Photosynthesis is the biological conversion of light energy into chemical energy. It occurs in green plants and photosynthetic bacteria through a series of many biochemical reactions. In higher plants and algae , light absorption by chlorophyll catalyzes the synthesis of carbohydrate (C6H12O6) and oxygen gas (O2) from carbon dioxide gas (CO2) and water (H2O). Thus, the overall chemical equation for photosynthesis in higher plants is expressed as:
The overall equation in photosynthetic bacteria is similar, although not identical.
History of research
People have long been interested in how plants obtain the nutrients they use for growth. The early Greek philosophers believed that plants obtained all of their nutrients from the soil . This was a common belief for many centuries.
In the first half of the seventeenth century, Jan Baptista van Helmont (1579-1644), a Dutch physician, chemist, and alchemist, performed important experiments which disproved this early view of photosynthesis. He grew a willow tree weighing 5 lb (2.5 kg) in a clay pot which had 200 lb (91 kg) of soil. Five years later, after watering his willow tree as needed, it weighed about 169 lb (76.5 kg) even though the soil in the pot lost only 2 oz (56 g) in weight. Van Helmont concluded that the tree gained weight from the water he added to the soil, and not from the soil itself. Although van Helmont did not understand the role of sunlight and atmospheric gases in plant growth, his early experiment advanced our understanding of photosynthesis.
In 1771, the noted English chemist Joseph Priestley performed a series of important experiments which implicated atmospheric gases in plant growth. Priestley and his contemporaries believed a noxious substance, which they called phlogiston, was released into the air when a flame burned. When Priestley burned a candle within an enclosed container until the flame went out, he found that a mouse could not survive in the "phlogistated" air of the container. However, when he placed a sprig of mint in the container after the flame had gone out, he found that a mouse could survive. Priestley concluded that the sprig of mint chemically altered the air by removing the "phlogiston." Shortly after Priestly's experiments, Dutch physician Jan Ingenhousz (1730-1799) demonstrated that plants "dephlogistate" the air only in sunlight, and not in darkness. Further, Ingenhousz demonstrated that the green parts of plants are necessary for" dephlogistation" and that sunlight by itself is ineffective.
As Ingenhousz was performing his experiments, the celebrated French chemist Antoine Lavoisier (1743-1794) disproved the phlogiston theory. He conclusively demonstrated that candles and animals both consume a gas in the air which he named oxygen. This implied that the plants in Priestley's and Ingenhousz's experiments produced oxygen when illuminated by sunlight. Considered by many as the founder of modern chemistry , Lavoisier was condemned to death and beheaded during the French revolution.
Lavoisier's experiments stimulated Ingenhousz to reinterpret his earlier studies of "dephlogistation." Following Lavoisier, Ingenhousz hypothesized that plants use sunlight to split carbon dioxide (CO2) and use its carbon (C) for growth while expelling its oxygen (O2) as waste. This model of photosynthesis was an improvement over Priestley's, but was not entirely accurate.
Ingenhousz's hypothesis that photosynthesis produces oxygen by splitting carbon dioxide was refuted about 150 years later by the Dutch-born microbiologist Cornelius van Niel (1897-1985) in America. Van Niel studied photosynthesis in anaerobic bacteria, rather than in higher plants. Like higher plants, these bacteria make carbohydrates during photosynthesis. Unlike plants, they do not produce oxygen during photosynthesis and they use bacteriochlorophyll rather than chlorophyll as a photosynthetic pigment. Van Niel found that all species of photosynthetic bacteria which he studied required an oxidizable substrate. For example, the purple sulfur bacteria use hydrogen sulfide as an oxidizable substrate and the overall equation for photosynthesis in these bacteria is:
On the basis of his studies with photosynthetic bacteria, van Niel proposed that the oxygen which plants produce during photosynthesis is derived from water, not from carbon dioxide. In the following years, this hypothesis has proven true. Van Niel's brilliant insight was a major contribution to our modern understanding of photosynthesis.
The study of photosynthesis is currently a very active area of research in biology . Hartmut Michel and Johann Deisenhofer recently made a very important contribution to our understanding of photosynthesis. They made crystals of the photosynthetic reaction center from Rhodopseudomonas viridis, an anaerobic photosynthetic bacterium, and then used x-ray crystallography to determine its three-dimensional structure. In 1988, they shared the Nobel Prize in Chemistry with Robert Huber for this ground-breaking research.
Modern plant physiologists commonly think of photosynthesis as consisting of two separate series of interconnected biochemical reactions, the light reactions and the dark reactions. The light reactions use the light energy absorbed by chlorophyll to synthesize labile high energy molecules. The dark reactions use these labile high energy molecules to synthesize carbohydrates, a stable form of chemical energy which can be stored by plants. 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 which 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 do not appear in the overall equation for photosynthesis because they are consumed during the subsequent dark reactions in the synthesis of carbohydrates.
Location of light reactions
In higher plants and algae, the light reactions occur on the thylakoid membranes of the chloroplasts. The thylakoid membranes are inner membranes of the chloroplasts which are arranged like flattened sacs. The thylakoids are often stacked on top of one another, like a roll of coins. A stack of thylakoids is referred to as a granum.
The light reactions of higher plants require photosynthetic pigments, chlorophyll-a, chlorophyll-b, and various types of carotenoids. These pigments are associated with special proteins which are embedded in the thylakoid membranes. Chlorophyll-a and chlorophyll-b strongly absorb light in the red and blue regions of the spectrum . Most carotenoids strongly absorb blue light. Thus, plant leaves are green simply because their photosynthetic pigments absorb blue and red light but not green light.
Non-cyclic energy transfer
Once light is absorbed by pigments in the chloroplast , its energy is transferred to one of two types of reaction centers, Photosystem-II (PS-II) or Photosystem-I (PS-I).
In non-cyclic energy transfer , light absorbed by PS-II splits a water molecule , producing oxygen and exciting chlorophyll to a higher energy level. Then, the excitation energy passes through a series of special electron carriers. Each electron carrier in the series is slightly lower in energy than the previous one. During electron transfer, the excitation energy is harnessed to synthesize ATP. This part of photosynthesis is referred to as non-cyclic photophosphorylation, where "photo-" refers to the light requirement and "-phosphorylation" refers to addition of a phosphate to ADP (adenosine diphosphate ) to make ATP.
Finally, one of the electron carriers of PS-II transfers electrons to PS-I. When chlorophyll transfers its excitation energy to PS-I, it is excited to higher energy levels. PS-I harnesses this excitation energy to make NADPH, analogous to the way PS-II harnessed excitation energy to make ATP.
In the 1950s, the botanist Robert Emerson (1903-1959) demonstrated that the rate of photosynthesis was much higher under simultaneous illumination by shorter wavelength red light (near 680 nm) and long wavelength red light (near 700 nm). We now know this is because PS-II absorbs shorter wavelength red light (680 nm) whereas PS-I absorbs long wavelength red light (700 nm) and both must be photoactivated to make the ATP and NADPH needed by the dark reactions.
Cyclic energy transfer
ATP can also be made by a special series of light reactions referred to as cyclic photophosphorylation. This also occurs in the thylakoid membranes of the chloroplast. In cyclic photophosphorylation, the excitation energy from PS-I is transferred to a special electron carrier and this energy is harnessed to make ATP.
The relative rates of cyclic and non-cyclic photophosphorylation determine the ratio of ATP and NADPH which become available for the dark reactions. Photosynthetic plant cells regulate cyclic and non-cyclic energy transfer by phosphorylating (adding a phosphate) to the pigment-protein complexes associated with PS-I and PS-II.
The photosynthetic dark reactions consist of a series of many enzymatic reactions which make carbohydrates from carbon dioxide. The dark reactions do not require light directly, but they are dependent upon ATP and NADPH which are synthesized in the light reactions. Thus, the dark reactions indirectly depend on light and usually occur in the light. The dark reactions occur in the aqueous region of the chloroplasts, referred to as the stroma.
The main part of the dark reactions is often referred to as the Calvin cycle, in honor of their discoverer, the chemist Melvin Calvin. The Calvin cycle consists of 13 different biochemical reactions, each catalyzed by a specific enzyme . The Calvin cycle can be summarized as consisting of carboxylation, reduction, and regeneration. Its final product is starch, a complex carbohydrate.
In carboxylation, a molecule of carbon dioxide (with one carbon atom) is combined with a molecule of RuBP (ribulose bisphosphate, with five carbon atoms ) to make two molecules of PGA (phosphoglycerate), each with three carbon atoms. This reaction is catalyzed by the enzyme RuBISCO (Ribulose bisphosphate carboxylase). RuBISCO accounts for about 20% of the total amount of protein in a plant leaf and is by far the most abundant enzyme on Earth .
In reduction, ATP and NADPH (made by the light reactions) supply energy for synthesis of high energy carbohydrates from the PGA made during carboxylation. Plants often store their chemical energy as carbohydrates because these are very stable and easily transported throughout the organism .
In regeneration, the carbohydrates made during reduction pass through a series of enzymatic reactions so that RuBP, the initial reactant in carboxylation, is regenerated. The regeneration of RuBP is the reason these reactions are considered a cycle. Once the Calvin cycle has gone around six times, six molecules of carbon dioxide have been fixed, and a molecule of glucose, a six-carbon carbohydrate, is produced.
The series of dark reactions described above is often referred to as C-3 photosynthesis because the first reaction product of carbon dioxide fixation is a 3-carbon molecule, PGA (phosphoglycerate).
In the early 1960s, plant physiologists discovered that sugarcane and several other plants did not produce the three-carbon molecule, PGA, as the first reaction product of their dark reactions. Instead, these other plants combined carbon dioxide with PEP (phosphoenol pyruvate), a three-carbon molecule, to make OAA (oxaloacetate), a four-carbon molecule. After a series of additional enzymatic reactions, carbon dioxide is introduced to the Calvin cycle, which functions more or less as described above.
This variant of photosynthesis is referred to as C-4 photosynthesis because carbon dioxide is first fixed into a four-carbon molecule, OAA. C-4 photosynthesis occurs in many species of tropical grasses and in many important agricultural plants such as corn, sugarcane, rice , and sorghum .
Plants which have C-4 photosynthesis partition their C-4 metabolism and their Calvin cycle metabolism into different cells within their leaves. Their C-4 metabolism occurs in mesophyll cells, which constitute the main body of the leaf. The Calvin cycle occurs in specialized cells referred to as bundle sheath cells. Bundle sheath cells surround the vascular tissue (veins ) which penetrate the main body of the leaf.
In at least 11 different genera of plants, some species have C-3 metabolism whereas other species have C-4 metabolism. Thus, plant physiologists believe that C-4 photosynthesis evolved independently many times in many different species. Recently, plant physiologists have found that some plant species are C-3/C-4 intermediates, in that they perform C-3 photosynthesis in some environments and C-4 photosynthesis in other environments. Study of these intermediates may help elucidate the evolution and physiological significance of C-4 photosynthesis.
Another variant of photosynthesis was originally found in many plants of the Crassulaceae family. The photosynthetic leaves of these plants accumulate malic acid or isocitric acid at night and metabolize these acidic compounds during the day. This type of photosynthesis is referred to as Crassulacean Acid Metabolism or more simply, CAM photosynthesis.
During the night, the following reactions occur in plants with CAM photosynthesis: (a) they open up special pores in their leaves, referred to as stomata, and the leaves take in carbon dioxide from the atmosphere; (b) they metabolize some of their stored starch to PEP (phosphoenol pyruvate), a 3-carbon molecule; (c) they combine carbon dioxide with PEP to form malic acid or isocitric acid, 4-carbon molecules; (d) they accumulate large amounts of malic acid or isocitric acid in their leaves, so that they taste somewhat sour if sampled at night or early morning.
During the day, the following reactions occur in plants with CAM photosynthesis: (a) they close their stomata; (b) they release carbon dioxide from the accumulated malic acid or isocitric acid; (c) they combine this released carbon dioxide with RuBP and the Calvin cycle operates more or less as described above.
Most plants with CAM photosynthesis grow in deserts and other arid environments. In such environments, evaporative loss of water is lower in CAM plants because they close their stomata during the day.
Species from over 20 different plant families, including Cactaceae, Orchidaceae, Liliaceae, and Bromeliaceae have been identified as having CAM photosynthesis. Thus, plant physiologists believe that CAM photosynthesis evolved independently many times. Many CAM plants are succulents, plants with thick leaves and a high ratio of volume to surface area. Interestingly, while CAM photosynthesis is genetically determined, some plants can switch from C-3 photosynthesis to CAM photosynthesis when they are transferred to an arid environment.
In the 1920s, the German biochemist Otto Warburg (1883-1970) discovered that plants consumed oxygen at a higher rate when they were illuminated. He also found that this increased rate of oxygen consumption inhibited photosynthesis. Stimulation of oxygen consumption by light is now referred to as photorespiration. Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high-energy molecules made by the light reactions. Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates.
RuBISCO, the enzyme which fixes carbon dioxide during the Calvin cycle, is also responsible for oxygen fixation during photorespiration. In particular, carbon dioxide and oxygen compete for access to RuBISCO. RuBISCO's affinity for carbon dioxide is much higher than its affinity for oxygen. Thus, fixation of carbon dioxide typically exceeds fixation of oxygen, even though atmospheric carbon dioxide levels are about 0.035% whereas oxygen is about 21%.
If photorespiration is so wasteful, why does it occur at all? Many plant physiologists believe that photorespiration is an artifact of the ancient evolutionary history of photosynthesis. In particular, RuBISCO originated in bacteria several billion years ago when there was very little atmospheric oxygen present. Thus, there was little selection pressure for the ancient RuBISCO to discriminate between carbon dioxide and oxygen and RuBISCO originated with a structure that reacts with both. Even though most modern plants are under great selection pressure to reduce photorespiration, evolution cannot easily alter RuBISCO's structure so that it fixes less oxygen yet still efficiently fixes carbon dioxide.
Interestingly, photorespiration has been observed in all C-3 plants which have been examined, but is virtually nonexistent in C-4 plants. This is because C-4 plants segregate their RuBISCO enzyme in bundle sheath cells deep within the leaf and the carbon dioxide concentration in these cells is maintained at very high levels. C-4 plants generally have higher growth rates than C-3 plants simply because they do not waste their ATP and NADPH in photorespiration.
Photosynthesis in lower organisms
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 PSII), and the same overall chemical reactions for photosynthesis (equation 1). They differ from higher plants in having different complements of additional chlorophylls. The Chlorophyta and Euglenophyta have chlorophyll-a and chlorophyll-b. The Chrysophyta, Pyrrophyta, and Phaeophyta have chlorophyll-a and chlorophyll-c. The 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.
This group was formerly called the blue-green algae and these organisms 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). The 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.
This is a group of bacteria represented by a single genus, Prochloron. Like the Cyanobacteria, the Chloroxybacteria are prokaryotes. 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 (equation 1). In general, Prochloron is rather like a free-living chloroplast from a higher plant.
Anaerobic photosynthetic bacteria
This is a group of bacteria which do not produce oxygen during photosynthesis and only photosynthesize in environments which are anaerobic (lacking oxygen). All these bacteria use carbon dioxide and another oxidizable substrate, such as hydrogen sulfide, to make carbohydrates (see equation 2). These bacteria have bacteriochlorophylls and other photosynthetic pigments which are similar to the chlorophylls used by higher plants. Their photosynthesis is different from that of higher plants, algae and cyanobacteria in that they only have one photosystem. This photosystem is similar to PS-I. Most biologists believe that photosynthesis first evolved in anaerobic bacteria several billion years ago.
There are two species in the genus Halobacterium. Most biologists now place this genus with methanogenic (methane-producing) bacteria in the Archaebacteria , a separate kingdom of organisms. Halobacteria thrive in very salty environments, such as the Dead Sea and the Great Salt Lake. In general, halobacteria prefer environments with NaCl concentration of about 5 Molar, and cannot tolerate environments with NaCl concentration below about 3 Molar.
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 which humans and other animals use 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, referred to as photoisomerization. Retinal photoisomerization leads to the synthesis of ATP, the same high-energy compound synthesized during the light reactions of higher plants. Interestingly, halobacteria also have two additional rhodopsins, sensory rhodopsin-I and sensory rhodopsin-II which regulate phototaxis, the directional movement in response to light. Bacteriorhodopsin and halorhodopsin seem to have an indirect role in phototaxis as well.
See also Plant pigment.
Attenborough, D. The Private Life of Plants. Princeton, NJ: Princeton University Press, 1995.
Buchanan, B.B., W. Gruissem, and R.L. Jones. Biochemistry and Molecular Biology of Plants. Rockville, MD: American Society of Plant Physiologists, 2000.
Corner, E.J. The Life of Plants. Chicago: University of Chicago Press, 1981.
Galston, A W. Life Processes of Plants: Mechanisms for Survival. New York: W. H. Freeman Press, 1993.
Kaufman, P.B., et al. Plants: Their Biology and Importance. New York: HarperCollins, 1990.
Wilkins, M. Plant Watching. New York: Facts on File, 1988.
Demmig-Adams, B., and W. W. Adams III. "Photosynthesis: Harvesting Sunlight Safely." Nature 403 (January 2000): 371-374.
Li, X. P., O. Bjorkman, C. Shih, et al. " A Pigment-binding Protein Essential for Regulation of Photosynthetic Light Harvesting." Nature 403 ; (January 2000): 391-395.
Peter A. Ensminger
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- Calvin cycle
—Dark reactions of photosynthesis which use the ATP and NADPH made by the light reactions to synthesize carbohydrates.
—Green organelle in higher plants and algae in which photosynthesis occurs.
- Cyanobacteria (singular, cyanobacterium)
—Photosynthetic bacteria, commonly known as blue-green alga.
—Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.
—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
—Membrane enclosed structure within a eukaryotic cell which is specialized for specific cellular functions.
—Cell without a nucleus, considered more primitive than a eukaryote.
—Pores in plant leaves which function in exchange of carbon dioxide, oxygen, and water during photosynthesis.
—The material that bathes the interior of chloroplasts in plant cells.
—A membranous structure that bisects the interior of a chloroplast.
Photosynthesis is the biological conversion of light energy into chemical energy. It occurs in green plants and photosynthetic bacteria through a series of many biochemical reactions.
In plants and algae, light absorption by chlorophyll catalyzes the synthesis of carbohydrate (C6H12O6) and oxygen gas (O2) from carbon dioxide gas (CO2) and water (H2O). The process in bacteria is similar but not identical.
Up until the early decades of the twentieth century, the prevailing scientific opinion was that photo-synthesis produced oxygen by splitting carbon dioxide. But this dogma was refuted by the Dutchborn American resident microbiologist Cornelius van Niel (1897-1985). Van Niel studied photosynthesis in anaerobic bacteria, rather than in higher plants. Like higher plants, these bacteria make carbohydrates during photosynthesis. Unlike plants, however, they do not produce oxygen during photosynthesis; they use bacteriochlorophyll rather than chlorophyll as a photosynthetic pigment. Van Niel established that photo-synthetic bacteria like purple sulfur bacteria require a compound that can accept electrons (in other words, the compound becomes oxidized).
Van Niel’s proposal that the oxygen which plants produce during photosynthesis is derived from water, not from carbon dioxide, was proven true. This brilliant insight was a major contribution to our modern understanding of photosynthesis.
Modern plant physiologists commonly think of photosynthesis as consisting of two separate series of interconnected biochemical reactions, the light reactions and the dark reactions. The light reactions use the light energy absorbed by chlorophyll to synthesize high-energy molecules that break down rapidly. The dark reactions use these molecules to synthesize carbohydrates, a more stable form of chemical energy that can be stored by plants. Although the dark reactions do not require light, hence their name, 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 which are enclosed by double membranes.
In the light reactions of photosynthesis, light energy excites photosynthetic pigments to higher energy levels. When liberated, the energy is used to make two other high energy compounds, ATP (adeno-sine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
In higher plants and algae, the light reactions occur on the thylakoid membranes of the chloroplasts. The thylakoid membranes are inner membranes of the chloroplasts, which are arranged like flattened sacs. The thylakoids are often stacked on top of one another, like a roll of coins. A stack of thylakoids is referred to as a granum.
The light reactions of higher plants require two photosynthetic compounds that contain chlorophyll and other compounds that are called carotenoids. These pigments are associated with special proteins that are embedded in the thylakoid membranes. The chlorophyll compounds strongly absorb light in the red and blue regions of the spectrum. Most carotenoids strongly absorb blue light. Thus, plant leaves are green simply because their photosynthetic pigments absorb blue and red light but not green light. The absorbed light is utilized in a series of chemical reactions that synthesize the high-energy ATP and NADPH compounds.
There are many different groups of photosynthetic algae. Like higher plants, they all have a chlorophyll compound as a photosynthetic pigment. They differ from higher plants in having different complements of additional chlorophylls. The different chlorophylls and other photosynthetic pigments allow algae to utilize different regions of the solar spectrum to drive photosynthesis.
Another photosynthetic microorganism are the cyanobacteria. This group was formerly called the blue-green algae and these organisms 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. Their photosynthetic mechanism is similar in some respects to that of higher plants. But, 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.
Anaerobic photosynthetic bacteria, which cannot grow in the presence of oxygen do not produce oxygen during photosynthesis and only photosynthesize in environments which are lack oxygen (anaerobic). These bacteria use carbon dioxide and another oxidizable substrate such as hydrogen sulfide to make carbohydrates, and have bacteriochlorophylls and other photosynthetic pigments which are similar to the chlorophylls used by higher plants.
Another type of photosynthetic bacteria are two species in the genus Halobacterium. Most biologists now place this genus with methanogenic (methane-producing) bacteria in the Archaebacteria, a separate kingdom of organisms. 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 that humans and other animals use 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, referred to as photoisomerization. Retinal
Calvin cycle— A series of energy-requiring photo-synthesis reactions used synthesize carbohydrates.
Chloroplast— Green organelle in higher plants and algae in which photosynthesis occurs.
Cyanobacteria (singular, cyanobacterium)— Photo-synthetic bacteria, commonly known as blue-green alga.
Enzyme— Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.
Eukaryote— A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
Organelle— Membrane enclosed structure within a eukaryotic cell which is specialized for specific cellular functions.
Prokaryote— Cell without a nucleus, considered more primitive than a eukaryote.
Stomata— Pores in plant leaves which function in exchange of carbon dioxide, oxygen, and water during photosynthesis.
Stroma— The material that bathes the interior of chloroplasts in plant cells.
Thylakoid— A membranous structure that bisects the interior of a chloroplast.
photoisomerization leads to the synthesis of ATP, the same high-energy compound synthesized during the light reactions of higher plants.
Blankenship, Robert E. Molecular Mechanisms of Photosynthesis. Boston: Blackwell Publishing, 2002.
Collings, Anthony F. and Christa Critchley. Artificial Photosynthesis: From Basic Biology to Industrial Application. New York: John Wiley & Sons, 2005.
Govindjee, J.T. Beatty, H. Gest, and J.F. Allen, eds. Discoveries in Photosynthesis (Advances in Photosynthesis and Respiration). New York: Springer, 2006.
Hopkins, William G. Photosynthesis And Respiration (The Green World). New York: Chelsea House Publications, 2006.
Peter A. Ensminger
Photosynthesis is the process by which plants use light energy to make food from simple chemicals. Photosynthesis is vital to all life on Earth since all food comes from this process, either directly or indirectly. People not only eat green plants and their fruit and grains, but they also eat the animals that feed on the green plants.
The word photosynthesis means "putting together with light," and perfectly describes a process by which a plant converts carbon dioxide and water into food by using light. The miracle of photosynthesis is that it captures light energy and converts it into chemical energy that can be used by organisms. Photosynthesis occurs inside the leaf of a plant at the cellular level. Plant cells contain chloroplasts. These chloroplasts contain a green pigment called chlorophyll. The flat leaf, acting as a solar collector, allows the light to strike the chlorophyll, which is stimulated to absorb it. In the chloroplasts, the light reacts with carbon dioxide (that the plant breathes in through microscopic holes in its leaves called stomata) and with water (that the plant takes in through its roots). During a series of complicated reactions, the water molecules are broken down into hydrogen and oxygen, and the hydrogen combines with carbon dioxide to produce glucose, a simple sugar that is used as a building block for starch and other complex carbohydrates. The excess oxygen is later released through the stomata into the atmosphere. The plants use the glucose as food. What they do not use they convert to starch for storage and to build cells walls.
If all life on Earth depends on photosynthesis, then all life really begins with what makes photosynthesis work—light. Sunlight is the energy that travels from the Sun. It arrives on Earth in waves of different lengths, and those lengths give it its different colors. As a combination of different wavelengths, sunlight is really a mixture of violet, blue, green, yellow, orange, and red light. These wavelengths can be observed by passing sunlight through a prism. Chlorophyll is extremely efficient and absorbs red, orange, and blue light, allowing only green light to pass through. This is what makes a leaf look green. When chlorophyll absorbs the Sun's light, the first part of photosynthesis begins as light energy splits up water molecules (hydrogen and oxygen). This process is called photolysis and produces energy-carrying molecules called adenosine triphosphate (ATP). In the second part of photosynthesis, energy from ATP and other energy carriers remove oxygen from carbon dioxide, allowing the carbon and hydrogen to combine and form glucose.
Dutch plant physiologist (a person who studies how an organism and its body parts work or function normally) Jan Ingenhousz (1730–1799) discovered photosynthesis and plant respiration. He demonstrated that plants use sunlight and carbon dioxide (atmospheric gas) to make their own food, and that they give off oxygen as a by-product. He was the first to indicate the close connection between animals and plants and to show how much animals depended on green plants.
Jan Ingenhousz was born in Breda, The Netherlands, where he received his basic education. He then attended the University of Louvain and later the University of Leyden. After receiving his medical degree, he worked at a private practice in Breda, but left for England when his father died in 1765. While at work in a hospital there, he became an expert in the new technique of smallpox inoculation. This was a hazardous occupation since he administered a live virus instead of today's weakened vaccine. However, his treatments were so effective that he was called to Vienna, Austria, to inoculate the royal family. He was then appointed court physician and given a lifetime income. This allowed him to pursue his research, and in 1779, after seven years in Vienna, he returned to England where he would remain for the remainder of his life.
It was also that year that he began experiments that led to his discovery of photosynthesis, the process by which plants convert sunlight into food. Ingenhousz found that green plants take in carbon dioxide and give off oxygen, but they do this only in the presence of sunlight. In the dark, he found that the opposite happens, and like animals, they absorb oxygen and give off carbon dioxide. This is called respiration. This was the first recognition that sunlight played a key role in the life of plants. Ingenhousz also proved that only the actual, visible light and not the Sun's heat, was necessary for photosynthesis to work. Others had been experimenting with air at this time, and the work of English chemist, Joseph Priestley (1733–1804), showed that a candle flame burning in a closed container eventually would go out. He also found that small animals placed in a similar space eventually died since all the oxygen was consumed and only carbon dioxide was left. Ingenhousz realized that since plants give off oxygen, which is essential to animal life, and then took in the carbon dioxide that animals breathed out as a waste product, there was a fundamental connection between plants and animals that no one before had realized. To Ingenhousz, photosynthesis meant that animals and plants are totally dependent on one another. We now know that photosynthesis is the key to all life on Earth since it provides food, either directly (for plants) or indirectly (for animals that eat the plants or eat other animals that have eaten plants) for virtually every living thing. For Ingenhousz, plants "purified" the air and "revitalized" it. His 1779 book detailed his plant discoveries and laid the foundation for the continued study of photosynthesis. Ingenhousz also broke new ground in physics and chemistry. For example, he improved phosphorous matches, invented a hydrogen-fueled lighter, and mixed an explosive propellant for firing pistols.
It has taken scientists hundreds of years to understand what happens during photosynthesis. Beginning in the early seventeenth century, the work of Flemish physician Jan Baptist van Helmont (1577–1644), and later the English botanist (a person specializing in the study of plants) Stephen Hales (1677–1761), demonstrated that plants needed air and water to grow. In the eighteenth century, chemists began to identify individual gases, and in 1779, the Dutch physician, Jan Ingenhousz (1730–1799), showed that plants take in carbon dioxide and release oxygen when light shines on them. By the 1880s, the German physiologist (a person specializing the study of the processes of living things), Theodor Wilhelm Engelmann (1843–1909), showed that the light reactions that capture solar energy and convert it into chemical energy occur in chloroplasts. It was not until the twentieth century, however, that scientists began to fully understand the complex biochemistry of photosynthesis.
Photosynthesis is a key part of a cycle that not only maintains life on Earth but keeps Earth's levels of carbon dioxide and oxygen in balance. Plants convert carbon dioxide into food and oxygen, which animals "burn" in a process called respiration (combining food with oxygen to release energy). Respiration is therefore the opposite or reverse of photosynthesis. In respiration, oxygen is used up and carbon dioxide and water are given off (which plants use to start photosynthesis again).
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.
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
Bishop, M. B., and C. B. Bishop. "Photosynthesis and Carbon Dioxide Fixation."Journal of Chemical Education 64 (1987): 302–305.
Govindjee, and W. J. Coleman. "How Plants Make O2." Scientific American 262 (February 1990): 50–58.
Youvan, D. C., and B. L. Marrs. "Molecular Mechanisms of Photosynthesis." Scientific American 256 (June 1987): 42–48.
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 (1897–1985). 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
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 molecules—so 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
Foyer, Christine H. (1984). Photosynthesis. New York: Wiley.
Govindjee, and Coleman, W. J. (1990). "How Plants Make Oxygen." Scientific American 262:50–59.
Wong, Kate (2000). "Photosynthesis's Purple Roots." Scientific American. Available from <http://www.sciam.com>.
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.
See I. Asimov, Photosynthesis (1969); R. M. Devlin and A. V. Barker, Photosynthesis (1972); O. Morton, Eating the Sun (2009).
Photosynthesis is the process by which green plants capture sunlight and convert its kinetic energy into chemical energy by manufacturing complex sugar molecules or carbohydrates. The plants use carbon dioxide from the air and water as the source materials for photosynthesis. Both carbon dioxide and water store relatively small amounts of energy. The carbohydrates manufactured are rich in energy. Later, during the process of respiration , the plant breaks down these carbohydrates, and the energy that is then released is used to fuel the growth and metabolism of the plant. The photosynthetic process also releases oxygen along with the formation of the carbohydrates. The water (H2O) that is used in the photosynthetic reaction contributes its hydrogen to the formation of the carbohydrate. The oxygen that is released comes from the remaining, unused portion of the water molecule. Therefore, water is just as essential a component as carbon dioxide to the photosynthetic process. The entire reaction is carried out with the help of green, light-sensitive pigments within the plant, known as chlorophylls.
Photosynthesis is vital to the earth in two ways. It is the process by which plant life is established, thereby providing food and supporting all the other consumers in the food chain/web . The release of oxygen also ensures a livable, breathable atmosphere for all other oxygen-dependent life forms. The maintenance of a stable balance between the processes of photosynthesis (which produces oxygen) and respiration (which consumes oxygen) is critical to the environment . For example, a large amount of organic matter goes into a lake receiving sewage. This organic matter will be used as a food source by bacteria which break it down by the process of respiration just as humans do. During the respiration process, the oxygen that is dissolved in the water is used up. If there is insufficient plant material in the lake to restore this used-up oxygen by photosynthesis, the total supply of dissolved oxygen in the waters of the lake may drop dangerously. Since fish are completely dependent on the dissolved oxygen for their breathing requirements, severe drops in the concentration of dissolved oxygen may kill the fish, leading to sudden, and sometimes massive, fish kills .
Another aspect of environmental pollution are the effects on the process of photosynthesis itself. A number of contaminants have been shown to affect plant growth and metabolism by inhibiting the plant's ability to photosynthesize. Frequently, as in the case of metals like copper , lead and cadmium , the mechanism of inhibition is due to the contaminant's effect on the chlorophyll pigment. Copper replaces the necessary magnesium in the chlorophyll molecule, and the copper-substituted chlorophyll cannot effectively capture light energy. Therefore, the effectiveness of the photosynthetic process is greatly diminished, which leads to a stunting of plant growth and a depletion of oxygen in the environment. The maintenance of a healthy level of photosynthesis is thus essential to the life of the planet, both on the global and the micro-environmental scales.
[Usha Vedagiri ]
Weier, T. E., et al. Botany: An Introduction to Plant Biology. New York: Wiley, 1982.
Connell, D. W., and G. J. Miller. Chemistry and Ecotoxicology of Pollution. New York: Wiley, 1984.
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 ]
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.