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Photosynthesis, the process by which green and certain other organisms transform energy into . During photosynthesis in green plants, light is captured and used to convert , , and minerals into and energy-rich organic .

plants: photosynthesisplants: photosynthesisThe location, importance, and mechanisms of photosynthesis.Encyclopædia Britannica, Inc.

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on . If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic , which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the (i.e., , , and ) that power . In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from , these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for and other products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major on Earth’s .

Requirements for food, materials, and energy in a world where population is rapidly growing have created a need to increase both the amount of photosynthesis and the of converting photosynthetic output into products useful to people. One response to those needs—the so-called , begun in the mid-20th century—achieved enormous improvements in agricultural yield through the use of chemical , pest and plant- control, , and mechanized tilling, harvesting, and crop processing. This effort limited severe to a few areas of the world despite rapid population , but it did not eliminate widespread . Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to decline. This was especially true for in Asia. Rising costs associated with sustaining high rates of agricultural production, which required ever-increasing inputs of fertilizers and pesticides and constant of new plant varieties, also became problematic for farmers in many countries.

A , based on plant , was forecast to lead to increases in plant productivity and thereby partially malnutrition. Since the 1970s, molecular biologists have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or ) with the aim of achieving improvements in disease and resistance, product yield and quality, hardiness, and other desirable properties. However, such traits are inherently complex, and the process of making changes to crop plants through genetic engineering has turned out to be more complicated than anticipated. In the future such genetic engineering may result in improvements in the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate that it could dramatically increase crop yields.

Another intriguing area in the study of photosynthesis has been the discovery that certain animals are able to convert light energy into chemical energy. The emerald green sea slug (), for example, acquires genes and chloroplasts from Vaucheria litorea, an it consumes, giving it a limited ability to produce . When enough chloroplasts are , the slug may forgo the ingestion of food. The (Acyrthosiphon pisum) can harness light to manufacture the energy-rich (ATP); this ability has been linked to the aphid’s manufacture of pigments.

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General characteristics

Development of the idea

The study of photosynthesis began in 1771 with observations made by the English clergyman and scientist . Priestley had burned a candle in a closed container until the air within the container could no longer support . He then placed a sprig of plant in the container and discovered that after several days the mint had produced some substance (later recognized as oxygen) that enabled the confined air to again support combustion. In 1779 the Dutch physician expanded upon Priestley’s work, showing that the plant had to be exposed to light if the combustible substance (i.e., oxygen) was to be restored. He also demonstrated that this process required the presence of the green tissues of the plant.

In 1782 it was demonstrated that the combustion-supporting gas (oxygen) was formed at the expense of another gas, or “fixed air,” which had been identified the year before as carbon dioxide. Gas-exchange experiments in 1804 showed that the gain in weight of a plant grown in a carefully weighed pot resulted from the uptake of carbon, which came entirely from absorbed carbon dioxide, and water taken up by plant roots; the balance is oxygen, released back to the atmosphere. Almost half a century passed before the concept of chemical energy had developed sufficiently to permit the discovery (in 1845) that light energy from the sun is stored as chemical energy in products formed during photosynthesis.

Overall reaction of photosynthesis

In chemical terms, photosynthesis is a light-energized . (Oxidation refers to the removal of electrons from a molecule; refers to the gain of electrons by a molecule.) In plant photosynthesis, the energy of light is used to drive the oxidation of water (H2O), producing oxygen gas (O2), (H+), and . Most of the removed electrons and hydrogen ions ultimately are transferred to carbon dioxide (CO2), which is reduced to organic products. Other electrons and hydrogen ions are used to reduce and to amino and sulfhydryl groups in , which are the building blocks of . In most green , —especially and the —are the major direct organic products of photosynthesis. The overall reaction in which carbohydrates—represented by the general formula (CH2O)—are formed during plant photosynthesis can be indicated by the following equation:

Chemical equation.

This equation is merely a summary statement, for the process of photosynthesis actually involves numerous reactions catalyzed by (organic ). These reactions occur in two stages: the “light” stage, consisting of (i.e., light-capturing) reactions; and the “dark” stage, chemical reactions controlled by . During the first stage, the energy of light is absorbed and used to drive a series of transfers, resulting in the synthesis of and the electron-donor-reduced (NADPH). During the dark stage, the ATP and NADPH formed in the light-capturing reactions are used to reduce carbon dioxide to organic carbon compounds. This assimilation of inorganic carbon into organic compounds is called carbon fixation.

During the 20th century, comparisons between photosynthetic processes in green plants and in certain photosynthetic provided important information about the photosynthetic mechanism. Sulfur bacteria use (H2S) as a source of and produce instead of oxygen during photosynthesis. The overall reaction is

Chemical equation.

In the 1930s Dutch biologist recognized that the utilization of carbon dioxide to form organic compounds was similar in the two types of photosynthetic organisms. Suggesting that differences existed in the light-dependent stage and in the nature of the compounds used as a source of hydrogen atoms, he proposed that hydrogen was transferred from hydrogen sulfide (in bacteria) or water (in green plants) to an unknown acceptor (called A), which was reduced to H2A. During the dark reactions, which are similar in both bacteria and green plants, the reduced acceptor (H2A) reacted with carbon dioxide (CO2) to form (CH2O) and to oxidize the unknown acceptor to A. This reaction can be represented as:

Chemical equation.

Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water, releasing oxygen) and that carbon then combined with water to form carbohydrate (rather than the hydrogen from water combining with CO2 to form CH2O).

By 1940 chemists were using to of photosynthesis. Water marked with an of oxygen (18O) was used in early experiments. Plants that photosynthesized in the presence of water containing H218O produced oxygen gas containing 18O; those that photosynthesized in the presence of normal water produced normal oxygen gas. These results provided definitive support for van Niel’s theory that the oxygen gas produced during photosynthesis is derived from water.

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