Photosynthesis Light And Dark Reaction Pdf
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- Light and Dark Reactions in Photosynthesis
- Light and Dark Reactions in Photosynthesis
- The Process of Photosynthesis The Dark Reactions
Photosynthesis in desert plants has evolved adaptations that conserve water.
Blankenship, Robert E. Department of Chemistry, Washington University, St. Louis, Missouri.
Light and Dark Reactions in Photosynthesis
Blankenship, Robert E. Department of Chemistry, Washington University, St. Louis, Missouri. Berkowitz, Gerald A. Portis, Archie R. Shopes, R. Last reviewed: March The synthesis of chemical compounds using light, especially the manufacture of organic compounds primarily carbohydrates from carbon dioxide and a hydrogen source such as water , most often with simultaneous liberation of oxygen, by chlorophyll- or bacteriochlorophyll-containing cells.
The term photosynthesis is used almost exclusively to designate one particularly important natural process: the use of light in the manufacture of organic compounds primarily certain carbohydrates from inorganic materials by chlorophyll- or bacteriochlorophyll-containing cells Fig.
This process requires a supply of energy in the form of light because its products contain much more chemical energy than its raw materials. This is clearly shown by the liberation of energy in the reverse process, namely the combustion of organic material with oxygen, which takes place during respiration.
Among chlorophyll-containing plant and algal cells, as well as in cyanobacteria formerly known as blue-green algae , photosynthesis involves the oxidation of water H 2 O to produce oxygen O 2 molecules, which are then released into the environment. This is called oxygenic photosynthesis Fig.
In contrast, bacterial photosynthesis does not involve O 2 evolution production. In this case, other electron donors, such as hydrogen sulfide H 2 S , are used instead of H 2 O. This process is called anoxygenic photosynthesis. Because all types of photosynthesis require light, photosynthetic organisms are generally restricted to the narrow region of the Earth near the surface that receives sunlight.
The only known exceptions are the anoxygenic photosynthetic bacteria that live near deep-sea hydrothermal vents and that utilize the very weak light given out by the hot vents. The light energy that is absorbed by the pigments of photosynthesizing cells, especially by chlorophyll or bacteriochlorophyll pigments, is efficiently converted into stored chemical energy.
Together, the two aspects of photosynthesis—the conversion of inorganic matter into organic matter, and the conversion of light energy into chemical energy—combine to create the fundamental process of life on Earth: it is the ultimate source of all living matter and of almost all of the energy of life. See also: Hydrothermal vent. The net overall chemical reaction of oxygenic photosynthesis by plants, algae, and cyanobacteria is shown in reaction 1 : 1. The photochemical reaction in photosynthesis belongs to the type known as oxidation-reduction, with carbon dioxide CO 2 acting as the oxidant electron acceptor and water acting as the reductant electron donor.
The unique characteristic of this particular oxidation-reduction is that it is energetically unfavorable; that is, it converts chemically stable materials into chemically unstable products.
A considerable part of the light energy used for this process is stored as chemical energy. See also: Carbon dioxide ; Oxidation-reduction.
Photosynthesis is a complex multistage process that consists of nearly a hundred physical processes and chemical reactions. To make this complex process more understandable, it is useful to divide it into four temporal stages.
Each phase is based roughly on the time scale in which it occurs. These phases are 1 photon absorption and energy transfer processes in antennas or antenna chlorophylls, that is, molecules that collect light quanta ; 2 primary electron transfer in photochemical reaction centers; 3 electron transport and adenosine triphosphate ATP formation; and 4 carbon fixation and export of stable products Fig. See also: Adenosine triphosphate ATP.
The photosynthetic process in plant cells and algae takes place within pigment-bearing subcellular organelles called chloroplasts which are cell plastids [ Fig. Ten to of them may be present in the average parenchyma cell of a leaf. Under the electron microscope, all chloroplasts show a layered structure with alternating lighter and darker layers of roughly 0. These layers are membranes, called thylakoid membranes thylakoid stands for a membrane sac , which contain proteins. These proteins bind all of the chlorophyll.
The thylakoid membranes are the sites of the first three phases of photosynthesis. In algae, the number and shape of the chloroplasts are much more variable. See also: Cell plastids ; Leaf ; Parenchyma ; Plant anatomy. The photochemical apparatus is less complex in cyanobacteria. These cells are prokaryotes and therefore lack a nucleus and other organelles, including chloroplasts and mitochondria.
The early phases of photosynthesis take place on thylakoid membranes, which extend throughout the interior of the cell. See also: Prokaryote. Two photochemical events cooperate to carry out oxygenic photosynthesis Fig. Experiments suggest that plants contained two pigment systems. One called photosystem I, or PSI; sensitizing reaction I is primarily composed of chlorophyll a ; the other called photosystem II, or PSII; sensitizing reaction II is also composed of chlorophyll a , but includes most of chlorophyll b or other auxiliary pigments including the carotenoids and the phycobilins.
This ensures that the excitation energy within both systems is absorbed by the antenna system and partitioned to each photosystem, where the energy drives the chemical reactions. The final result of this set of reactions is the oxidation of water to O 2 and the reduction of a plastoquinone an oxidation-reduction catalyst. Current evidence suggests that light absorbed by the major part of the accessory pigments is ultimately transferred to a special chlorophyll a molecule in the PSII reaction center, which is in a favorable position to act as an energy trap.
Note that the membrane has an inherent asymmetry in that the protein complexes are oriented in a particular way in the membrane. This orientation is essential to the proper functioning of photosynthesis.
Figure 5 shows the Z scheme, which delineates the way in which the two photosystems cooperate to carry out the electron transfer reactions involved in photosynthesis. It is an energetic diagram, in that the energy of the component, which is measured as the midpoint redox potential E m , is shown on the y axis and the progress of the reaction is shown on the x axis.
The two vertical arrows in the diagram represent energy input to the system due to photon absorption. See also: Photon. When illuminated in the presence of adenosine diphosphate ADP and inorganic phosphate P i , the inner cytoplasmic membranes from photosynthetic bacteria, cyanobacterial cells, and chloroplasts from green plants and algae use light energy to synthesize adenosine triphosphate ATP.
About 42 kilojoules kJ of converted light energy in this reaction is stored in each mole of the high-energy phosphate, ATP. This type of cyclic electron flow, mediated by added cofactors as well as ADP and inorganic phosphate, also leads to the production of ATP and has been termed cyclic phosphorylation; it has been shown to exist under certain experimental conditions in vivo.
See also: Nicotinamide adenine dinucleotide NAD. Besides chlorophyll a which is present in nearly all oxygenic photosynthetic organisms , there are other chlorophylls, including chlorophyll b in the green algae and higher plants. In addition, chlorophyll c replaces chlorophyll b in brown algae, whereas most of the chlorophyll a in the marine cyanobacterium Acaryochloris marina is replaced by chlorophyll d.
There are also nonchlorophyllous pigments belonging to two groups: the carotenoids and the phycobilins. The carotenoids named because of their similarity to the orange pigment of carrots are a variable assortment of pigments found in all photosynthetic higher plants and algae. The phycobilins, or vegetable bile pigments, are chemically related to animal bile pigments. They are either red phycoerythrin or blue phycocyanin.
All of these pigments are associated with specific proteins to form so-called antenna pigment proteins. The structures of many of these antenna complexes are known, and their absorption spectra have been analyzed Fig.
See also: Carotenoids ; Phycobilin. The light-dependent conversion of radiant energy into chemical energy as adenosine triphosphate ATP and reduced nicotinamide adenine dinucleotide phosphate NADPH serves as a prelude to the utilization of these compounds for the reductive fixation of CO 2 into organic molecules.
Such molecules, broadly designated as photosynthates, are usually but not invariably in the form of carbohydrates for example, glucose polymers or sucrose and form the base for the nutrition of all living things, as well as serving as the starting material for fuel, fiber, animal feed, oil, and other compounds used by humans. Collectively, the biochemical processes by which CO 2 is assimilated into organic molecules are known as the photosynthetic dark reactions, which are so named because light is not required in contrast to the photosynthetic light reactions.
Approximately pentagrams of carbon 1 pentagram equals 10 9 metric tons as CO 2 is assimilated annually into organic molecules by photosynthesis about half of this amount is assimilated by photosynthetic marine algae. The essential details of C 3 photosynthesis can be seen in Figure 7. The entire cycle can be separated into three phases—carboxylation, reduction, and regeneration. For the purposes of understanding, it is easiest to start with three molecules of CO 2 because the smallest intermediate in the cycle consists of three carbons.
These unstable molecules are hydrolyzed further into six molecules of the three-carbon compound phosphoglyceric acid PGA. These products of the carboxylation phase, that is, the six three-carbon PGA molecules, are phosphorylated by six molecules of ATP releasing ADP to be used for photophosphorylation via the light reactions to form six 1,3-bisphosphoglycerate 1,3-BP molecules.
The resulting compounds are reduced that is, in the reduction phase of the C 3 cycle by the NADPH formed in the photosynthetic light reactions to form six molecules of the three-carbon compound phosphoglyceraldehyde PGAL. PGAL the aldehyde and DHAP the ketone are energetically equivalent, reduced compounds and can be considered to be the products of the reductive phase of the C 3 photosynthetic cycle. The rest of the C 3 photosynthetic cycle the regeneration phase involves enzymatic steps that allow regeneration of RuBP, the initial carboxylation substrate.
The other five PGAL molecules, through a complex series of enzymatic reactions, are rearranged into three molecules of RuBP, which can again be carboxylated with CO 2 to continue the cycle. This reaction initiates the process called photorespiration, which results in the release of one previously incorporated molecule of CO 2 for every two molecules of O 2 that are allowed to react. Due to its low catalytic efficiency, Rubisco can be up to half of the soluble protein in C 3 chloroplasts, and it is likely the most abundant protein found in nature.
Structurally, Rubisco is a large and complex enzyme, comprising eight large polypeptide subunits and eight small subunits. See also: Enzyme ; Photorespiration. Factors within the photosynthesizing cell, such as energy requirements in different compartments mitochondria, cytoplasm, and chloroplasts , along with energy needs of the plant for example, increased sink requirements during different developmental stages , and external environmental factors for example, light intensity and duration ultimately regulate the partitioning of the newly formed photosynthetic product PGAL into starch or sucrose.
See also: Plant metabolism. Initially, the C 3 cycle was thought to be the only route for CO 2 assimilation, although it was recognized by plant anatomists that some rapidly growing plants such as maize, sugarcane, and sorghum possessed an unusual organization of the photosynthetic tissues in their leaves Kranz morphology. Further work demonstrated that plants having the Kranz anatomy utilized an additional CO 2 assimilation route, which is now known as the C 4 -dicarboxylic acid pathway Fig.
Carbon dioxide enters a mesophyll cell, where it is combined in the form of bicarbonate with the three-carbon compound phosphoenolpyruvate PEP via the enzyme PEP carboxylase to form a four-carbon acid, oxaloacetate, which is reduced to malic acid or transaminated to aspartic acid.
The four-carbon acid moves into bundle sheath cells, where the acid is decarboxylated and the CO 2 reassimilated via the C 3 cycle. To complete the cycle, the resulting three-carbon compound, pyruvic acid, moves back into the mesophyll cell and is transformed into PEP at the cost of 2 ATP molecules via the enzyme pyruvate phosphate dikinase located in the mesophyll chloroplasts.
The net effect of this cycle is to increase the CO 2 concentration around Rubisco, thereby reducing photorespiration via the competing oxygenase activity of this enzyme. C 4 metabolism is classified into three types, depending on the primary decarboxylation reaction used with the four-carbon acid in the bundle sheath cells.
Under arid and desert conditions, where soil water is in short supply, transpiration during the day when temperatures are high and humidity is low may rapidly deplete the plant of water, leading to desiccation and death. By keeping stomata closed during the day, water can be conserved; however, the uptake of CO 2 , which occurs entirely through the stomata, is prevented.
Therefore, many desert plants including those in the Crassulaceae, Cactaceae, and Euphorbiaceae families have evolved, apparently independently of C 4 plants, a similar strategy of concentrating and assimilating CO 2 by which the CO 2 is taken in at night when the stomata open; in general, water loss is low because of the reduced temperatures and correspondingly higher humidities.
The biochemical understanding of the mechanisms involved in this process was first studied in plants of the Crassulaceae family; thus, the process has been called crassulacean acid metabolism CAM. See also: Plant-water relations.
Light and Dark Reactions in Photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration , can later be released to fuel the organism's metabolic activities. Most plants , algae , and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts , which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate NADPH and adenosine triphosphate ATP , the "energy currency" of cells.
Photosynthesis is the process by which green plants absorb light energy from the sun with the assistance of water and carbon dioxide, and transform it into chemical energy to make synthesize carbohydrate specifically glucose and oxygen. If you need a longer review, check out our lesson on photosynthesis before reading on. The "light-independent" or dark reactions happen in the stroma of the chloroplasts. This is also known as the Calvin Cycle. Since these processes can only happen in the chloroplast a chlorophyll filled plastid in green plants , photosynthesis can only happen in green plants! The second overall principle of photosynthesis is that carbon, oxygen, and hydrogen atoms are taken from carbon dioxide and water molecules and are broken up and rearranged into new substances: carbohydrate specifically glucose and oxygen gas so we can breathe, whew!
The light-dependent or “Light” Reactions: • convert sunlight energy into chemical energy. (stored in ATP &. NADPH). “Dark” Reactions. (Calvin cycle).
The Process of Photosynthesis The Dark Reactions
Photosynthesis sustains virtually all life on planet Earth providing the oxygen we breathe and the food we eat; it forms the basis of global food chains and meets the majority of humankind's current energy needs through fossilized photosynthetic fuels. The process of photosynthesis in plants is based on two reactions that are carried out by separate parts of the chloroplast. The light reactions occur in the chloroplast thylakoid membrane and involve the splitting of water into oxygen, protons and electrons.
This analysis suggests that a model of the temperature dependence of carbon exchange by a plant can be developed based upon absolute reaction rate theory. Component temperature-dependent physiological processes necessary to describe net photosynthesis over the biological temperature range include the light reaction, dark reaction carboxylase CO 2 uptake, oxygenase photorespiration and mitochondrial dark respiration.
Chloroplasts pp Cite as. Rather, these direct products of photosynthesis are used to synthesize more appropriate storage forms of energy. The final achievement of photosynthesis, therefore, is the biosynthesis of carbohydrate, as glucose , from CO 2 and H 2 O. Glucose is the key carbohydrate around which the metabolism of most organisms revolves.
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- Вы же учились в колледжах.