Photosynthesis

This entry was compiled, edited and written by: Cutler Cleveland

Photosynthesis is the conversion of light energy to chemical energy and its subsequent storage in the bonds of sugar. This process occurs in plants and some algae that use only solar radiation, carbon dioxide (CO2), and water (H2O) to make sugar. Photosynthesis is arguably the most important energy conversion process on Earth because the chemical energy it yields is the base of food chains that sustain the overwhelming majority of other life forms.

Origin of photosynthesis

Cyanobacteria were among the first photosynthetic organisms. Source: NOAA. Photosynthesis evolved over three billion years ago, shortly after the appearance of the first living organisms. The development of the biochemical process of photosynthesis is one of the most important events in the history of the Earth. Prior to photosynthesis, organisms obtained energy from oxidation-reduction reactions associated with weathering and hydrothermal activity. These energy sources were reduced forms of S, Fe2+, Mn2+, H2, and CH4. Hydrothermal sources of these compounds could support about 0.2 to 2.0 x 1012 mol C year-1 of organic carbon production by microorganisms capable of using hydrothermal energy as their energy source.

In contrast, global photosynthetic productivity is estimated at 9000 x 1012 mol C year-1. Thus, photosynthesis probablyincreased global organic productivity by at least two to three orders of magnitude. This enormous productivity resulted principally from the ability of oxygenic photosynthetic bacteria to capture hydrogen by cleaving water. This virtually unlimited supply of hydrogen freed life from its sole dependence upon abiotic chemical sources of reducing power, such as hydrothermal sources and weathering. Communities sustained by oxygenic photosynthesis could thrive wherever supplies of sunlight, moisture, and nutrients were sufficient.

At the present time, no known chemical system can be made to serve as a substitute for this process. Each CO2 molecule in the atmosphere is incorporated into a plant structure every 200 years and that all the O2 in air is renewed by plants every 2000 years. Coal, oil, and gas originated directly or indirectly from photosynthesis, since these fossil fuels were derived from the remains of living organisms.

Importance of photosynthesis1

It would be difficult to overstate the biological, ecological, economic, and technological importance of photosynthesis. By releasing oxygen and consuming carbon dioxide, it transformed the world into the hospitable environment we know today. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials. The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world. New forms of biomass energy may play important roles in our energy future.

Photosynthesis is the basis for all food production. Source: USDA.

Photosynthesis and food. All of our biological energy needs are met by the plant kingdom, either directly or through herbivorous animals. Plants in turn obtain the energy to synthesize foodstuffs via photosynthesis. Although plants draw necessary materials from the soil and water and carbon dioxide from the air, the energy needs of the plant are filled by sunlight.

Photosynthesis and energy. One of the carbohydrates resulting from photosynthesis is cellulose, which makes up the bulk of dry wood and other plant material. When we burn wood, we convert the cellulose back to carbon dioxide and release the stored energy as heat. Burning fuel is basically the same oxidation process that occurs in our bodies; it liberates the energy of "stored sunlight" in a useful form, and returns carbon dioxide to the atmosphere. Energy from burning "biomass" is important in many parts of the world. In developing countries, firewood continues to be critical to survival. Ethanol (grain alcohol) produced from sugars and starches by fermentation is a major automobile fuel in Brazil, and is added to gasoline in some parts of the United States to help reduce emissions of harmful pollutants. Ethanol is also readily converted to ethylene, which serves as a feedstock to a large part of the petrochemical industry.

Photosynthesis, fiber, and materials. Wood, of course, is not only burned, but is an important material for building and many other purposes. Paper, for example, is nearly pure photosynthetically produced cellulose, as is cotton and many other natural fibers. Even wool production depends on photosynthetically-derived energy. In fact, all plant and animal products including many medicines and drugs require energy to produce, and that energy comes ultimately from sunlight via photosynthesis. Many of our other materials needs are filled by plastics and synthetic fibers which are produced from petroleum, and are thus also photosynthetic in origin. Even much of our metal refining depends ultimately on coal or other photosynthetic products. Indeed, it is difficult to name an economically important material or substance whose existence and usefulness is not in some way tied to photosynthesis.

Photosynthesis and the environment. There is a lot of discussion concerning the possible effects of carbon dioxide and other "greenhouse gases" on the environment. The magnitiude of such effects will depend strongly on the effect of photosynthesis carried out by land and sea organisms. As photosynthesis consumes carbon dioxide and releases oxygen, it helps counteract the effect of combustion of fossil fuels. The burning of fossil fuels releases not only carbon dioxide, but also hydrocarbons, nitrogen oxides, and other trace materials that pollute the atmosphere and contribute to long-term health and environmental problems. These problems are a consequence of the fact that nature has chosen to implement photosynthesis through conversion of carbon dioxide to energy-rich materials such as carbohydrates.

The process of photosynthesis

Organisms that use solar energy to convert inorganic forms of energy to organic forms of energy are termed phototrophs; they are a type of autotroph, from the Greek autos = self and trophe = nutrition. The term autotroph is slightly misleading. The first law of thermodynamics states that energy cannot be created, therefore autotrophs do not really create their own food. Rather, autotrophs use inorganic forms of energy to power chemical reactions that store energy in the bonds of organic molecules. Phototrophs generate organic forms of energy via photosynthesis.

The general formula for photosynthesis is given by:

$6CO_2 + 6H_{2}O \rightarrow C_{6}H_{12}O_{6} + 6O_2$

Solar energy is critical to photosynthesis. Carbon dioxide and water do not combine spontaneously to form the sugar glucose. Rather, solar energy is used to break the bond between the hydrogen and oxygen atoms in the water molecule and to incorporate the hydrogen atoms in the carbon dioxide molecule to form glucose. As a work process, photosynthesis is relatively inefficient; only about 1.0 percent of the solar energy absorbed by a leaf is converted to organic energy in the glucose molecule.

Chlorophyll

In plants, photosynthesis occurs in organelles called chloroplasts. All of the green structures in plants, including stems and unripened fruit, contain chloroplasts, but the majority of photosynthesis activity in most plants occurs in the leaves. On the average, the chloroplast density on the surface of a leaf is about one-half million per square millimeter. Chloroplasts are one of several different types of plastids, plant cell organelles that are involved in energy storage and the synthesis of metabolic materials.

A cross-section of leaf showing the location of chloroplasts.

In the chloroplasts is chlorophyll, a green pigment that absorbs light most strongly in the blue and red but poorly in the green portions of the electromagnetic spectrum. Chlorophyll gives hence the green color of chlorophyll-containing tissues like plant leaves.

The basic structure of a chlorophyll molecule is a porphyrin ring, a multi-ring‚ carbon-based molecule with nitrogens at its central corners. This is very similar in structure to the heme group found in hemoglobin, except that in heme the central atom is iron, whereas in chlorophyll it is magnesium.

In the photosynthetic reaction, carbon dioxide is reduced by water. that is, electrons are transferred from water to carbon dioxide. Chlorophyll assists this transfer. When chlorophyll absorbs light energy, an electron in chlorophyll is excited from a lower energy state to a higher energy state. In this higher energy state, this electron is more readily transferred to another molecule. This starts a chain of electron-transfer steps, which ends with an electron transferred to carbon dioxide. Meanwhile, the chlorophyll which gave up an electron can accept an electron from another molecule. This is the end of a process which starts with the removal of an electron from water. Thus, chlorophyll is at the center of the photosynthetic oxidation-reduction reaction between carbon dioxide and water.

Light and dark reactions

The light reaction convert light energy into chemical energy in the form of ATP & NADPH that carry energy to the second part of photosynthesis, known as the dark reactions. The light reactions are defined by these steps:

• Light absorption & splitting of water.
• Production of ATP.
• Movement of electrons through electron acceptors to power a hydrogen pump.
• Re-energizing electrons so they can produce NADPH.

The reactions of photosynthesis that are not directly dependent upon light are known as the dark reactions; they occur whether there is light present or not. The dark reactions occur in the part of the chloroplast known as the stroma. Dark reactions the energy from ATP and energized electrons and hydrogen ions from NADPH and add them to CO2 to make glucose or sugar.

History

 Highlights in photosynthesis research 1771 Joseph Priestley, England, discovers that plants can "purify" air that has been "burned out" by a candle. 1779 Jan Ingenhousz, The Netherlands, demonstrates that the plant in Priestley's experiment is dependent on light and its green parts. 1782-1804 Several researchers show that carbon dioxide and water are stored as organic matter by plants. 1845 Robert Mayer, Germany, points out that plants store solar energy in organic matter. ca 1915 Richard Willstätter, Germany, (Nobel Prize 1915) suggests that chlorophyll plays an active role in plants. ca 1930 Cornelis van Niel, USA, proposes that photosynthesis is based on oxidation-reduction reactions and that the primary reaction is a photolysis of water followed by oxygen evolution. 1932 Robert Emerson and William Arnold, USA, conclude that several hundred chlorophyll molecules cooperate in photosynthesis. 1939 Robert Hill, England, demonstrates that photolysis of water and carbon dioxide fixation are separate processes. 1940 Hans Fischer, Germany, solves the chemical structure of chlorophyll. (Nobel Prize 1930 for his investigations of hemes and chlorophyll.) 1954 Melvin Calvin, USA, (Nobel Prize 1961) and coworkers unravel the reactions of carbon dioxide fixation. 1954 Daniel Arnon, USA, discovers light-dependent synthesis of ATP (photophosphorylation). 1960-1961 Robert Hill and Fay Bendall, England, and independently Louis Duysens, The Netherlands, show how two separate photosystems cooperate in plants. 1968 William Parson, USA, confirms Duysens' hypothesis (1956) that chlorophyll is oxidized in the primary reaction of photosynthesis. 1984 Johann Deisenhofer, Robert Huber and Hartmut Michel,reaction center from a bacterium. The Federal Republic of Germany, solve the structure of a photosynthetic

1 This section is drawn from: Gust, Devens, Why Study Photosynthesis? Department of Chemistry and Biochemistry, Arizona State University, Accessed 13 January 2008.

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