Question: Complete the assignment using the assigned readings from your textbook

Question

Question: Complete the assignment using the assigned readings from your textbook (Chapter 10: pages 165-170... Complete the assignment using the assigned readings from your textbook (Chapter 10: pages 165-170, 180-184,188). Yo need to use outside sources (i.e., Internet, journal articles, other texts, etc.) to complete this assignment. Any additional research should be cited in APA format, and listed at the end of the assignment.

Short Answer Questions: Plant Metabolism Copy and paste these questions and complete them using your textbook readings and the following videos about photosynthesis and cellular respiration. When you are finished please put your answers in the proper dropbox in .doc, .docx or .rtf format. Photosynthesis video Cellular Respiration Video

1.) Define Metabolism:

2.) Put the following molecules into the correct stage of photosynthesis:

Carbon Dioxide Gas (Used) Oxygen Gas (Produced) Water (used) Chlorophyll Sunlight ATP (produced) ATP (Used) NADPH (Produced) NADPH (Used) Glucose (Produced) Light Dependent Reactions Light Independent Reactions (Calvin Cycle)

3.) What is the ultimate goal of photosynthesis?

4.) Where does each stage of photosynthesis occurs in the cell?

5.) Explain why we, as humans, are dependent on photosynthesis.

6.) Put the following molecules into the correct stage of cellular respiration (NOTE: some molecule may be produced or used in more than one stage):

Glucose (Used) Water (Produced) Carbon Dioxide (produced) Oxygen (Used) NADH (produced) FADH2 (produced) FADH2 (Used) NADH (Used) ATP (Produced) Glycolysis Transition Stage Citric Acid (Krebs) Cycle Electron Transport

7.) Where does each stage of cellular respiration take place?

8.) In plants, where does the glucose that starts Cellular Respiration come from?

9.) Animals, such as humans also use cellular respiration, where do we get glucose to start this metabolic process?

(pgs from book) OVERVIEW Enzymes and energy transfer are explained first to introduce general metabolic concepts. Photosynthesis and respiration are then presented at three different levels: the essence of the process is examined, the major steps are briefly introduced, and the processes are explored in greater detail. One or two levels may be sufficient for some readers; others will want to explore all three. The chapter discusses the importance of the main features of each process and summarizes the light-dependent reactions, light-independent reactions, glycolysis, the citric acid cycle, and the electron transport system. It concludes with a tabular comparison between photosynthesis and respiration and makes a few brief observations on additional metabolic pathways. One of the most beautiful and breathtaking scenes in nature is the appearance of sunlight rays beaming through a canopy of trees. This vision is often encountered at dawn or dusk, or maybe even in the middle of the day, depending on the angle of the sun and the way in which plants are positioned in the forest. It reminds us of the necessity of the sun for virtually all life on earth and the essential process known as photosynthesis that produces the building blocks of life and generates oxygen for organisms to breathe. In this process—which involves little more than the air we breathe, water, green pigment, and light—parts of the water and air are combined in cells and stored as sugar. Also, as long as any cell remains alive, stored energy is released by another process, respiration. Without the proper balance between photosynthesis and respiration found in nature, life on earth as we know it would not exist (Fig. 10.1). Photosynthesis and respiration form the basis for most of our discussion in this chapter. Each living cell of a plant contains genetic information that programs all the metabolic activities that take place in the cells and plant as a whole. All forms of metabolism, which may be defined as the sum of all the interrelated biochemical processes that take place in a living organism, require energy to occur. When rosebushes flower, apples are produced on an apple tree, leaves appear on a maple tree in the spring, or any other form of life activity occurs, the dynamic process of acquiring, releasing, and transferring energy from one form to another takes place. Plants and animals release energy during their life cycles; the energy is then recycled or used by other living organisms. Photosynthetic cells can convert light energy to a usable form, and that usable energy may then be released during respiration, facilitating growth, development, and reproduction. Although photosynthetic organisms carry on both photosynthesis and respiration, most animals, including humans, carry on only respiration and rely upon green plants for oxygen, food, shelter, and many other items. f0165-01.png Figure 10.1 Summary of photo-synthesis and aerobic respiration. Photosynthesis uses the energy from the sun to form carbohydrates from carbon dioxide and water. Aerobic respiration uses oxygen in breaking down carbohydrates to release carbon dioxide and water. © Ingram Publishing RF page 166 Enzymes and Energy Transfer Enzymes (proteins that speed up chemical reactions in cells without being used up in the reactions) regulate just about every metabolic activity. In biochemical reactions, one or more specific enzymes are associated with the myriad forms of energy conversion that take place within cells. In some cases, these enzymes help form the chemical bonds needed to build molecules through a process called anabolism. Enzymes also break chemical bonds through the process of catabolism. Most reactions of photosynthesis are anabolic because they involve the construction of molecules that are stored for energy, whereas reactions in cellular respiration (referred to as “respiration” in the text) are generally catabolic, because they release energy held in chemical bonds. Photosynthesis builds organic compounds by combining carbon dioxide and water, forming carbohydrates. Respiration, on the other hand, breaks down those carbohydrates, producing carbon dioxide and water, which may be used once again in photosynthesis. As we will see in the sections to follow, this photosynthesis-respiration cycle is keyed by an enzyme complex that splits water molecules and releases electrons that function in temporarily storing biochemical energy. These electrons transfer energy from one form to another through oxidation-reduction reactions. Oxidation-Reduction Reactions The processes of both photosynthesis and respiration include many oxidation-reduction reactions. Oxidation is the loss of one or more electrons; it involves removal of electrons from a compound. Reduction is the gain of one or more electrons; it involves the addition of electrons to a compound. In most oxidation-reduction reactions, oxidation of one compound is coupled with reduction of another compound catalyzed by the same enzyme or enzyme complex. When an electron is removed, a proton may follow, with the result that a hydrogen atom is often removed during oxidation and added during reduction. Oxygen is usually the oxidizing agent (i.e., the final acceptor of the electron), but oxidations can occur without oxygen being involved. Photosynthesis Oil and coal today provide about 90% of the energy needed to power trains, trucks, ships, airplanes, factories, computers, communication systems, and a multitude of electrically energized appliances. The energy within that oil and coal was originally captured from the sun by plants and algae growing millions of years ago and then transformed into fossil fuels by geological forces. The energy needs of transportation, industry, and homes seem insignificant, however, when compared with the combined energy requirements of all living organisms. Every living cell requires energy just to remain alive, and more energy is needed for the cell to reproduce, grow, or do physical work as part of an organism. In addition, oxygen is vital to nearly all life in processes that release stored energy. Photosynthesis, at least indirectly, is not only the principal means of keeping all forms of humanity functioning but also the sole means of sustaining life at any level—except for a few bacteria that derive their energy from sulfur salts and other inorganic compounds. This unique manufacturing process of green plants furnishes raw material, energy, and oxygen. In photosynthesis, energy from the sun is harnessed and, with the aid of chlorophyll, is transformed from light energy to biochemical energy in the bonds between the atoms in a sugar molecule. Oxygen is given off as a by-product of the process. It has been estimated that all of the world’s green organisms (including those in the oceans) together produce between 100 billion and 200 billion metric tons (between 110 billion and 220 billion tons) of sugar each year. To visualize that much sugar, consider that it is enough to make about 300 quadrillion sugar cubes, with a total volume exceeding that of 2 million Empire State Buildings.1 Much of the sugar produced by plants is converted to wood, fibers (such as cotton and linen), and other structural materials. The first products of photosynthesis may also be converted to disaccharides, such as sucrose; polysaccharides, such as starch; and other storage forms of carbohydrates. The digestive activities of living organisms break down the carbohydrates to smaller molecules. Sugars produced by photosynthesis are also involved in the synthesis of amino acids for proteins and a host of other cell constituents. In fact, photosynthesis produces more than 94% of the dry weight of green organisms, with the remainder coming from the soil or dissolved matter. The capacity of plants to meet our energy needs may well determine the ultimate size of human populations. In some heavily populated parts of the world, the food supply already is falling short of providing enough energy to sustain life, and starvation is widespread. Meanwhile, in the Western world, significant numbers of persons consume too much food and are spending large sums on weight reduction. We will, however, eventually approach a point at which human populations in general will need to stabilize, or even those in the most affluent areas could exceed the capacity of the plants to sustain them. A great deal of photosynthesis occurs in organisms living in the oceans. It is estimated that between 40% and 50% of the oxygen in the atmosphere originates in oceans and lakes. Laboratory tests have shown, however, that pollutants—such as the PCBs (polychlorinated biphenyls) used in electrical insulators—are capable, in concentrations as low as 20 parts per billion, of stopping many delicate algae from carrying on photosynthesis. The concentration of such substances in ocean waters at present is considerably less than one part per billion, but PCB concentrations of up to five or more parts per billion have already been reported in some estuaries. The use of PCBs and related chemicals has been curtailed in the United States, but other countries are still using them, and residues are still washing off into rivers and on into the oceans. It is important to make the world community fully aware of the dangers of allowing pollutant concentrations to build up to the point of creating significant problems that could ultimately adversely affect all life. page 167 NOTE: To the Reader Photosynthesis is undoubtedly the most important process on earth to life as we know it. It is also a complex process that can be summarized briefly or examined in detail. What follows is a discussion of the subject at three different levels: (1) the essence of photosynthesis; (2) a brief introduction to the major steps of photosynthesis; and (3) a closer look at photosynthesis. The process of respiration, which is discussed after photosynthesis, is treated in similar fashion. The third level, in particular, contains detail that may or may not be discussed in your course. 1. The Essence of Photosynthesis Cells need energy to do their work and reproduce. The energy for most of this cellular activity involves energy-storing molecules commonly known as ATP (adenosine triphosphate).2 ATP doesn’t last more than a few seconds and has to be produced constantly. Instead of ingesting food in order to obtain energy as animals do, plants can make ATP, using light as the source of energy, just like a solar-powered engine. Without light, however, ATP production stops, and the cells could quickly die. In a solar-powered engine, this problem is avoided by using some of the generated electricity to charge batteries before the sun goes down. Plants also accumulate energy for later use by building sugar molecules for short-term energy storage or starch for longer-term energy storage. The energy-storing process of photosynthesis takes place in chloroplasts and other parts of green organisms in the presence of light. The light energy stored in a simple sugar molecule is produced from carbon dioxide (CO2) present in the air and water (H2O) absorbed by the plant. When the carbon dioxide and water (H2O) are combined and ultimately a sugar molecule (C6H12O6, glucose) is produced in a chloroplast, oxygen gas (O2) is released as a by-product. The oxygen diffuses out into the atmosphere. The overall process can be depicted by the equation that follows. The equation should not, however, be taken literally because there are many intermediate steps to the process, and glucose is not the immediate first product of photosynthesis. f0167-01.png Photosynthesis takes place in chloroplasts (see Figs. 3.4 and 3.11) or in cells with membranes in which chlorophyll is embedded. The principals in the process are carbon dioxide, water, light, and chlorophyll; a brief examination of each follows. Carbon Dioxide Our atmosphere consists of approximately 78% nitrogen and about 21% oxygen. The remaining 1% is made up of a mixture of less common gases, including 0.039% (390 parts per million) carbon dioxide and a little hydrogen, helium, argon, and neon. The carbon dioxide in the air surrounding the leaves of plants reaches the chloroplasts in the mesophyll cells by diffusing through the stomata into the leaf interior. The carbon dioxide goes into solution in a thin film of water on the outside walls of cells. It then diffuses through the cell walls, across cell membranes, and into the cytoplasm, where it finally reaches the chloroplasts. The amount of carbon dioxide constantly being taken from the atmosphere during daylight hours by all green plants is enormous. Just four-tenths of a hectare (1 acre) of corn (10,000 plants) accumulates more than 2,500 kilograms (5,512 pounds) of carbon from the atmosphere during a growing season. Over 10 metric tons (11 tons) of carbon dioxide are needed to furnish this much carbon. It also has been calculated that the total present atmospheric supply of carbon dioxide (more than 2.2 billion metric tons, or about 50 metric tons over each hectare of the earth’s surface) would be completely used up in about 22 years if it were not constantly being replenished. A large reservoir of carbon dioxide in the oceans has helped maintain atmospheric carbon dioxide levels throughout most of recorded history, but in the last 50 years, those levels have begun to climb. Plant and animal respiration, decomposition, natural fires, volcanoes, and similar sources replace carbon dioxide at roughly the same rate at which it is removed during photosynthesis. The use of fossil fuels, pollution, deforestation, and other human activities, however, have disrupted this cycle by adding more carbon dioxide to the atmosphere than is removed. Measurements of atmospheric carbon dioxide at the Mauna Loa research station on the big island of Hawaii have demonstrated this steady increase. Increased carbon dioxide levels have the potential to cause global increases in temperatures because, in the atmosphere, this gas acts much like a glass panel in a greenhouse by trapping solar radiation from the sun. Climatic models project that the earth’s average atmospheric temperature will rise 1.0°C to 3.5°C (1.8°F to 6.3°F) by the year 2100. Since the 1980s, global warming, page 168presumably as a result of this increase, has become a major political and scientific issue. See the section “Global Warming” in Chapter 25 for a discussion of this problem. Although increases in carbon dioxide levels may enhance photosynthesis and increase food production, insects, bacteria, and viruses that proliferate with warmer temperatures can offset these potential gains. Under carefully monitored conditions, commercial greenhouses have pumped carbon dioxide through pipes placed over plant beds to supplement their natural supply and have found that fertilizing plants with this gas has increased yields by more than 20%. Indeed, this indicates that plants have the potential to limit elevated carbon dioxide levels in the atmosphere. However, recent evidence suggests that many plants develop fewer stomata when carbon dioxide levels increase, thereby adapting them to such changes and reducing photosynthetic efficiency. The abundance of photosynthetic organisms that may or may not proliferate under these conditions is a key factor affecting global warming. Some scientists note that increased carbon dioxide levels will cause temperatures to rise globally, which, in turn, will result in longer growing seasons in middle and high latitudes and, hence, increase global photosynthesis. Nonetheless, the same temperature increases may accelerate plant and animal respiration and decomposition, which would add carbon dioxide to the environment. Respiration at higher temperatures may also diminish benefits to be anticipated from an increase in numbers of photosynthetic organisms during global warming. Through careful evaluation of global trends and physiological processes, scientists should be better able to predict what is in store for our planet’s future and how to improve management of the environment. Water Less than 1% of all the water absorbed by plants is used in photosynthesis; most of the remainder is transpired or incorporated into cytoplasm, vacuoles, and other materials. The water used is the source of electrons involved in photosynthesis, and the oxygen released is a by-product, even though carbon dioxide also contains oxygen. This has been demonstrated by conducting photosynthetic experiments using either carbon dioxide or water containing isotopes of oxygen. When the isotope is used only in the water, it appears in the oxygen gas released. If, however, it is used only in the carbon dioxide, it is confined to the sugar and water produced and never appears in the oxygen gas, demonstrating clearly that the water is the sole source of the oxygen released. If water is in short supply, it may indirectly become a limiting factor in photosynthesis; under such circumstances, the stomata usually close and sharply reduce the carbon dioxide supply. Light Light exhibits properties of both waves and particles. Energy reaches the earth from the sun in waves of different lengths, the longest waves being radio waves and the shortest being gamma rays. About 40% of the radiant energy we receive is in the form of visible light. If this visible light is passed through a glass prism, it splits into its component colors. Reds are on the longer wavelength end and violets on the shorter wave end, with yellows, greens, and blues between (Fig. 10.2). Although nearly all of the visible light colors can be used in photosynthesis, those in the violet to blue and red-orange to red wavelengths are used most extensively. Light in the green range is reflected. Leaves commonly absorb about 80% of the visible light that reaches them. The intensity of light varies with the time of day, season of the year, altitude, latitude, and atmospheric conditions. On a clear summer day at noon in a temperate zone, sunlight reaches an intensity of about 2,000 µmols3 per square meter of surface per second. In contrast, consider that a room with fluorescent lighting produces only about 40 µmols per square meter of surface per second. Plants vary considerably in the light intensities they need for photosynthesis to occur at optimal rates. Factors such as temperature and amount of carbon dioxide available can also be limiting (Fig. 10.3). For example, an increase in photosynthesis will not occur in some plants receiving more than 670 µmols of light per square meter unless supplemental carbon dioxide is provided. With supplements, however, rates of photosynthesis will continue to increase up to about 1,000 µmols. Herbaceous plants on a forest floor can survive with less than 2% of full daylight, and some mosses are reported to thrive on intensities as low as 0.05% to 0.01%. Most land plants that grow naturally in the open need at least 30% of full daylight to thrive. The optimal amount for some species of trees approaches full daylight, while shade plants often do well in 10% of full daylight. f0168-01.png Figure 10.2 Visible light that is passed through a prism is broken up into individual colors with wavelengths ranging from 390 nanometers (violet) to 780 nanometers (red). page 169 Light that is too intense may change the way in which some of a cell’s metabolism takes place. For example, higher light intensities and temperatures may change the ratio of carbon dioxide to oxygen in the interiors of leaves, which, in turn, may accelerate photorespiration (discussed on page 178). Photorespiration is typically considered to be a wasteful process that uses oxygen and releases carbon dioxide, although it may help some plants survive under adverse conditions. It differs from common aerobic respiration in its chemical pathways. Photooxidation, which involves the destruction (“bleaching”) of chlorophyll by light, may also occur. High light intensities may cause chlorophyll molecules to go to a different, excited state. The energy released from the excited chlorophyll is passed to oxygen molecules, which become highly reactive and bleach the chlorophyll. In the fall, photooxidation plays a significant role in the breakdown of chlorophyll in leaves, resulting in the autumn colors discussed in Chapter 7. High light intensities may also cause an increase in transpiration, resulting in the closing of stomata. A sharp reduction in the available carbon dioxide supply inevitably follows. Chlorophyll There are several different types of chlorophyll molecules, all of which contain one atom of magnesium. They are very similar in structure to the heme of hemoglobin, the iron containing red pigment that transports oxygen in blood. Each molecule has a long lipid tail, which anchors the chlorophyll molecule in the lipid layers of the thylakoid membranes (Fig. 10.4). Chloroplasts of most plants contain two major types of chlorophyll associated with the thylakoid membranes. Chlorophyll a is blue-green in color and has the formula C55H72MgN4O5. Chlorophyll b is yellow-green in color and has the formula C55H70MgN4O6. Usually, a chloroplast has about three times more chlorophyll a than b.The more chlorophyll a there is in a cell, the brighter green the cell and the tissue of which it is a part appear to be. When a molecule of chlorophyll b absorbs light, it transfers the energy to a molecule of chlorophyll a. Chlorophyll b, then, makes it possible for photosynthesis to take place over a broader spectrum of light than would be possible with chlorophyll a alone (see Fig. 10.7). f0169-01.png Figure 10.3 Effects of light and temperature on two forms of photosynthesis. Both forms of photosynthesis, known as C3 and C4, respectively, are discussed in this chapter. (a) In C3, the rate of photosynthesis will not increase beyond a certain intensity of light. In C4 plants, when additional carbon dioxide is available, photosynthetic rates undergo up to a 30% increase in light intensity. (b) In C3plants, quantum yield of photosynthesis decreases as temperatures increase, whereas in C4 plants, the quantum yield of photosynthesis is not significantly affected by temperature fluctuations between 10°C and 40°C. f0169-02.png Figure 10.4 The structure of a molecule of chlorophyll a, the most important of the pigments involved in photosynthesis. The boxlike ring structure on the left, with magnesium and nitrogen inside, functions in capturing light energy. The tail, which extends into the interior of a thylakoid membrane, is insoluble in water; all chlorophyll molecules are, however, fat soluble. Other photosynthetic pigments include carotenoids (yellowish to orange pigments), phycobilins (blue or red pigments found in cyanobacteria and red algae), and several other types of chlorophyll. Chlorophylls c, d,and possibly e take the place of chlorophyll b in certain algae, and several other photosynthetic pigments are found in bacteria. The various chlorophylls are all closely related and differ from one another only slightly in their molecular structure. In chloroplasts, about 250 to 400 pigment molecules are grouped as a light-harvesting complex called a photosynthetic unit, with countless numbers of these units in each granum. Two types of these photosynthetic units function together in the chloroplasts of green plants, bringing about the first phase of photosynthesis, the light--dependent reactions, which are discussed in the next section. 2. Introduction to the Major Steps of Photosynthesis The process of photosynthesis takes place in two series of steps called the light-dependent reactions and the light-independent reactions. Although the light-independent reactions use products of the light-dependent reactions, both processes occur simultaneously. The Light-Dependent Reactions The light-dependent reactions are the first major steps in the conversion of light energy to biochemical energy. The reactions are initiated when units of light energy (photons) strike chlorophyll molecules embedded in the thylakoid membranes of chloroplasts. Our knowledge of the light-dependent reactions essentially began in the 1930s in England through a discovery by Robin Hill, a biochemist. He found that a solution of fragmented and whole chloroplasts, isolated from leaves that had been ground up and centrifuged, could briefly produce oxygen if an electron acceptor were present to receive electrons from water. In 1951, it was shown that NADP (nicotin amide adenine dinucleotide phosphate), which is derived from the B vitamin niacin, was a natural electron acceptor in this reaction. In honor of its discoverer, the process became known as the Hill reaction. During the light-dependent reactions, water molecules are split apart, releasing electrons and hydrogen ions, and oxygen gas is released; the electrons from the split water molecules are passed along an electron transport system; energy-storing ATP molecules are produced; some hydrogen from the split water molecules is involved in the reduction of NADP to form NADPH (reduced nicotinamide adenine dinucleotide phosphate), which carries hydrogen and is used in the second phase of photosynthesis, the light-independent reactions. The Light-Independent Reactions The light-independent reactions (or carbon-fixing and reducing reactions) complete the conversion of light energy to chemical energy by utilizing ATP and NADPH to form sugars. Some scientists refer to the light-independent reactions as the dark reactionsbecause they don’t directly require light, but darkness has nothing to do with their functioning. In fact, even though light is not directly required in the same sense as it is for the light-dependent reactions, light is nevertheless required for the activation of the enzymes involved, and the processes normally can occur only in daylight. The light-independent reactions are a series of reactions that take place outside of the grana in the stroma of the chloroplast (see page 41), if the products of the light-dependent reactions are available. They may initially proceed in different ways, depending on the particular kind of plant involved, but they all go through the Calvin cycle, discovered and elucidated by Dr. Melvin Calvin of the University of California. In 1961, Calvin received a Nobel Prize for unraveling how this most widespread type of light-independent reactions takes place. In this cycle, carbon dioxide (CO2) from the air is combined with a 5-carbon sugar (RuBP, ribulose bisphos-phate), and then the combined molecules are converted, through several steps, to sugars, such as glucose (C6H12O6). Energy and electrons involved in these steps are furnished by the ATP molecules and NADPH produced during the light-dependent reactions. Some of the sugars that are produced during the light-independent reactions are recycled, while others are stored as starch or other polysaccharides (simple sugars strung together in chains). A summary of simplified photosynthetic reactions is shown in Figure 10.5. More detailed diagrams of the light-dependent and light-independent reactions are shown in Figures 10.8, 10.10, and 10.13. Two molecules of a 3-carbon sugar compound (3PGA—3-phosphoglyceric acid) are shown as the first stable substance produced when carbon dioxide from the air and RuBP are combined and then converted during the light-independent reactions (Fig. 10.5). Some grasses and many plants of arid regions fix carbon differently. They produce a 4-carbon acid as the first product, followed by the Calvin cycle. This 4-carbon pathway is discussed, along with another variation found mostly in desert plants, in the next section. CAM Photosynthesis Crassulacean acid metabolism (CAM) photosynthesis is found in plants of about 30 families, including cacti, stonecrops, orchids, bromeliads, and other succulents that are often stressed by limited availability of water. A few succulents do not have CAM photosynthesis, however, and several nonsucculent plants do. Many CAM plants are facultative C3 plants that can switch to C3photosynthesis during the day after a good rain or when night temperatures are high. Plants with CAM photosynthesis typically do not have a well-defined palisade mesophyll in the leaves, and, in contrast to the chloroplasts of the bundle sheath cells of C4 plants, those of CAM photosynthesis plants resemble the mesophyll cell chloroplasts of C3plants. CAM photosynthesis is similar to C4 photosynthesis in that 4-carbon compounds are produced during the light-independent reactions. In these plants, however, the organic acids (mainly malic acid) accumulate at night and break down during the day, releasing carbon dioxide. The enzyme PEP carboxylase is responsible for converting the carbon dioxide plus PEP to organic acids at night when the stomata are open. During daylight, the organic acids diffuse out of the cell vacuoles in which they are stored and are converted back to carbon dioxide for use in the Calvin cycle. A much larger amount of carbon dioxide can be converted to carbohydrate each day than would otherwise be possible, because the stomata of such plants are closed during the day to conserve water. This arrangement allows the plants to function well under conditions of both limited water supply and high light intensity (Fig. 10.13). Other Significant Processes That Occur in Chloroplasts In addition to photosynthesis, two very important sets of biochemical reactions take place in chloroplasts (and in the proplastids of roots). Sulfates are reduced to sulfide via several steps involving ATP and enzymes. The sulfide is rapidly converted into important sulfur-containing amino acids, such as methionine and cysteine, which are part of the building blocks for proteins, anthocyanin pigments, chlorophylls, and several other cellular components. Nitrates are reduced to organic compounds. Initially, the nitrates are reduced to nitrites in the cytoplasm. The nitrites are then transported into chloroplasts (or root proplastids) where, through several enzyme-mediated steps, they are converted to ammonia. The ammonia is then converted to amino groups that are integral parts of several important amino acids such as glutamine and aspartic acid. Glutamine is an important form of nitrogen storage in roots or specialized stems such as those of carrots, beets, and potato tubers. f0180-01.png Figure 10.13 Crassulacean acid metabolism (CAM) photosynthesis found in orchids, pineapples, and many desert plants such as cacti. CAM is similar to C4photosynthesis, but the plants have their stomata closed during daylight heat, thus conserving water. The organic acids accumulate at night and break down during the day, releasing carbon dioxide, which then enters the Calvin cycle and C3metabolism while the stomata are closed. Respiration The solar energy that is converted into biochemical energy by the process of photosynthesis today is stored in various organic compounds such as wood, while coal and oil contain energy originally captured by green organisms in the geological past. If the organic compounds are burned, the energy is released very rapidly in the form of heat and light, and much of the usable energy is lost. Living organisms, however, “burn” their energy-containing compounds in numerous, small, enzyme-controlled steps that release tiny amounts of immediately usable energy. The released energy is usually stored in ATP molecules, which allows the available energy to be used more efficiently and the process to be controlled more precisely. 1. The Essence of Respiration Respiration is essentially the release of energy from glucose molecules that are broken down to individual carbon dioxide molecules. The process takes place in all active cells 24 hours a day, regardless of whether or not photosynthesis happens to be occurring simultaneously in the same cells. It is initiated in the cytoplasm and completed in the mitochondria. The energy, stored in chemical bonds containing high-energy electrons, is released from simple page 181sugar molecules that are broken down during a series of steps controlled by enzymes. No oxygen is needed to initiate the process, but in aerobic respiration (the most widespread form of respiration), the process cannot be completed without oxygen gas (O2). The controlled release of energy is the significant event; carbon dioxide (CO2) and water (H2O) are the by-products. Aerobic respiration can be summed up in the following equation, but bear in mind that respiration, like photosynthesis, is a complex process that involves many steps not reflected in a simplified equation. f0181-01.png Anaerobic respiration and fermentation are two forms of respiration that were probably carried on in the geological past when there was no oxygen in the atmosphere. These forms of respiration are still carried on today by certain bacteria and other organisms in the absence of oxygen gas. Anaerobic respiration and fermentation release less than 6% of the energy released from a molecule of glucose by aerobic respiration. The two forms differ from one another in the manner in which hydrogen released from the glucose is combined with other substances (see the discussion on page 183). Fermentation is very important industrially, particularly in the brewing and baking industries. Two well-known forms of fermentation are illustrated by the following equations: f0181-02.png The relatively small amount of energy released during these inefficient forms of respiration is partly stored in two ATP molecules. The actual amount of energy stored is roughly only 29% of the approximately 48 kcals6 of energy released in anaerobic respiration. 2. Introduction to the Major Steps of Respiration Glycolysis In most forms of carbohydrate respiration, the first major phase takes place in the cytoplasm and requires no oxygen gas (O2). This phase, called glycolysis, involves three main steps and several smaller ones, each controlled by an enzyme. During the process, a small amount of energy is released, and some hydrogen atoms are removed from compounds derived from a glucose molecule. The essence of this complex series of steps is as follows: In a series of reactions, the glucose molecule becomes a fructose molecule carrying two phosphates (P). This sugar (fructose) molecule is split into two 3-carbon fragments called glyceraldehyde 3-phosphate (GA3P). Some hydrogen, energy, and water are removed from these 3-carbon fragments, leaving pyruvic acid (Fig. 10.14). Two ATP molecules supply the energy needed to start the process of glycolysis. By the time pyruvic acid has been formed, however, four ATP molecules have been produced from the energy released along the way, for a net gain of two ATP molecules. A great deal of the energy originally in the glucose molecule remains in the pyruvic acid. The hydrogen ions and high-energy electrons released during the process are picked up and temporarily held by an acceptor molecule, NAD (nicotinamide adenine dinucleotide). What happens to them next depends on the kind of respiration involved: aerobic respiration, true anaerobic respiration, or fermentation. Aerobic Respiration In aerobic respiration (the most common type of respiration), glycolysis is followed by two major stages: the citric acid cycle and electron transport. Both stages occur in the mitochondria and involve many smaller steps, each of which is controlled by enzymes (see Fig. 10.14). The Citric Acid (Krebs) Cycle??The citric acid cycle was originally named the Krebs cycle after Hans Krebs, a British biochemist who received a Nobel Prize in 1953 for his unraveling of many of the complex reactions that take place in respiration. The name citric acid cycle, or tricarboxylic acid (TCA) cycle, reflects the important role played by several organic acids during the process. Before entering the citric acid cycle, which takes place in the fluid matrix located within the compartments formed by the cristae of mitochondria (see Fig. 3.13), carbon dioxide is released from pyruvic acid that was produced by glycolysis. What remains is restructured to a 2-carbon acetyl group. This acetyl group combines with an acceptor molecule called coenzyme A (CoA). This combination (called acetyl CoA) then enters the citric acid cycle, which is a series of biochemical reactions that are catalyzed by enzymes. Little of the energy originally trapped in the glucose molecule is released during glycolysis. As the citric acid cycle proceeds, however, high-energy electrons and hydrogen are successively removed. This removal takes place from a series of organic acids and, after transfer, ultimately produces compounds such as NADH (reduced nicotinamide adenine dinucleotide) and FADH2(reduced flavin adenine dinucleotide), as well as a small amount of ATP. Carbon dioxide is produced as a by-product while the cycle is proceeding. page 182 f0182-01.png Figure 10.14 A summary of cellular respiration. In the first phase, glycolysis, which takes place in the cytoplasm, a sugar molecule is converted to two pyruvic acid molecules. The subsequent phases take place within the matrix of a mitochondrion. In aerobic respiration, the pyruvic acid is broken down in the citric acid cycle, and energy is transferred to compounds such as NADH, ATP, and FADH2. Carbon dioxide and hydrogen are also released. The released hydrogen is carried by an electron transport system and combined with oxygen, forming water. In anaerobic respiration and fermentation, pyruvic acid is converted in the absence of oxygen gas to ethyl alcohol or lactic acid with little release of energy. Electron Transport Much of the energy originally in the glucose molecule now has been transferred to the acceptors NAD and FAD, which became NADH and FADH2, respectively. NADH and FADH2 are electron donors to an electron transport systemconsisting of special acceptor molecules arranged in a precise sequence on the inner membranes of mitochondria. The electrons flow through a series of carrier molecules, many of which are part of protein complexes, down an energy gradient. Some of these electron carriers also accept protons and release them to the intermembrane space of the mitochondrion. Shuttling of protons in this way causes a buildup of protons outside the mitochondrial matrix, thereby establishing an electrochemical gradient. Through the process of chemiosmosis, additional protein complexes couple the transport of protons back into the matrix with phosphorylation of ADP to form ATP. The production of ATP stops if there are no electron donors or electron acceptor oxygen molecules. The acceptor molecules include iron-containing proteins called cytochromes. Energy is released in small increments at each step along the system, and ATP is produced from ADP and P. As the final step in aerobic respiration (see Fig. 10.14), oxygen acts as the ultimate electron acceptor, producing water as it combines with hydrogen. By the time the process is complete, the recoverable energy locked in a molecule of glucose has been used to create high-energy ATP molecules. The energy in ATP is then available for use in the synthesis of other molecules and for growth, active transport, and a host of other metabolic processes. Aerobic respiration produces a net gain of 36 ATP molecules from one glucose molecule, using up six molecules of oxygen and producing six molecules of carbon dioxide and a net total of six molecules of water. For each mole (180 grams) of glucose aerobically respired, 686 kcal of energy is released, with about 39% of it being stored in ATP molecules and the remainder being released as heat. Anaerobic Respiration and Fermentation In living organisms, glucose molecules often may undergo glycolysis without enough oxygen being available to complete aerobic respiration. In such cases, the hydrogen released during glycolysis is simply transferred from the hydrogen acceptor molecules back to the pyruvic acid after it has been formed, creating ethyl alcohol in some organisms, and lactic acid or similar substances in others. A little energy is released during either fermentation or true anaerobic respiration, but most of it remains locked up in the alcohol, lactic acid, or other compounds produced. In true anaerobic respiration, the hydrogen removed from the glucose molecule during glycolysis is typically combined with an inorganic ion, as, for example, when sulfur bacteria (discussed in Chapter 17) convert sulfate (SO4) to sulfur (S) or another sulfur compound, or when certain cellulose bacteria produce methane gas (CH4) by combining the hydrogen with carbon dioxide. Oxygen gas is not required to make these compounds, but few organisms can live long without oxygen, and many that carry on fermentation can also respire aerobically. If oxygen becomes available, the remaining energy can be released by further breakdown of these compounds. About 7% of the total energy in a glucose molecule is removed during anaerobic respiration or fermentation. So much of that energy goes into the making of the alcohol or the lactic acid or is dissipated as heat that there is a net gain of only two ATP molecules (compared with 36 ATP molecules produced in aerobic respiration). The forms of anaerobic respiration are adaptive to the organisms that have them in that they recycle NAD and allow glycolysis to continue. Living cells can tolerate only certain concentrations of alcohol. In media in which yeasts are fermenting sugars, for example, once the alcohol concentration builds up beyond 12%, the cells die and fermentation ceases. This is why most wines have an alcohol concentration of about 12% (24 proof). Many bacteria carry on both fermentation and true anaerobic respiration simultaneously, making it difficult to distinguish between the two processes. Some texts use the terms anaerobic respiration and fermentation interchangeably to designate respiration occurring in the presence of little or no oxygen gas. Factors Affecting the Rate of Respiration Temperature Temperature plays a major role in the rate at which the respiratory reactions occur. For example, when air temperatures rise from 20°C (68°F) to 30°C (86°F), the respiration rates of plants double and sometimes even triple. The faster respiration occurs, the faster the energy is released from sugar molecules, with an accompanying decrease in weight. In growing plants, this weight loss is more than offset by the production of new sugar by photosynthesis. In harvested fruits, seeds, and vegetables, however, respiration continues without sugar replacement, and some water loss occurs. Respiring cells convert energy stored as starch or sugar primarily to ATP, but much of the energy is lost in the form of heat, with only 39% being stored as ATP. Most fresh foods are kept under refrigeration, not only to lower the respiration rate and retard water loss but also to dissipate the heat. Keeping the temperatures down is also important to prevent the growth and reproduction of food-spoiling molds and bacteria, which may thrive at warmer temperatures. Heat inactivates most enzymes at temperatures above 40°C (104°F), but a few organisms, such as cyanobacteria and algae in the hot springs of Yellowstone National Park and similar places, have adapted in such a way that they are able to thrive at temperatures exceeding 60°C (140°F)—heat that would kill other organisms of comparable size almost instantly. Water Water inside cells and their organelles acts as a medium in which the enzymatic reactions can take place. Living cells often have a water content of more than 90%, but page 184the cells of mature seeds may have a water content of less than 10%. When water content becomes this low, respiration does not cease completely, but it continues at a drastically reduced rate, resulting in only very tiny amounts of heat being released and of carbon dioxide being given off. Seeds may remain viable (capable of germinating) for many years if stored under dry conditions. If they come in contact with water, however, they swell by imbibition. Respiration rates then increase rapidly. If the wet seeds happen to be in an unrefrigerated storage bin, the temperature may increase to the point of killing the seeds. In fact, if fungi and bacteria begin to grow on the seeds, temperatures from their respiration can become so high that spontaneous combustion can occur. Oxygen If flooding reduces the oxygen supply available to the roots of trees and house plants, their respiration and growth rates may decline. They may even die if the condition persists too long. When foods are stored in a low-oxygen environment, however, low respiration rates are beneficial. In fact, it is a common commercial practice to reduce the oxygen present in warehouses where crops are stored. The oxygen content is reduced to as little as 1% to 3% by pumping in nitrogen gas, while maintaining low temperatures and humidity. Oxygen concentration is not reduced below 1% because that can result in an undesirable increase in fermentation. 3. A Closer Look at Respiration Respiration, like photosynthesis, is a very complex process, and, as with photosynthesis, it is beyond the scope of this book to explore the subject in great detail. The following amplification of information already discussed is modest, and those who wish further information are referred to the reading list at the end of the chapter. Glycolysis Reexamined As previously discussed, this initial phase of all forms of respiration brings about the conversion of each 6-carbon glucose molecule to two 3-carbon pyruvic acid molecules via three main steps, each mediated by enzymes. The three main steps are as follows: Phosphorylation, whereby the 6-carbon sugars receive phosphates Sugar cleavage, which involves the splitting of 6-carbon fructose into two 3-carbon sugar fragments Pyruvic acid formation, which involves the oxidation of the sugar fragments Energy needed to initiate the process is furnished by an ATP molecule, which also furnishes the phosphate group for the phosphorylation of the sugar glucose to yield glucose 6-phosphate. Another ATP, with the aid of the enzyme fructokinase, yields fructose bisphosphate (fructose 1,6-diphosphate). As a result of the cleavage of the fructose bisphosphate, two different 3-carbon sugars are produced, but ultimately, only two glyceraldehyde 3-phosphate (GA3P) molecules remain. These two 3-carbon sugars are oxidized to two 3-carbon acids, and, in the successive production of several of these acids, phosphate groups are removed from the acids. The phosphate groups combine with ADP, producing a net direct gain of two ATP molecules during glycolysis. In addition, hydrogen is removed as GA3P is oxidized. This hydrogen is picked up by the acceptor molecule, NAD, which becomes NADH. Glycolysis, which requires no oxygen gas, is summarized in Figure 10.14. Transition Step to the Citric Acid (Krebs) Cycle Before a pyruvic acid molecule enters the citric acid cycle, which takes place in the mitochondria, a molecule of carbon dioxide is removed and a molecule of NADH is produced, leaving an acetyl fragment. The 2-carbon fragment is then bonded to a large molecule called coenzyme A. Coenzyme A consists of a combination of the B vitamin pantothenic acid and a nucleotide. Pantothenic acid is one of several B vitamins essential to respiration in both plants and animals; others include thiamine (vitamin B1), niacin, and riboflavin. The bonded acetyl fragment and coenzyme A molecule is referred to as acetyl CoA. The following equation summarizes the fate of the two pyruvic acid molecules following glycolysis and leading to the citric acid cycle: f0184-01.png In addition to pyruvic acid, fats and amino acids can also be converted to acetyl CoA and enter the process at this point. The NADH molecules donate their hydrogen to an electron transport system (discussed in the section “Electron Transport and Oxidative Phosphorylation”), and the acetyl CoA enters the citric acid cycle (see Fig. 10.14). The Citric Acid (Krebs) Cycle Reexamined In the citric acid cycle, acetyl CoA is first combined with oxaloacetic acid, a 4-carbon compound, producing citric acid, a 6-carbon compound. The citric acid cycle is kept going by oxaloacetic acid, which is produced in small amounts, but is an intermediate product rather than a starting substance or an end product of the cycle. As the cycle progresses, a carbon dioxide is removed, producing a 5-carbon compound. Then another carbon dioxide is removed, producing a 4-carbon compound. This 4-carbon compound, through additional steps, is converted back to oxaloacetic acid, the substance with which the cycle began, and the cycle is repeated. Each full cycle uses up a 2-carbon acetyl group and releases two carbon dioxide molecules while regenerating an oxaloacetic acid molecule for the next turn of the cycle. Some hydrogen is removed during the process and is picked up by FAD and NAD. One molecule of ATP, three molecules of NADH, and one molecule of FADH2 page 185are produced for each turn of the cycle. The citric acid cycle may be summarized as follows: f0185-01.png The hydrogen carried by NAD and FAD can mostly be traced to the acetyl groups and to water molecules added to some compounds in the citric acid cycle. The FAD and FADH2 are now known to be intermediate compounds. Ubiquinol, a component of the electron transport system, receives electrons from either NADH or FADH2. Y THEME: ecology Photosynthesis, Global Warming, and Tropical Rain Forests In much the same way that the windows of your car trap heat during the summer, carbon dioxide in our atmosphere traps radiation from the sun, which helps keep our planet warm. Without CO2 and other greenhouse gases, the temperature on earth would be about 33°C (60°F) cooler than it is now. Conversely, increasing the concentration of these gases in our atmosphere can cause escalating global temperatures. For the past 1,000 years, a careful balance maintained by the oceans and photosynthetic organisms has kept atmospheric CO2 levels fairly constant at 0.028%. Since the 1850s, fossil fuel combustion and deforestation have contributed to increasing atmospheric CO2 to a current level of 0.039%. Increases in greenhouse gases such as CO2 have caused a global warming of 0.6°C (1.1°F) since 1900, and predictions suggest an overall global warming of 1.0 to 3.5°C (1.8 to 6.3°F) (or even more, according to recent estimates) by the end of the 21st century. Billions of metric tons of carbon are converted to organic matter through carbon fixation by continental and marine ecosystems each year. Some of this carbon balance is regulated by the ability of oceans and seas to equilibrate CO2, and most scientists agree that a large amount of the CO2 is absorbed by terrestrial ecosystems. Among these ecosystems, tropical rain forests (Box Figure 10.2) are the most productive, with photosynthetic efficiencies that average 2.2 kilograms (4.8 pounds) of dry organic matter produced per square meter per year, which is almost three times the average efficiency of all other continental ecosystems. Although this superior efficiency may provide a substantial contribution to global carbon fixation, the amount of earth’s landmass covered with tropical rain forests declined from 30% in the 1950s to about 5% in 2010. In general, as you move toward the tropics, there is an increase in the diversity of all organisms. This includes plants. Even though tropical rain forests currently occupy only 5% of the world’s land surface, they contain over half of the world’s plant and animal species. For example, Costa Rica is about one-fourth the size of Great Britain, but it has almost six times the number of plant species. Experts estimate that deforestation contributes to the extinction of over 100 plant, animal, and insect species each day. For many reasons it is unfortunate to suffer such a rapid loss of life. For example, 25% of Western drugs are derived from rain-forest ingredients, and some may contain possible cures for life-threatening diseases. While the photosynthetic capacity of tropical rain forests may increase as a consequence of high CO2 levels in the atmosphere, this does not necessarily mean that these ecosystems are a panacea to the looming threat of global warming. Between 10 and 30 million hectares (25 and 74 million acres) of rain forest are lost every year to ranching, logging, mining, and otherwise developing areas for human needs. For each hectare (2.5 acres) of forest that is cleared and burned, about 220 metric tons (242 tons) of carbon are released, which directly increases the amount of CO2 in the atmosphere. Each year, deforestation in tropical rain forests accounts for 20% to 30% of all carbon dioxide introduced into the atmosphere. The consequences are magnified because the hectares of tropical rain forest that are lost to deforestation no longer remain to absorb excess CO2. Some evidence suggests that, as the CO2 content in the air rises, tropical rain forests will likely increase photosynthetic capacity, thereby removing excess carbon from the atmosphere. However, increasing temperatures that accompany the accumulation of greenhouses gases may, at some point, hinder the ability of plants to increase photosynthetic efficiency. This is because plant respiration, which releases CO2, increases exponentially with increasing temperature, whereas photosynthetic rates generally increase to a temperature optimum and then decline. The results of a study conducted from 1984 through 2000 in La Selva, Costa Rica, have shed new light on possible scenarios for changes in atmospheric CO2 concentrations during global warming. In this 16-year study, the investigators measured annual diameter increments of tree girth along with daily temperatures and atmospheric CO2 levels. Even though the tree species studied were adapted to tropical rain-forest ecosystems, the data collected demonstrated relatively lower forest productivity at higher temperatures. These findings suggest that, as temperatures rise, increased plant respiration in tropical rain forests may add to ongoing atmospheric CO2accumulation and accelerate global warming. These studies provide a compelling argument to do what we can now to slow the process of global warming before it gets out of hand. Some countries are doing what they can to combat the problem of deforestation. In the mid-1970s, Costa Rica established a system of national parks and reserves to protect 12% of the country’s land area from degradation. The current Costa Rican government continues to promote conservation programs through the preservation of rain forests and has a goal of expanding protected areas to 25% in the near future. Similar efforts may, indeed, slow the ever-increasing threat of global warming. f0188-01.png Box Figure 10.2 Lush vegetation in a Costa Rican rain forest. © Joe Vogt

Explanation / Answer

1. Metabolism - It is sum total of all chemical reactions that occur in living organisms in order to maintain their living state by converting food (taken) into energy .

3. The ultimate goal of photosynthesis is fixation of carbon-dioxide in the form of sugar like glucose ( reserve food ) .

4. All the reactions of photosynthesis occur in chloroplast (thylakoids) .

5. We ( human ) are dependent on photosynthesis for food . Photosynthesis fix carbon dioxide in the form of reserve food in the presence of sunlight . This food is utilised by human for their survival as they are heterotrophic and can not synthesize their own food .

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