Photosynthesis

The leaf is the primary site of photosynthesis in plants.

Photosynthesis is the conversion of energy from sunlight into a usable form of potential chemical energy. It is arguably the most important biochemical pathway known: all free energy consumed by biological systems arises from solar energy trapped by the process of photosynthesis.

Photosynthesis occurs in higher plants, phytoplankton, algae, some bacteria, and some protists. These organisms are collectively referred to as photoautotrophs, as they synthesize food directly from inorganic compounds using light energy. In green plants and algae, photosynthesis takes place in specialized cellular compartment (organelle) called chloroplasts. In photosynthetic bacteria, which lack membrane-bound organelles, photosynthesis takes place directly in the cell.

Although photosynthesis consists of a complex array of coordinated biochemical reactions, it occurs in two broadly defined stages:

1. In the first phase, known as the light-dependent reactions), energy captured from sunlight is harvested as the high-energy molecules ATP and NADPH.
2. During the second phase, referred to as the light-independent reactions (also called the Calvin-Benson Cycle), these high-energy molecules are used to drive the synthesis of glucose from carbon dioxide (CO₂) and water. These organic materials – primarily glucose and starch are used in cellular functions such as biosynthesis and respiration. Oxygen is a waste product of the light-independent reactions, but the majority of organisms on Earth use oxygen for cellular respiration, including photosynthetic organisms.

What else? Variations on the theme? Stress importance?

Overview of reactions

In chemical terms, photosynthesis is an example of an oxidation-reduction process. In plants, photosynthesis uses light energy to power the oxidation (removal of electrons) of water, producing oxygen, hydrogen ions, and electrons. Most of the hydrogen ions and electrons are then transferred to carbon dioxide, which is reduced (i.e., it gains electrons) to organic products.

Specifically, carbon dioxide is reduced to make triose phospate (G3P), which is generally considered the prime end-product of photosynthesis. It can be used as an immediate food nutrient, or combined and rearranged to form monosaccharide sugars, such as glucose, which can be transported to other cells, or packaged for storage as an insoluble polysaccharide such as starch.

The general chemical equation for photosynthesis is often presented in introductory chemistry texts in simplified form as:

CO_2(gas) + H₂O_(liquid) + photons → CH₂O _(aqueous) + O_2(gas)

carbon dioxide + water + light energy → triose phosphate + oxygen + water

where (CH₂O) refers to the general formula for a carbohydrate.

The site of photosynthesis

Photosynthesis occurs in the chloroplasts of green plants and algae

Plant cells with visible chloroplasts.

The reactions of photosynthesis occur in the chloroplasts, which consist of an inner and outer membrane with an intervening intermembrane space.

The inner membrane surrounds the stroma, containing enzymes and membranous structures called thylakoids, flattened sacs at which the light-dependent reactions of photosynthesis occur. The thylakoids are arranged in stacks called grana (singular: granum).

Embedded in the thylakoid membrane is the antenna complex, which consists of proteins and light-absorbing pigments. Plants absorb light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. The function of chlorophyll is often supported by other accessory pigments such as carotenes and xanthophylls. This complex both increases the surface area for light capture, and allows capture of photons with a wider range of wavelengths.

The internal structure of a chloroplast. One of the stacks of thykaloids (called a granum) is circled.

Although all cells in the green parts of a plant have chloroplasts, most light energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells, where most of the process of photosynthesis takes place.

Like plants, algae, which come in multiple forms from multicellular organisms like kelp, to microscopic, single-celled organisms, contain chloroplasts and produce chlorophyll; however, various accessory pigments are also present in some algae, such as phyverdin in green algae and phycoerythrin in red algae (rhodophytes), resulting in a wide variety of colors.

Bacteria do not have specialized compartments for photosynthesis

Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles). Instead, photosynthesis takes place directly within the cell. Cyanobacteria contain thylakoid membranes very similar to those in chloroplasts and are the only prokaryotes that perform oxygen-generating photosynthesis. The other photosynthetic bacteria contain a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen. Some bacteria, such as Chromatium, oxidize hydrogen sulfide instead of water for photosynthesis, producing sulfur as a waste product.

Photosynthesis occurs in two stages

The light reactions convert solar energy to chemical energy

The light-dependent reactions of photosynthesis occur at the thylakoid membrane.

Photosynthesis begins when light is absorbed by chlorophyll and accessory pigments. Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with peaks for violet-blue and red light. In red algae, the action spectrum overlaps with the absorption spectrum of phycobilins for blue-green light, which allows these algae to grow in deeper waters that filter out the longer wavelengths used by green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g. green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms. (chlorophylls absorb all visible light of wavelengths other than green)

File:Photosystems.png

A light-harvesting cluster of photosynthetic pigments (called a photosystem) is present in the thylakoid membrane of chloroplasts.

The electronic excitation caused by light absorption passes from one chlorophyll molecule to the next until it is trapped by a chlorophyll pair with special properties. At this site, known as the ‘’reaction center’’, the energy of the electron is converted into chemical energy; i.e., light is used to crate a reducing potential. There are two kinds of light reactions that occur in these reaction centers, which are called photosystems:

1. Photosystem I generates reducing power in the form of NADPH. (photoreduction)
2. Photosystem II transfers the electrons of water to a quinone (define), at the same time that it forms oxygen from the oxidation of water.

Electron flow within and between each photosystem generates a transmembrane proton gradient that drives the synthesis of ATP (a process called photophosphorylation). Light causes electrons to flow from water to NADPH and leas to the generation of a proton-motive force; so-called because the redox diagram looks like a Z. When a chlorophyll molecule at the core of the photosystem II reaction center obtains sufficient excitation energy from the adjacent antenna pigments, an electron is transferred to the primary electron-acceptor molecule, Pheophytin, through a process called Photoinduced charge separation. These electrons are shuttled through an electron transport chain, the so-called Z-scheme shown in the diagram, that initially functions to generate a chemiosmotic potential across the membrane. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation while NADPH is a product of the terminal redox reaction in the Z-scheme.

The "Z scheme" is an electron transport chain that generates the chemioosmotic potential used to synthesize ATP.

NADPH is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. However, its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The source of these electrons in green-plant and cyanobacterial photosynthesis is water. Two water molecules are oxidized by four successive charge-separation reactions by photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions; the electron yielded in each step is transferred to a redox-active tyrosine residue that then reduces the photoxidized paired-chlorophyll a species called P680 that serves as the primary (light-driven) electron donor in the photosystem II reaction center. The hydrogen ions contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis.

An alternative pathway for electrons from photosystem I is cyclic photophosphorylation: ATP generated without the concomitant formation of NADPH; takes place when nadp plus is unavailable to accept electrons. The cyclic reaction is similar to that of the non-cyclic, but differs in the form that it only generates ATP and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns back to photosystem I, from where it was emitted; hence the name cyclic reaction.

In the dark reactions, carbon fixation enables the synthesis of organic compounds

Overview of the Calvin cycle and carbon fixation.

Plants use chemical energy generated from ATP and NADPH to fix carbon dioxide (a process also known as carbon reduction) into carbohydrates and other organic compounds through light-independent reactions. In a series of reactions called the Calvin cycle, they reduce carbon dioxide and convert it into 3-photosphoglycerate in a series of reactions, which occur in the stroma (the fluid-filled interior) of the chloroplast. Hexoses (six-carbon sugars) such as glucose are then formed from 3-phosphoglycerate by the gluconeogenic pathway.

Specifically, the fixation of carbon dioxide is a light-independent process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), to form a six-carbon compound. This compound is hydrolyzed to two molecules of a three-carbon compound, glycerate 3-phosphate (GP), also known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehyde 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate. Triose is a 3-carbon sugar. This reaction is catalyzed by an enzyme commonly called rubisco (after ribulose 1,5-bisphosphate carboxylase/oxygenase), located on the stromal surface of thyklakoid membrane; rubisco is the most abundant enzyme, and probably the most abundant protein, in the biosphere, accounting for more than sixteen percept of the total protein of chloroplasts.

Most (5 out of 6 molecules) of the G3P produced is used to regenerate RuBP so that the process can continue. One out of six molecules of the triose phosphates not "recycled" often condenses to form hexose phosphate, which ultimately yields sucrose, starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

Three molecules of ATP and 2 molecules of NADPH are consumed in converting carbon dioxide into a one molecule of a hexose such as glucose or fructose.

Alternative methods of carbon fixation have evolved to meet environmental conditions

Overview of C₄ carbon fixation.

In hot and dry conditions, plants will close their stomata (small openings on the underside of leaves used for gas exchange) to prevent loss of water. Under these conditions, oxygen gas, produced by the light reactions of photosynthesis, will concentrate in the leaves, causing photorespiration to occur. Photorespiration is a wasteful reaction; organic carbon is converted into carbon dioxide without the production of ATP, NADPH, or another energy-rich metabolite. Rubisco’s tendency to catalyze this oxygenase activity increases more rapidly with temperature than its carboxylase activity.

The solution arrived at by the C₄ plants (which include many important crop plants such as maize, sorghum, sugarcane, and millet) is to achieve a high concentration of carbon dioxide in the leaves (the site of the Calvin cycle) under these conditions.

C₄ plants capture carbon dioxide using an enzyme called PEP Carboxylase that adds carbon dioxide to the three carbon molecule Phosphoenolpyruvate (PEP) creating the 4-carbon molecule oxaloacetic acid. Plants without this enzyme are called C₃ plants because the primary carboxylation reaction produces the three-carbon sugar 3-phosphoglycerate directly in the Calvin-Benson Cycle. When oxygen levels rise in the leaf, C₄ plants plants reverse the reaction to release carbon dioxide, thus preventing photorespiration. By preventing photorespiration, C₄ plants can produce more sugar than C₃ plants in conditions of strong light and high temperature. These C₄ plants compounds carry carbon dioxide from mesophyll cells, which are in contact with air, to bundle-sheath cells, which are major sites of photosynthesis.

Needs better explanation: Xerophytes such as Cacti and most succulents can also use PEP Carboxylase to capture carbon dioxide in a process called Crassulacean acid metabolism (CAM). They store the CO₂ in different molecules than the C₄ plants. They also have a different leaf anatomy than C₄ plants. They grab the CO₂ at night when their stomata are open, and they release it into the leaves during the day to increase their photosynthetic rate.

The pineapple is an example of a CAM plant.

Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some photosynthetic plants. CAM is usually found in plants living in arid conditions, including those found in the desert (for example, cacti or pineapple).

CAM plants close their stomata during the day in order to conserve water by preventing evapotranspiration. Their stomata then open during the cooler and more humid nighttime hours, allowing uptake of carbon dioxide for use in carbon fixation. This is begun when the three-carbon compound phosphoenolpyruvate is carboxylated into oxaloacetate which is then reduced to form malate. CAM plants store these four-carbon intermediates and other simple organic compounds in their vacuoles. Malate is easily broken down into pyruvate and CO₂, after which pyruvate is phosphorylated to regenerate phosphoenolpyruvate (PEP). In the daytime, the malic acid is removed from the vacuoles and cleaved to produce CO₂ so that it can be utilized by the enzyme RuBisCO in the Calvin-Benson cycle in the chloroplast stroma. By thus reducing evapotranspiration rates during gas exchange, CAM allows plants to grow in environments that would otherwise be far too dry for plant growth or, at best, subject them to severe drought stress.

In sum, C₄ plants metabolism physically separates CO₂ fixation from the Calvin cycle, while CAM metabolism temporally separates CO₂ fixation from the Calvin cycle.

Photosynthesis in algae and bacteria

The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by Cornelis Van Niel in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They obtain electrons from a variety of different inorganic chemicals including sulfide or hydrogen; thus, for most of these bacteria oxygen is not a by-product of photosynthesis. Others, such as the halophiles (an Archaea) produced so-called purple membranes where the bacteriorhodopsin (?) can harvest light and produce energy (what does this have to do w/ anything?).

The energy efficiency of photosynthesis

Through photosynthesis, sunlight energy is transferred to molecular reaction centers for conversion into chemical energy with great efficiency. The transfer of the solar energy takes place almost instantaneously, so little energy is wasted as heat. This chemical energy production is more than 90% efficient with only 5-8% of the energy transferred thermally. In contrast, commercial solar panels use less than 30% of the light energy that strikes them .

A study led by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley suggests that long-lived wavelike electronic quantum coherence plays an important part in this instantaneous transfer of energy by allowing the photosynthetic system to simultaneously try each potential energy pathway and choose the most efficient option. Results of the study are presented in the April 12, 2007 issue of the journal Nature.

Factors affecting photosynthesis

There are three main factors affecting the rate of photosynthesis, as well as several corollary factors:

• Light irradiance and wavelength. In the early 1900s Frederick Frost Blackman along with Gabrielle Matthaei investigated the effects of light intensity (irradiance) and temperature on the rate of carbon assimilation. At constant irradiance, the rate of carbon assimilation increases as the temperature is increased over a limited range. This effect is only seen at high irradiance levels. At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. Another limiting factor is the wavelength of light. Cyanobacteria, which reside several meters underwater, cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, a series of proteins with different pigments surround the reaction center.
• Temperature. At constant temperature, the rate of carbon assimilation varies with irradiance, initially increasing as the irradiance increases. However, at higher irradiance, this relationship no longer holds and the rate of carbon assimilation reaches a plateau. [Photochemical]] reactions are not generally affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation,
• Carbon dioxide concentration. As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the oxygen concentration is high, RuBisCO will bind oxygen instead of carbon dioxide. This process, called photorespiration, uses energy, but does not make sugar.

The evolution of photosynthesis

The ability to convert light energy to chemical energy confers a significant evolutionary advantage to living organisms. Early photosynthetic systems, such as those from green and purple sulfur and green and purple non-sulfur bacteria, are thought to have been anoxygenic, using various molecules other than oxygen, such as hydrogen and sulfur, as electron donors.

The oxygen in the atmosphere today exists due to the evolution of oxygenic photosynthesis, sometimes referred to as the oxygen catastrophe (explain). Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic (i.e., oxygen is produced).

In fact, chloroplasts are now considered to have evolved from an endosymbiotic bacterium, which was also an ancestor of and later gave rise to cyanobacterium. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction centre.

The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis or fusion) by early eukaryotic cells to form the first plant cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, while plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex animals, including humans, whose cells depend upon mitochondria as their energy source; mitochondria are thought to have evolved from endosymbiotic bacteria, related to modern rickettsia bacteria. Both chloroplasts and mitochondria actually have their own DNA, separate from the nuclear DNA of their animal or plant host cells (which further supports this idea because…).

Scientific discoveries about photosynthesis

Joseph Priestley in 1794.

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the late 18th century.

In the mid-1600s, Jan van Helmont laid the foundations of research on photosynthesis when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from water, the only substance he added to the potted plant. His hypothesis was partially accurate: much of the gain in mass comes from carbon dioxide as well as water. However, the important discovery here was that the bulk of a plant's biomass comes from the inputs of photosynthesis, not from the soil itself.

In 1780, Joseph Priestley, a chemist and minister, discovered that oxygen is produced during photosynthesis. In a famous experiment, he isolated a volume of air under an inverted glass jar, and burned a candle in it. The candle would burn out very quickly, long before it ran out of wax. When he placed a sprig of mint in the jar in a vessel of water, he found that several days later, the air would not extinguish the candle and wasn’t harmful to a mouse put into the vessel.

In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestley's experiments. He discovered that it was the influence of sunlight on the plant that could cause it to rescue a mouse in a matter of hours.

In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light.

Soon afterwards, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO₂, but must also involve the incorporation of water. Thus, the basic reaction of photosynthesis was outlined.

Modern scientists built on this foundational knowledge. In the 1930s, Cornelis Van Niel, for example, was the first scientist to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces carbon dioxide. He noticed the common pattern of photosynthesis in green plants and sulfur bacteria, in which sulfur plays an analogous role to oxygen in green plants.

Melvin Calvin, along with Andrew Benson and James Bassham, discovered the path of carbon fixation in plants.

Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water were performed by Robert Hill in 1937 and 1939. He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The Hill reaction is as follows:

2 H₂O + 2 A + (light, chloroplasts) → 2 AH₂ + O₂

where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved. His work confirmed that oxygen comes from water rather than carbon dioxide, and that a primary event in photosynthesis is the light-driven transfer of an electron from one substance to another in a thermodynamically unfavorable direction.

Samuel Ruben and Martin Kamen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.

Melvin Calvin and Andrew Benson, along with James Bassham, elucidated the path of carbon fixation in plants. The carbon reduction cycle is known as the Calvin cycle, which ignores the contribution of Bassham and Benson. Many scientists refer to the cycle as the Calvin-Benson Cycle, Benson-Calvin, and some even call it the Calvin-Benson-Bassham (or CBB) Cycle.

A Nobel Prize winning scientist, Rudolph A. Marcus, was able to discover the function and significance of the electron transport chain.

References

ISBN links support NWE through referral fees

• Blankenship, R.E. 2002. Molecular Mechanisms of Photosynthesis. Oxford, UK: Blackwell Science.
• Campbell, N. and J. Reece. 2005. Biology, 7th ed. San Francisco: Benjamin Cummings.
• Cooper, G. M., and R. E. Hausman. 2004. The Cell: A Molecular Approach, 3rd edition. Washington, D.C.: ASM Press & Sunderland, M.A.: Sinauer Associates. ISBN 0878932143
• Gregory, R.P.F. 1971. Biochemistry of Photosynthesis. Belfast: Universities Press.
• Govindjee, B.J.T. 1975. Bioenergetics of Photosynthesis. New York: Academic Press.
• Govindjee, B.J.T., Gest, H. and J.F. Allen, J.F., eds. 2005. Discoveries in Photosynthesis. Advances in Photosynthesis and Respiration, Volume 20. New York: Springer.
• Rabinowitch, E. and B.J.T. Govindjee. 1969. Photosynthesis. New York: John Wiley & Sons.
• Raven, P.H., Evert, R.F. and S.E. Eichhorn 2005. Biology of Plants, 7th ed. New York: W.H. Freeman. ISBN 0-7167-1007-2
• Stern, K.R., Jansky, S., and J.E. Bidlack. 2003. Introductory Plant Biology. New York: McGraw Hill. ISBN 0-07-290941-2
• Stryer, L. 1995. Biochemistry, 4th edition. New York: W.H. Freeman. ISBN 0-716-72009-4
• Brown, T.L., LeMay, H.E., Bursten, B.E., and J.R. Burdge. 2002. Chemistry: The Central Science, 9th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-048450-4, p. 958

External links

• Liverpool John Moores University, Dr.David Wilkinson
• Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology
• Overall examination of Photosynthesis at an intermediate level
• Overall Energetics of Photosynthesis

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