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Photosynthesis

2007 Schools Wikipedia Selection. Related subjects: General Biology

   The leaf is the primary site of photosynthesis in plants.
   Enlarge
   The leaf is the primary site of photosynthesis in plants.

   Photosynthesis (photo=light, synthesis=putting together), generally, is
   the synthesis of sugar from light, carbon dioxide and water, with
   oxygen as a waste product. It is arguably the most important
   biochemical pathway known; nearly all life depends on it. It is an
   extremely complex process, comprised of many coordinated biochemical
   reactions. It occurs in higher plants, algae, some bacteria, and some
   protists, organisms collectively referred to as photoautotrophs.

Overview

   Photosynthesis uses the energy of light to make the sugar, glucose. A
   simple general equation for photosynthesis follows.

       6 CO[2] + 12 H[2]O + light → C[6]H[12]O[6] + 6 O[2] + 6 H[2]O

     carbon dioxide + water + light energy → glucose + oxygen + water

   Photosynthesis occurs in two stages. In the first phase light-dependent
   reactions or photosynthetic reactions (also called the Light reactions)
   capture the energy of light and use it to make high-energy molecules.
   During the second phase, the light-independent reactions (also called
   the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use
   the high-energy molecules to capture carbon dioxide (CO[2]) and make
   the precursors of glucose.

   In the light-dependent reactions the pigment chlorophyll absorbs light
   and loses an electron that travels down an electron transport chain
   producing the high energy molecules NADPH and ATP. The chlorophyll
   molecule regains its electron by taking one from a water molecule
   through a process called photolysis, that releases oxygen gas as a
   byproduct.

   In the Light-independent or dark reactions the enzyme RuBisCO captures
   CO[2] from the atmosphere and in a complex process called the
   Calvin-Benson cycle releases 3-carbon sugars which are later combined
   to form glucose.

   Photosynthesis may simply be defined as the conversion of light energy
   into chemical energy by living organisms. It is affected by its
   surroundings and the rate of photosynthesis is affected by the
   concentration of carbon dioxide, light intensity and the temperature.

In plants

   Most plants are photoautotrophs, which means that they are able to
   synthesize food directly from inorganic compounds using light energy -
   for example the sun, instead of eating other organisms or relying on
   nutrients derived from them. This is distinct from chemoautotrophs that
   do not depend on light energy, but use energy from inorganic compounds.

   The energy for photosynthesis ultimately comes from absorbed photons
   and involves a reducing agent, which is water in the case of plants,
   releasing oxygen as a waste product. The light energy is converted to
   chemical energy (known as light-dependent reactions), in the form of
   ATP and NADPH, which is used for synthetic reactions in
   photoautotrophs. Most notably plants use the chemical energy to fix
   carbon dioxide into carbohydrates and other organic compounds through
   light-independent reactions. The overall equation for carbon fixation
   (sometimes referred to as carbon reduction) in green plants is

          n CO[2] + 2n H[2]O + ATP + NADPH → (CH[2]O)[n] + n H[2]O + n
          O[2],

          Where n is defined according to the structure of the resulting
          carbohydrate.

   More specifically, carbon fixation produces an intermediate product,
   which is then converted to the final hexose carbohydrate products.
   These carbohydrate products are then variously used to form other
   organic compounds, such as the building material cellulose, as
   precursors for lipid and amino acid biosynthesis or as a fuel in
   cellular respiration. The latter not only occurs in plants, but also in
   animals when the energy from plants get passed through a food chain.
   Organisms dependent on photosynthetic and chemosynthetic organisms are
   called heterotrophs. In general outline, cellular respiration is the
   opposite of photosynthesis: glucose and other compounds are oxidised to
   produce carbon dioxide, water, and chemical energy. However, both
   processes actually take place through a different sequence of reactions
   and in different cellular compartments.

   Plants absorb light primarily using the pigment chlorophyll, which is
   the reason that most plants have a green colour. The function of
   chlorophyll is often supported by other accessory pigments such as
   carotenes and xanthophylls. Both chlorophyll and accessory pigments are
   contained in organelles (compartments within the cell) called
   chloroplasts. Although all cells in the green parts of a plant have
   chloroplasts, most of the energy is captured in the leaves. The cells
   in the interior tissues of a leaf, called the mesophyll, contain about
   half a million 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 photosynthesis takes
   place.

In algae and bacteria

   Algae is a range from multicellular forms like kelp to microscopic,
   single-celled organisms. Although they are not as complex as land
   plants, photosynthesis takes place biochemically the same way. Very
   much like plants, algae have chloroplasts and chlorophyll, but various
   accessory pigments are present in some algae such as phycoerythrin in
   red algae (rhodophytes), resulting in a wide variety of colours. All
   algae produce oxygen, and many are autotrophic. However, some are
   heterotrophic, relying on materials produced by other organisms. For
   example, in coral reefs, there is a symbiotic relationship between
   zooxanthellae and the coral polyps.

   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, 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. The other
   photosynthetic bacteria have 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 waste.

The evolution of photosynthesis

   The ability to convert light energy to chemical energy is a significant
   evolutionary advantage. Early photosynthetic systems, such as those
   from green and purple sulphur and green and purple non-sulphur
   bacteria, were anoxygenic using various molecules as electron donors.
   Green and purple sulphur bacteria used hydrogen and sulphur as an
   electron donor. Green nonsulphur bacteria used various amino and other
   organic acids. Purple nonsulphur bacteria used a variety of
   non-specific organic molecules. The use of these molecules is
   consistent with the geological evidence that the atmosphere was highly
   reduced at that time.

   Fossils have been found of filamentous photosynthetic organisms dating
   from 3400 million years ago ( New Scientist, 19 Aug., 2006).

   The oxygen in the atmosphere today exists due to the evolution of
   oxygenic photosynthesis, sometimes referred to as the oxygen
   catastrophe. 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. Oxygenic
   photosynthesis uses water as an electron donor which is oxidized into
   molecular oxygen by the absorption of a photon by the photosynthetic
   reaction centre.

Origin of chloroplasts

   In plants the process of photosynthesis is compartmentalized in
   organelles called chloroplasts. Chloroplasts have many similarities
   with photosynthetic bacteria including a circular chromosome,
   prokaryotic ribosomes, and similar proteins in the photosynthetic
   reaction centre.

   The endosymbiotic theory suggest that photosynthetic bacteria were
   ingested by early eukaryotic cells to form the first plant cells.

Molecular production

Light to chemical energy

   A Photosystem: A light-harvesting cluster of photosynthetic pigments
   present in the thylakoid membrane of chloroplasts.
   Enlarge
   A Photosystem: A light-harvesting cluster of photosynthetic pigments
   present in the thylakoid membrane of chloroplasts.

   The light energy is converted to chemical energy using the
   light-dependent reactions. The products of the light dependent
   reactions are ATP from photophosphorylation and NADPH from
   photoreduction. Both are then utilized as an energy source for the
   Light independent reactions.

Z scheme

   In plants, light dependent reactions occur in the thylakoid membranes
   of the chloroplasts 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 photons are captured in the
   light-harvesting antenna complexes of photosystem II by chlorophyll and
   other accessory pigments (see diagram at right). When a chlorophyll
   molecule at the core of the photosystem II reaction centre 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 electron enters the Photosystem I
   molecule. The electron is emitted 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 NADH, 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.

Water photolysis

   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 photooxidized
   paired-chlorophyll a species called P680 that serves as the primary
   (light-driven) electron donor in the photosystem II reaction centre.
   The oxidation of water is catalyzed in photosystem II by a redox-active
   structure that contains four manganese ions; this 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.

Oxygen and photosynthesis

   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.

Bacterial variation

   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 get electrons from a variety of different
   inorganic chemicals including sulfide or hydrogen, so for most of these
   bacteria oxygen is not produced.

   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.

Carbon fixation

   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 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
   carbohydrates). 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.

C[4] and CAM

   In hot and dry conditions, plants will close their stomata (pores 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. Some
   plants have devised mechanisms to increase the CO[2] concentration in
   the leaves under these conditions.

   C[4] 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 C[3] 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 this reaction to release
   carbon dioxide thus preventing photorespiration. By preventing
   photorespiration, C[4] plants can produce more sugar than C[3] plants
   in conditions of strong light and high temperature. Many important crop
   plants are C[4] plants including maize, sorghum, sugarcane, and millet.

   Cacti and most succulents also can use PEP Carboxylase to capture
   carbon dioxide in a process called Crassulacean acid metabolism (CAM).
   They store the CO[2] in different molecules than the C[4] plants
   (mostly they store it in malic acid). They also have a different leaf
   anatomy than C[4] plants. They grab the CO[2] at night when their
   stomata are open, and they release it into the leaves during the day to
   increase their photosynthetic rate.

Discovery

   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 signalling
   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 Austrian 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 French pastor, showed that CO[2] was the
   "fixed" or "injured" air and that it was taken up by plants in
   photosynthesis. 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 CO[2], 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 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 H[2]O + 2 A + (light, chloroplasts) → 2 AH[2] + O[2]

   where A is the electron acceptor. Therefore, in light the electron
   acceptor is reduced and oxygen is evolved.

   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.

Factors affecting photosynthesis

   There are three main factors affecting photosynthesis and several
   corollary factors. The three main are:
     * Light irradiance and wavelength
     * Carbon dioxide concentration
     * Temperature

Light intensity (Irradiance), wavelength and temperature

   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 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 'photochemical' stage and the
   light-independent, temperature-dependent stage. Secondly, Blackman's
   experiments illustrate the concept of limiting factors. 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 centre. This unit is
   called a phycobilisome.

Carbon dioxide levels and Photorespiration

   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

   RuBisCO oxygenase activity is disadvantageous to plants for several
   reasons:
    1. 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.
    2. Phosphoglycolate is quickly metabolized to glycolate that is toxic
       to a plant at a high concentration; it inhibits photosynthesis.
    3. 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.

                A highly simplified summary is:

                      2 glycolate + ATP → 3-phophoglycerate + carbon
                      dioxide + ADP +NH[3]

   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.

Corollary factors

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