Definition (Encyclopedia of Global Warming)
Photosynthesis is the process by which plants convert light energy from the Sun into chemical energy in the form of carbohydrates. The name carbohydrate literally means “carbon plus water,” a vivid descriptive term for the sugar formed from combining carbon dioxide (CO2) with water. The process of photosynthesis begins with the absorption of photons by plant pigments. As photons reach a reaction center made up of chlorophylls, light energy excites electrons from the splitting of water molecules. A portion of the energy released by the energized electrons is used to produce adenosine triphosphate (ATP). ATP is then used to produce carbohydrates from CO2 and water. In the process, water acts as the source of both electrons and oxygen gas. Photosynthesis produces food for all living organisms, directly or indirectly, in all ecosystems. In addition, it releases oxygen and utilizes CO2 and thus becomes an indispensable link in the carbon cycle.
Several pathways of photosynthesis are employed by different plants as a response to different climatic conditions, primarily temperature and water availability. In order to survive terrestrial environments, all land plants must cope with water deficits from time to time. When plants open their numerous microscopic pores, calledstomata, to admit CO2 for photosynthesis, they risk losing water through these openings by evaporation. Plants will thus at times close stomata in order to conserve...
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Significance for Climate Change (Encyclopedia of Global Warming)
Both water and CO2 serve as essential raw materials in photosynthesis. Therefore, any climate change that affects the availability of either substance will affect photosynthesis. Given the various pathways different plants employ for photosynthesis, the same climate change will have different effects on different plants. Rising CO2 concentrations in the atmosphere would in theory increase the photosynthetic rates of all plants. This phenomenon is described as CO2 fertilization. However, the increase in photosynthesis from rising atmospheric CO2 concentrations is short-lived: The response decreases under long-term exposure, because plants acclimate to elevated CO2 concentrations through a process known as down regulation.
Plants’ response to rising temperatures is complex, may be positive or negative, and is often compounded by other climatic factors. Moisture in the environment is particularly influential, as changes in temperature may correlate to changes in dew point and evaporation rate, significantly affecting the availability of water. Depending upon the degree of increase, warming temperatures may drive some plants out of their natural habitats, causing biodiversity declines or species extinction.
Carbon 3 plants tend to increase their photosynthetic rate as a result of CO2 fertilization to a point where the associated temperature increase may offset the positive effect of rising CO2. If the...
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Further Reading (Encyclopedia of Global Warming)
Margaris, N. S., and H. A. Mooney, eds. Components of Productivity of Mediterranean-Climate Regions: Basic and Applied Aspects. Boston: W. Junk, 1981. Compendium of papers from a conference on photosynthesis, primary production, and biomass utilization. Details the effects of Mediterranean climate on photosynthesis and thus gives insight into the general relationship between photosynthesis and climate.
Morison, James I. L., and Michael D. Morecroft, eds. Plant Growth and Climate Change. Ames, Iowa: Blackwell, 2006. Includes extensive discussion of the relationship between global and regional climate change and photosynthesis.
Morton, Oliver. Eating the Sun: How Plants Power the Planet. New York: HarperCollins, 2008. Extensive scientific analysis of photosynthesis from the points of view of both geologic time and human history. Emphasizes the role of the process in human societies and its particular relation to contemporary environmental challenges.
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Photosynthesis (Encyclopedia of Science)
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.
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Photosynthesis (Science Experiments)
How it works
Experiment 1 Photosynthesis: How does light affect plant growth?
Design Your Own Experiment
To get our food, we go to the supermarket, pick vegetables or fruit from our gardens, or cast a rod in our favorite fishing hole. A plant, however, makes its own food using sunlight as its major energy source in a process called . In fact, the term photosynthesisChemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product. means "putting together by light."
In the eighteenth century, Jan Ingenhousz, a Dutch physician and plant , proved that sunlight was essential to the life activities of green plants. In 1779, he published experiments showing that plants have two respiratory cycles. At night, plants absorb oxygen and exhale carbon dioxide, just as animals do, but during the day the cycle is reversed. Another eighteenth-century scientist, Englishman Joseph Priestley, made similar discoveries about plant
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Photosynthesis (World of Microbiology and Immunology)
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 (1897985). 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