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[[Image:Leaf 1 web.jpg|thumb|right|300px|The [[leaf]] is the primary site of photosynthesis in plants.]]

[[Image:Leaf 1 web.jpg|thumb|right|300px|The [[leaf]] is the primary site of photosynthesis in plants.]]

[[Image:Photosystems.png|thumb|right|250px|A Photosystem: A '''light-harvesting''' cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.]]

[[Image:Photosystems.png|thumb|right|250px|A Photosystem: A '''light-harvesting''' cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.]]

<!-- Unsourced image removed: [[Image:NewZScheme.jpg|thumb|right|250px|The Z scheme]] -->

Photosynthesis. Green plants exposed to sunlight at a growing temperature are able to manufacture organic food substances, that is, carbohydrates. The term photosynthesis, derived from Greek words signifying "light" and "putting together," is applied to this process of food manufacture. Green plants manufacture not only their own food carbohydrates but also are the sources of practically all of the organic matter which may eventually furnish food for botn plants and animals. It may be said, therefore, that life today is dependent upon the green leaf. The first carbon-containing compound made is a relatively simple substance, but the first recognizable material is sugar. The crude materials out of which organic substance is made in the cells of the green tissues are CO2 (carbon dioxide) and water. The leaf green, chlorophyl, and the protoplasm of the cell may be regarded as the important mechanism, while the source of energy for the chemical change induced is radiant energy, light. Air ordinarily contains about .03 per cent of CO2, yet the ordinary green plant obtains all of its carbon for the making of organic matter from this extremely small quantity in the atmosphere. The chlorophyl is important inasmuch as it absorbs the radiant energy which is directly or indirectly responsible for the process. Chlorophyl is distributed within the cells in definite granules, or small bodies, protoplasmic in nature, commonly ovoidal in form. The light absorbed is largely from the red or red-orange portion of the spectrum. It is possible that the energy so derived is first transformed into electrical energy, yet little is known upon this point. It is certain, however, that green plants are unable to utilize energy derived, for example, from the absorption of heat. The process may be briefly pictured in the following manner: The cell-sap absorbs the CO2 which diffuses into the tissues from the air. By means of the energy absorbed by the chlorophyl bodies, within the cells, the CO2 is supposed to be reduced to CO (carbon monoxide), and the same means resolves the water into its constituents. The products of these molecular changes form new substances, perhaps formaldehyde (CH2O) and oxygen (O2). The formation of formaldehyde is still somewhat uncertain; but in any case sugar is soon recognized. In all probability the formaldehyde molecules are immediately condensed to sugar (C6H12O6). It will be noted that the surplus oxygen is in reality a by-product and during active photosynthesis it is produced in such quantity as to be actively eliminated from the plant by diffusion. The usual test for photosynthesis is carried out by counting the bubbles given off from the cut stem of a water plant exposed to sunlight in a well-aerated vessel of spring-water. The content of oxygen in these bubbles is greater than that of normal air, and the rate of bubble-production is a fair estimate of the rate of photo-synthesis.

The light energy is converted to chemical energy using the [[light-dependent reaction]]s. The products of the light dependent reactions are [[adenosine triphosphate|ATP]] from [[photophosphorylation]] and [[NADPH]] from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

 

In plants, [[light dependent reaction]]s occur in the [[thylakoid membrane]]s of the [[chloroplast]]s and use light energy to synthesize ATP and NADPH. The light dependent reaction has two forms; cyclic and non-cyclic reaction. In the non-cyclic reaction, the [[photon]]s are captured in the light-harvesting [[antenna complex]]es of [[Photosystem|photosystem II]] by [[chlorophyll]] and other [[accessory pigments]] (see diagram at right). When a chlorophyll molecule at the core of the photosystem II reaction center obtains sufficient excitat

ion 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 transfer chain|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 electron enters the Photosystem I molecule. The electron is excited due to the light absorbed by the photosystem. A second electron carrier accepts the electron, which again is passed down lowering energies of electron acceptors. The energy created by the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is used to reduce the co-enzyme NADP, which has functions in the light-independent reaction. 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.

 

The NADPH is the main [[reducing agent]] in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in photosystem I are replaced from the electron transport chain by [[plastocyanin]]. However, since photosystem II includes the first steps of the ''Z-scheme'', an external source of electrons is required to reduce its oxidized '''chlorophyll ''a''''' molecules. The source of 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 oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four [[manganese]] ions; this [[Oxygen evolution|oxygen-evolving complex]] binds two water molecules and stores the four oxidizing equivalents that are required to drive the water-oxidizing reaction. Photosystem II is the only known biological [[enzyme]] that carries out this oxidation of water. The hydrogen ions contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-independent reactions, but the majority of organisms on Earth use oxygen for [[cellular respiration]], including photosynthetic organisms.

 

With respect to oxygen and photosynthesis, there are two important concepts.

* Plant and cyanobacterial (blue-green algae) cells ''also use oxygen'' for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.

* Oxygen is a ''product of the light-driven water-oxidation reaction catalyzed by photosystem II''; it is not generated by the fixation of carbon dioxide. Consequently, the source of oxygen during photosynthesis is water, not carbon dioxide.

 

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 r

 

Others, such as the halophiles (an [[Archaea]]) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane.

 

The fixation or reduction of carbon dioxide is a light-independent process in which [[carbon dioxide]] combines with a five-carbon sugar, [[ribulose 1,5-bisphosphate]] (RuBP), to yield two molecules of a three-carbon compound, [[glycerate 3-phosphate]] (GP), also known as 3-phosphoglycerate (PGA). GP, in the presence of [[Adenosine triphosphate|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 (see [[carbohydrate]]s). Most (5 out of 6 molecules) of the G3P produced is used to regenerate RuBP so the process can continue (see [[Calvin-Benson cycle]]). The 1 out of 6 molecules of the triose phosphates not "recycled" often condense to form [[hexose]] phosphates, which ultimately yield [[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]].

 

 

[[C4 carbon fixation|'''C₄ plants''']] capture carbon dioxide using an enzyme called [[PEP Carboxylase]] that adds carbon dioxide to the three carbon molecule [[Phosphoenolpyruvate|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 reverse the reaction to release carbon dioxide thus preventing photorespiration. By preventing photorespiration, C₄plants can produce more sugar than C₃ plants in conditions of strong light and high temperature. Many important crop plants are C₄ plants including maize, sorghum, sugarcane, and millet.

 

[[Xerophytes]] such as [[Cacti]] and most [[succulents]] also can use PEP Carboxylase to capture carbon dioxide in a process called [[CAM photosynthesis|Crassulacean acid metabolism (CAM)]]. They store the CO₂ in different molecules than the C₄ plants (mostly they store it in the form of [[malic acid]] via carboxylation of [[phosphoenolpyruvate]] to oxaloacetate which is then reduced to malate). Nevertheless, C₄ plants capture the CO₂ in one type of cell tissue ([[mesophyll]]) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin cycle. 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. C4 metabolism ''physically'' separates CO₂ fixation from the Calvin cycle, while CAM metabolism ''temporally'' separates CO₂ fixation from the Calvin cycle.

 

 

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the [[1800s]].

 

[[Jan van Helmont]] began the research of the process in the mid-1600s 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 the water, the only substance he added to the potted plant. His hypothesis was partially accurate - much of the gained mass also comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's [[biomass]] comes from the inputs of photosynthesis, not the soil itself.

 

[[Joseph Priestley]], a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant.

 

In [[1778]], [[Jan Ingenhousz]], court physician to the [[Austria]]n Empress, repeated Priestley's experiments. He discovered that it was the influence of sun and light 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 who 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 also to the incorporation of water. Thus the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined.

 

Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things.

 

[[Cornelis Van Niel]] made key discoveries explaining the chemistry of photosynthesis. By studying [[purple sulfur bacteria]] and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent [[redox]] reaction, in which hydrogen reduces carbon dioxide.

 

Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by [[Robert Hill (plant biochemist)|Robert Hill]] in [[1937]] and [[1939]]. He showed that isolated [[chloroplast]]s 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.

 

[[Sam Ruben|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 assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the [[Calvin cycle]], which inappropriately 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.

 

There are three main factors affecting photosynthesis and several corollary factors. The three main are:

 

=== Light intensity (Irradiance), wavelength and temperature ===

In the early 1900s [[Frederick Blackman|Frederick Frost Blackman]] along with Gabrielle Matthaei investigated the effects of light intensity ([[irradiance]]) and temperature on the rate of carbon assimilation.

* 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.

* 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.

 

These two experiments illustrate vital points: firstly, from [[research]] it is known that [[photochemical]] reactions are not generally affected by [[temperature]]. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are of course the [[Light-dependent reaction|light-dependent 'photochemical']] stage and the [[Light-independent reaction|light-independent, temperature-dependent]] stage. Secondly, Blackman's experiments illustrate the concept of [[limiting factor]]s. 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. This unit is called a [[phycobilisome]].

 

As carbon dioxide concentrations rise, the rate at which sugars are made by the [[light-independent reaction]]s 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 [[Carbon fixation|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

 

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:

# One product of oxygenase activity is [[phosphoglycolate]] (2 carbon) instead of [[3-phosphoglycerate]] (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the [[Calvin-Benson cycle]].

# Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.

# Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate.

 

 

 

The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as [[photorespiration]] since it is characterized by light dependent oxygen consumption and the release of carbon dioxide.

 

{{section-stub}}

 

* Blankenship, R.E., 2002. ''Molecular Mechanisms of Photosynthesis''. Blackwell Science.

* Gregory, R.P.F., 1971. ''Biochemistry of Photosynthesis''. Belfast: Universities Press.

* Govindjee; Beatty, J.T., Gest, H. and Allen, J.F. (Eds.), 2005. Discoveries in Photosynthesis. ''Advances in Photosynthesis and Respiration'', Volume 20, Springer.

 

 

 

As a rule the sugar formed in the leaf does not accumulate to any large extent, but is transformed into starch. Some of the sugar, however, may be immediately diffused to other cells or "transported," supplying the needs of this substance in growth. The starch which is deposited is in the form of insoluble granules, and the formation of these bodies on exposure of the green leaf to sunlight is so rapid as to make it possible in some cases to use starch formation as an index to rate of photosynthesis. During the night, when no photosynthesis occurs, the transformation and removal of the starch usually goes on rapidly, so that within an interval of twelve hours most of that formed during the day seems to have disappeared from the leaf. It is, in fact, changed to sugar prior to transportation but may be removed to other organs of the plant, as, for example, to fleshy roots or tubers, where it may again be converted into starch, accumulating at times to a very considerable extent.

* [http://www.ljmu.ac.uk/NewsCentre/63012.htm Liverpool John Moores University, Dr.David Wilkinson]

* [http://www.biochemweb.org/metabolism.shtml Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology]

* [http://www.chemsoc.org/networks/learnnet/cfb/Photosynthesis.htm Overall examination of Photosynthesis at an intermediate level]

* [http://www.life.uiuc.edu/govindjee/photosynBook.html Overall Energetics of Photosynthesis]

Photosynthesis is most rapid under those conditions of temperature which are favorable for growth. Under strong light and favorable temperature, however, a slight increase in the amount of CO2 gives a higher rate of starch-production. The presence in the leaf or stem of other color bodies, such as browns and reds, is no indication that chlorophyl is absent. As a matter of fact, chlorophyl is generally present in such cases, but may be veiled by the more prominent color. In showy flowers, however, chlorophyl seldom occurs. Photosynthesis is inhibited by any condition affecting the general health of the plant, and it is low during cold and dark weather. The larger number of plants are most active in the brightest sunlight, but certain shade-loving species are injured by such exposures, and are adjusted to conditions of half-shade, such as obtain in the shade of trees or bushes. B.M.Duggar

[[Category:Photosynthesis|Photosynthesis]]