Which photosynthetic process is responsible for making atp

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light- dependent reaction.

The light-independent stage, also known as the Calvin Cycle , takes place in the stroma , the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light- independent reaction. C3 and C4 photosynthesis. Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose.

C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water.

Used by the majority of plants, it involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become a sugar called glucose. Involves producing a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle in plants that do not get a lot of light or water. In a plant cell, the protein-containing matrix between the thylakoid membranes and the chloroplast membrane.

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As the protons move down the F0 component, the gamma subunit of F1 ATPase brings out the conformational change of the nucleotide-binding sites in the F1 beta subunits. As a result, a proton motive force is experienced by the F0 F1 complex that further catalyse the generation of ATP. The ATP synthesis occurs by the two successive stages. The diagram below depicts the one stage of the active cycle.

The first stage explains the conformational changes in the F1 alpha and beta-subunits. A gamma subunit revolves in anticlockwise direction at an angle of degrees. Thus, the free energy produced while the proton translocation is availed by the cell for the interconversion of O, L and T state. Therefore, we can conclude that the ATP synthase performs a significant role in the generation of ATP during the light-dependent phase of photosynthesis.

Photosynthetic membranes effectively limit electron transport to two dimensions. For mobile electron carriers, limiting diffusion to two dimensions increases the number of random encounters Whitmarsh, Furthermore, because plastocyanin is mobile, any one cytochrome bf complex can interact with a number of photosystem I complexes.

The same is true for plastoquinone, which commonly operates at a stoichiometry of about six molecules per photosystem II complex. Although the value of DY across the photosynthetic membrane in chloroplasts can be as large as mV, under normal conditions the proton gradient dominates. For example, during photosynthesis the outer pH is typically near 8 and the inner pH is typically near 6, giving a pH difference of 2 across the membrane that is equivalent to mV.

The CF0 subunit spans the photosynthetic membrane and forms a proton channel through the membrane. The CF1 subunit is attached to the top of the CF0 on the outside of the membrane and is located in the aqueous space. CF1 is composed of several different protein subunits, referred to as a, b, g, d and e.

The top portion of the CF1 subunit is composed of three ab-dimers that contain the catalytic sites for ATP synthesis. A recent major breakthrough has been the elucidation of the structure of ATPase of beef heart mitochondria by Abrahams et al.

The molecular processes that couple proton transfer through the protein to the chemical addition of phosphate to ADP are poorly understood. It is known that phosphorylation can be driven by a pH gradient, a transmembrane electric field, or a combination of the two.

Experiments indicate that three protons must pass through the ATP synthase complex for the synthesis of one molecule of ATP. However, the protons are not involved in the chemistry of adding phosphate to ADP. Paul Boyer and coworkers have proposed an alternating binding site mechanism for ATP synthesis Boyer, One model based on their proposal is that there are three catalytic sites on each CF1 that cycle among three different states Fig.

At any one time, each site is in a different state. This model is supported by the structure of ATPase elucidated by Abrahams et al.

Initially, one catalytic site on CF1 binds one ADP and one inorganic phosphate molecule relatively loosely. Due to a conformational change of the protein, the site becomes a tight binding site, that stabilizes ATP. Next, proton transfer induces an alteration in protein conformation that causes the site to release the ATP molecule into the aqueous phase.

In this model, the energy from the proton electrochemical gradient is used to lower the affinity of the site for ATP, allowing its release to the water phase. The three sites on CF1 act cooperatively, i. It has been proposed that protons affect the conformational change by driving the rotation of the top part the three ab-dimers of CF1.

Such a rotating model has recently been supported by recording of a rotation of the gamma subunit relative to the alpha-beta subunits by Sabbert et al. This revolving site mechanism would require rates as high as revolutions per second. It is worth noting that flagella that propel some bacteria are driven by a proton pump and can rotate at 60 revolutions per second.

The process is a sequence of biochemical reactions that reduce carbon and rearrange bonds to produce carbohydrate from CO2 molecules. The first step is the addition of CO2 to a five-carbon compound ribulose 1,5-bisphosphate Fig. The six-carbon compound is split, giving two molecules of a three-carbon compound 3-phosphoglycerate. This key reaction is catalyzed by Rubisco, a large water soluble protein complex.

The 3-dimensional structure has been determined by X-ray analysis for Rubisco isolated from tobacco Schreuder et al. The carboxylation reaction is energetically downhill. The main energy input in the Calvin cycle is the phosphorylation by ATP and subsequent reduction by NADPH of the initial three-carbon compound forming a three-carbon sugar, triosephosphate. Some of the triosephosphate is exported from the chloroplast and provides the building block for synthesizing more complex molecules.

In a process known as regeneration, the Calvin cycle uses some of the triosephosphate molecules to synthesize the energy rich ribulose 1,5-bisphosphate needed for the initial carboxylation reaction. This reaction requires the input of energy in the form of one ATP.

Overall, thirteen enzymes are required to catalyze the reactions in the Calvin cycle. The reactions do not involve energy transduction, but rather the rearrangement of chemical energy. Rubisco is a bifunctional enzyme that, in addition to binding CO2 to ribulose bisphosphate, can also bind O2.

This oxygenation reaction produces the 3-phosphoglycerate that is used in the Calvin cycle and a two-carbon compound 2-phosphoglycolate that is not useful for the plant.

In response, a complicated set of reactions known as photorespiration are initiated that serve to recover reduced carbon and to remove phosphoglycolate. The Rubisco oxygenation reaction appears to serve no useful purpose for the plant. Some plants have evolved specialized structures and biochemical pathways that concentrate CO2 near Rubisco. These pathways C4 and CAM , serve to decrease the fraction of oxygenation reactions see Chapter this volume on carbon reduction.

Measurements in algal cells and leaves under optimal conditions e. However, under normal growing conditions the actual performance of the plant is far below these theoretical values. The factors that conspire to lower the quantum yield of photosynthesis include limitations imposed by biochemical reactions in the plant and environmental conditions that limit photosynthetic performance.

However, most crops are less productive. The annual conversion efficiency of corn, wheat, rice, potatoes, and soybeans typically ranges from 0. They represent a diverse group that include the dinoflagellates, the euglenoids, yellow-green algae, golden-brown algae, diatoms, red algae, brown algae, and green algae. The photosynthetic apparatus and biochemical pathways of carbon reduction of algae are similar to plants.

Photosynthesis occurs in chloroplasts that contain photosystems II and I, the cytochrome bf complex, the Calvin cycle enzymes and pigment-protein complexes containing chlorophyll a, and other antenna pigments e. Green algae are thought to be the ancestral group from which land plants evolved see Douglas, Algae are abundant and widespread on the earth, living mainly in fresh and sea water.

Some algae live as single celled organisms, while others form multicellular organisms some of which can grow quite large, like kelp and seaweed. Phytoplankton in the ocean is made up of algae and oxygenic photosynthetic bacteria. Most photosynthesis in the ocean is due to phytoplankton, which is an important source of food for marine life.

Fossil evidence indicates that cyanobacteria existed over 3 billion years ago and it is thought that they were the first oxygen evolving organisms on earth Wilmotte, Cyanobacteria are presumed to have evolved in water in an atmosphere that lacked O2. Initially, the O2 released by cyanobacteria reacted with ferrous iron in the oceans and was not released into the atmosphere.

Geological evidence indicates that the ferrous Fe was depleted around 2 billion years ago, and earth's atmosphere became aerobic. The release of O2 into the atmosphere by cyanobacteria has had a profound affect on the evolution of life. The photosynthetic apparatus of cyanobacteria is similar to that of chloroplasts.

The main difference is in the antenna system. Cyanobacteria depend on chlorophyll a and specialized protein complexes phycobilisomes to gather light energy Sidler, They do not contain chlorophyll b. As in chloroplasts, the chlorophyll a is located in membrane bound proteins. The phycobilisomes are bound to the outer side of the photosynthetic membrane and act to funnel exciton energy to the photosystem II reaction center.

They are composed of phycobiliproteins, protein subunits that contain covalently attached open ring structures known as bilins that are the light absorbing pigments.

Primary photochemistry, electron transport, phosphorylation and carbon reduction occur much as they do in chloroplasts. Cyanobacteria have a simpler genetic system than plants and algae that enable them to be easily modified genetically. Because of this cyanobacteria have been used as a model to understand photosynthesis in plants.

By genetically altering photosynthetic proteins, researchers can investigate the relationship between molecular structure and mechanism Barry et al. Over the past three decades several types of oxygenic bacteria known as prochlorophytes or oxychlorobacteria have been discovered that have light harvesting protein complexes that contain chlorophyll a and b, but do not contain phycobilisomes Palenik and Haselkorn , Urbach et al. Anoxygenic photosynthetic bacteria differ from oxygenic organisms in that each species has only one type of reaction center Blankenship et al.

In some photosynthetic bacteria the reaction center is similar to photosystem II and in others it is similar to photosystem I. However, neither of these two types of bacterial reaction center is capable of extracting electrons from water, so they do not evolve O2.

Many species can only survive in environments that have a low concentration of O2. To provide electrons for the reduction of CO2, anoxygenic photosynthetic bacteria must oxidize inorganic or organic molecules available in their environment. Although many photosynthetic bacteria depend on Rubisco and the Calvin cycle for the reduction of CO2, some are able to fix atmospheric CO2 by other biochemical pathways. Despite these differences, the general principles of energy transduction are the same in anoxygenic and oxygenic photosynthesis.

Anoxygenic photosynthetic bacteria depend on bacteriochlorophyll, a family of molecules that are similar to the chlorophyll, that absorb strongly in the infrared between and nm. The antenna system consists of bacteriochlorophyll and carotenoids that serve a reaction center where primary charge separation occurs. The electron carriers include quinone e. Non-sulfur purple bacteria typically use an organic electron donor, such as succinate or malate, but they can also use hydrogen gas.

The sulfur bacteria use an inorganic sulfur compound, such as hydrogen sulfide as the electron donor. The only pathway for carbon fixation by purple bacteria is the Calvin cycle. Sulfur purple bacteria must fix CO2 to live, whereas non-sulfur purple bacteria can grow aerobically in the dark by respiration on an organic carbon source. The determination of the three-dimensional structures of the reaction center of the non- sulfur purple bacteria, Rhodopseudomonas viridis and Rhodobacter sphaeroides, has provided an unprecedented opportunity to understand the structure and function of photosynthetic reaction centers Deisenhofer et al.

The positions of the electron transfer components in the reaction center of Rhodobacter sphaeroides are shown in Fig. The reaction center contains four bacteriochlorophyll and two bacteriopheophytin molecules. Two of the bacteriochlorophyll molecules form the primary donor P At present, there is controversy over whether a bacteriochlorophyll molecule is an intermediate in electron transfer from the P to bacteriopheophytin.

However, there is agreement that the remaining steps involve two quinone molecules QA and QB and that two turnovers of the reaction center results in the release of reduced quinone QH2 into the photosynthetic membrane. Although there is a non-heme Fe between the two quinone molecules, there is convincing evidence that this Fe is not involved directly in transferring an electron from QA to QB. Because the primary donor P , bacteriopheophytin and quinone acceptors of the purple bacterial reaction center are similar to the photosystem II reaction center, the bacterial reaction center is used as guide to understand the structure and function of photosystem II.

Light driven electron transfer is cyclic in Rhodobacter sphaeroides and other purple bacteria Fig. The reaction center produces reduced quinone, which is oxidized by the cytochrome bc complex. The product of the light driven electron transfer reactions is ATP. The electrons for the reduction of carbon are extracted from an organic donor, such as succinate or malate or from hydrogen gas, but not by the reaction center.

The energy transformation pathway is complicated. As shown in Fig. The antenna system of the green sulfur bacteria is composed of bacteriochlorophyll and carotenoids and is contained in complexes known as a chlorosomes that are attached to the surface of the photosynthetic membrane.

This antenna arrangement is similar to the phycobilisomes of cyanobacteria. Green sulfur bacteria can fix CO2 without Rubisco. It has been proposed that they accomplish this by using the respiratory chain that normally oxidizes carbon known as the Krebs cycle , resulting in the release of CO2. With the input of energy this process can be run in the reverse direction, resulting the uptake and reduction of CO2. Like the green sulfur bacteria, green gliding bacteria harvest light using chlorosomes.

The green gliding bacteria appear to have reaction centers similar to those of the purple bacteria Fig. For example, instead of two monomer bacteriochlorophyll molecules, C. The three-dimensional structure of the reaction center of Rhodopseudomonas viridis and Rhodobacter sphaeroides reveals the distances between the electron donors and acceptors Deisenhofer et al.

There is currently a controversy concerning the importance of specific amino acid composition of the protein on the rate of intraprotein electron transfer. In part, the disagreement centers on whether the protein between the donor and acceptor molecules can be treated as a uniform material, or whether the specific amino acid composition of the protein significantly alters the rate.

For example, it has been proposed that aromatic amino acids may provide a particular pathway that facilitates electron transfer between a donor and acceptor pair. However, in other cases, replacement of an aromatic by another non-aromatic residue has resulted in relatively minor changes in the rate of electron transfer. Dutton and coworkers Moser et al. Marcus and others DeVault, Dutton and coworkers argue that protein provides a uniform electronic barrier to electron tunneling and a uniform nuclear characteristic frequency.

They suggest that the specific amino acid residues between an electron transfer pair is generally of less importance than the distance in determining the rate of pairwise electron transfer. In their view, protein controls the rate of electron transfer mainly through the distance between the donor and acceptor molecules, the free energy, and the reorganization energy of the reaction.

The importance of distance is demonstrated by electron transfer data from biological and synthetic systems showing that the dependence of the electron transport rate on the edge to edge distance is exponential over orders of magnitude when the free energy is optimized Moser et al. Increasing the distance between two carriers by 1. The extent to which this view is generally applicable for intraprotein transfer remains to be established Williams, One of the challenges in understanding pairwise electron transfer rates from first principles is illustrated by the reaction centers of Rhodopsuedobacter sphaeroides in which the redox components are arranged along two-fold axis of symmetry that extends from the primary donor P to the non heme Fe.

Despite the fact that the reaction center presents two spatially similar pathways for electron transfer from P to quinone, nearly all electrons are transferred down the right-arm of the reaction center as shown in Fig. The same is true for the reaction center of Rhodopseudomonas viridis, in which it is estimated that electron transfer down the left-arm is less than Kellogg et al. The challenge to theorists is to explain the surprisingly high probability that electron flow goes down the right-arm.

Since the distances are similar, it has been suggested that electron transfer down the left-arm is less probable due to an endothermic free energy change Parson et al. The amount of CO2 removed from the atmosphere each year by oxygenic photosynthetic organisms is massive. This is equivalent to 4 x kJ of free energy stored in reduced carbon, which is roughly 0. Each year the photosynthetically reduced carbon is oxidized, either by living organisms for their survival, or by combustion.

The result is that more CO2 is released into the atmosphere from the biota than is taken up by photosynthesis. The oceans mitigate this increase by acting as a sink for atmospheric CO2. This carbon is eventually stored on the ocean floor. Although these estimates of sources and sinks are uncertain, the net global CO2 concentration is increasing.

Direct measurements show that each year the atmospheric carbon content is currently increasing by about 3 x grams. Over the past two hundred years, CO2 in the atmosphere has increased from about parts per million ppm to its current level of ppm. Based on predicted fossil fuel use and land management, it is estimated that the amount of CO2 in the atmosphere will reach ppm within the next century.

The consequences of this rapid change in our atmosphere are unknown. Such a large temperature increase would lead to significant changes in rainfall patterns. Little is known about the impact of such drastic atmospheric and climatic changes on plant communities and crops.

Current research is directed at understanding the interaction between global climate change and photosynthetic organisms. This text is a revised and modified version of "Photosynthesis" by J.

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