Light-dependent reactions refers to certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b6f and then to PSI. The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP+ to NADPH.

In anoxygenic photosynthesis various electron donors are used. Cytochrome b6f and ATP synthase work together to produce ATP (photophosphorylation) in two distinct ways.

The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses electrons and energy from PSI to create more ATP and to stop the production of NADPH.

The net-reaction of all light-dependent reactions in oxygenic photosynthesis is:. PSI and PSII are light-harvesting complexes.

This reaction, called photoinduced charge separation, is the start of the electron flow and transforms light energy into chemical forms.

Such reactions range from the silver halide reactions used in photographic film to the creation and destruction of ozone in the upper atmosphere. This article discusses a specific subset of these, the series of light-dependent reactions related to photosynthesis in living organisms.

The reaction center is in the thylakoid membrane. It transfers absorbed light energy to a dimer of chlorophyll pigment molecules near the periplasmic (or thylakoid lumen) side of the membrane.

This special pair is slightly different in PSI and PSII reaction centers. In PSII, it absorbs photons with a wavelength of 680 nm, and is therefore called P680.

In bacteria, the special pair is called P760, P840, P870, or P960. “P” here means pigment, and the number following it is the wavelength of light absorbed.

Electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they are at the lowest possible energy level, the ground state.

Any light that has too little or too much energy cannot be absorbed and is reflected. The electron in the higher energy level is unstable and will quickly return to its normal lower energy level.

This can happen in various ways. The extra energy can be converted into molecular motion and lost as heat, or re-emitted by the electron as light (fluorescence).

this is called resonance energy transfer. If an electron of the special pair in the reaction center becomes excited, it cannot transfer this energy to another pigment using resonance energy transfer.

The loss of the electron gives the special pair a positive charge and, as an ionization process, further boosts its energy.[citation needed] The formation of a positive charge on the special pair and a negative charge on the acceptor is referred to as photoinduced charge separation. The electron can be transferred to another molecule.

Plant pigments usually utilize the last two of these reactions to convert the sun’s energy into their own.

In their high-energy states, the special pigment and the acceptor could undergo charge recombination. that is, the electron on the acceptor could move back to neutralize the positive charge on the special pair.

In the case of PSII, this backflow of electrons can produce reactive oxygen species leading to photoinhibition. Three factors in the structure of the reaction center work together to suppress charge recombination nearly completely:.

The photosynthesis process in chloroplasts begins when an electron of P680 of PSII attains a higher-energy level. This energy is used to reduce a chain of electron acceptors that have subsequently higher redox potentials.

When this chain reaches PSI, an electron is again excited, creating a high redox-potential. The electron transport chain of photosynthesis is often put in a diagram called the Z-scheme, because the redox diagram from P680 to P700 resembles the letter Z.

The final product of PSII is plastoquinol, a mobile electron carrier in the membrane. Plastoquinol transfers the electron from PSII to the proton pump, cytochrome b6f.

Cytochrome b6f transfers the electron chain to PSI through plastocyanin molecules. PSI can continue the electron transfer in two different ways.

PSI releases FNR into the stroma, where it reduces NADP+ to NADPH.

The resulting transmembrane proton gradient is used to make ATP via ATP synthase.

PSII is extremely complex, a highly organized transmembrane structure that contains a water splitting complex, chlorophylls and carotenoid pigments, a reaction center (P680), pheophytin (a pigment similar to chlorophyll), and two quinones. It uses the energy of sunlight to transfer electrons from water to a mobile electron carrier in the membrane called plastoquinone:.

The step H2O → P680 is performed by an imperfectly understood structure embedded within PSII called the water-splitting complex or oxygen-evolving complex (OEC). It catalyzes a reaction that splits water into electrons, protons and oxygen,.

The actual steps of the above reaction possibly occur in the following way (Kok’s diagram of S-states): (I) 2H2O (monoxide) (II) OH. H2O (hydroxide) (III) H2O2 (peroxide) (IV)HO2 (super oxide)(V) O2 (di-oxygen).[citation needed] (Dolai’s mechanism).

The excitation P680 → P680* of the reaction center pigment P680 occurs here. These special chlorophyll molecules embedded in PSII absorb the energy of photons, with maximal absorption at 680 nm.

This is one of two core processes in photosynthesis, and it occurs with astonishing efficiency (greater than 90%) because, in addition to direct excitation by light at 680 nm, the energy of light first harvested by antenna proteins at other wavelengths in the light-harvesting system is also transferred to these special chlorophyll molecules.

This is followed by the electron transfer P680*→ pheophytin, and then on to plastoquinol, which occurs within the reaction center of PSII. The electrons are transferred to plastoquinone and two protons, generating plastoquinol, which released into the membrane as a mobile electron carrier.

The initial stages occur within picoseconds, with an efficiency of 100%. The seemingly impossible efficiency is due to the precise positioning of molecules within the reaction center.

It occurs within an essentially crystalline environment created by the macromolecular structure of PSII. The usual rules of chemistry (which involve random collisions and random energy distributions) do not apply in solid-state environments.

When the excited chlorophyll P680* passes the electron to pheophytin, it converts to high-energy P680+, which can oxidize the tyrosineZ (or YZ) molecule by ripping off one of its hydrogen atoms. The high-energy oxidized tyrosine gives off its energy and returns to the groun.

Within plant cells, chloroplasts are specialized organelles that serve as the sites of photosynthesis. The reactions that make up the process of photosynthesis can be divided into light-dependent reactions, which take place in the thylakoids, and light-independent reactions (also known as dark reactions or the Calvin cycle), which take place in the stroma.

Surrounding the chloroplast is a double membrane, consisting of an outer membrane and an inner membrane. This is similar in structure to the double membrane of mitochondria.

The light-independent reactions of photosynthesis take place within the stroma. It contains enzymes that work with ATP and NADPH to “fix” carbon from carbon dioxide into molecules that can be used to build glucose.

The interior of the chloroplast contains another membrane—the thylakoid membrane—which is folded to form numerous connected stacks of discs. Each disc is a thylakoid and each stack is a granum (pl.

The light-dependent reactions of photosynthesis take place within the thylakoids. These reactions occur when the pigment chlorophyll, located within the thylakoid membranes, captures energy from the sun (photons) to initiate the breakdown of water molecules.

These two energy-storing molecules are then used in the light-independent reactions. Within chloroplasts, chlorophyll is the pigment that absorbs sunlight.

The series of light-dependent reactions begins when sunlight hits a molecule of chlorophyll, located in photosystem II. This excites an electron, which leaves the chlorophyll molecule and travels along the thylakoid membrane via a series of carrier proteins (known as the electron transport chain).

This is a process humans haven’t been able to replicate exactly in a lab.

The oxygen is released as a waste product—oxygen atoms from disassembled water molecules join up in pairs to form oxygen gas (O2). The hydrogen ions build up in high concentration in the lumen of the thylakoid.

This energy-storing molecule powers many cellular processes. In fact, the glucose made during photosynthesis is broken down to produce more ATP later, during cellular respiration.

Energy from the sun excites the electron again, giving it enough energy to pass across the membrane and into the stroma, where it joins with a hydrogen ion and an NADP+ to create the energy-carrying molecule NADPH. ATP and NADPH move from the thylakoid into the stroma, where the energy they store is used to power the light-independent reactions.

This is the part of photosynthesis that requires the CO2 the plant gets from the air. Essentially, the plant needs the carbon from the CO2 to create the building blocks for glucose.

This creates a six-carbon molecule that is broken down into two three-carbon molecules (3-phosphoglycerate). This part of the light-independent reactions is referred to as carbon fixation.

ATP and NADPH give each 3-phosphoglycerate a hydrogen atom, creating two molecules of the simple sugar G3P (glyceraldehyde-3-phosphate). Ultimately, these two molecules of G3P are used to build one molecule of glucose.

It is important to note that the Calvin cycle typically uses six molecules of carbon dioxide at a time. This means that twelve molecules of G3P are generated.

The light-dependent reaction is an important part of photosynthesis, the process by which plants and other organisms convert light energy into chemical energy. This reaction requires light energy to reduce NADP (nicotinamide adenine dinucleotide phosphate) and H+ ions to NDPH, synthesise ATP (adenosine triphosphate) from inorganic phosphate (Pi) and ADP (adenosine diphosphate), and split water into H+ ions, electrons, and oxygen.

The light-dependent reaction is a redox reaction, meaning that substances both lose and gain electrons, hydrogen, and oxygen in the process. Oxidation is when a substance loses electrons, loses hydrogen, or gains oxygen, while reduction is when a substance gains electrons, gains hydrogen, or loses oxygen.

In the light-dependent reaction, the reactants are water, NADP+, ADP, and inorganic phosphate (Pi). Water is a crucial part of photosynthesis, as it donates its electrons and H+ ions through a process called photolysis.

Photolysis refers to the process in which light energy (direct) or radiant energy (indirect) breaks the bonds between atoms. NADP+ is a type of co that binds with catal a reaction.

It combines with electrons and H+ ions to form NADPH, a molecule that is necessary for the light-independent reaction. ATP is often called the cell’s energy currency, and its formation from ADP is a critical part of photosynthesis.

There are three stages in light-dependent reaction: oxidation, reduction and generation of ATP. Photosynthesis takes place in the chloroplast (you can refresh your memory on the strcture in the photosynthesis article).

When light energy is absorbed by chlorophyll molecules in photosystem II, the electrons within the molecule are raised to a higher energy level. These then leave the chlorophyll molecule, causing it to become ionised.

To replace the missing electrons in the chlorophyll molecule, water acts as an electron donor. This leads to the oxidation of water, where it loses electrons.

These electrons are carried from photosystem II to photosystem I by plastocyanin, a protein thatates electron transfer reaction electrons also pass through plastoquinone, a molecule involved in the electron transport chain, and cytochrome b6f, an enzyme. However, these are not typically necessary to know for A-levels.

With the help of the enzyme NADP dehydrogenase, they combine with an H+ ion and NADP+. This reaction results in the production of NADPH (nicotinamide adenine dinucleotide phosphate hydrogen) and is called a reduction reaction since NADP+ gains electrons.

The equation for this reaction is: Various inhibitors can slow down this process, such as ammonium hydroxide.

You can read more about this and other substances that affect the rate of theInvestigating the Rate of Photosynthesis Practical” article. The final stage of the light-dependent reaction involves the production of ATP.

During earlier stages of the light-dependent reaction, H+ produced through photolysis, creating a high concentration of protons in the thylakoid lumen behind the membrane that separates this space from the stroma. The production of ATP can be explained by the chemiosmotic theory proposed by Peter D.

According to this theory, most ATP synthesis is generated from an electrochemical gradient established across the thylakoid disc membrane. This gradient is created by the high concentration of H+ ions in the thylakoid lumen and the low concentration of H+ ions in the stroma.

As protons pass through ATP synthase, they cause the enzyme to change shape, catalyzing the production of ATP from ADP and phosphate. Figure 1 will help you visualise the light-dependent reaction.

The light-dependent reaction produces oxygen, ATP, and NADPH. Oxygen is released back into the atmosphere during photosynthesis, while ATP and NADPH are used in the light-independent reaction.

It is composed of a nucleotide, which consists of an adenine base attached to a ribose sugar and three phosphate groups (Figure 2). These three phosphate groups are connected by two high-energy bonds, known as phosphoanhydride bonds.

This energy is utilized in the light-independent reaction. NADPH serves as both an electron donor and an energy source for various stages of the light-independent reaction.

In summary, the light-dependent reaction is a crucial in photosynthesis that requires light energy. It serves three functions: producing NADPH, synthesizing ATP, and breaking down water.

NADPH and ATP are both essential molecules for the light-independent reaction. Where does a light-dependent reaction take place.

The light-dependent reaction takes place along the thylakoid membrane. This is the membrane of the thylakoid discs, which are found in the structure of the chloroplast.

What happens in the light-dependent reactions of photosynthesis.

In oxidation, water is oxidised through photolysis, meaning that light is used to split water into oxygen, H+ ions, and electrons. Oxygen is produced as a result, and the H+ ions go into the thylakoid lumen in order to facilitate the conversion of ADP to ATP.

How is oxygen produced in light-dependent reactions.

This involves the use of light energy to split water into its basic compounds. The end products of photolysis are oxygen, 2 electrons, and 2H+ ions.

The light-dependent reactions of photosynthesis produce three essential molecules. These are oxygen, NADPH (or reduced NADP), and ATP.

How does ammonium hydroxide affect the light-dependent reaction.

Ammonium hydroxide inhibits the enzyme that catalyses the reaction that turns NADP into NADPH, NADP dehydrogenase. This means that NADP cannot be reduced to NADPH at the end of the electron chain.

Ammonium hydroxide also has a highly alkaline pH (around 10.09), which further inhibits the rate of the light-dependent reaction. Most of the light-dependent reactions are enzyme-controlled, so if the pH is too acidic or too alkaline, they will denature, and the reaction rate will sharply decrease.

The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria.

These reactions take the products (ATP and NADPH) of light-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and reducing power of NADPH from the light dependent reactions to produce sugars for the plant to use.

there is no direct reaction that converts several molecules of CO2 to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

Though it is called the “dark reaction”, the Calvin cycle does not actually occur in the dark or during night time. This is because the process requires NADPH, which is short-lived and comes from the light-dependent reactions.

The Calvin cycle thus happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism (CAM)). CAM plants store malic acid in their vacuoles every night and release it by day to make this process work.

The reactions of the Calvin cycle are closely coupled to the thylakoid electron transport chain, as the energy required to reduce the carbon dioxide is provided by NADPH produced during the light dependent reactions. The process of photorespiration, also known as C2 cycle, is also coupled to the Calvin cycle, as it results from an alternative reaction of the RuBisCO enzyme, and its final byproduct is another glyceraldehyde-3-P molecule.

The Calvin cycle, Calvin–Benson–Bassham (CBB) cycle, reductive pentose phosphate cycle (RPP cycle) or C3 cycle is a series of biochemical redox reactions that take place in the stroma of chloroplast in photosynthetic organisms. The cycle was discovered in 1950 by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14.[citation needed].

In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecule ATP and the moderate-energy hydrogen carrier NADPH. The Calvin cycle uses these compounds to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals that feed on it).

The key enzyme of the cycle is called RuBisCO. In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.[citation needed].

They are activated in the light (which is why the name “dark reaction” is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from being respired to carbon dioxide.

The sum of reactions in the Calvin cycle is the following:[citation needed]. Hexose (six-carbon) sugars are not products of the Calvin cycle.

The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or “triose phosphates”, namely, glyceraldehyde-3-phosphate (G3P).[citation needed]. In the first stage of the Calvin cycle, a CO2 molecule is incorporated into one of two three-carbon molecules (glyceraldehyde 3-phosphate or G3P), where it uses up two molecules of ATP and two molecules of NADPH, which had been produced in the light-dependent stage.

The next stage in the Calvin cycle is to regenerate RuBP. Five G3P molecules produce three RuBP molecules, using up three molecules of ATP.

The regeneration stage can be broken down into a series of steps.

This requires nine ATP molecules and six NADPH molecules per three CO2 molecules. The equation of the overall Calvin cycle is shown diagrammatically below.[citation needed].

The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3-PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine.

Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3-PGA.

C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example. Furthermore, RuBisCOs catalyzing the light-independent reactions of photosynthesis generally exhibit an improved specificity for CO2 relative to O2, in order to minimize the oxygenation reaction.

The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2 NADP+. (ADP and NADP+ are not really “products”.

Each G3P molecule is composed of 3 carbons. For the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated.

Therefore, there is only 1 net carbon produced to play with for each turn. To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle.

Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs.

There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH.

and the RuBisCo enzyme activation, active in the Calvin cycle, which involves its own activase.

This happens when light is available, as the ferredoxin protein is reduced in the photosystem I complex of the thylakoid electron chain when electrons are circulating through it. Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle enzymes by severing a cystine bond found in all these enzymes.

The implications of this process are that the enzymes remain mostly activated by day and are deactivated in the dark when there is no more reduced ferredoxin available.[citation needed]. The enzyme RuBisCo has its own, more complex activation process.

This lysine binds to RuBP and leads to a non-functional state if left uncarbamylated. A specific activase enzyme, called RuBisCo activase, helps this carbamylation process by removing one proton from the lysine and making the binding of the carbon dioxide molecule possible.

This magnesium ion is released from the thylakoid lumen when the inner pH drops due to the active pumping of protons from the electron flow. RuBisCo activase itself is activated by increased concentrations of ATP in the stroma caused by its phosphorylation.

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 Chlorella, which was illuminated with red light alone, with blue light alone, and with red and blue light at the same time.

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.

It is now known that light reaction I can use light of a slightly longer wavelength than red (λ = 680 nm), while light reaction II requires light with a wavelength of 680 nm or shorter. 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.

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.

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 detergents, lamellae can be broken down into smaller particles capable of carrying out single light reactions.

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).

To synthesise organic energy molecules, light processes are required (ATP and NADPH). Colored pigments, primarily green chlorophyll, initiate them.

The photolysis process is known as the light reaction. It occurs when light is present.

An autotroph is an organism with metabolic activities that produce organic molecules by using energy from either inorganic chemicals or light. An organism that uses light energy to create organic compounds from inorganic sources is known as a photoautotroph.

Chlorophyll is one of the major components in plants that carry out an operation in light responses, such as photosynthesis. Carotenoids are one of the additional pigments.

This energy is used to generate ATP and NADPH through the electron transport chain (ETC).

Photosynthesis is a chemical reaction through which plants, bacteria, and Protista use the energy from sunlight to make carbohydrates, which are then processed into ATP by cellular respiration. This is the “fuel” that all life forms need.

Photosynthesis is complete in two stages, which are light-dependent reactions and light-independent reactions (Calvin cycle).

Photosynthesis in the chloroplast.

The plants use light energy to produce the coenzyme, nicotinamide adenine dinucleotide phosphate, or NADPH, and the energy-carrying molecules, ATP. These compounds’ chemical bonds store energy, and are used during the dark phase.

Pigment molecules in thylakoids absorb light and convert it to chemical energy during light reactions.

oxygen gets released into the atmosphere, and the plant utilises ATP to produce sugars through light-independent reactions.

Light-independent reactions are also understood as the Calvin Cycle, named after Melvin Calvin, who revealed these reactions.

Sugar is made up of energy from ATP, electrons from NADPH (both generated by light processes), and carbon from CO2 (taken in from outside). This process of photosynthesis is also known as carbon fixation and is critical for maintaining stable atmospheric carbon dioxide levels.

When ATP and NADPH are consumed, they produce ADP and NADP+, which are then returned to the light processes to produce new ATP and NADPH.

This results in the synthesis of two high-energy chemical compounds: ATP and NADPH, the latter of which has chemical energy that can be easily transferred to other compounds. This series of reactions necessitates the use of water, from which oxygen is released during the process.

The light reaction uses water, light (brought in from outside the cell), NADP+, phosphorus, and ADP (all created by the Calvin cycle) to produce ATP (both used in the Calvin cycle), and oxygen (a waste product). The light excites the electrons, and the excited electrons’ energy is then used to join ADP and phosphate together to form ATP.

Water is split in the process, and oxygen is generated as waste and is discharged. In light reactions, sunlight energy is used to oxidise water (an electron donor) to oxygen, and then to pass these electrons to NADP+, resulting in NADPH.

The NADPH and ATP created, are then used to run the Calvin cycle, which produces sugar.

Chlorophyll is a green pigment that is found in the chloroplasts of plants, algae, and some bacteria that are photosynthetic.

The light processes occur inside the chloroplast on the chlorophyll-containing thylakoid membranes.

It possesses chlorophyll and serves as the site for photosynthesis’ light-dependent reactions. The Calvin cycle reactions occur in the aqueous stroma, which is located between the inner envelope membrane and the thylakoid membranes.

Photosystems are complex combinations of chlorophyll as well as other pigments, such as chlorophyll b, xanthophyll, and carotenoids, that capture light energy and use it to energise an electron, which is removed from a water molecule. Photosystems in plants are found within the chloroplast’s thylakoid membrane.

Between the two photosystems, photosystem II oxidises water and reduces the electron transport chain. P680 is oxidised (which oxidises water) in PS II (the first photosystem in the sequence), and the PS II main electron acceptor is reduced (which reduces the electron transport chain between the photosystems).

Between the two photosystems, photosystem I reduce NADP+ and oxidises the electron transport chain. The PS I primary electron acceptor is reduced (which reduces other molecules, reducing NADP+ to NADPH), and P700 is oxidised in PS I (which in turn, oxidises the electron transport chain between the photosystems).

A series of membrane-bound carriers that transfer electrons from one to the next.

High-energy electrons are delivered, while low-energy electrons are expelled.

The stroma becomes negatively charged in relation to the space within the thylakoids when H+ ions accumulate. H+ ions are unable to pass through the thylakoid membranes directly.

The gradient propels H+ ions through ATP synthase, which causes it to rotate. As it rotates to make ATP, ATP synthase joins ADP and a phosphate group together.

Water is oxidised, electrons from water are transported to NADP+, and ATP is generated during the light processes. Equations can be used to summarise the three elements of the light reaction.

$2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{O}_{2}+4 \mathrm{e}^{-}+4 \mathrm{H}^{+}$. NADP reduction.

Synthesis of ATP. $\mathrm{ATP} \rightarrow \mathrm{ADP}+\mathrm{Pi}$.

$2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{NADP}^{+}+\mathrm{nADP}+\mathrm{nPi}+\mathrm{hv} \rightarrow \mathrm{O}_{2}+2 \mathrm{NADPH}+2 \mathrm{H}^{+}+\mathrm{nATP}$. The light reaction of photosynthesis, which takes place on the chloroplast’s thylakoid membranes and transforms solar energy into the chemical energy of ATP and NADPH while also releasing oxygen.

This article has given all the useful details about the light reaction with respect to the NEET syllabus. These also help in clearing the doubts about this topic and help in qualifying for the exam.

Heterotrophic organisms ranging from E. coli to humans rely on the chemical energy found mainly in carbohydrate molecules.

Although photosynthesis is most commonly associated with plants, microbial photosynthesis is also a significant supplier of chemical energy, fueling many diverse ecosystems. In this section, we will focus on microbial photosynthesis.

In the light-dependent reactions, energy from sunlight is absorbed by pigment molecules in photosynthetic membranes and converted into stored chemical energy. In the light-independent reactions, the chemical energy produced by the light-dependent reactions is used to drive the assembly of sugar molecules using CO2.

The light-dependent reactions produce ATP and either NADPH or NADH to temporarily store energy. These energy carriers are used in the light-independent reactions to drive the energetically unfavorable process of “fixing” inorganic CO2 in an organic form, sugar.

The light-dependent reactions of photosynthesis (left) convert light energy into chemical energy, forming ATP and NADPH. These products are used by the light-independent reactions to fix CO2, producing organic carbon molecules.

These chloroplasts are enclosed by a double membrane with inner and outer layers. Within the chloroplast is a third membrane that forms stacked, disc-shaped photosynthetic structures called thylakoids (Figure 2).

Figure 2. (a) Photosynthesis in eukaryotes takes place in chloroplasts, which contain thylakoids stacked into grana.

(credit: scale bar data from Matt Russell.). Photosynthetic membranes in prokaryotes, by contrast, are not organized into distinct membrane-enclosed organelles.

In cyanobacteria, for example, these infolded regions are also referred to as thylakoids. In either case, embedded within the thylakoid membranes or other photosynthetic bacterial membranes are photosynthetic pigment molecules organized into one or more photosystems, where light energy is actually converted into chemical energy.

The light-harvesting complex consists of multiple proteins and associated pigments that each may absorb light energy and, thus, become excited. This energy is transferred from one pigment molecule to another until eventually (after about a millionth of a second) it is delivered to the reaction center.

The reaction center contains a pigment molecule that can undergo oxidation upon excitation, actually giving up an electron. It is at this step in photosynthesis that light energy is converted into an excited electron.

Pigments reflect or transmit the wavelengths they cannot absorb, making them appear the corresponding color. Examples of photosynthetic pigments (molecules used to absorb solar energy) are bacteriochlorophylls (green, purple, or red), carotenoids (orange, red, or yellow), chlorophylls (green), phycocyanins (blue), and phycoerythrins (red).

Because photosynthetic bacteria commonly grow in competition for sunlight, each type of photosynthetic bacteria is optimized for harvesting the wavelengths of light to which it is commonly exposed, leading to stratification of microbial communities in aquatic and soil ecosystems by light quality and penetration.

The ETS is similar to that used in cellular respiration and is embedded within the photosynthetic membrane. Ultimately, the electron is used to produce NADH or NADPH.

Figure 3. This figure summarizes how a photosystem works.

The energy is passed from one LH pigment to another until it reaches a reaction center (RC) pigment, exciting an electron. This high-energy electron is lost from the RC pigment and passed through an electron transport system (ETS), ultimately producing NADH or NADPH and ATP.

For photosynthesis to continue, the electron lost from the reaction center pigment must be replaced. The source of this electron (H2A) differentiates the oxygenic photosynthesis of plants and cyanobacteria from anoxygenic photosynthesis carried out by other types of bacterial phototrophs (Figure 4).

Because oxygen is generated as a byproduct and is released, this type of photosynthesis is referred to as oxygenic photosynthesis. However, when other reduced compounds serve as the electron donor, oxygen is not generated.

Hydrogen sulfide (H2S) or thiosulfate [latex]\left({\text{S}}_{2}\text{O}_{3}^{2-}\right)[/latex] can serve as the electron donor, generating elemental sulfur and sulfate [latex]\left({\text{SO}}_{4}^{2-}\right)[/latex] ions, respectively, as a result. Figure 4.

Photosystems have been classified into two types: photosystem I (PSI) and photosystem II (PSII) (Figure 5). Cyanobacteria and plant chloroplasts have both photosystems, whereas anoxygenic photosynthetic bacteria use only one of the photosystems.

If the cell requires both ATP and NADPH for biosynthesis, then it will carry out noncyclic photophosphorylation. Upon passing of the PSII reaction center electron to the ETS that connects PSII and PSI, the lost electron from the PSII reaction center is replaced by the splitting of water.

The flow of electrons in this way is called the Z-scheme. If a cell’s need for ATP is significantly greater than its need for NADPH, it may bypass the production of reducing power through cyclic photophosphorylation.

the high-energy electron of the PSI reaction center is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it. Figure 6.

(a) PSI and PSII are found on the thylakoid membrane. The high-energy electron from PSII is passed to an ETS, which generates a proton motive force for ATP synthesis by chemiosmosis, and ultimately replaces the electron lost by the PSI reaction center.

(b) When both ATP and NADPH are required, noncyclic photophosphorylation (in cyanobacteria and plants) provides both. The electron flow described here is referred to as the Z-scheme (shown in yellow in [a]).

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage.

The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the biochemical pathway used for fixation of CO2, is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts.

Light-independent reaction n., [lʌɪtˌɪndɪˈpɛndənt rɪˈækʃən] Definition: dark reactions of photosynthesis. Table of Contents.

The photosynthesis cycle is made up of two stages. Which set of reactions uses H2O and produces O2.

The energy is saved in the form of NADPH and ATP. Therefore in photosynthesis, NADPH is used for storing energy.

The light reactions of photosynthesis supply the Calvin cycle with carbon dioxide to produce sugars. What are light-independent reactions in photosynthesis.

They do not require light or energy from the sun to initiate the reaction. Light-independent reactions are also identified as the Calvin cycle for the reason that the process is cyclic.

The Calvin cycle or light-independent reaction of photosynthesis happens in the stroma of the chloroplast. It has certain enzymes that work with NADPH and ATP.

The chloroplast has its genetic makeup, which is separate from the cell. This genetic material is also stored in the stroma.

In light-dependent reactions, solar energy is used to produce NADPH and ATP. This is the fuel that is further used by light-independent reactions to form carbohydrate molecules.

These carbon atoms are provided by carbon dioxide. The carbon dioxide diffuses into the leaves through stomata.

Light-independent reaction (biology definition):. Light-independent reaction is a series of biochemical reactions in photosynthesis not requiring light to proceed, and ultimately produce organic molecules from carbon dioxide.

It is described to be light-independent as it proceeds regardless of the amount of light available. The term is used in contrast to the light-dependent reaction of photosynthesis that as the name implies depends on and requires light to take place.

What is the Calvin cycle or Calvin cycle definition in biology. In green plants, leaves have small openings known as stomata.

From stomata, it reaches into mesophyll cells through the intercellular spaces. When carbon dioxide gets into the mesophyll cells, it circulates into the stroma of the chloroplast.

The light-independent reaction is also recognized as the Calvin-Benson cycle, Calvin Cycle, and dark reaction. The name dark reaction creates confusion as the name shows the reaction takes place in darkness but it’s not true.

Calvin cycle location/ where does the Calvin cycle take place. The Calvin cycle takes place in the stroma (the inner space of chloroplasts).

Following are the three steps of Calvin Cycle Reaction:.

In the stroma of chloroplast, carbon fixation takes place. Here, along with carbon dioxide, some other components are also present that help in the reaction.

In the fixation stage, one molecule of RuBP reacts with CO2. As a result, 2 molecules of 3-phosphoglyceric acid (3-PGA) are formed.

During the reaction, the number of carbon atoms remains the same despite the formation of new bonds. 15 atoms from 3RuBP + 3 atoms from 3CO2 = 18 atoms in 3 atoms of 3-PGA.

For the conversion of 6 molecules of 3-PGA, NADPH and ATP are used. 3-PGA gets converted into G3P- glyceraldehyde 3-phosphate.

3-PGA gains electrons. that’s why it is a reduction reaction.

ATP converts into ADP due to the loss of phosphate atoms and energy is released. NADP+ is formed from NADPH due to the loss of H+ and energy.

Is NADPH a reducing agent. The source of energy is ATP but NADPH is the reducing agent because it adds high-energy electrons for the formation of sugar.

G3P comes from the chloroplast and has 3-C atoms. It takes three rounds of Calvin cycle for exporting one G3P.

So, 6-G3Ps form in three rounds. One of the G3P is exported and the remaining 5-GEPs are used for RuBP regeneration.

In the regeneration reaction further 3 more molecules of ATP are used. For the process of photosynthesis: Plant Metabolism – Photosynthesis Tutorial.

ADP and NADP+ are not actual products. They are further utilized by light-dependent reaction.

For the Calvin cycle to continue to work, RuBP needs to be regenerated. For this, 5 carbon atoms from 6 of 2 G3P molecules are used.

For the formation of 1 extra G3P, 3 carbon atoms are needed and consequently 3 turns of the Calvin cycle. Therefore, 6 rounds of the Calvin cycle are needed for the formation of one molecule of glucose.

These are the products of the light-independent reactions:.

In C3, the first carbon compound which is produced has 3 carbon atoms. But some plants have evolved another form of photosynthesis which is C4.

Due to C4 photosynthesis, the leaves which are formed have a different and unique structure. This structure causes the CO2 to concentrate in ‘bundle sheath’ cells around Rubisco.

Due to this the requirement of photorespiration is not required. Because of this adaptability, the stomata remain close and there is less loss of water.

Try to answer the quiz below to check what you have learned so far about light-independent reaction. Choose the best answer.

Time is Up.

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The processes in all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating, that is, by ingesting other organisms.

All of this energy can be traced back to photosynthesis. Photosynthesis is essential to all life on earth.

It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules.

The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis around 300 million years ago. Figure 1.

Cyanobacteria and planktonic algae can grow over enormous areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds.

(credit a: modification of work by Steve Hillebrand, U.S. Fish and Wildlife Service.

credit c: modification of work by NASA. credit d: University of Washington, NOAA.

Figure 2. The energy stored in carbohydrate molecules from photosynthesis passes through the food chain.

(credit: modification of work by Steve VanRiper, U.S. Fish and Wildlife Service).

Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs.

hence, they are referred to as chemoautotrophs. The importance of photosynthesis is not just that it can capture sunlight’s energy.

Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo” part) as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules (the “synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration.

When a top predator, such as a wolf, preys on a deer (Figure 2), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf. Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 3).

These sugar molecules contain energy and the energized carbon that all living things need to survive. Figure 3.

Oxygen is generated as a waste product of photosynthesis. The following is the chemical equation for photosynthesis (Figure 4):

The basic equation for photosynthesis is deceptively simple. In reality, the process takes place in many steps involving intermediate reactants and products.

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.

The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance.

Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes. In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast.

Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). Within the chloroplast are stacked, disc-shaped structures called thylakoids.

The thylakoid membrane encloses an internal space called the thylakoid lumen. As shown in Figure 5, a stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).

Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

On a hot, dry day, plants close their stomata to conserve water. What impact will this have on photosynthesis.

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy.

Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light.

The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy.

Figure 6. Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle.

The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2. Figure 7.

(credit: Associação Brasileira de Supermercados). Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth.

Although there is a large variety, each item links back to photosynthesis. Meats and dairy link because the animals were fed plant-based foods.

What about desserts and drinks. All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis.

Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.

When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work.

However, autotrophs only use a few specific components of sunlight. The sun emits an enormous amount of electromagnetic radiation (sola.

The key difference between light reaction and Calvin cycle is the dependence of light in each type of reaction in photosynthesis. Light reaction in photosynthesis is light-dependent whereas the Calvin cycle (or dark reaction in photosynthesis) is light-independent.

It is an anabolic process of producing food. Photosynthesis in plants takes place in two main procedures.

The light reaction of photosynthesis is a light-dependent process which converts solar energy into chemical energy. In contrast, the Calvin cycle also called the dark reaction of photosynthesis is a light-independent process.

Overview and Key Difference 2. What is Light Reaction in Photosynthesis 3.

Similarities Between Light Reaction and Calvin Cycle 5. Side by Side Comparison – Light Reaction vs Calvin Cycle in Tabular Form 6.

Light reactions of photosynthesis take place in the thylakoid membranes of chloroplasts. They are light-dependent reactions.

There are two photosystems involved in light reactions. Namely, they are photosystem I and photosystem II.

They absorb different wavelengths in light. Afterwards, the electrons in the photosystems absorb this energy and get excited.

Figure 01: Light Reaction. Hence, through the transfer of electrons, phosphorylation takes place to produce adenosine tri phosphate (ATP).

In addition to this, water involves in the process. This is known as photolysis of water which gives free oxygen and hydrogen ions.

Light reaction of photosynthesis has two categories cyclic reactions and non-cyclic reactions. Calvin cycle also referred to as dark reaction of photosynthesis is a light independent reaction.

Accordingly, the formation of sugar compounds with carbon-dioxide as the starting compound drives Calvin cycle. However, there is no activation of electrons in the Calvin cycle.

They are energy dependent reactions. There are three main phases of the Calvin cycle.

Figure 02: Calvin Cycle. The first carbon acceptor in the light-independent reactions is the 5 carbon sugar known as Rubisco bisphosphate (RuBP).

PGA then splits to produce glyceraldehyde – 3 – phosphate and regenerates RuBP. The produced glyceraldehyde – 3 – phosphate is thus used to produce glucose.

They are the C3 pathway that takes place in C3 plants and the C4 pathway which takes place in C4 plants. Light reaction in photosynthesis depends on the light energy while Calvin cycle (or dark reaction in photosynthesis) does not depend on the light energy.

Further difference between light reaction and Calvin cycle is that the light reaction takes place in the thylakoid membrane while the Calvin cycle occurs in the stroma of the chloroplast. Moreover, there is a difference between light reaction and Calvin cycle in the end products too.

the end products of the light reaction are ATP and NADPH while the end product of Calvin cycle is glucose. The below infographic on difference between light reaction and Calvin cycle provides more differences between both reactions.

Photosynthesis occurs in photoautotrophic organisms. There are two types of photosynthesis based on their dependence to light such as light-dependent reactions and light-independent reactions.

On the other hand, light-dependent reactions take place through the involvement of photosystems. Thus, it takes place in the thylakoid membranes of the chloroplast.

Accordingly, this takes place in the stroma of the chloroplast. This is the difference between light reaction and Calvin cycle.

Available here 2.“The Calvin Cycle.” Khan Academy, Khan Academy. Available here.

1.”4619809768″ by BlueRidgeKitties (CC BY 2.0) via Flickr 2.”Calvin cycle”By Yikrazuul – Own work, (CC BY-SA 3.0) via Commons Wikimedia.

Figure 6.13: Photophosphorylation. Figure 6.14: Photophosphorylation Experiment Setup.

This quick change in pH should mimic conditions found in the chloroplasts and provide the means necessary for ATP generation. To test this hypothesis, both scientists placed isolated mitochondria in a weakly acidic medium (pH 4) for a minute before the pH was raised to 8.

The collapse of the gradient is the energy transducing medium – the chemical potential of a concentration difference is transduced into the synthesis of ATP. Figure 6.15: Cyclic Photophosphorylation.

This pathway diverts activated electrons lost from PSI back to the PQ pool, the cytochrome b6f complex, and plastocyanin to re-reduce P700+. Here, ATP is the sole product of energy conversion.

Light-dependent reaction n., [laɪt dɪˈpɛndənt ɹiˈækʃən] Definition: photosynthetic reaction requiring light. Table of Contents.

In this process, green plants capture light energy and form organic compounds, which are rich in energy and oxygen (O) by using minerals, carbon dioxide (CO2), and water (H2O). What is the light-dependent reaction.

Those reactions which occur in the presence of light (photons) are known as light-dependent reactions. The reaction starts when the sunlight or any other source of light causes the excitation of sensitive molecules.

They transfer sunlight (solar energy) into potential energy and store it as chemical energy in sugars. In biology, light-dependent reactions are very important as many green plants are carrying them out when light is available, such as during day time.

If photosynthesis stops, the organic matter and the food on earth will come to an end. These organisms that rely on light energy for making food are called photoautotrophs.

They don’t require light energy but rather utilize chemical sources to energize the process of making food. To understand the difference between photoautotrophs and chemoautotrophs, read: Autotorophs Definition and Examples.

Like other forms of energy, light is used to do work. It not only travels but also changes form.

Although the sun gives a different level of electromagnetic radiation (solar energy), autotrophic organisms use a specific component of sunlight. Visible light is only a small fraction of energy that is seen by humans.

In a series of waves, the distance between two identical points like the crest to crest or trough to trough is known as wavelength. Scientists use wavelength to evaluate the amount of energy wave.

Each wavelength carries a different level of energy and has different characteristics. The longer or stretched wavelength carries less energy but tight and short waves have more energy.

For photosynthesis, a plant’s pigment molecule absorbs visible light. Humans see visible light as white but in reality, it is in rainbow colors.

In the rainbow, violet and blue have short wavelengths so they have high energy. Red has a longer wavelength so has low energy.

The wavelengths that cannot be absorbed by the pigments will be reflected. Plants are color green due to the presence of a photosynthetic pigment, which is a chlorophyll molecule.

Chlorophyll reflects the green and that’s why it appears green in color. The nature of pigment is recognized by the pattern of the wavelength, which is absorbed from the absorption spectrum of visible white light.

Such organisms can absorb an extensive series of visible light wavelengths. Each photosynthetic organism does not have a complete approach to solar light.

Most of the sunlight will be absorbed by the water and a certain wavelength reaches the photosynthetic organism. Some organisms grow in a competitive environment like on the rainforest floor.

Light-dependent reactions are responsible for converting light energy into chemical energy in the Calvin cycle. Thus, the light reactions of photosynthesis supply the Calvin cycle with light energy.

This chemical energy is utilized as fuel, for the formation of molecules of sugar. The light-dependent reactions take place in the photosystem.

In membranes of thylakoids of chloroplast, photosystems are found. At a time, a molecule of pigment from the photosystems absorbs a photon-quantity or “packet” of light energy.

It is also known as photolysis. This reaction occurs in the grana of chloroplasts.

Carotenoids act as an accessory pigment. In the thylakoid membrane of the chloroplast, chlorophyll is present which absorbs energy from the sun.

In this process, water (H20) is consumed and oxygen (O2) is released. A photon travels till it reaches chlorophyll.

The energy causes the electron to get separated freely from the atom of chlorophyll. This is the reason chlorophyll is known as a donor.

In thylakoid space, this splitting not only releases an electron but also leads to the formation of hydrogen ions (H+) and oxygen (O2). The splitting of water molecules gives a pair of electrons.

The chlorophyll responds to another photon due to the replacing of an electron. The oxygen which is a by-product of this reaction goes into the external environment.

Two stages carry out the process of photosynthesis. These are:

In this reaction, by using water, chlorophyll absorbs the energy from the sun and transforms it into chemical energy. Water (H2O) is split and oxygen (O2) comes out as a by-product.

The chemical energy which is obtained from a light-dependent reaction carries out the formation of a sugar molecule and also the capture of carbon (C) in carbon dioxide (CO2) molecules. For transferring energy, these two reactions use carrier molecules.

After releasing the energy which is now termed as “empty” energy goes back to light-dependent reactions to gain more energy. In cellular respiration, NADH and FADH2 are the two energy carrier molecules.

NADPH is formed when a lower energy molecule NADP+ takes a high energy proton and electron. NADP+ formed again when NADPH releases an electron.

This energy is stored in a bond that holds the only atom to the molecule. In ATP and NADPH, phosphate and hydrogen atoms are present.

They become lower energy particles ADP and NADP+ when releasing the energy into the Calvin cycle. An electrochemical gradient is formed in thylakoid space due to hydrogen ions.

By chemiosmosis, the energy is stored in the form of ATP. Like mitochondria, there is an electrochemical gradient due to the movement of hydrogen ions.

In mitochondria, this protein generates ATP from ADP. In photophosphorylation, a molecule of ATP is formed.

What is chemiosmosis.

NADPH, which is an energy carrier molecule, is also formed by the light-dependent reaction. When the electrons from the electron transport chain (ETC) reach the photosystem, the electron again gains energy with additional photons which are captured by the chlorophyll.

The energy from the light of the sun is stored in the form of energy carriers. This can be utilized for the formation of glucose molecules.

The seven steps are as follows: The process of light reaction is given below:

In the Calvin cycle, these reactions help in the manufacturing of glucose in photosynthesis. In cyclic photophosphorylation, which is another form of these reactions, the path of the electron is different and circular and at the end of the reaction, only ATP is produced.

Cyclic and noncyclic photophosphorylation are two different pathways involved in the light-dependent reactions of photosynthesis, occurring in the thylakoid membranes of chloroplasts.

This pathway occurs in the thylakoid membranes and requires participation of two light-gathering units: photosystem I (PS I) and photosystem II (PS II). 2.

solar energy is absorbed and high-energy electrons are generated. 3.

Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a. 5.

they escape to electron-acceptor molecule. 6.

electrons move from H2O through PS II to PS I and then on to NADP+. 7.

high-energy electrons (e-) leave the reaction-center chlorophyll a molecule. 8.

Oxygen is released as oxygen gas (O2). 10.

As H+ flow down electrochemical gradient through ATP synthase complexes, chemiosmosis occurs. 12.

When the PS I pigment complex absorbs solar energy, high-energy electrons leave reaction-center chlorophyll a and are captured by an electron acceptor. 14.

NADP+ takes on an H+ to become NADPH: NADP+ + 2 e- + H+ NADPH. 16.

Outcome: The key products of noncyclic photophosphorylation are ATP and NADPH, which are essential for the Calvin Cycle (light-independent reactions) to convert carbon dioxide into sugars.

It’s a simpler pathway that can occur under specific conditions, such as when there’s a shortage of NADP⁺ or when the light intensity is high.

The cyclic electron pathway begins when the PS I antenna complex absorbs solar energy. 2.

Before they return, the electrons enter and travel down an electron transport system. a.

Energy released is stored in form of a hydrogen (H+) gradient. c.

Because the electrons return to PSI rather than move on to NADP+, this is why it is called cyclic and also why no NADPH is produced. Outcome: Cyclic photophosphorylation produces ATP but does not result in the production of NADPH or the release of oxygen.

Noncyclic Photophosphorylation: Produces both ATP and NADPH, requires both Photosystem I and Photosystem II, and contributes to the generation of oxygen. Cyclic Photophosphorylation: Produces only ATP, involves Photosystem I alone, and does not result in the production of NADPH or oxygen release.

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage.

But where does the carbon come from. It comes from carbon dioxide—the gas that is a waste product of respiration in microbes, fungi, plants, and animals.

Once in the mesophyll cells, CO2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have several names associated with them.

Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is “dark reaction,” which was used because light is not directly required (figure 2.2.16).

The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.

RuBP has five atoms of carbon, flanked by two phosphates.

RuBisCO catalyzes a reaction between CO2 and RuBP. Each RuBP molecule combines with one CO2 molecule producing one molecule of 1, 3-bisphosphoglycerate.

PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA.

3 C atoms from 3CO2 + 15 C atoms from 3RuBP = 18 C atoms in 6 molecules of 3-PGA. This process is called carbon fixation because CO2 is “fixed” from an inorganic form into organic molecules.

some examples are rice, wheat, soybeans, and all trees.

That is a reduction reaction because it involves the gain of electrons by 3-PGA. (Recall that a reduction is the gain of an electron by an atom or molecule.) Six molecules of both ATP and NADPH are used.

for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and re-energized.

Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P.

One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.

It is only about 3% on cloudy days. Why is so much solar energy lost.

During photorespiration, the key photosynthetic enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) uses O2 as a substrate instead of CO2. This process uses up a considerable amount of energy without making sugars (Figure 2.2.18).

However, when a plant closes its stomata during times of water stress and O2 from respiration builds up inside the cell, the rate of photorespiration increases because O2 is now more abundant inside the mesophyll. So, there is a tradeoff.

In addition, Rubisco has a higher affinity for O2 when temperatures increase, which means that C3 plants use more energy (ATP) for photorespiration at higher temperatures.

Understanding: • Absorption of light by photosystems generates excited electrons.

The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH)These reactions occur within specialised membrane discs within the chloroplast called thylakoids and involve three steps:.

Understanding: • Excited electrons from Photosystem II are used to contribute to generate a proton gradient.

Step 2: Production of ATP via an Electron Transport Chain.

Understanding: • Excited electrons from Photosystem I are used to reduce NADP.

Step 3: Reduction of NADP+ and the Photolysis of Water.

Overview of the Light Dependent Reactions.

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The energy changes (oxidation / reduction) that occur during photosynthesis may be represented as a Z scheme:.

We are all aware that the process of photosynthesis requires sunlight. But did you know chloroplast only absorb the blue and red light wavelengths from the sunlight.

Let us learn about Light Reaction and how it functions.

Co2, water, chlorophyll, and sunlight are four important requirements for this process. Photosynthesis occurs in two steps: Light reaction and Dark Reaction.

(Image Source: actforlibraries.com). The light reaction of light dependent reaction occurs in the chloroplast of the mesophyll cells of the leaves.

The pigment, chlorophyll, which is required for the process is present on the membrane of these thylakoids and this is where the light reaction occurs. The main purpose of the light reaction is to generate organic energy molecules such as ATP and NADPH which are needed for the subsequent dark reaction.

2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP. For any plant performing photosynthesis, four factors influence this process.

But, in case of light dependent reaction or light reaction of photosynthesis, it is most influenced by presence or absence of light. The other three factors do not play a critical role in it.

Sol: Blue and Red are the colour which chlorophylls are most sensitive to. Leaves are green and so they reflect the green wavelength of white light.

I remember being confused by the numbers when I first studies it primarily because most sources seem to differ on the stoichiometry. I have tried to explain why is it difficult to comment upon such large equations, hope it helps.

There are two faults in your assumptions.

As you might notice, the above scheme does not account for many things. For starters, since the 4$H^+$ from water splitting are produced and not pumped, only 2 of those need to be pumped back via ATP-Synthase to restore equilibrium.

Furthermore, some sources also try to account for the acidity of the inorganic $Pi$ in the ATP-synthesis which will further allow for changes in the $H^+/e^-$ ratio.

As a result, some experiments give us some parts of the stoichiometry (we know that the $NADPH/ATP$ ratio is around $2/3$) and others give us the reactions. Often times, many possible reactions are proposed, from which, the ones which can best be integrated with results from different experiments are thought to represent the actual truth.

Furthermore, biological processes often are stochastic and do not have perfect stoichiometries. This is all overlooking the actual disagreements between experiments (the $H^+/ATP$ ratio for the synthase had significant variations in different experiments).

Don’t loose hope. We do know quite a bit about photosynthesis.

There are three distinct processes that can produce a chemiosmotic gradient of $H^+$. a.

This constitutes the popular Z-scheme involving a linear electron transport from water to the final acceptor $NADPH$, and this process pumps about $12H^+$ per $2H_2O$. b.

Cyclic Photophosphorylation which allows for recycling an electron, that is, proton pumping without there being a final acceptor of electron solely by internal shuffling of the electron between different states (primarily involved Plastoquinone (Q) cycle. This can pump $2H^+$ extra for 1 electron.

The number of protons per ATP depends on the rotational symmetry of the synthase. (details in the references below).

The number of Photosystem 1 units(where the cyclic version operates) is more than that of Photosystem 2 units. This means, that cyclic photophosphorylation goes on simultaneously at these centres, and will account for the extra $2H^+$ required to meet the experimentally observed ratio (1 in every 5 electron is “recycled”).

A good overall paper addressing all the main issues. Photosynthesis of ATP—Electrons, Proton Pumps, Rotors, and Poise.

Cell Volume 110, Issue 3, 9 August 2002, Pages 273-276. General resources on these reactions.

Studies on the $H^+/ATP$ ratio.

Petersen, Jan et al. “Comparison of the H+/ATP ratios of the H+-ATP synthases from yeast and from chloroplast” Proceedings of the National Academy of Sciences of the United States of America vol.

Watt IN, Montgomery MG, Runswick MJ, Leslie AG, Walker JE. Bioenergetic cost of making an adenosine triphosphate molecule in animal mitochondria.

107(39):16823-7.

Function of the Two Cytochrome Components in Chloroplasts: A Working Hypothesis. R.

Nature – volume 186, pages136–137 (1960). b.

DANIEL I. ARNON, M.

ALLEN & F. R.

Nature – volume 174, pages394–396 (1954).

What is used in the light reactions, and what is used in the Calvin cycle.

Photosynthesis is essential for all living beings that cannot produce their own energy (heterotrophs) like us. We are dependent on photosynthetic plants, not only to provide us with the oxygen we breathe but also to provide us with the food that we eat.

Photosynthesis occurs in plants, algae, phytoplankton and cyanobacteria. They are said to be photoautotrophic organisms, as they produce their own food with the help of the sun’s energy.

It comprises two groups of reactions: the light reactions, and the dark reactions (also known as light-independent reactions, or the Calvin cycle). But ultimately, even the dark reactions are dependent on light, or rather, the products of the light reactions.

Create your account. The light reactions need: photons, ADP, NADP+, and water.

The light reactions source their energy from the.. See full answer below.