Cellular Respiration and Photosynthesis ( Read ) | Biology | CK Foundation
in a considerable difference between gross and net photosyn- thesis. This is why . the interplay between photosynthesis and photorespiration in. | Timm et al. .. Engineering photosynthesis in plants and synthetic. How the C4 and CAM pathways help minimize photorespiration. Some plants, unlike wheat and soybean, can escape the worst effects of photorespiration. .. plant species that use CAM photosynthesis not only avoid photorespiration, but. The main difference between photosynthesis and photorespiration is that that occur during the production of energy using sunlight in plants.
These stages can be distinguished by studying the rates of photosynthesis at various degrees of light saturation i. Over a range of moderate temperatures and at low to medium light intensities relative to the normal range of the plant speciesthe rate of photosynthesis increases as the intensity increases and is relatively independent of temperature. In the light-dependent range before saturation, therefore, the rate of photosynthesis is determined by the rates of photochemical steps.
At high light intensities, some of the chemical reactions of the dark stage become rate-limiting. In many land plants, a process called photorespiration occurs, and its influence upon photosynthesis increases with rising temperatures. More specifically, photorespiration competes with photosynthesis and limits further increases in the rate of photosynthesis, especially if the supply of water is limited see below Photorespiration. Carbon dioxide Included among the rate-limiting steps of the dark stage of photosynthesis are the chemical reactions by which organic compounds are formed by using carbon dioxide as a carbon source.
The rates of these reactions can be increased somewhat by increasing the carbon dioxide concentration. Since the middle of the 19th century, the level of carbon dioxide in the atmosphere has been rising because of the extensive combustion of fossil fuels, cement production, and land-use changes associated with deforestation.
The atmospheric level of carbon dioxide climbed from about 0. This increase in carbon dioxide directly increases plant photosynthesis, but the size of the increase depends on the species and physiological condition of the plant. Furthermore, most scientists maintain that increasing levels of atmospheric carbon dioxide affect climate, increasing global temperatures and changing rainfall patterns. Such changes will also affect photosynthesis rates. Water For land plants, water availability can function as a limiting factor in photosynthesis and plant growth.
Besides the requirement for a small amount of water in the photosynthetic reaction itself, large amounts of water are transpired from the leaves ; that is, water evaporates from the leaves to the atmosphere via the stomata.
Stomata are small openings through the leaf epidermisor outer skin; they permit the entry of carbon dioxide but inevitably also allow the exit of water vapour. The stomata open and close according to the physiological needs of the leaf. In hot and arid climates the stomata may close to conserve water, but this closure limits the entry of carbon dioxide and hence the rate of photosynthesis. The decreased transpiration means there is less cooling of the leaves and hence leaf temperatures rise.
The decreased carbon dioxide concentration inside the leaves and the increased leaf temperatures favour the wasteful process of photorespiration. If the level of carbon dioxide in the atmosphere increases, more carbon dioxide could enter through a smaller opening of the stomata, so more photosynthesis could occur with a given supply of water. Minerals Several minerals are required for healthy plant growth and for maximum rates of photosynthesis. Nitrogen, sulfate, phosphateironmagnesiumcalcium, and potassium are required in substantial amounts for the synthesis of amino acids, proteins, coenzymesdeoxyribonucleic acid DNA and ribonucleic acid RNAchlorophyll and other pigments, and other essential plant constituents.
Smaller amounts of such elements as manganesecopperand chloride are required in photosynthesis. Some other trace elements are needed for various nonphotosynthetic functions in plants. Internal factors Each plant species is adapted to a range of environmental factors. These adjustments maintain a balance in the overall photosynthetic process and control it in accordance with the needs of the whole plant. With a given plant species, for example, doubling the carbon dioxide level might cause a temporary increase of nearly twofold in the rate of photosynthesis; a few hours or days later, however, the rate might fall to the original level because photosynthesis produced more sucrose than the rest of the plant could use.
By contrast, another plant species provided with such carbon dioxide enrichment might be able to use more sucrose, because it had more carbon-demanding organs, and would continue to photosynthesize and to grow faster throughout most of its life cycle. Energy efficiency of photosynthesis The energy efficiency of photosynthesis is the ratio of the energy stored to the energy of light absorbed.
The chemical energy stored is the difference between that contained in gaseous oxygen and organic compound products and the energy of water, carbon dioxide, and other reactants. The amount of energy stored can only be estimated because many products are formed, and these vary with the plant species and environmental conditions. If the equation for glucose formation given earlier is used to approximate the actual storage process, the production of one mole i.
This amount must then be compared with the energy of light absorbed to produce one mole of oxygen in order to calculate the efficiency of photosynthesis. Light can be described as a wave of particles known as photons ; these are units of energy, or light quanta.
The quantity N photons is called an einstein. The energy of light varies inversely with the length of the photon waves; that is, the shorter the wavelength, the greater the energy content. An einstein of red light with a wavelength of nm has an energy of about 42 kcal.
Blue light has a shorter wavelength and therefore more energy than red light. Regardless of whether the light is blue or red, however, the same number of einsteins are required for photosynthesis per mole of oxygen formed. In order to compute the amount of light energy involved in photosynthesis, one other value is needed: This is called the quantum requirement. The minimum quantum requirement for photosynthesis under optimal conditions is about nine.
The actual percentage of solar energy stored by plants is much less than the maximum energy efficiency of photosynthesis. An agricultural crop in which the biomass total dry weight stores as much as 1 percent of total solar energy received on an annual areawide basis is exceptional, although a few cases of higher yields perhaps as much as 3.
There are several reasons for this difference between the predicted maximum efficiency of photosynthesis and the actual energy stored in biomass. First, more than half of the incident sunlight is composed of wavelengths too long to be absorbed, and some of the remainder is reflected or lost to the leaves.
Consequently, plants can at best absorb only about 34 percent of the incident sunlight. Second, plants must carry out a variety of physiological processes in such nonphotosynthetic tissues as roots and stems; these processes, as well as cellular respiration in all parts of the plant, use up stored energy.
Third, rates of photosynthesis in bright sunlight sometimes exceed the needs of the plants, resulting in the formation of excess sugars and starch. When this happens, the regulatory mechanisms of the plant slow down the process of photosynthesis, allowing more absorbed sunlight to go unused. Fourth, in many plants, energy is wasted by the process of photorespiration. Finally, the growing season may last only a few months of the year; sunlight received during other seasons is not used.
Furthermore, it should be noted that if only agricultural products e. The process of plant photosynthesis takes place entirely within the chloroplasts. Detailed studies of the role of these organelles date from the work of British biochemist Robert Hill. About Hill discovered that green particles obtained from broken cells could produce oxygen from water in the presence of light and a chemical compound, such as ferric oxalate, able to serve as an electron acceptor.
This process is known as the Hill reaction. During the s Daniel Arnon and other American biochemists prepared plant cell fragments in which not only the Hill reaction but also the synthesis of the energy-storage compound ATP occurred. In addition, the coenzyme NADP was used as the final acceptor of electrons, replacing the nonphysiological electron acceptors used by Hill.
His procedures were refined further so that small individual pieces of isolated chloroplast membranes, or lamellae, could perform the Hill reaction.
These small pieces of lamellae were then fragmented into pieces so small that they performed only the light reactions of the photosynthetic process. It is now possible also to isolate the entire chloroplast so that it can carry out the complete process of photosynthesis, from light absorption, oxygen formation, and the reduction of carbon dioxide to the formation of glucose and other products. Structural features The intricate structural organization of the photosynthetic apparatus is essential for the efficient performance of the complex process of photosynthesis.
The chloroplast is enclosed in a double outer membrane, and its size approximates a spheroid about 2, nm thick and 5, nm long. Some single-celled algae have one chloroplast that occupies more than half the cell volume. Leaf cells of higher plants contain many chloroplasts, each approximately the size of the one in some algal cells.
When thin sections of a chloroplast are examined under the electron microscopeseveral features are apparent. Chief among these are the intricate internal membranes i. Also visible are starch granules, which appear as dense bodies. The stroma is basically a solution of enzymes and small molecules. The dark reactions occur in the stroma, the soluble enzymes of which catalyze the conversion of carbon dioxide and minerals to carbohydrates and other organic compounds.
The capacity for carbon fixation and reduction is lost if the outer membrane of the chloroplast is broken, allowing the stroma enzymes to leak out. A single lamella, which contains all the photosynthetic pigments, is approximately 10—15 nm thick. The lamellae exist in more-or-less flat sheets, a few of which extend through much of the length of the chloroplast. The chloroplasts of most higher plants have regions, called grana, in which the thylakoids are very tightly stacked. When viewed by electron microscopy at an oblique angle, the grana appear as stacks of disks.
When viewed in cross section, it is apparent that some thylakoids extend from one grana through the stroma into other grana. The thin aqueous spaces inside the thylakoids are believed to be connected with each other via these stroma thylakoids. These thylakoid spaces are isolated from the stroma spaces by the relatively impermeable lamellae. The light reactions occur exclusively in the thylakoids. The complex structural organization of lamellae is required for proper thylakoid function; intact thylakoids are necessary for the formation of ATP.
Thylakoids that have been broken down to smaller units can no longer form ATP, even when the conversion of light into chemical energy occurs during electron transport in these units. Chemical composition of lamellae Lipids Lamellae consist of about equal amounts of lipids and proteins.
About one-fourth of the lipid portion of the lamellae consists of pigments and coenzymes; the remainder consists of various lipids, including polar compounds such as phospholipids and galactolipids. When polar lipids are placed in an aqueous environment, they can line up with the fatty acid tails side by side.
A second layer of phospholipids forms tail-to-tail with the first, establishing a lipid bilayer in which the hydrophilic heads are in contact with the aqueous solution on each side of the bilayer. Sandwiched between the heads are the hydrophobic tails, creating a hydrophobic environment from which water is excluded. This lipid bilayer is an essential feature of all biological membranes see cell: The hydrophobic parts of proteins and lipid-soluble cofactors and pigments are dissolved or embedded in the lipid bilayer.
Lamellar membranes can function as electrical insulating material and permit a charge, or potential difference, to develop across the membrane. Such a charge can be a source of chemical or electrical energy. Approximately one-fifth of the lamellar lipids are chlorophyll molecules; one type, chlorophyll a, is more abundant than the second type, chlorophyll b. The chlorophyll molecules are specifically bound to small protein molecules.
These absorb light and pass its energy on to special chlorophyll a molecules that are directly involved in the conversion of light energy to chemical energy. When one of these special chlorophyll a molecules is excited by light energy as described laterit gives up an electron.
There are two types of these special chlorophyll a molecules: Although chlorophylls are the main light-absorbing molecules in green plants, there are other pigments such as carotenes and carotenoids which are responsible for the yellow-orange colour of carrots. Carotenes can also absorb light and may supplement chlorophyll as the light-absorbing molecules in some plant cells.
The light energy absorbed by carotenes must be passed to chlorophyll before conversion to chemical energy can occur.
Proteins Many of the lamellar proteins are components of the chlorophyll—protein complexes described above. Other proteins include enzymes and protein-containing coenzymes. Enzymes are required as organic catalysts for specific reactions within the lamellae.
Protein coenzymes, also called cofactorsinclude important electron carrier molecules called cytochromeswhich are iron-containing pigments with the pigment portions attached to protein molecules.
During electron transfer, an electron is accepted by an iron atom in the pigment portion of a cytochrome molecule, which thus is reduced; then the electron is transferred to the iron atom in the next cytochrome carrier in the electron transfer chain, thus oxidizing the first cytochrome and reducing the next one in the chain.
In addition to the metal atoms found in the pigment portions of cytochrome molecules, metal atoms also are found in other protein molecules of the lamellae. In proteins with a total molecular weight ofbased on the weight of hydrogen as onethere are 2 atoms of manganese10 atoms of ironand 6 atoms of copper. These metal atoms are required for the catalytic activity of some of the enzymes important in photosynthesis.
C3, C4, and CAM plants
The manganese atoms are involved in water-splitting and oxygen formation. Both copper- and iron-containing proteins function in electron transport between water and the final electron-acceptor molecule of the light stage of photosynthesis, an iron-containing protein called ferredoxin. Ferredoxin is a soluble component in the chloroplasts. In its reduced form, it gives electrons directly to the systems that reduce nitrate and sulfate and via NADPH to the system that reduces carbon dioxide.
A copper-containing protein called plastocyanin PC carries electrons at one point in the electron transport chain. PC molecules are water soluble and can move through the inner space of the thylakoids, carrying electrons from one place to another. Quinones Small molecules called plastoquinones are found in substantial numbers in the lamellae.
Like the cytochromes, quinones have important roles in carrying electrons between the components of the light reactions. Since they are lipid soluble, they can diffuse through the membrane.
They can carry one or two electrons, and, in their reduced form with added electronsthey carry hydrogen atoms that can be released as hydrogen ions when the added electrons are passed on, for example, to a cytochrome. The process of photosynthesis: Light of shorter wavelength such as blue has more energy than light of longer wavelength such as redso absorption of blue light creates an excited state of higher energy.
This lowest excited state is similar to that of a molecule that has just absorbed the longest wavelength light capable of exciting it. In the case of chlorophyll a, this lowest excited state corresponds to that of a molecule that has absorbed red light of about nm.
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The return of a chlorophyll a molecule from its lowest excited state to its original low-energy state ground state requires the release of the extra energy of the excited state. This can occur in one of several ways. In photosynthesis, most of this energy is conserved as chemical energy by the transfer of an electron from a special chlorophyll a molecule P or P to an electron acceptor.
When this electron transfer is blocked by inhibitors, such as the herbicide dichlorophenylmethylurea DCMUor by low temperature, the energy can be released as red light. Such reemission of light is called fluorescence. The examination of fluorescence from photosynthetic material in which electron transfer has been blocked has proved to be a valuable tool for scientists studying the light reactions. The general features of a widely accepted mechanism for photoelectron transfer, in which two light reactions light reaction I and light reaction II occur during the transfer of electrons from water to carbon dioxide, were proposed by Robert Hill and Fay Bendall in This mechanism is based on the relative potential in volts of various cofactors of the electron-transfer chain to be oxidized or reduced.
Molecules that in their oxidized form have the strongest affinity for electrons i.
In contrast, molecules that in their oxidized form are difficult to reduce have a high relative potential once they have accepted electrons. The molecules with a low relative potential are considered to be strong oxidizing agents, and those with a high relative potential are considered to be strong reducing agents.
In diagrams that describe the light reaction stage of photosynthesis, the actual photochemical steps are typically represented by two vertical arrows. These arrows signify that the special pigments P and P receive light energy from the light-harvesting chlorophyll-protein molecules and are raised in energy from their ground state to excited states.
In their excited state, these pigments are extremely strong reducing agents that quickly transfer electrons to the first acceptor. These first acceptors also are strong reducing agents and rapidly pass electrons to more stable carriers.
In light reaction II, the first acceptor may be pheophytin, which is a molecule similar to chlorophyll that also has a strong reducing potential and quickly transfers electrons to the next acceptor. Special quinones are next in the series. These molecules are similar to plastoquinone; they receive electrons from pheophytin and pass them to the intermediate electron carriers, which include the plastoquinone pool and the cytochromes b and f associated in a complex with an iron-sulfur protein.
In light reaction I, electrons are passed on to iron-sulfur proteins in the lamellar membrane, after which the electrons flow to ferredoxin, a small water-soluble iron-sulfur protein. Each time a P or P molecule gives up an electron, it returns to its ground unexcited state, but with a positive charge due to the loss of the electron.
These positively charged ions are extremely strong oxidizing agents that remove an electron from a suitable donor. There is good evidence that two or more manganese atoms complexed with protein are involved in this catalysis, taking four electrons from two water molecules with release of four hydrogen ions. When manganese is selectively removed by chemical treatment, the thylakoids lose the capacity to oxidize water, but all other parts of the electron pathway remain intact.
The pool of intermediate carriers may receive electrons from water via light reaction II and the quinones. Transfer of electrons from water to ferredoxin via the two light reactions and intermediate carriers is called noncyclic electron flow. Alternatively, electrons may be transferred only by light reaction I, in which case they are recycled from ferredoxin back to the intermediate carriers.
This process is called cyclic electron flow. Evidence of two light reactions Many lines of evidence support the concept of electron flow via two light reactions. An early study by American biochemist Robert Emerson employed the algae Chlorellawhich was illuminated with red light alone, with blue light alone, and with red and blue light at the same time.
Oxygen evolution was measured in each case. It was substantial with blue light alone but not with red light alone. With both red and blue light together, the amount of oxygen evolved far exceeded the sum of that seen with blue and red light alone. These experimental data pointed to the existence of two types of light reactions that, when operating in tandem, would yield the highest rate of oxygen evolution. Since those early studies, the two light reactions have been separated in many ways, including separation of the membrane particles in which each reaction occurs.
As discussed previously, lamellae can be disrupted mechanically into fragments that absorb light energy and break the bonds of water molecules i. These electrons can be transferred to ferredoxin, the final electron acceptor of the light stage.
No transfer of electrons from water to ferredoxin occurs if the herbicide DCMU is present. The subsequent addition of certain reduced dyes i. It is now known that DCMU blocks the transfer of electrons between the first quinone and the plastoquinone pool in light reaction II. When treated with certain detergentslamellae can be broken down into smaller particles capable of carrying out single light reactions.
One type of particle can absorb light energy, oxidize water, and produce oxygen light reaction IIbut a special dye molecule must be supplied to accept the electrons. In the presence of electron donors, such as a reduced dye, a second type of lamellar particle can absorb light and transfer electrons from the electron donor to ferredoxin light reaction I.
Photosystems I and II The structural and photochemical properties of the minimum particles capable of performing light reactions I and II have received much study. Photosystem II absorbs light energy and transport into photocenters, allowing the production of high energy electrons.
These high energy electrons move into the photosystem I through the cytochrome b6f complex. The electron deficiency that occurs in the photosystems is filled by splitting water molecules in a process called photolysis. The resultant hydrogen ions are used in the production of ATP. Light Reaction Dark Reaction Light reaction is followed by the dark reaction.
C3, C4, and CAM plants (article) | Khan Academy
Dark reaction, which occurs through the C3 cycle, is also called the Calvin cycle and it occurs in the stroma of the chloroplast without the use of light. Some of the 3-phosphoglycerate molecules reduce to form glucose while rest is recycled to produce RuBP.
What is Photorespiration Photorespiration is the inhibition of the Calvin cycle in the presence of excess oxygen. It leads to the loss of already-fixed carbon dioxide; hence, photorespiration decreases the sugar synthesis and wastes the energy of the cell. Two molecules are produced in this reaction: On that account, photorespiration steals or removes carbons from the Calvin cycle. Furthermore, plants use a series of reactions to recover phosphoglycolate, which steals the energy of the cell as well.
Therefore, photorespiration is considered as an inefficient method of producing energy. Then, oxaloacetate is converted into malate and is transported to the bundle-sheath cells. Malate dissociates into carbon dioxide and pyruvate inside the bundle sheath cells, increasing the carbon dioxide concentration inside the cell.
In the presence of high carbon dioxide concentration, RuBisCO does not bind with oxygen. Similarities Between Photosynthesis and Photorespiration Photosynthesis and photorespiration are two processes which occur during the production of glucose in plants.
They undergo light reaction. Both processes get the use of RuBisCO enzyme.