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Photosynthesis

Photosynthesis

Photosynthesis is the biological conversion of light energy into chemical energy. This occurs in green plants, algae, and photosynthetic bacteria .

Much of the early knowledge of bacterial photosynthesis came from the work of Dutch-born microbiologist Cornelius van Neil (18971985). During his career at the Marine Research Station in Monterey, California, van Neil studied photosynthesis in anaerobic bacteria. Like higher plants, these bacteria manufacture carbohydrates during photosynthesis. But, unlike plants, they do not produce oxygen during the photosynthetic process. Furthermore, the bacteria use a compound called bacteriochlorophyll rather than chlorophyll as a photosynthetic pigment. Van Neil found that all species of photosynthetic bacteria require a compound that the bacteria can oxidize (i.e., remove an electron from). For example, the purple sulfur bacteria use hydrogen sulfide.

Since van Neil's time, the structure of the photosynthetic apparatus has been deduced. The study of photosynthesis is currently an active area of research in biology. Crystals of the photosynthetic reaction center from the anaerobic photosynthetic bacterium Rhodopseudomonas viridis were created in the 1980s by Hartmut Michel and Johann Deisenhofer, who then used x-ray crystallography to determine the three-dimensional structure of the photosynthetic protein. In 1988, the two scientists shared the Nobel Prize in Chemistry with Robert Huber for this research.

Photosynthesis consists of two series of biochemical reactions, called the light reactions and the dark reactions. The light reactions use the light energy absorbed by chlorophyll to synthesize structurally unstable high-energy molecules. The dark reactions use these high-energy molecules to manufacture carbohydrates. The carbohydrates are stable structures that can be stored by plants and by bacteria. Although the dark reactions do not require light, they often occur in the light because they are dependent upon the light reactions. In higher plants and algae, the light and dark reactions of photosynthesis occur in chloroplasts, specialized chlorophyll-containing intracellular structures that are enclosed by double membranes.

In the light reactions of photosynthesis, light energy excites photosynthetic pigments to higher energy levels and this energy is used to make two high energy compounds, ATP (adenosine triphosphate) and NADPH ( nicotinamide adenine dinucleotide phosphate). ATP and NADPH are consumed during the subsequent dark reactions in the synthesis of carbohydrates.

In algae, the light reactions occur on the so-called thylakoid membranes of the chloroplasts. The thylakoid membranes are inner membranes of the chloroplasts. These membranes are arranged like flattened sacs. The thylakoids are often stacked on top of one another, like a roll of coins. Such a stack is referred to as a granum. ATP can also be made by a special series of light reactions, referred to as cyclic photophosphorylation, which occurs in the thylakoid membranes of the chloroplast .

Algae are capable of photosynthetic generation of energy. There are many different groups of photosynthetic algae. Like higher plants, they all have chlorophyll-a as a photosynthetic pigment, two photosystems (PS-I and PS-II), and the same overall chemical reactions for photosynthesis. Algae differ from higher plants in having different complements of additional chlorophylls. Chlorophyta and Euglenophyta have chlorophyll-a and chlorophyll-b. Chrysophyta, Pyrrophyta, and Phaeophyta have chlorophyll-a and chlorophyll-c. Rhodophyta have chlorophyll-a and chlorophyll-d. The different chlorophylls and other photosynthetic pigments allow algae to utilize different regions of the solar spectrum to drive photosynthesis.

A number of photosynthetic bacteria are known. One example are the bacteria of the genus Cyanobacteria. These bacteria were formerly called the blue-green algae and were once considered members of the plant kingdom. However, unlike the true algae, cyanobacteria are prokaryotes, in that their DNA is not sequestered within a nucleus . Like higher plants, they have chlorophyll-a as a photosynthetic pigment, two photosystems (PS-I and PS-II), and the same overall equation for photosynthesis (equation 1). Cyanobacteria differ from higher plants in that they have additional photosynthetic pigments, referred to as phycobilins. Phycobilins absorb different wavelengths of light than chlorophyll and thus increase the wavelength range, which can drive photosynthesis. Phycobilins are also present in the Rhodophyte algae, suggesting a possible evolutionary relationship between these two groups.

Cyanobacteria are the predominant photosynthetic organism in anaerobic fresh and marine water.

Another photosynthetic bacterial group is called cloroxybacteria. This group is represented by a single genus called Prochloron. Like higher plants, Prochloron has chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments, two photosystems (PS-I and PS-II), and the same overall equation for photosynthesis. Prochloron is rather like a free-living chloroplast from a higher plant.

Another group of photosynthetic bacteria are known as the purple non-sulfur bacteria (e.g., Rhodospirillum rubrum. The bacteria contain bacteriochlorophyll a or b positioned on specialized membranes that are extensions of the cytoplasmic membrane.

Anaerobic photosynthetic bacteria is a group of bacteria that do not produce oxygen during photosynthesis and only photosynthesize in environments that are devoid of oxygen. These bacteria use carbon dioxide and a substrate such as hydrogen sulfide to make carbohydrates. They have bacteriochlorophylls and other photosynthetic pigments that are similar to the chlorophylls used by higher plants. But, in contrast to higher plants, algae and cyanobacteria, the anaerobic photosynthetic bacteria have just one photosystem that is similar to PS-I. These bacteria likely represent a very ancient photosynthetic microbe.

The final photosynthetic bacteria are in the genus Halobacterium. Halobacteria thrive in very salty environments, such as the Dead Sea and the Great Salt Lake. Halobacteria are unique in that they perform photosynthesis without chlorophyll. Instead, their photosynthetic pigments are bacteriorhodopsin and halorhodopsin. These pigments are similar to sensory rhodopsin, the pigment used by humans and other animals for vision. Bacteriorhodopsin and halorhodopsin are embedded in the cell membranes of halobacteria and each pigment consists of retinal, a vitamin-A derivative, bound to a protein. Irradiation of these pigments causes a structural change in their retinal. This is referred to as photoisomerization. Retinal photoisomerization leads to the synthesis of ATP. Halobacteria have two additional rhodopsins, sensory rhodopsin-I and sensory rhodopsin-II. These compounds regulate phototaxis, the directional movement in response to light.

See also Evolutionary origin of bacteria and viruses

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Photosynthesis

Photosynthesis


No chemical process is more important to life on Earth than photosynthesis the series of chemical reactions that allow plants to harvest sunlight and create carbohydrate molecules. Without photosynthesis, not only would there be no plants, the planet could not sustain life of any kind. In plants, photosynthesis occurs in the thykaloid membrane system of chloroplasts. Many of the enzymes that allow photosynthesis to occur are transmembrane proteins embedded in the thykaloid membranes. What then is the chemistry involved?

The most basic summary of the photosynthesis process can be shown with a net chemical equation

6CO2(g) + 6 H2O(l) + hν C6H12O6(s ) + 6O2(g )

The symbol hν is used to depict the energy input from light (in the case of most plants, sunlight). This chemical equation, however, is a dramatic simplification of the very complicated series of chemical reactions that photo-synthesis involves. It also implies that the only product is glucose , C6H12O6 (s ), which is also a simplification.

Still, take a moment to look at this chemical equation. If one were to guess where the various atoms in the reactants end up when products are produced, it would be reasonable to suggest that the oxygen atoms in the O2 (g ) were those originally associated with carbon dioxide. Most scientists believed this to be true until the 1930s when experiments by American biologist Cornelius van Niel suggested that oxygen-hydrogen bonds in water must be broken in photosynthesis. Further research confirmed his hypothesis and ultimately revealed that many reactions are involved in photosynthesis.

There are two major components of photosynthesis: the light cycle and the dark cycle. As implied by these names, the reactions in the light cycle require energy input from sunlight (or some artificial light source) to take place. The reactions in the dark cycle do not have to take place in the dark, but they can progress when sunlight is not present.

The critical step of the light cycle is the absorption of electromagnet radiation by a pigment molecule. The most famous pigment is chlorophyll , but other molecules, such as β- carotene, also absorb light (see Figure 1). Together, these pigment molecules form a type of light harvesting antennae that is more efficient at interacting with sunlight than would be possible with

the pigments acting alone. When the light is absorbed, electrons in the pigment molecule are excited to high energy states. A series of enzymes called electron transport systems help channel the energy present in these electrons into reactions that store it in chemical bonds.

For example, one major chemical reaction that results from the absorbed light energy (and excited electrons) involves water and nicotinamide adenine dinucleotide phosphate (NADP+). The net reaction is shown by the chemical equation

2 NADP+ + 2 H2O NADPH + O2 + 2H+

This is an example of an oxidation reduction reaction, and it shows that the light cycle is the stage of photosynthesis when water breaks up. The amount of energy required to make this reaction proceed is greater than what can be provided by a single photon of visible light. Therefore, there must be at least two ways that plants harvest light energy in photosynthesis. These two systems are referred to as photosystem I (PSI) and photosystem II (PSII), although the numbers associated with these names do not imply which one happens "first."

At the same time that NADPH is being produced, the combination of the photo systems also produces a concentration gradient of protons. Enzymes in the cell use this proton gradient to produce ATP from ADP. Thus, the light cycle produces two "high energy" molecules: NADPH and ATP.

With the high energy products provided by the light cycle, plants then use reactions that do not require light to actually produce carbohydrates. The initial steps in the dark cycle are collectively called the Calvin cycle, named after American chemist Melvin Calvin who along with his coworkers determined the nature of these reactions during the late 1940s and early 1950s.

The Calvin cycle essentially has two stages. In the first part of the cycle, several enzymes act in concert to produce a molecule called glyceraldehyde-3-phosphate (GAP). (See Figure 2). Note in the illustration that this molecule has three carbon atoms. Each of these carbon atoms comes originally from carbon dioxide moleculesso photosynthesis completes the amazing task of manufacturing carbohydrates out of air (the source of the carbon dioxide). This stage of the Calvin cycle is sometimes called carbon fixing. In order to carry out this synthesis of GAP, the Calvin cycle consumes some of the NADPH and ATP that was produced during the light cycle.

The carbon dioxide needed for this step enters through pores in the photosynthetic leaf (called stromata). Plants close these pores during hot, dry times of the day (to prevent water loss) so the details of carbon fixing vary for plants from different climates. In hot climates, where stomata are closed for a higher percentage of time, the trapping of carbon dioxide has to be more efficient than in cooler climates. This biochemical difference in photosynthesis helps explain why plants from one climate do not grow as well in warmer (or cooler) places.

The second stage of the cycle builds even larger carbohydrate molecules. With more than half a dozen enzyme-catalyzed reactions in this portion of the dark cycle, five-and six-carbon carbohydrates are produced. The five-carbon molecules continue in the cycle to help produce additional GAP, thus perpetuating the cyclic process.

Photosynthesis is central to all life on the planet and has been for many thousands of years. As a result, there are numerous variations in the way it occurs in different cells. The efficient collection of carbon dioxide mentioned earlier is one example of variation in photosynthesis. Other differences occur when the process takes place in bacteria rather than plants. Nonetheless, the description provided here outlines the basic concepts that would be noted in all photosynthesis. These differences pose the research questions that continue to challenge scientists today.

see also Calvin, Melvin; Concentration Gradient.

Thomas A. Holme

Bibliography

Foyer, Christine H. (1984). Photosynthesis. New York: Wiley.

Govindjee, and Coleman, W. J. (1990). "How Plants Make Oxygen." Scientific American 262:5059.

Internet Resources

Wong, Kate (2000). "Photosynthesis's Purple Roots." Scientific American. Available from <http://www.sciam.com>.

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Photosynthesis

Photosynthesis

Photosynthesis is the process by which plants use the energy of light to produce carbohydrates and molecular oxygen (O2) from carbon dioxide (CO2) and water:

Virtually all ecosystems on Earth depend on photosynthesis as their source of energy, and all free oxygen on the planet, including that in the atmosphere, originates from photosynthesis. The overall reaction is the reverse of respiration, which releases energy by oxidizing carbohydrates to produce CO2 and water. Photosynthesis and respiration are linked ecologically, being the cellular metabolic processes that drive the carbon and oxygen cycles.

Photosynthesis occurs in plants, photosynthetic protist (algae), and some bacteria. In plants and algae, it takes place within chloroplasts, whereas in bacteria it occurs on the plasma membrane and in the cytosol . The remainder of this discussion will refer to photosynthesis in chloroplasts of plants.

Overview

Photosynthesis is divided into two sets of reactions: the light-dependent (light) reactions and the light-independent (dark) reactions. As their names imply, the first set depends directly on light, whereas the second set does not. Nevertheless, even the dark reactions will cease if the plants are deprived of light for too long because they rely on the products of the light reactions.

The light reactions, which convert the energy in light into chemical energy, take place within the thylakoid membranes of the chloroplasts, whereas the dark reactions, which use that chemical energy to fix CO2 into organic molecules, take place in the stroma of the chloroplast. In the light reactions, the energy of light is used to "split water," stripping a pair of electrons from it (and causing the two hydrogens to be lost), thus generating molecular oxygen. The energy in light is transferred to these electrons, and is then used to generate adenosine triphosphate (ATP ) and the electron carrier NADPH. These two products carry the energy and electrons generated in the light reactions to the stroma, where they are used by the dark reactions to synthesize sugars from CO2.

The Light Reaction

The light reactions rely on colored molecules called pigments to capture the energy of light. The most important pigments are the green chlorophylls, but accessory pigments called carotenoids are also present, which are yellow or orange. The accessory pigments capture wavelengths of light that chlorophylls cannot, and then transfer the energy to chlorophyll, which uses this energy to carry out the light reactions. These pigments are arranged in the thylakoid membranes in clusters, along with proteins and electron carriers, to form light-harvesting complexes referred to as photosystems. Each photosystem has about two hundred chlorophyll molecules and a variable number of accessory pigments.

In most plants there are two photosystems, which differ slightly in how they absorb light. At the center of each photosystem is a special chlorophyll molecule called the reaction center, to which all the other pigments molecules pass the energy they harvest from sunlight. When the reaction-center chlorophyll absorbs light or receives energy from its accessory molecules, a pair of electrons on it becomes excited. These electrons now carry the energy from light, and are passed to an electron acceptor molecule.

The fate of these electrons depends on which photosystem they arose from. Electrons from photosystem I are passed down a short electron transport chain to reduce NADP+ to NADPH (which also gains an H+ ion ). Electrons from photosystem II are passed down a longer electron transport chain, eventually arriving at photosystem I, where they replace the electrons given up by photosystem I's reaction center. Along the way, the energy released by the electrons is used to make ATP in a process called photophosphorylation. Many of the molecular details of this ATP-generating system are similar to those used by the mitochondrion in oxidative phosphorylation . (Phosphorylation refers to the addition of a phosphate group to adenosine diphosphate [ADP] to form ATP.) Like the mitochondrion, the chloroplast uses an electron transport chain, and ATP synthetase to create ATP.

The end result of excitation of both photosystems is that electrons have been transferred from chlorophyll to NADP+, forming NADPH, and some of their energy has been used to generate ATP. While photosystem I gains electrons from photosystem II, the electrons lost by photosystem II have not been replaced yet. Its reaction center acquires these electrons by splitting water. During this process, the electrons in water are removed and passed to the reaction center chlorophyll. The associated hydrogen ions are released from the water molecule, and after two water molecules are thus split, the oxygen atoms join to form molecular oxygen (O2), a waste product of photosynthesis. The reaction is:

The Dark Reactions

The NADPH and ATP generated in the light reactions enter the stroma, where they participate in the dark reactions. Energy and electrons provided by ATP and NADPH, respectively, are used to incorporate CO2 into carbohydrate via a cyclic pathway called the Calvin-Benson cycle. In this complex pathway, the CO2 is added to the five-carbon sugar ribulose bisphosphate to form a six-carbon unstable intermediate, which immediately breaks down to two three-carbon molecules. These then go through the rest of the cycle, regenerating ribulose bisphosphate as well as the three-carbon sugar glyceraldehyde phosphate. It takes three turns of the cycle to produce one glyceraldehyde phosphate, which leaves the cycle to form glucose or other sugars.

Some plants bind CO2 into a four-carbon compound before performing the Calvin-Benson cycle. Such plants are known as C4 plants or CAM plants, depending on the details of the CO2 capture process.

see also Biogeochemical Cycles; C4 and CAM Plants; Chloroplast; Oxidative Phosphorylation

David W. Tapley

Bibliography

Bishop, M. B., and C. B. Bishop. "Photosynthesis and Carbon Dioxide Fixation."Journal of Chemical Education 64 (1987): 302305.

Govindjee, and W. J. Coleman. "How Plants Make O2." Scientific American 262 (February 1990): 5058.

Youvan, D. C., and B. L. Marrs. "Molecular Mechanisms of Photosynthesis." Scientific American 256 (June 1987): 4248.

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photosynthesis

photosynthesis (fō´tōsĬn´thəsĬs), process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g., the Indian pipe, secure their nutrients from organic material, as do animals, and a few bacteria manufacture their own carbohydrates with hydrogen and energy obtained from inorganic compounds (e.g., hydrogen sulfide) in a process called chemosynthesis. However, the vast majority of plants contain chlorophyll—concentrated, in the higher land plants, in the leaves.

In these plants water is absorbed by the roots and carried to the leaves by the xylem, and carbon dioxide is obtained from air that enters the leaves through the stomata and diffuses to the cells containing chlorophyll. The green pigment chlorophyll is uniquely capable of converting the active energy of light into a latent form that can be stored (in food) and used when needed.

The Photosynthetic Process

The initial process in photosynthesis is the decomposition of water (H2O) into oxygen, which is released, and hydrogen; direct light is required for this process. The hydrogen and the carbon and oxygen of carbon dioxide (CO2) are then converted into a series of increasingly complex compounds that result finally in a stable organic compound, glucose (C6H12O6), and water. This phase of photosynthesis utilizes stored energy and therefore can proceed in the dark. The simplified equation used to represent this overall process is 6CO2+12H2O+energy=C6H12O6+6O2+6H2O. In general, the results of this process are the reverse of those in respiration, in which carbohydrates are oxidized to release energy, with the production of carbon dioxide and water.

The intermediary reactions before glucose is formed involve several enzymes, which react with the coenzyme ATP (see adenosine triphosphate) to produce various molecules. Studies using radioactive carbon have indicated that among the intermediate products are three-carbon molecules from which acids and amino acids, as well as glucose, are derived. This suggests that fats and proteins are also products of photosynthesis. The main product, glucose, is the fundamental building block of carbohydrates (e.g., sugars, starches, and cellulose). The water-soluble sugars (e.g., sucrose and maltose) are used for immediate energy. The insoluble starches are stored as tiny granules in various parts of the plant—chiefly the leaves, roots (including tubers), and fruits—and can be broken down again when energy is needed. Cellulose is used to build the rigid cell walls that are the principal supporting structure of plants.

Importance of Photosynthesis

Animals and plants both synthesize fats and proteins from carbohydrates; thus glucose is a basic energy source for all living organisms. The oxygen released (with water vapor, in transpiration) as a photosynthetic byproduct, principally of phytoplankton, provides most of the atmospheric oxygen vital to respiration in plants and animals, and animals in turn produce carbon dioxide necessary to plants. Photosynthesis can therefore be considered the ultimate source of life for nearly all plants and animals by providing the source of energy that drives all their metabolic processes.

Bibliography

See I. Asimov, Photosynthesis (1969); R. M. Devlin and A. V. Barker, Photosynthesis (1972); O. Morton, Eating the Sun (2009).

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Photosynthesis

Photosynthesis

Photosynthesis is the process by which green plants and certain types of bacteria make carbohydrates, beginning only with carbon dioxide (CO2) and water (H2O). Carbohydrates are complex chemical compounds that occur widely in plants and that serve as an important food source for animals. Sugar, starch, and cellulose are among the most common carbohydrates. The energy needed to make photosynthesis possible comes from sunlight, which explains the term photo ("light") synthesis ("to make"). The absorption of sunlight in plants takes place in specific molecules known as chlorophyll (KLOR-uh-fill) that give plants their green color.

Photosynthesis can be represented by means of a simple chemical equation:

In this equation, C6H12O6 represents a simple sugar known as glucose. Molecules of glucose later combine with each other to form more complex carbohydrates, such as starch and cellulose. The oxygen formed during photosynthesis is released to the air. It is because of this oxygen that animal life on Earth is possible.

Words to Know

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Chlorophyll: A compound in plants that makes possible the conversion of light energy to chemical energy.

Dark reactions: Those reactions in the photosynthesis process that can occur in the absence of sunlight.

Glucose: A sugar, or simple carbohydrate, that serves as an energy source for cells.

Light reactions: Those reactions in the photosynthesis process that can occur only in the presence of sunlight.

The stages of photosynthesis

The equation for photosynthesis shown above is very misleading. It suggests that changing carbon dioxide and water into carbohydrates is a simple, one-step process. Nothing could be further from the truth. Scientists have been working for well over 200 years trying to find out exactly what happens during photosynthesis. Although the major steps of the process are understood, researchers are still unable to duplicate the process in the laboratory.

The equation above seems to say that six carbon dioxide molecules (6 CO2) and six water molecules (6 H2O) somehow get joined to each other to form one carbohydrate molecule (C6H12O6). Instead, the process occurs one small step at a time. During each of the many stages of photosynthesis, a single atom or an electron is transferred from one compound to another. Only after dozens of steps have taken place has the overall reaction shown above been completed.

What scientists have learned is that two general kinds of reactions are involved in photosynthesis: the light reactions and the dark reactions. Light reactions, as their name suggests, can take place only in the presence of sunlight. In those reactions, light energy is used to generate certain kinds of energy-rich compounds. These compounds do not themselves become part of the final carbohydrate product. Instead, they are used to "carry" energy from one compound to another in the process of photosynthesis.

The dark reactions are able to take place in the absence of sunlight, although they often occur during the daylight hours. During the dark reactions, the energy-rich compounds produced in the light reactions generate the compounds from which carbohydrates are eventually produced.

[See also Plant ]

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photosynthesis

photosynthesis The chemical process by which green plants and other phototrophs synthesize organic compounds from carbon dioxide and water in the presence of sunlight. In plants and most algae it occurs in the chloroplasts and there are two principal types of reactions. In the light-dependent reactions, which require the presence of light, energy from sunlight is absorbed by photosynthetic pigments (chiefly the green pigment chlorophyll) and used to bring about the photolysis of water: H2O → 2H+ + 2e + ½O2

The electrons released by this reaction pass along a series of electron carrier molecules; as they do so they lose their energy, which is used to convert ADP to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of water are used to reduce NADP: 2H+ + 2e + NADP+ → NADPH + H+

The ATP and NADPH produced during the light-dependent reactions provide energy and reducing power, respectively, for the ensuing light-independent reactions (formerly called the ‘dark reaction’), which nevertheless cannot be sustained without the ATP generated by the light-dependent reactions. During these reactions carbon dioxide is reduced to carbohydrate in a metabolic pathway known as the Calvin cycle. Photosynthesis can be summarized by the equation: CO2 + 2H2O → [CH2O] + H2O + O2

Since virtually all other forms of life are directly or indirectly dependent on plants for food, photosynthesis is the basis for all life on earth. Furthermore virtually all the atmospheric oxygen has originated from oxygen released during photosynthesis.

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photosynthesis

photosynthesis Term given to the series of metabolic reactions that occur in certain autotrophic organisms, whereby organic compounds are synthesized by the reduction of carbon dioxide using energy absorbed by chlorophyll from sunlight. In green plants, where water acts both as a hydrogen donor and as a source of released oxygen, photosynthesis may be summarized by the empirical equation: chlorophyll
CO2 + 2H2O → [CH2O] + H2O + O2
light
(oxygen being released as a gas). Photosynthetic bacteria are unable to utilize water and therefore do not produce oxygen. Instead they may use hydrogen sulphide (purple and green sulphur bacteria) or organic compounds (purple non-sulphur bacteria) as a source of hydrogen.

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photosynthesis

photosynthesis Chemical process occurring in green plants, algae, and many bacteria, by which water and carbon dioxide convert into food and oxygen using energy absorbed from sunlight. The reactions take place in the chloroplasts, which are microscopic green structures. During the first part of the process, the green pigment chlorophyll absorbs light and splits water into hydrogen and oxygen. The hydrogen attaches to a carrier molecule and the oxygen is set free. The hydrogen and light energy build a supply of cellular chemical energy, adenosine triphosphate (ATP). Hydrogen and ATP convert the carbon dioxide into sugars, including glucose and starch.

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photosynthesis

photosynthesis The series of metabolic reactions that occur in certain autotrophs, whereby organic compounds are synthesized by the reduction of carbon dioxide using energy absorbed by chlorophyll from sunlight. In green plants, where water acts as both a hydrogen donor and a source of released oxygen, photosynthesis may be summarized by the empirical equation:(oxygen being released as a gas). Photosynthetic bacteria are unable to utilize water and therefore do not produce oxygen. Instead they may use hydrogen sulphide (purple and green sulphur bacteria) or organic compounds (purple non-sulphur bacteria) as a source of hydrogen.

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photosynthesis

pho·to·syn·the·sis / ˌfōtōˈsin[unvoicedth]əsis/ • n. the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a byproduct. DERIVATIVES: pho·to·syn·thet·ic / -ˌsinˈ[unvoicedth]etik/ adj. pho·to·syn·thet·i·cal·ly / -ˌsinˈ[unvoicedth]etik(ə)lē/ adv.

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"photosynthesis." The Oxford Pocket Dictionary of Current English. . Retrieved October 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/photosynthesis-0

photosynthesis

photosynthesis The series of metabolic reactions occurring in certain autotrophs, whereby the energy of sunlight, absorbed by chlorophyll, powers the reduction of carbon dioxide (CO2) and the synthesis of organic compounds. In green plants, where water (H2O) acts as both a hydrogen donor and a source of released oxygen, photosynthesis may be summarized by the empirical equation:See dark reactions; light reactions; and photo-inhibition.

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"photosynthesis." A Dictionary of Ecology. . Encyclopedia.com. 22 Oct. 2017 <http://www.encyclopedia.com>.

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"photosynthesis." A Dictionary of Ecology. . Retrieved October 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/photosynthesis-0

photosynthesis

photosynthesis The synthesis of carbohydrates from carbon dioxide and water by plants in sunlight, with the release of oxygen.

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"photosynthesis." A Dictionary of Food and Nutrition. . Encyclopedia.com. 22 Oct. 2017 <http://www.encyclopedia.com>.

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"photosynthesis." A Dictionary of Food and Nutrition. . Retrieved October 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/photosynthesis

photosynthesis

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"photosynthesis." Oxford Dictionary of Rhymes. . Encyclopedia.com. 22 Oct. 2017 <http://www.encyclopedia.com>.

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"photosynthesis." Oxford Dictionary of Rhymes. . Retrieved October 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/photosynthesis