the light reactions, to reduce CO2 to carbohydrates. Because there is no stockpiling of these energy-rich coenzymes, these Calvin–Benson cycle reactions take place only in the light, when these coenzymes are being generated.

 

Isotope labeling experiments revealed the steps of the Calvin–Benson cycle

To identify the sequence of reactions by which CO2 ends up in carbohydrates, it was necessary to label CO2 so that it could be followed after being taken up by a photosynthetic cell. In the 1950s, Melvin Calvin, Andrew Benson, and their colleagues used radioactively labeled CO2 in which some of the carbon atoms were not the normal 12C, but its radioisotope 14C. Although 14C is distinguished by its emission of radiation, chemically it behaves virtually identically to nonradioactive 12C. In general, enzymes do not distinguish between isotopes of an element in their substrates, so 14CO2 is treated the same way by photosynthesizing cells as 12CO2. Calvin and his colleagues exposed cultures of the unicellular green alga Chlorella to 14CO2 for 30 seconds. They then rapidly killed the cells, extracted their carbohydrates and separated the different compounds from one another by paper chromatography. Many compounds, including monosaccharides and amino acids, contained 14C (Figure). However, if they stopped the exposure after just 3 seconds, only one compound was labeled—a three-carbon sugar phosphate called 3-phosphoglycerate (3PG):

By tracing the steps in this manner, they soon discovered a cycle that “fixes” CO2 in a larger molecule, produces a carbohydrate, and regenerates the initial CO2 acceptor. This cycle was appropriately named the Calvin–Benson cycle. The initial reaction in the Calvin–Benson cycle adds the one-carbon CO2 to a receptor, the five-carbon compound ribulose 1,5-bisphosphate (RuBP). The product is an intermediate six-carbon compound, which quickly breaks down and forms two three-carbon molecules of 3PG (as Calvin and colleagues observed). The enzyme that catalyzes this fixation reaction, ribulose bisphosphate carboxylase/ oxygenase (rubisco), is the most abundant protein in the world comprising about 20 percent of all the protein in every plant leaf.

 

The Calvin–Benson cycle is made up of three processes

The Calvin–Benson cycle uses the high-energy coenzymes made in the thylakoids during the light reactions (ATP and NADPH) to reduce CO2 to a carbohydrate. There are three processes that make up the cycle:

Fixation of CO2. As we saw, this reaction is catalyzed by rubisco and its product is 3PG.

Reduction of 3PG to form a carbohydrate, glyceraldehydes 3-phosphate (G3P). This series of reactions involves a phosphorylation (using the ATP made in the light reactions) and a reduction (using the NADPH made in the light reactions). Regeneration of the CO2 acceptor, RuBP. Most of the G3P ends up as RuMP (ribulose monophosphate), and ATP is used to convert this compound to RuBP. So for every “turn” of the cycle, with one CO2 fixed, the CO2 acceptor is regenerated. The end product of this cycle is glyceraldehyde 3-phosphate (G3P), which is a three-carbon sugar phosphate, also called triose phosphate. In a typical leaf, there are two fates for the G3P:

One-third of it ends up in the polysaccharide starch, which is stored in the chloroplast.

Two-thirds of it is converted in the cytosol to the disaccharide sucrose, which is transported out of the leaf to other organs in the plant, where it is hydrolyzed to its constituent monosaccharides: glucose and fructose. The G3P produced in photosynthesis is subsequently used by the plant to make other compounds. Its carbon is thus incorporated into amino acids, lipids, and the building blocks of the nucleic acids. The products of the Calvin–Benson cycle are of crucial importance to the entire biosphere, for the covalent bonds of the carbohydrate generated in the cycle represent the total energy yield from the harvesting of light by photosynthetic organisms. Most of this stored energy is released by glycolysis and cellular respiration and used to support plant growth, development, and reproduction. Much plant matter ends up being consumed by animals, supplying them with both raw materials and energy sources. Glycolysis and cellular respiration in the animals release free energy from the plant matter for use in the animal cells.

 

Photorespiration and Its Consequences

The enzyme rubisco, used by the Calvin-Benson cycle to fix CO2 during photosynthesis, is probably the most abundant enzyme on the planet. Its properties are remarkably identical in all photosynthetic organisms, from bacteria to flowering →follows, we will identify and explore some of these limitations and see how evolution has constructed metabolic bypasses around them. First we’ll look at photorespiration, a process in which rubisco reacts with O2 instead of CO2, lowering the overall rate of CO2 fixation. Then we’ll examine some biochemical pathways and features of plant anatomy that compensate for the limitations of rubisco.

 

Rubisco catalyzes RuBP reaction with O2 as well as CO2

As its full name indicates, rubisco is a carboxylase (addingCalvin–Benson cycle. The phosphoglycolate forms glycerate which diffuses into membrane-enclosed organelles called peroxisomes. There, a series of reactions converts it to the amino acid glycine: glycolate →→glycine The glycine then diffuses into a mitochondrion, where two glycine molecules are converted to glycerate (a three-carbon molecule), and CO2: 2 glycine →glycerate + CO2 This pathway is called photorespiration because it consumes O2 and releases CO2. It uses ATP and NADPH produced in the light reactions, just like the Calvin–Benson cycle. The net effect is to take two two-carbon molecules and make one threecarbon molecule. So one carbon of the four is released as CO2 and three of the carbons (75%) are recovered as fixed carbon. In other words, photorespiration reduces net carbon fixation by 25 percent compared with the Calvin–Benson cycle. How does rubisco “decide” whether to act as an oxygenase or a carboxylase? First, rubisco has 10 times more affinity for CO2 than O2, and so favors CO2 fixation. Another consideration is the relative concentrations of CO2 and O2 in the leaf. If O2 is relatively abundant, rubisco acts as an oxygenase, and photorespiration ensues. If CO2 predominates, rubisco fixes it and the Calvin–Benson cycle occurs. Temperature is also a factor: photorespiration is more likely at high CO2 to the acceptor molecule RuBP) as well as an oxygenase (adding O2 to RuBP). These two reactions compete with each other. So when RuBP reacts with O2, it cannot react with CO2. This reaction reduces the overall CO2 that is converted to carbohydrates, and therefore limits plant growth. When O2 is added to RuBP, one of the products is a twocarbon compound, phosphoglycolate: RuBP + O2 →phosphoglycolate + 3PG Plants have evolved a metabolic pathway that partially recovers the carbon that has been channeled away from the temperatures.

Rubisco acts as an oxygenase, and photorespiration occurs, under these conditions. Because the first product of CO2 fixation in these plants is the three-carbon molecule 3PG, they are called C3 plants. Corn, sugarcane, and other tropical grasses also close their stomata on a hot day, but their rate of photosynthesis does not fall, nor does photorespiration occur. They keep the ratio of CO2 to O2 around rubisco high so that rubisco continues to act as a carboxylase. They do this in part by making a four-carbon compound, oxaloacetate, as the first product of CO2 fixation, and so are called C4 plants. C4 plants perform the normal Calvin–Benson cycle, but they have an additional early reaction that fixes CO2 without losing carbon to photorespiration, greatly increasing the overall photosynthetic yield. Because this initial CO2 fixation step can function even at low levels of CO2 and high temperatures, C4 plants very effectively optimize photosynthesis under conditions that inhibit it in C3 plants. C4 plants have two separate enzymes for CO2 fixation located in two different parts of the leaf (Figure ). One enzyme, present in the cytosol of mesophyll cells near the surface of the leaf, fixes CO2 to a three-carbon acceptor compound, phosphoenolpyruvate (PEP), to produce the fourcarbon fixation product, oxaloacetate. This enzyme, PEP carboxylase, has two advantages over rubisco:

It does not have oxygenase activity.

It fixes CO2 even at very low CO2 levels.

So even on a hot day when the stomata are closed, the CO2 concentration in the leaf is low, and the O2 concentration is high, PEP carboxylase just keeps on fixing CO2. Oxaloacetate diffuses out of the mesophyll cells and through plasmodesmata into the bundle sheath cells, located in the interior of the leaf. The chloroplasts in bundle sheath cells contain abundant rubisco. There, the four-carbon oxaloacetate loses one carbon, forming CO2 and regenerating the threecarbon acceptor compound, PEP, in the mesophyll cells. Thus, the role of PEP is to bind CO2 from the air in the leaf and carry it to the bundle sheath cells, where it is “dropped off” at rubisco. This process essentially pumps up the CO2 concentration around rubisco, so that it acts as a carboxylase and begins the Calvin–Benson cycle. Kentucky bluegrass, a C3 plant, thrives on lawns in April and May. But in the heat of summer, it does not do as well, and crabgrass, a C4 plant, takes over the lawn. The same is true on a global scale for crops: C3 plants, such as soybeans, rice, wheat and barley, have been adapted for human food production in temperate climates, while C4 plants, such as corn and sugarcane, originated and are grown in the tropics. C3 plants are certainly more ancient than C4 plants. While C3 photosynthesis appears to have begun about 3.5 billion years ago, C4 plants appeared about 12 million years ago. A possible factor in the emergence of the C4 pathway is the decline in atmospheric CO2. When dinosaurs ruled Earth 100 million years ago, the concentration of CO2 in the atmosphere was four times what it is now. As CO2 levels declined thereafter, the more efficient C4 plants would have had an advantage over their C3 counterparts.

 

CAM plants also use PEP carboxylase

Other plants besides the C4 species use PEP carboxylase to fix and accumulate CO2. Such plants include some water-storing plants (called succulents) of the family Crassulaceae, many cacti, pineapples and several other kinds of flowering plants. The CO2 metabolism of these plants is called crassulacean acid metabolism, or CAM, after the family of succulents in which it was discovered. CAM is much like the metabolism of C4 plants in that CO2 is initially fixed into a four-carbon compound. In CAM plants, however, the processes of initial CO2 fixation and the Calvin–Benson cycle are separated in time, rather than in space.

At night, when it is cooler and water loss in minimized, the stomata open. CO2 is fixed in mesophyll cells to form the four-carbon compound oxaloacetate which is converted to malic acid.

During the day, the accumulated malic acid is shipped to the chloroplasts, where decarboxylation supplies the CO2 for operation of the Calvin–Benson cycle and the light reactions supply the necessary ATP and NADPH + H+.

 

Metabolic Pathways in Plants

 

Green plants are autotrophs and can synthesize all the molecules they need from simple starting materials: CO2, H2O, phosphate, sulfate and ammonium ions (NH4 +). NH4 + is needed for amino acids and comes either from the conversion of nitrogen-containing molecules in soil water taken up by the plant’s roots or from the bacterial conversion of N2 gas from the atmosphere. The light reactions of photosynthesis generate ATP and NADPH which are used to synthesize carbohydrates. These compounds can then be used in cellular respiration to provide energy for processes such as active transport and anabolism. Both cellular respiration and fermentation can occur in plants, although the former is far more common. Plant cellular respiration, unlike photosynthesis, takes place both in the light and in the dark. Because glycolysis occurs in the cytosol, respiration in the mitochondria, and photosynthesis in the chloroplasts, all these processes can proceed simultaneously. Photosynthesis and respiration are closely linked through the Calvin–Benson cycle. The partitioning of G3P is particularly important:

Some G3P from the Calvin–Benson cycle can be converted to pyruvate, the end product of glycolysis. This pyruvate can be used in cellular respiration for energy or its carbon skeletons can be used anabolically to make lipids, proteins, and other carbohydrates.

Some G3P can enter a pathway that is the reverse of glycolysis (the gluconeogenic pathway). In this case, sucrose is formed and transported to the nonphotosynthetic tissues of the plant, such as the root.

Energy flows from sunlight to reduced carbon in photosynthesis to ATP in respiration. Energy can also be stored in the bonds of macromolecules such as polysaccharides, lipids, and proteins. For a plant to grow, energy storage (as body structures) must exceed energy release; that is, overall carbon fixation by photosynthesis must exceed respiration.

 

Using Energy for Life Processes

All cells need energy to carry out their life processes. Even the one-celled organisms needs energy for defense and to catch food. For example, the Paramecium used energy to swim and to shoot tiny darks at its enemies and prey.

In complex organisms, such as a human being cells use energy for life processes. Cells need energy to transport food and waste. They need energy for growth and repair. If you cut your finger, for example, your body’s cells must have energy to repair the injury.


Date: 2014-12-22; view: 1085


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