Photosynthesis

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.

Light reactions

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.

Dark reactions

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.

Calvin cycle

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).

C-4 photosynthesis

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.

CAM 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.

Photorespiration

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

Algae

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.

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. 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.

Chloroxybacteria

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.

Halobacterium

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.

Resources

books

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.

periodicals

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

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Calvin cycle

—Dark reactions of photosynthesis which use the ATP and NADPH made by the light reactions to synthesize carbohydrates.

Chloroplast

—Green organelle in higher plants and algae in which photosynthesis occurs.

Cyanobacteria (singular, cyanobacterium)

—Photosynthetic 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.