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| <!-- Introduction section. It is meant to be very nontechnial and accessible. Details come later. --> | |||
| ] | |||
| '''Photosynthesis''' is an important ] process in which ]s, ]e, ], and some ] harness the energy of ] to chemical energy and store it in the bonds of sugar, ]. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on ]. It is also responsible for producing the ] that makes up a large portion of the ]. Organisms that produce energy through photosynthesis are called ]s. Plants are the most visible representatives of photoautotrophs, but it should be emphasized that bacteria and algae as well contribute to the conversion of free energy into usable energy. | |||
| == Plant photosynthesis == | |||
| Most plants are ]s (exceptions include the infamous ]), which means they are able to synthesize food directly from inorganic compounds using light energy, instead of eating other organisms or relying on material derived from them. This is distinct from ]s that do ''not'' depend on light energy, but use energy from inorganic compounds. | |||
| The energy for photosynthesis ultimately comes from absorbed ]s and involves a reducing agent, which is ] in the case of plants, releasing ] as a waste product. The light energy is converted to chemical energy, in the form of ] and ], using the ]s and is then available for ]. Most notably plants use the chemical energy to fix ] into ]s and other organic compounds through ]s. The overall equation for photosynthesis in green plants is: | |||
| :''n'' CO<sub>2</sub> + ''2n'' H<sub>2</sub>O + light energy → (CH<sub>2</sub>O)''<sub>n</sub>'' + ''n'' O<sub>2</sub> + ''n'' H<sub>2</sub>O | |||
| Where n is defined according to the structure of the resulting carbohydrate. However, ] ]s and ] are the primary products, so the following generalised equation is often used to represent photosynthesis: | |||
| :6 CO<sub>2</sub> + 12 H<sub>2</sub>O + light energy → C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6 O<sub>2</sub> + 6 H<sub>2</sub>O | |||
| More specifically, photosynthetic reactions usually 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 ], as precursors for ] and ] biosynthesis or as a fuel in ]. The latter not only occurs in plants, but also in ]s when the energy from plants get passed through a ]. 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 capture light primarily using the ] ], which is the reason that most plants have a green color. The function of chlorophyll is often supported by other ]s such as ]s and ]s. Both chlorophyll and accessory pigments are contained in ]s (compartments within the ]) called ]s. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the ]. The cells in the interior tissues of a leaf, called the ], contain about half a million chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant, ]y ], that protects the leaf from excessive ] of water as well as decreasing the absorption of ] or ] ] to reduce ]ing. The transparent, colourless ] layer allows light to pass through to the ] mesophyll cells where most of the photosynthesis takes place. | |||
| == Photosynthesis in algae and bacteria == | |||
| {{section-stub}} | |||
| Algae range from multicellular forms like ] to ], single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. 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 ]ic, relying on materials produced by other organisms. | |||
| Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles), instead, photosynthesis takes place directly within the cell. ] 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 ] from an ] bacterium, which was also an ancestor of and later gave rise to cyanobacterium. The other photosynthetic bacteria have a variety of different pigments, called ]s, and do not produce oxygen. Some bacteria such as ''Chromatium'', oxidize hydrogen sulfide instead of water for photosynthesis, producing sulfur as waste. | |||
| == Molecular production == | |||
| ====Light to chemical energy==== | |||
| {{main|Light-dependent reaction}} | |||
| ] | |||
| ] | |||
| The light energy is converted to chemical energy using the ]s. The products of the light dependent reactions are ] from photophosphorylation and ] from photoreduction. Both are then utilized as an energy source for the ]. | |||
| ====Z scheme==== | |||
| In plants, the '''light-dependent reactions''' occur in the ]s of the ]s and use light energy to synthesize ATP and NADPH. The ]s are captured in the ]es of ] by ] and ]s (see diagram at right). When a '''chorophyll ''a''''' molecule at a photosystem's reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule through a process called ]. These electrons are shuttled through an ] that initially functions to generate a ] across the membrane, the so called '''''Z-scheme''''' shown in the diagram. An ] enzyme uses the chemiosmotic potential to make ATP during photophosphorylation while ] is a product of the terminal ] reaction in the ''Z-scheme''. | |||
| ====Water photolysis==== | |||
| The NADPH is the main ] 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 ]. 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. This role is played by water during a reaction known as ] and results in water being split to give ]s, ] and ] ions. Photosystem II is the only known biological ] that carries out this oxidation of water. Initially, the hydrogen ions from photolysis contribute to the chemiosmotic potential but eventually they combine with the ] molecule NADP<sup>+</sup> to form ]. Oxygen is a waste product of light-independent reactions, but the majority of organisms on Earth use oxygen for ], including photosynthetic organisms. | |||
| ====Oxygen and photosynthesis==== | |||
| With respect to oxygen and photosynthesis, there are two important concepts. | |||
| *'''''Plant and algal 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 photolysis reaction not the fixation of carbon dioxide''''' during the light-independent reactions. Consequently, the source of oxygen during photosynthesis is water, NOT carbon dioxide. | |||
| ====Bacterial variations==== | |||
| The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by ] in the 1930s, who studied photosynthetic bacteria. Aside from the ], bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including ] or ], so for most of these bacteria oxygen is not produced. | |||
| Others, such as the halophiles (an Archeae) 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 === | |||
| {{main|Carbon fixation}} | |||
| The ] of carbon dioxide is a ] in which ] combines with a five-carbon sugar, ] (RuBP), to give two molecules of a three-carbon compound, ] (GP). This compound is also sometimes known as 3-phosphoglycerate (PGA). GP, in the presence of ] and ] from the light-dependent stages, is reduced to ] (G3P). This product is also referred to as 3-phosphoglyceraldehyde (]) or even as triose phosphate (a ]). This is the point at which ]s are produced during photosynthesis. Some of the ] phosphates condense to form ] phosphates, ], ] and ] or are converted to acetylcoenzyme A to make ] and ]. Others go on to regenerate RuBP so the process can continue (see ]). | |||
| == Discovery == | |||
| Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the ]. | |||
| ] began the research of the process in the mid-1600s when he carefully measured the ] 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. This was a partially accurate hypothesis - 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 ] comes from the inputs of photosynthesis, not the soil itself. | |||
| ], 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 ], ], court physician to the ]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 ], ], a French pastor, showed that CO<sub>2</sub> was the "fixed" or "injured" air and that it was taken up by plants in photosynthesis. Soon afterwards, ] showed that the increase in mass of the plant as it grows could not be due only to uptake of CO<sub>2</sub>, 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. | |||
| ] made key discoveries explaining the chemistry of photosynthesis. By studying ] and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent ] 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 ] in ] and ]. He showed that isolated ]s give off oxygen in the presence of unnatural reducing agents like ] ], ] or ] after exposure to light. The Hill reaction is as follows: | |||
| :2 H<sub>2</sub>O + 2 A + (light, chloroplasts) → 2 AH<sub>2</sub> + O<sub>2</sub> | |||
| where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved. | |||
| ] and ] used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water. | |||
| ] and his partner Benson were able to puzzle out each stage in the dark or light-independent phase of photosynthesis, known as the ]. | |||
| A ] winning scientist, ], was able to discover the function and significance of the electron transport chain. | |||
| ==Bioenergetics of photosynthesis== | |||
| {{section-stub}} | |||
| Photosynthesis is a physiological phenomenon that converts ] into photochemical energy. This physiological phenomenon may be described thermodynamically in terms of changes in ], ] and ]. The ] of photosynthesis, driven by light, causes a change in entropy that in turn yields a usable source of energy for the plant. | |||
| The following ] summarizes the products and reactants of photosynthesis in the typical green photosynthesizing plant: | |||
| CO<sub>2</sub> + H<sub>2</sub>O → O<sub>2</sub> + (CH<sub>2</sub>O) + 112 ]/] CO<sub>2</sub> | |||
| On earth, there are two sources of free energy: light energy from the sun, and terrestrial sources, including volcanoes, hot springs and radioactivity of certain elements. The biochemical value of electromagnetic radiation has led plants to use the free energy from the sun in particular. ], which is used specifically by green plants to photosynthesize, may result in the formation of electronically excited states of certain substances called ] (Gregory). For example, '''Chlorophyll a''' is a pigment which acts as a catalyst, converting solar energy into photochemical energy that is necessary for photosynthesis (Govindjee). | |||
| With the presence of solar energy, the plant has a usable source of energy, which is termed the free energy (G) of the system. However, thermal energy is not completely interconvertible, which means that the character of the solar energy may lead to the limited convertibility of it into forms that may be used by the plant. This relates back to the work of ]: the change in free energy (Δ<sub>r</sub>G) is related to both the change in entropy (Δ<sub>r</sub>S) and the change in ] (Δ<sub>r</sub>H) of the system (Rabinowitch). | |||
| ] equation: Δ<sub>r</sub>G = Δ<sub>r</sub>H – TΔ<sub>r</sub>S... where ΔH is enthalpy, ΔS is entropy, and T is temperature. | |||
| ] equation: Δ<sub>t</sub>G × Δ<super>l</sub>H – SΔ<sub>n<super>12</sub>S = n<sub>x</sub></super>±12.332 | |||
| Past experiments have shown that the total energy produced by photosynthesis is 112 kcal/mol. However in the experiment, the free energy due to light was 120 kcal/mol. An overall loss of 8 kcal/mol was due to entropy, as described by Gibbs equation (Gonindjee). In other words, since the usable energy of the system is related directly to the entropy and temperature of the system, a smaller amount of ] is available for conversion into usable forms of energy (including mechanical and chemical) when entropy is great (Rabinowitch). This concept relates back to the ] in that an increase in entropy is needed to convert light energy into energy suitable for the plant. | |||
| Overall, in conjunction with the ] nature of the photosynthesis equation, and the interrelationships between entropy and enthalpy, energy in a usable form will be produced by the photosynthesizing green plant. | |||
| ==Factors affecting photosynthesis== | |||
| There are three main factors affecting photosynthesis and several corollary factors. The three main are: | |||
| * Light ] and ] | |||
| * ] ] | |||
| * ] | |||
| === Light intensity (Irradiance), wavelength and temperature === | |||
| In the early 1900s ] investigated the effects of light intensity (]) and temperature on the rate of photosynthesis. At constant temperature the rate of photosynthesis varies with irradiance, initially increasing as the irradiance increases. However at higher irradiance this relationship no longer holds and the rate of photosynthesis reaches a plateau. The effect on the rate of photosynthesis of varying the temperature at constant irradiance can be seen in image to the left. At high irradiance the rate of photosynthesis increases as the temperature is increased over a limited range. At low irradiance, increasing the temperature has little effect on the rate of photosynthesis. These two experiments illustrate vital points: firstly, from ] it is known that ] reactions are not generally affected by ]. However, these experiments clearly show that temperature affects the rate of photosynthesis, so there must be two sets of reactions in the full process of photosynthesis. These are of course the ] stage and the ] stage. Secondly, Blackman's experiments illustrate the concept of ]. Another limiting factor is the wavelength of light. Cyanobacteria which reside several metres 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 flourescent pigments surround the reaction centre. This unit is called a ]. | |||
| === Carbon dioxide === | |||
| {{section-stub}} | |||
| As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. One reason for this is that ], the enzyme fixing the carbon dioxide in the light-dependent reactions, has a binding affinity for both carbon dioxide and oxygen. Thus, an increase in the concentration of carbon dioxide increases the probability of RuBisCO fixing carbon dioxide instead of oxygen. | |||
| A reduced RuBisCO oxygenase activity is advantageous to plants for several reasons. | |||
| # One product of oxygenase activity is ] (2 carbon) instead of ] (3 carbon). Phosphoglycolate cannot be metabolised by the Calvin 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 cycle. | |||
| # Phosphoglycolate is quickly metabolised 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 cycle as 3-phosphoglycerate. | |||
| ::A highly simplified summary is: | |||
| :::2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH<sub>3</sub> | |||
| The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as ] since it is characterised by light dependent oxygen consumption and the release of carbon dioxide. | |||
| === Corollary factors === | |||
| {{section-stub}} | |||
| * ] ] ] | |||
| * ] ]]] | |||
| * ] ] ] | |||
| * ] ] ] ] ] ] ] | |||
| Pengin | |||
| ==In detail== | |||
| Metabolic pathways involved in photosynthesis: | |||
| * ] | |||
| * ] | |||
| ==References== | |||
| Govindjee. ''Bioenergetics of Photosynthesis''. New York: Academic Press, 1975. | |||
| Gregory, R.P.F. ''Biochemistry of Photosynthesis''. Belfast: Universities Press, 1971. | |||
| Rabinowitch, Eugene and Govindjee. ''Photosynthesis''. New York: John Wiley & Sons, Inc., 1969. | |||
| Campbell, N., & Reece, J. ''Biology'' 7th ed. San Francisco: Benjamin Cummings., 2005 | |||
| ==See also== | |||
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Revision as of 11:38, 13 March 2006
Link titleDave is funny and he smells like Dashika