BIOS 1700 Exam 2: Chapter 8 Notes
How is ATP generated in the photosynthetic electron transport chain?
In chloroplasts, as in mitochondria, ATP is synthesized by ATP synthase, a transmembrane protein powered by the movement of protons across the membrane. In chloroplasts, the ATP synthase is oriented such that the movement of protons from the thylakoid lumen to the stroma results in the synthesis of ATP.
What is the second major problem that photosynthesis faces?
A major challenge to photosynthetic efficiency is the fact that rubisco can add O2 to RuBP instead of CO2. When rubisco adds oxygen instead of carbon dioxide to RuBP, the result is one molecule with three carbon atoms (3-PGA) and one molecule with only two carbon atoms (2-phosphoglycolate). The production of 2-phosphoglycolate creates a serious problem because this molecule cannot be utilized by the Calvin cycle either to produce triose phosphate or to regenerate RuBP.
What was a major event in photosynthetic life history?
A major event in the history of life was the evolution of photosynthetic electron transport chains that could bridge the energy difference between the oxidation of water and the reduction of CO2. The first organisms to accomplish this feat were the cyanobacteria. These photosynthetic bacteria incorporated two different photosystems into a single photosynthetic electron transport chain. The addition of light energy in the first step allows electrons to be pulled from a water molecule, while the addition of light energy in the second step raises the energy level of these electrons so that they can be used to reduce CO2. Stripping electrons from water results in the release of oxygen. Indeed, all the oxygen in Earth's atmosphere results from photosynthesis by organisms containing two photosystems.
What is photorespiration? Why is it bad?
A metabolic pathway to recycle 2-phosphoglycolate is present in photosynthetic cells. However, this pathway is not able to return all of the carbon atoms in 2-phosphoglycolate to the Calvin cycle; some are released as carbon dioxide. Because the overall effect of oxygenation is a release of carbon dioxide in the presence of light, this process is referred to as photorespiration. However, unlike respiration, which produces ATP, photorespiration actually consumes ATP. In photorespiration, ATP is used to drive the reactions that convert a portion of the carbon atoms in 2-phosphoglycolate into 3-PGA, which can reenter the Calvin cycle. Thus, photorespiration represents a net energy drain on two accounts; first, it results in the oxidation and loss, in the form of carbon dioxide, of carbon atoms that had previously been incorporated and reduced by the Calvin cycle, and second, it consumes ATP.
Do photosynthetic cells also have mitochondria?
A photosynthetic cell can have more than 100 chloroplasts. Photosynthetic cells also contain mitochondria. Although photosynthetic organisms are correctly described as autotrophs because they can form carbohydrates from CO2, they also require mitochondrial respiration. Cellular respiration is therefore one of several features that heterotrophic organisms like ourselves share with photosynthetic organisms.
What is the second line of defense that deals with reactive oxygen species?
A second line of defense is to prevent reactive oxygen species from forming in the first place. Xanthophylls are yellow-orange pigments that slow the formation of reactive oxygen species by reducing excess light energy. These pigments accept absorbed light energy directly from chlorophyll and then convert this energy to heat. Photosynthetic organisms that live in extreme environments often appear brown or yellow because of high levels of xanthophyll pigments. Plants that lack xanthophylls grow poorly when exposed to moderate light levels and die in full sunlight. Converting absorbed light energy into heat is beneficial at high light levels, but at low light levels it would decrease the production of carbohydrates. Therefore, this ability is switched on only when the photosynthetic electron transport chain in working at high capacity. The creation of a strong proton gradient across the thylakoid membrane at high light levels activates the enzyme that converts inactive xanthophyll molecules into their light-absorbing form.
What are the Calvin cycle's energy requirements?
ATP is required for the regeneration of RuBP, raising the Calvin cycle's total energy requirements to two molecules of NADPH and three molecules of ATP for each molecule of CO2 incorporated by rubisco. Thus, NADPH and ATP play distinct roles in the formation of carbohydrates from CO2. NADPH provides energy that is incorporated into the higher-energy bonds that characterize carbohydrate molecules, whereas ATP provides energy that is needed to regenerate RuBP.
Where are Photosystem II and the cytochrome b6f complex located?
In contrast, photosystem II is located primarily on the closely packed inner regions of the thylakoid membrane that have little contact with the stroma. The cytochrome b6f complex, which is located between the two photosystems, has a relatively even distribution.
What happens when a chlorophyll absorbs visible light?
Absorption of visible light by a chlorophyll molecule results in one of its electrons being elevated to a higher energy state. For chlorophyll molecules that have been isolated in the laboratory, this absorbed light energy is rapidly released, allowing the electron to return to its initial "ground" energy state. Most of the energy (>95%) is converted into heat; a small amount is reemitted as light (fluorescence). By contrast, for chlorophyll molecules situated within the photosystems of the thylakoid, something entirely different occurs: the absorbed light energy is transferred to another chlorophyll molecule and then on to another.
What happens in the reduction step of the Calvin cycle?
Before rubisco can act as a carboxylase, RuBP and CO2 must diffuse into its active site. Once the active site is occupied, carboxylation proceeds spontaneously. The product is a 6-carbon compound that immediately breaks into two molecules of 3-phosphoglycerate (3-PGA). These 3-carbon molecules are the first stable products of the Calvin cycle.
Where does carbohydrate synthesis occur?
Carbohydrate synthesis takes place in the stroma, whereas sunlight is captured and transformed via the photosynthetic electron transport chain on the thylakoid membrane.
Where does energy come from in photosynthesis? What is it used for?
Carbohydrates have more energy stored in their chemical bonds than is contained in the bonds of the CO2 molecules from which they are synthesized during photosynthesis. Therefore, to build carbohydrates using CO2 requires an input of energy. In photosynthesis, this energy comes from sunlight. Energy is added to molecules during carbohydrate synthesis through the transfer of high-energy electrons. It is the addition of energy and electrons that allows the incoming CO2 molecules to form the higher-energy bonds found in a carbohydrate molecule. During photosynthesis, CO2 molecules are reduced to form higher-energy carbohydrate molecules.
How are cellular respiration and photosynthesis complementary?
Cellular respiration and photosynthesis are complementary metabolic processes. Cellular respiration breaks down carbohydrates in the presence of oxygen to supply the energy needs of the cell, producing carbon dioxide and water as byproducts, while photosynthesis uses carbon dioxide and water in the presence of sunlight to build carbohydrates, releasing oxygen as a byproduct.
What is chlorophyll? What is its structure?
Chlorophyll is the major photosynthetic pigment; it appears green because it is poor at absorbing green wavelengths. The chlorophyll molecule consists of a large, light-absorbing "head" containing a magnesium atom at its center and a long hydrocarbon "tail" that allows the pigment to be anchored in the lipid membrane.
What is the thylakoid membrane? What does it contain?
Chloroplasts are enclosed by a double membrane. Filling much of the center of the chloroplast is a third, highly folded membrane known as the thylakoid membrane. The photosynthetic electron transport chain is located on the thylakoid membrane.
Why is rubisco significant?
Compared to other enzymes, rubisco is surprisingly slow. As a result, photosynthetic cells must produce large amounts of this single enzyme. In fact, rubisco is the most abundant protein on Earth. As the principle gatekeeper through which CO2 becomes incorporated into carbohydrates, rubisco plays a key role in the processes that permit life on Earth.
How does cyclic electron transport work?
Cyclic electron transport leads to the production of ATP because as the high-energy electrons from ferredoxin pass through the cytochrome b6f complex, additional protons are pumped into the lumen. As a result, there are more protons in the lumen that can be used to drive the synthesis of ATP. In addition, a subset of the photosystem II antennae migrate through the thylakoid membrane to become associated with photosystem I. This redistribution of light-harvesting capacity increases the rate of cyclic electron transport relative to linear electron transport, further enhancing the production of ATP.
What is horizontal gene transfer?
Each of the two photosystems present in cyanobacteria has a high degree of similarity to the single photosystem found in other lineages of photosynthetic bacteria. An alternative hypothesis is that the genetic material associated with one photosystem was transferred to a bacterium with the other photosystem, resulting in a single bacterium with the genetic material to produce both types of photosystems. The transfer of genetic material between organisms that are not parent and offspring is called horizontal gene transfer, and it is common among prokaryotes, where it provides an important means of producing genetic diversity.
What happens when a chlorophyll absorbs visible light?
Each year, photosynthesis removes more than 100 billion metric tons of carbon from the atmosphere, while incorporating more than 100 terrawatts of solar energy into chemical bonds.
More facts about rubisco
For land plants, rubisco's low catalytic rate means that photosynthetic cells must produce huge amounts of this enzyme; as much as 50% of the total protein within a leaf is rubisco. At the same time, the low concentration of carbon dioxide in the atmosphere relative to oxygen means that as much as one-quarter of the reduced carbon formed in photosynthesis can be lost via photorespiration.
What do Photosystems II and I in the photosynthetic electron transport chain do?
For the two photosystems to work together to move electrons from water to NADPH, they must have distinct chemical properties. Photosystem II supplies electrons to the beginning of the electron transport chain. When photosystem II loses an electron (that is, when it is itself oxidized) it is able to pull electrons from water. In contrast, photosystem I energizes electrons with a second input of energy so they have enough energy to reduce NADP+. The key point here is that photosystem I when oxidized is not a sufficiently strong oxidant to split water, whereas photosystem II cannot produce electrons with enough energy to form NADPH.
How is the photosynthetic electron transport chain constrained?
High-energy electrons moving through the photosynthetic electron transport chain have the potential to damage the cell. To prevent damage, the movement of electrons through the photosynthetic electron transport chain is constrained: the electron follows a defined network of pathways through large protein complexes embedded in specialized "photosynthetic" membranes. These protein complexes provide a scaffold along which the pigments and other elements of the photosynthetic electron transport chain are arrayed.
How do protons accumulate in the thylakoid lumen?
How do protons accumulate in the thylakoid lumen? Two features of the photosynthetic electron transport chain are responsible for the build-up of protons in the thylakoid lumen. First, the oxidation of water releases protons and O2 into the lumen. Second, the cytochrome-b6f complex, the protein complex situated between photosystem II and photosystem I, functions as a proton pump. As electrons transit through the cytochrome b6f complex, some of the energy that is released by each redox reaction is used to drive protons from the stroma side of the thylakoid membrane to the lumen.
What happens if rubisco adds O2 instead of CO2?
However, if O2 instead of CO2 diffuses into the active site, the reaction can still proceed, although O2 is added to RuBP in place of CO2. An enzyme like rubisco that adds O2 to a substrate molecule is called an oxygenase. The addition of O2 by rubisco creates a major challenge for photosynthesis.
What is the Z scheme?
If you follow the flow of electrons from water, through both photosystems, and on to NADP+, you can see a large increase in the energy level of the electrons as they pass through each of the two photosystems. You can also see that every other step along the photosynthetic electron transport chain is associated with a small decrease in the energy level of the electrons. Because the overall energy trajectory has an up-down configuration resembling a "Z," the photosynthetic electron transport chain is sometimes referred to as the Z scheme.
What are pigments? Why are they colored?
Pigments are molecules that absorb some wavelengths of visible light. Pigments look colored because they reflect light enriched in the wavelengths that they do not absorb.
How is the movement of electrons through the photosynthetic electron transport chain controlled?
In fact, the movement of electrons through the photosynthetic electron transport chain is not controlled by the specificity of individual enzymes. Instead, it depends upon the spatial arrangement of the sites where the redox reactions take place within and between these major protein complexes.
What are the two steps involved in the reduction of 3-PGA?
In the Calvin cycle, the reduction of 3-PGA involves two steps: (1) ATP is used to phosphorylate 3-PGA, and (2) NADPH transfers two high-energy electrons to the phosphorylated compound. Because two molecules of 3-PGA are formed each time rubisco catalyzes the incorporation of one molecule of CO2, two ATP and two NADPH are required for each molecule of CO2 incorporated by rubisco.
What are triose phosphates? Why are they important?
Larger sugars, such as glucose and sucrose, are assembled from triose phosphates in the cytoplasm. However, if every triose phosphate molecule produced by the Calvin cycle were exported from the chloroplast, RuBP could not be regenerated and the Calvin cycle would grind to a halt. Thus, for every six triose phosphate molecules that are produced, only one can be withdrawn from the Calvin cycle.
What is visible light?
Light is a type of electromagnetic radiation, as are radio waves, X-rays, and other forms of radiation. Electromagnetic radiation is energy in the form of waves; the type of electromagnetic energy depends on the wavelength. Visible light is the portion of the electromagnetic spectrum apparent to our eyes, and it includes the range of wavelengths used in photosynthesis. The wavelengths of visible light are from 400 nm to 700 nm.
What is the reaction center?
Most of the chlorophyll molecules in the thylakoid membrane function as an antenna: absorbed light energy is transferred from one chlorophyll molecule to another until it is finally transferred to a specially configured pair of chlorophyll molecules known as the reaction center. The reaction center is where light energy is converted into electron transport. We now know that several hundred antenna chlorophyll molecules are associated with each reaction center. The antenna chlorophylls allow the photosynthetic electron transport chain to operate efficiently. Without the antenna to gather light energy, reaction centers would sit idle much of the time, even in bright sunlight.
Why would reactive oxygen species form?
NADP+ is returned to the photosynthetic electron transport chain by the Calvin cycle's consumption of NADPH. Thus, any factor that causes the rate of NADP+ regeneration to fall behind the rate of light-driven electron transport can potentially lead to damage. Such an imbalance is likely to occur, for example, in the middle of the day when light intensity is highest. The rate at which the Calvin cycle can utilize NADPH is also influenced by a number of factors that are independent of light intensity. For example, cold temperatures cause the enzymes of the Calvin cycle to function more slowly, but they have little impact on the absorption of light energy by the photosynthetic electron transport chain. Other factors that depress the rate at which the Calvin cycle can function include shortages of nitrogen, which reduces protein levels overall, and of CO2.
How is NADPH formed?
NADPH is formed when high-energy electrons are passed from photosystem I to a membrane-associated protein called ferredoxin. The enzyme ferredoxin-NADP+ reductase then catalyzes the formation of NADPH by transferring two high-energy electrons to NADP+ as well as a proton from the surrounding solution: NADP+ + 2e- + H+ → NADPH.
How is ATP important in the Calvin cycle?
NADPH provides most of the energy incorporated in the bonds of the carbohydrate molecules produced by the Calvin cycle. Nevertheless, ATP plays an essential role in preparing 3-PGA for the addition of energy and electrons from NADPH.
What is the regeneration step of the Calvin cycle?
Of the 15 chemical reactions that make up the Calvin cycle, 12 are directly involved in the last step, the regeneration of RuBP. The large number of reactions involved in this step reflects the degree of reshuffling of carbon atoms needed to produce three 5-carbon RuBP molecules from five 3-carbon triose phosphate molecules.
What organisms carry out land photosynthesis?
On land, photosynthesis is dominated by multicellular plants. On land, photosynthesis occurs most readily in environments that are both moist and warm. However, photosynthetic organisms have evolved adaptations that allow them to tolerate a wide range of environmental conditions. In very dry regions, a combination of photosynthetic bacteria and unicellular algae forms an easily distributed layer on the surface of the soil known as desert crust. At the other extreme, unicellular eukaryotic algae can grow on the surfaces of glaciers, causing the surface of the snow to appear red.
What happens once the reaction center gives up its electron?
Once the reaction center has lost an electron, it is no longer able to absorb light. From a visual perspective, it has been bleached. Thus, for the photosystem to continue contributing electrons to the photosynthetic electron transport chain, another electron must be delivered to take the place of the one that has entered the transport chain. These replacement electrons ultimately come from water.
What is the chemical equation for photosynthesis?
Overall, then, the equation for photosynthesis can be described as follows: CO2 + H2O → C6H12O6 + O2.
What did the first forms of photosynthesis probably entail?
Photosynthesis is a highly complex process, involving the coordination of many reactions and enzymes. This complexity is the result of several billion years of evolution. The first organisms that were able to draw upon sunlight as an energy source would have been much simpler. The earliest reaction centers may have used light energy to drive the movement of electrons from a soluble inorganic electron donor in the surrounding medium to an electron-acceptor molecule within the cell. Reduced iron, Fe2+, is thought to have been abundant in the early ocean, and therefore could have served as the first electron donor. Alternatively, the first forms of light-driven electron transport may have been coupled to the net movement of protons across the membrane, allowing for the synthesis of ATP.
How did photosynthesis gain a foothold in eukaryotic cells?
Photosynthesis is hypothesized to have gained a foothold among eukaryotic organisms when a free-living cyanobacterium was engulfed by a eukaryotic cell. Over time, the engulfed cyanobacterium lost its ability to survive outside of its host cell and evolved into the chloroplast. The process in which one cell takes up residence inside of another cell is called endosymbiosis.
What is the distribution of photosynthesis?
Photosynthesis occurs among prokaryotic as well as eukaryotic organisms, on land as well as in the sea. Approximately 50% of global photosynthesis is carried out by terrestrial organisms, with the other half taking place in the ocean.
What is the photic zone?
Photosynthesis takes place almost everywhere sunlight is available to serve as a source of energy. In the ocean, photosynthesis occurs in the surface layer about 100 m deep, called the photic zone, through which enough sunlight penetrates to enable photosynthesis.
Why is the efficiency of the photosynthetic electron transport chain lower than would be expected?
Photosynthetic cells use light energy at only half the maximum efficiency predicted. The reason that the efficiency is lower than predicted is that the photosynthetic electron transport chain includes not one but two photosystems arranged in series. Two photosystems are necessary to provide enough energy to pull electrons from water and then use them to reduce NADP+.
How is this possible?
Photosynthetic efficiency is typically calculated relative to the total energy output of the sun. However, only visible light has the appropriate energy levels to produce the high-energy electrons required by the photosynthetic electron transport chain. Most of the sun's output (about 60%) is not absorbed by chlorophyll and thus cannot be used in photosynthesis. In addition, leaves are not perfect at absorbing visible light—about 8% is either reflected or passes through the leaf. Finally, even under optimal conditions, not all of the light energy absorbed by chlorophyll can be transferred to the reaction center and instead is given off as heat (also about 8%). As we have seen, when light levels are high, excess light is actively converted into heat by xanthophyll pigments. The photosynthetic electron transport chain therefore captures at most about 24% of the sun's usable energy arriving at the surface of a leaf. However, energy is lost at another step as well. The incorporation of carbon dioxide into carbohydrates results in considerable loss in free energy, equivalent to about 20% of the total incoming solar radiation. Much of this loss in free energy is due to photorespiration. In total, therefore, the maximum energy conversion efficiency of photosynthesis is calculated to be around 4%. Efficiencies achieved by real plants growing in nature, however, are typically much lower, on the order of 1% to 2%.
What is the first line of defense that deals with reactive oxygen species?
Photosynthetic organisms employ two major lines of defense to deal with the stresses that occur when the Calvin cycle cannot keep up with light harvesting. First among these are chemicals that detoxify reactive oxygen species. Ascorbate (vitamin C), beta-carotene, and other antioxidants are able to neutralize reactive oxygen species. These compounds exist in high concentration in chloroplasts. Some of these antioxidant molecules are brightly colored, like the red pigments found in algae that live on snow. The presence of antioxidant compounds is one of the many reasons that eating photosynthetic tissues is good for your health.
What is the reducing agent used in the Calvin cycle?
Rubisco is responsible for the addition of the carbon atoms needed for the formation of carbohydrates, but by itself rubisco does not increase the amount of energy stored within the newly formed bonds. For this energy increase to take place, the carbon compounds formed by rubisco must be reduced. Nicotinamide adenine dinucleotide phosphate (NADPH) is the reducing agent used in the Calvin cycle. NADPH transfer the energy and electrons that allow carbohydrates to be synthesized from CO2, the most oxidized of all carbon compounds. NADPH is produced by the photosynthetic electron transport chain. Like all components of the Calvin cycle, NADPH can move freely within the stroma of the chloroplast. Although NADPH is a powerful reducing agent, energy and electrons are transferred from NADPH only under the catalysis of a specific enzyme, thus providing a high degree of control over the fate of these high-energy electrons.
If rubisco makes mistakes, why is it still around?
Rubisco plays a key role in photosynthesis, and yet it is an enzyme that makes mistakes. What accounts for its evolutionary success? A partial answer is that rubisco first evolved long before oxygen appeared in Earth's atmosphere. The difficulty is that for rubisco to favor the addition of carbon dioxide over oxygen, requires a high degree of selectivity, and the price of selectivity is speed. Nowhere is this trade-off more evident than in land plants, whose photosynthetic cells acquire carbon dioxide from an oxygen-rich and carbon dioxide-poor atmosphere. The rubiscos of land plants are highly selective, but incredibly slow, with catalytic rates on the order of three reactions per second.
Why can sunlight be a problem for photosynthesis?
Sunlight varies in intensity throughout the day. When light levels are low, light is shared evenly between the two photosystems. However, as light levels increase, the light energy absorbed begins to overwhelm the capacity of the Calvin cycle to make use of NADPH. The danger is that when there is no NADP+ returning from the Calvin cycle to act as the terminal electron acceptor for the photosynthetic electron transport chain, the high-energy electrons can damage the cell.
What did Melvin Calvin's experiment test?
The American chemist Melvin Calvin and colleagues supplied radioactively labeled CO2 to the unicellular green alga Chlorella and then plunged the cells into boiling alcohol, thus halting all enzymatic reactions. By examining the compounds that became radioactively labeled, Calvin and his colleagues were able to determine the identity of the carbon compounds produced in photosynthesis. By using a very short exposure to CO2, Calvin and colleagues determined that the incorporation of CO2 results in the production of 3-PGA. To determine how 3-PGA is formed, they supplied radioactive CO2 to label the products, but then cut off the supply of CO2 to block the carboxylation reaction. In this experiment, the amount of RuBP increased relative to the amount seen in the first experiment. They concluded that the first step was the addition of CO2 to RuBP.
What are the steps of the Calvin cycle?
The Calvin cycle consists of 15 chemical reactions that synthesize carbohydrates from CO2. These reactions can be grouped into three main steps: (1) carboxylation, in which CO2 absorbed from the air is added to a 5-carbon molecule; (2) reduction, in which energy and electrons are transferred to the molecules formed from carboxylation; and (3) regeneration of the 5-carbon molecule needed for carboxylation.
How is the Calvin cycle connected to the photosynthetic electron transport chain?
The Calvin cycle does not utilize sunlight directly. However, this pathway cannot operate without the energy input provided by a steady supply of NADPH and ATP. Both are supplied by the photosynthetic electron transport chain, in which light is captured and transformed into chemical energy. Thus, under natural conditions, photosynthesis, including the Calvin cycle, occurs only in the light.
What happens if the Calvin cycle produces too many carbohydrates?
The Calvin cycle is capable of producing more carbohydrates than the cells need or, in a multicellular organism, more than the cell is able to export. Excess carbohydrates are converted to starch, a storage form of carbohydrates. Because starch molecules are not soluble, they provide a means of carbohydrate storage that does not affect the cell's osmotic balance. The formation of starch during the day provides photosynthetic cells with a source of carbohydrates that they can use during the night.
Why is the spacing of chlorophyll antennas important?
The chlorophyll molecules that make up the antenna are precisely spaced so that when a chlorophyll molecule absorbs light it transfers energy, but not electrons, to an adjacent chlorophyll molecule. The transfer of energy between antenna chlorophyll molecules is highly efficient, so little energy is lost as heat. Light energy absorbed by the antenna is passed from one chlorophyll molecule to the next until eventually it is transferred to the reaction center.
Where are Photosystem I and ATP synthase concentrated?
The components of the photosynthetic electron transport chain are distributed in such a way as to make tight pacing possible increasing surface area without sacrificing function. Photosystem I and the ATP synthase are concentrated on the outer regions of the granal stacks. From there they can readily supply NADPH and ATP to the stroma, where these molecules are used to power the Calvin cycle.
What are the two major problems that photosynthesis faces?
The efficient functioning of photosynthesis faces two major challenges. The first is that if more energy is generated than the Calvin cycle can use, excess light energy can damage the cell. The second challenge stems from rubisco's ability to catalyze the addition of either carbon dioxide or oxygen to RuBP. The addition of oxygen instead of carbon dioxide can substantially reduce the amount of carbohydrate produced.
What happens in the carboxylation reaction of the Calvin cycle?
The first step of the Calvin cycle is a carboxylation reaction. Specifically, CO2 is added to a 5-carbon sugar called ribulose-1,5-biphosphate (RuBP). This step is catalyzed by the enzyme ribulose biphosphate carboxylase oxygenase, or rubisco for short. Rubisco has a disproportionate influence over the functioning of the entire pathway.
What is the function of a photosystem?
The function of a photosystem is to convert absorbed light energy into the movement of electrons, making it a key element in the photosynthetic electron transport chain. Physically, a photosystem is a complex of proteins and pigments that is embedded in the thylakoid membrane.
What is the cytochrome b6f complex?
The major protein complexes of the photosynthetic electron transport chain include the two photosystems as well as the cytochrome b6f complex, through which electrons pass between photosystem II and photosystem I. Small, relatively mobile compounds convey electrons between these protein complexes. Plastoquinone, a lipid-soluble mobile compound, carries electrons from photosystem II to the cytochrome b6f complex, while plastocyanin, a water-soluble protein, carries electrons from the cytochrome b6f complex to photosystem I by diffusing through the thylakoid lumen. The modular nature of the photosynthetic electron transport chain, interconnected by diffusible elements, enables this pathway to adjust in response to changes in the availability of light.
What organisms carry out marine photosynthesis?
The majority of photosynthetic organisms in marine environments are unicellular. About half of oceanic photosynthesis is carried out by phytoplankton (single-celled marine eukaryotes), while the other half is carried out by cyanobacteria (photosynthetic bacteria).
How did ancient forms of photosynthesis work?
The most ancient forms of photosynthesis have simple photosynthetic electron transport chains with only a single photosystem. However, a single photosystem cannot capture enough energy to pull electrons from water and then use them to reduce CO2. Thus, photosynthetic organisms with a single photosystem must use more easily oxidized compounds, such as H2S, as electron donors. These organisms are limited to environments where the electron-donor molecules are abundant. Because these organisms do not use water as an electron donor, they do not produce O2 during photosynthesis.
How ancient are the parts of the chloroplast, according to endosymbiotic theory?
The outer membrane of the chloroplast double membrane is thought to have originated from the plasma membrane of the ancestral eukaryotic cell, which surrounded the ancestral cyanobacterium as it was being engulfed. The inner chloroplast membrane is thought to correspond to the plasma membrane of the ancestral free-living cyanobacterium. The thylakoid membrane then corresponds to the internal photosynthetic membrane found in cyanobacteria. Finally, the stroma corresponds to the cytoplasm of the ancestral cyanobacterium.
What is the photosynthetic electron transport chain?
The oxidation of water is linked with the reduction of CO2 through a series of redox reactions in which electrons are passed from one compound to another. This series of reactions constitutes the photosynthetic electron transport chain.
What does the oxidation of water produce?
The oxidation of water results in the production of electrons, protons, and O2. Thus, oxygen is formed in photosynthesis as a by-product of water's role as a source of electrons. Water is the source of the oxygen released during photosynthesis using isotopes, molecules that can be distinguished in the basis of their molecular mass.
What is the basic process of photosynthesis?
The process begins with the absorption of light by protein-pigment complexes known as photosystems. Photosystems use absorbed light energy to drive redox reactions and thereby set the photosynthetic electron transport chain in motion. In turn, the movement of electrons through this transport chain is used to drive the synthesis of ATP and NADPH. And finally, ATP and NADPH are the energy sources needed to synthesize carbohydrates using CO2 in a process called the Calvin cycle.
What is photosynthesis?
The process that allowed Van Helmont's tree to increase in mass using substances pulled from the air is called photosynthesis. Photosynthesis is a biochemical process for building carbohydrates from sunlight and carbon dioxide taken from the air. These carbohydrates are used as both structural components of the plant and as a source of energy used to produce ATP.
What sets the photosynthetic electron transport chain in motion?
The reaction center chlorophylls have a configuration distinct from that of the antenna chlorophylls. As a result, reaction center chlorophylls are able to transfer both absorbed light energy and an associated high-energy electron to an adjacent molecule that acts as an electron acceptor. When the transfer takes place, the reaction center becomes oxidized and the adjacent electron acceptor molecule is reduced. The result is the conversion of light energy into a chemical form. This electron transfer sets in motion the light-driven chain of redox reactions that constitute the photosynthetic electron transport chain.
What are accessory pigments? Why are they important? What are the major ones?
The thylakoid membrane also contains other pigments, most notably the orange-yellow carotenoids. Carotenoids are able to absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll. Thus, the presence of these accessory pigments allows photosynthetic cells to absorb a broader range of visible light than would be possible with just chlorophyll alone. Carotenoids play an important role in protecting the photosynthetic electron transport chain from damage.
Why is the structure of the thylakoid membrane important?
The thylakoid membrane forms a single large sac, with the lumen on the inside and the stroma on the outside. The convoluted arrangement of interlinked granal sacs increases the total surface area of the thylakoid that can be accommodated within a single chloroplast. Nevertheless, the tight packing of the granal stacks means that only membranes on the outside of each granal stack and the links between grana are in direct contact with the stroma.
How are ATP and NADPH produced?
To use sunlight to power the Calvin cycle, the cell must be able to convert light energy into both NADPH and ATP. NADPH is the reducing agent used to synthesize carbohydrates from CO2, whereas ATP is required for the regeneration of RuBP. The production of both molecules is the task of the photosynthetic electron transport chain. NADPH is produced by the linear flow of electrons through the photosynthetic electron transport chain from water to NADP+. To produce ATP, the photosynthetic electron transport chain functions as a proton pump, leading to the accumulation of protons in the thylakoid lumen. The resulting proton gradient is used to drive the synthesis of ATP.
Where do the electrons used to reduce CO2 come from?
These electrons can only come from the oxidation of other molecules, illustrating once again that reduction-oxidation (or redox) reactions always come in pairs. In photosynthesis carried out by plants and many algae, the ultimate electron donor is water. However, photosynthetic bacteria can use a variety of other electron donors.
What happens after NADPH transfers electrons to 3-PGA?
These energy transfer steps result in the formation of glyceraldehyde 3-phosphate (GAP), which is reversibly interconverted to dihydroxyacetone phosphate (DHAP) by the enzyme triose phosphate isomerase. Together GAP and DHAP constitute a pool of 3-carbon carbohydrate molecules known as triose phosphates. Triose phosphates are the true products of the Calvin cycle because they are the molecules exported from the chloroplast.
How powerful is the ATP synthase?
These two mechanisms are quite powerful. When the photosynthetic electron transport chain is operating at full capacity, the concentration of protons in the lumen can be more than 1000 times greater than that in the stroma. This accumulation of protons on one side of the thylakoid membrane can then be used to power the synthesis of ATP. The pathway most readily available is through a channel in the ATP synthase. When protons pass through this enzyme, some of the energy that is released is used to drive the synthesis of ATP.
How is the distribution of components of the photosynthetic electron transport chain possible?
This distribution of components to different regions is made possible by the presence of the mobile compounds that carry electrons between the major components of the photosynthetic electron transport chain. In particular, the diffusion of the protein plastocyanin though the lumen allows photosystem II and photosystem I to be located in different regions of the thylakoid membrane.
What are other important parts of the chloroplast?
Thylakoid membranes form structures that resemble flattened sacs, which are grouped into structures called grana. Grana are connected to one another by membrane bridges so that the thylakoid forms a single continuous membrane. The thylakoid membrane encloses a common fluid-filled interior compartment called the lumen. The region surrounding the thylakoid membrane is called the stroma.
What is the solution to this problem?
To prevent this from happening, electrons are shunted into an alternative pathway that increases the production of ATP while decreasing the production of NADPH. Electrons from photosystem I are redirected from ferredoxin back into the electron transport chain. These electrons reenter the photosynthetic electron transport chain upstream of the cytochrome b6f proton pump. Because these electrons eventually return to photosystem I, this alternative pathway is referred to as cyclic electron transport.
How much of the sun's energy ends up in carbohydrates?
Typically, only 1% to 2% of the sun's energy that lands on a leaf ends up in carbohydrates.
What are reactive oxygen species?
Unless the photosynthetic reactions are carefully controlled, molecules will be formed that can damage cells through the indiscriminate oxidization of lipids, proteins, and nucleic acids. Under normal conditions, the redox reactions that make up the photosynthetic electron transport chain do not allow either absorbed light energy or the resulting high-energy electrons to stray. However, when NADP+ is in short supply, either the absorbed light energy or the energy and the associated electron can be transferred to O2, resulting in the formation of highly reactive forms of oxygen known collectively as reactive oxygen species.
What sits at the two ends of the photosynthetic electron transport chain?
Water, as the electron donor, sits at one end of the photosynthetic electron transport chain, whereas NADP+, the electron acceptor that transports high-energy electrons to the Calvin cycle, sits at the other.
Where do xanthophyll pigments come from?
Xanthophyll pigments are predominantly associated with photosystem II. When activated by high light, these light-absorbing pigments reduce linear electron transport, but allow cyclic electron transport to continue. Light absorbed by photosystem I thus remains available to power the synthesis of ATP, which can then be used to help repair cellular components that have been damaged by reactive oxygen species.
