Photolysis and Light Reactions in Photosynthesis

Photosynthesis is a redox reaction in which carbon dioxide is reduced to sugars and water is oxidised to release oxygen.

Photosynthesis is chemically a redox reaction in which carbon dioxide is reduced and water is oxidised. During light reaction phase of photosynthesis, the electron that has been donated to the electron transport chain is replaced through photolysis (light dependent splitting) of water, a process that not only yields electrons but also is the source of almost all the oxygen in Earths atmosphere. 

Photolysis, the chemical process by which molecules are broken down into smaller units through the absorption of light.

Light reaction occur inside the thylakoids, especially those of grana region. It involves two types of reaction photosynthesis of water and production of assimilation power.  The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplasts is called photolysis or photocatalytic , splitting of water, Water splitting complex is associated with the PSII, which itself is physically located on the inner surface of thylakoid  membrane. 
$4H _{2} O \rightleftharpoons 4H^{+} + 4OH^-$
$ 4OH^- \xrightarrow[Mn^{2+}, Ca^{2+}, Cl^-]{Oxygen ,,evolving,, complex} 2H _{2} + O _{2}\uparrow + 4e^- $ , So, the correct answer is 'inner surface of thylakoid membrane.'.

Photolysis, or the light-dependent splitting of water, occurs during the light reactions of photosynthesis to provide electrons, protons, and oxygen.

Cycling flow of electrons during light reactions is known as cyclic photophosphorylation. It is performed by photosystem I independently. PS II is not involved in the process. In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called $P _{700}$, while in PS II, reaction centre has an absorption maxima at 680 nm and is called $P _{680}$. As cyclic photophosphorylation requires only PS I, so it takes place only when light of wavelength beyond 700 nm is available for excitation. This process takes place in stroma lamellae membrane. An external source of electrons is not required. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. It operates under low light intensity, anaerobic conditions or when $CO _{2}$ availability is low. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to cyclic flow of electrons. The membrane and lamella of the grana have both PS I and PS II, the stroma lamella membrane lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $NADP^{+}$ and is cycled back to the PS I complex through the electron transport chain.

Splitting up of water or photolysis is associated with Photosystem 2 and this photosystem 2 is present only in the membrane of thylakoids. 

In order to split water, four photochemical reactions are needed to take out four electrons and four protons from two water molecules. This leads to the formation of one oxygen molecule. Protons form water are released into the lumen (donor side of PSII) and electrons are moved to the plastoquinone (QB) binding site. So, the correct option is "A" (2).

The light reaction, convert energy from the sun into chemical energy in the form of NADPH and ATP. These reactions must take place in the light and in chloroplasts of plants. Chlorophyll, which is a light-absorbing organelle in plant cells, is a very complex molecule that works in conjunction with the metal magnesium. There are primarily two types of chlorophyll: chlorophyll a and chlorophyll b. These chlorophyll molecules absorb light in the red and blue wavelengths, making the plants in which they are stored look green. Thus, the correct answer is option C.

Robin Hill in the year 1939 showed that all chloroplasts extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptor, e.g., potassium ferrioxalate and potassium ferricyanide, oxygen is released due to the photochemical splitting of water. Under these conditions, no CO$ _2$ is consumed and no carbohydrate is produced.

The photolysis of water occurs during light reaction. The site for occurrence of photolysis is thylakoid lumen. The photolysis of water releases protons which accumulate in the lumen and contribute to the electrochemical gradient of protons. The electrons released are used up to fill the electron hole in the reaction centre of PSII and oxygen molecule is released as by product of photosynthesis. The electrons released during photolysis of water are ultimately used to synthesize the reducing power for reducing carbon dioxide later on. The mineral ions required for photolysis of water are calcium, manganese and chlorine. 

The photolysis of water occurs during light reaction. The site for occurrence of photolysis is thylakoid lumen. The photolysis of water releases protons, which accumulate in the lumen and contribute to the electrochemical gradient of protons. The electrons released are used up to fill the electron hole in the reaction centre of PSII and oxygen molecule is released as by product of photosynthesis. The electrons released during photolysis of water are ultimately used to synthesize the reducing power for reducing carbon dioxide later on. The mineral ions required for photolysis of water are calcium, manganese and chlorine. 

Experiments with isolates chloroplasts, in presence of oxidants like ferricyanides and benzoquinone (Hill reagents) were conducted by Hill in 1937 and 1939. By placing the isolated chloroplasts in sunlight, he notices evolution of oxygen from water. Hill concluded that if there was suitable hydrogen acceptor in the medium it would have been reduced. In absence of light Hill reaction can not be possible. So, it can say it occurs only in day time.

Photolysis means splitting of water in presence of sunlight. During photosynthesis, light reaction occurs in chloroplasts. In light reaction the water is oxidised and the electrons from water are passed on through a series of electron acceptors and ultimately used to reduce NADP. Since electrons from water molecule are never returned or cycled back, this process is called as non cyclic flow of electrons. It involves PS-II and PS-I both.

Chloroplasts contain light trapping chlorophyll molecules. The chlorophyll molecules are an essential requirement for light reaction. The light energy is trapped by chlorophyll molecules for carrying out photolysis of water i.e., light dependent split of water molecules and photophosphorylation i.e., light dependent synthesis of adenosine triphosphate (ATP) by phosphorylation of adenosine diphosphate (ADP).

Experiments with isolated chloroplasts, in presence of oxidants like ferricyanides and benzoquinone (Hill reagents) were conducted by Hill in 1937 and 1939. By placing the isolated chloroplasts in sunlight, he noticed evolution of oxygen from water. Hill concluded that if there was suitable hydrogen acceptor in the medium it would have been reduced. Arnon 1951 and 1954 repeated the experiment of isolated chloroplast and concluded that, the hydrogen acceptor present in plants is NADP (Nicotinamide adenine dinucleotide phosphate). It is reduced to $NADPH _2$ and an ADP molecule is converted to ATP in light by utilizing inorganic phosphate. This phenomenon is called as photophosphorylation.

The process of photolysis is light dependent split of water molecule releasing oxygen as a by product. The essential minerals for this process are calcium, manganese and chlorine.

Hill reaction occurs in light or can be called as light reaction. Hill first of all demonstrated that isolated chloroplasts can release oxygen in presence of suitable inorganic electron acceptors. The Hill reaction is the light-driven transfer of electrons from water to Hill reagents (non-physiological oxidants) against a chemical potential gradient. 

Photolysis of water refers to the light dependent split of water, which occurs during the light phase of photosynthesis in the chloroplast. The minerals essential for the process of photolysis are calcium, chlorine and manganese.

Photolysis of water occurs during non cyclic electron flow. During non cyclic electron flow, the ultimate source of electrons is water. The water molecule is split in a process, called as photolysis. The electrons are ultimately used in reduction of oxidised NADP to form reduced $NADPH _2$. The oxygen is released as a byproduct of photosynthesis. The mineral ions essential for photolysis are manganese, chlorine and calcium.

The process of photosynthesis occurs in two stages light reaction and dark reaction. The light reaction involves oxidation of water (photolysis i.e., light dependent splitting of water), which releases oxygen as a by product. The photolysis of water also releases electrons, which are used to drive electron flow in the thylakoid membrane ultimately this electron flow results in the synthesis of ATP and $NADPH _2$. These products ATP and $NADPH _2$ are used up subsequently in the process of dark reaction.

As we know that the PS-II or photosystem II constantly supplies electrons during the light reaction.

During the light-dependent reactions, the electrons released from the photolysis of water are used to reduce NADP+ to NADPH.

The process of water dissociation during photosynthesis is called as photo-oxidation of water. It requires the presence of Mn++, Ca+ and Cl- , a water oxidising enzyme and an unknown substance Z.

When the chlorophyll molecule is excited by light, the energy level of an electron in its structure is boosted to a higher energy level and this excited chlorophyll moves rapidly the reaction center of the photosystem I where it transfers its extra energy to an electron which is then expelled from the reaction center and is accepted by the first member of a chain of electron carriers and ultimately reaches NADP$^+$, reducing it to NADPH. The reaction center has lost an electron and this electron hole is filled by stripping electrons from water which leaves hydrogen ion (H$^+$) and molecular oxygen (O$ _2$). The pathway of electrons from water to NADP$^+$ has Z shape called as Z scheme. 

Photolysis, as the name suggests, is the breakdown of water in the presence of sunlight to form H⁺ and  O₂ and electron. This takes place during the light phase of photosynthesis in the PS-II. This creates the oxygen which is the net product of photolysis.

Chemiosmotic hypothesis explains how ATP is synthesized in the chloroplast. ATP synthesis is linked to the development of a proton gradient across the membrane of the thylakoid of chloroplast and proton accumulation is towards the inside of the membrane i.e., in the lumen. Source of protons within the chloroplasts is water. The splitting of water molecule takes place on the inner side of the membrane and so the protons (hydrogen ions) are produced, they accumulate within the lumen of the thylakoids. Thus, the correct answer is option A.

In the process of photosynthesis, water acts as a stable oxygen donor. To prove the theory radioisotopic technique was used. 

A)In 1930, C.B.Van Niel proved that the sulphur bacteria use H$ _2$S and CO$ _2$ to synthesize carbohydrates.

The oxygen in photosynthesis is released from photolysis of water in which water get split into hydrogen ions oxygen in the presence of sunlight.

Photolysis as the name suggests is the process, which happens in the presence of light. Photolysis is the process of splitting of water molecule to liberate oxygen in the presence of light. It takes place in the light phase of photosyntehsis and closely in proximation of PS-II.

Photolysis means to break down of the water molecule in the presence of light into H$^+$ nascent oxygen and an electron this process constitute in the PS II and the electron release is utilized by it as the compensation of electrons given by it to PS I during the Z scheme.

During this process, the electrons in water are removed and passed to the reaction center chlorophyll. The associated hydrogen ions are released from the water molecule, and after two water molecules are thus split, the oxygen atoms join to form molecular oxygen (O ₂ ), a waste product of photosynthesis.

Robin Hill demonstrated Hill reaction in 1939. Hill's reaction proves that oxygen is evolved during the light requiring steps of photosynthesis. It is isolated in chloroplasts.

A. Ruben and Kamen used the radioactive isotope of oxygen ($^1$$^8$O) in water and found that O$ _2$ liberated in the process was isotopically labelled. On the contrary, when $^1$$^8$O in CO$ _2$ was used, the evolved O$ _2$ did not show radioactivity.

Photolysis of water means splitting of water in the presence of light to get $H^+$ nascent oxygen and electron that compensate the loss of an electron from PS II during Z scheme and oxygen is released a the by-product with $H^+$ that acts as a proton and collects in the stroma.

Photolysis of water is mainly carried out to compensate the electrons that are transferred to photosystem I by photosystem II during the non-cyclic photophosphorylation that reduce NADP$^+$ into NADPH.

A) As the name suggests, photolysis of water requires light.

Photolysis of water: Occurs at grana i.e., lumen side of grana thylakoid membrane with the help of water splitting complex or OEC (oxygen evolving complex). This step is associated with PS- II of Z- scheme.

$2H _2O\rightarrow4{H}^++ O _2+4{e}^-$

It is believed that oxygen evolves as oxygen gas, electrons are accepted by PS II reaction centre through unknown substance and hydrogen ions temporarily stay in the thylakoid space.

Hence, the correct answer is 'Water'.

• The photolysis of water (H$ _2$O) in the light reactions of photosynthesis occurs in the water-splitting complex of photosystem II.
• photosystem II  in thylakoid membranes of chloroplasts where light is used to split water molecules as represented by the following equation: 2H$ _2$O → 4H+ + 4e- + O$ _2$.
• Oxygen (O$ _2$) is liberated to the atmosphere as a waste product in this process and the electrons (e-) are gained to replace those lost through photoexcitation from chlorophyll a in the reaction center of photosystem II which begins noncyclic photophosphorylation. Hence four electrons produced during photolysis of water will enter PS  II.
• So, the correct answer is 'PS  II'.

Photolysis is part of the light-dependent reactions of photosynthesis. The general reaction of photosynthetic photolysis can be given as 

H2AH2A + 2 photons (light) → 2 e−e− + 2 H+H+ + A

So, the correct option is 'Option D'.

A) Mg is responsible for the activation of many enzymes, but is not involved in photolysis. Cl ions help in the photolysis of water.

Non-cyclic phosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and then PS I. The two photosystems are connected through an electron transport chain. $NADPH$ is synthesized by this kind of electron flow. Electrons are transferred from the PS II and then passed on to the pigments of PS I. Then, electrons get excited and transferred to another acceptor. These electrons are then moved downhill again to a molecule of $NADP^{+}$. The addition of these electrons results in the formation of $NADPH$. Extrusion of the electron from reaction centre of PS II leaves a hole which is filled by electron released from $H _{2}O$. This is achieved by electrons available due to the splitting of water. The water splitting complex is associated with PS II, which itself is physically located on the inner side of the membrane of the thylakoid. Water is split into $H^{+}$, $O$ and electrons. The protons and oxygen formed by splitting of water is released within the lumen of the thylakoids. The oxygen produced is released as one of the net products of photosynthesis.

A) $ _{ \quad  }^{ 16 }{ O }$ is not used to study photolysis.
B) $ _{ \quad  }^{ 15 }{ O }$ is not used to study photolysis.

A) Mn and Ca are involved in the photolysis of water.
B) Mn, Cl and Ca are involved in the photolysis of water.

A) Absorption of light is carried out by photosynthetic pigments.

During ATP synthesis in the chloroplast, the proton gradient is formed. There are several processes that take place during activation of electrons and their transport which lead to the development of a proton gradient. Photolysis of water is the first process of this activation. The splitting of the water molecule takes place on the inner side of the thylakoid membrane and so the hydrogen ions (protons) that are produced, they accumulate within the lumen of the thylakoids. Oxygen-evolving complex called Z-enzyme is attached to the inner surface of the thylakoid membrane. The enzyme has four Mn ions. Light energized  Mn and removes electrons from $OH^{-}$ component of water forming oxygen. So, the essential element required for water splitting in photosynthesis leading to release of oxygen is Mn.

In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. In anoxygenic photosynthesis, various electron donors are used. Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation.

The oxygen evolution is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere.

In the light reaction of photosynthesis, chlorophyll molecule absorbs light energy, results in the splitting of water molecules, results in the release of protons, electrons and nascent oxygen. This process is called Photolysis of water. It occur in the lumen of thylakoid.

So,
the correct option is ‘photolysis of water’

Photolysis of water occurs at grana i.e., lumen side of grana thylakoid membrane with the help of water splitting complex or OEC (oxygen evolvong complex). This step is associated with PS- II of Z- scheme.

This part of photosynthesis occurs in the granum of a chloroplast where light is absorbed by chlorophyll; a type of photosynthetic pigment that converts the light to chemical energy. This reacts with water (H2O) and splits the oxygen and hydrogen molecules apart. In the first stage of photosynthesis light energy is absorbed by chlorophyll in plant cells. This light energy is then used to produce ATP and to split water into hydrogen and oxygen. The splitting of water is referred to as photolysis.