Tag: photosystems

Questions Related to photosystems

Evidence for the liberation of O$ _2$ from water during photosynthesis comes from

botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

  1. Release of $^{18}$O if water contains the same

  2. Isolated chloroplast supplied with reducing agent like potassium ferrocyanide evolves O$ _2$ even in absence of CO$ _2$

  3. Photosynthetic bacteria do not Iiberate O$ _2$ and they use H$ _2$S for obtaining reducing power

  4. All of the above

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Correct Option: D
Explanation:

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.

B. Hill and Bendall isolated chloroplast from leaves of Stellaria media and experimented in the absence of CO$ _2$.
$ 2A + 2H _2O\rightarrow 2AH _2+O _2$
In the presence of Artificial Hill reagent Potassium ferricyanide and sunlight, this experiment is performed.
C.  Van Niel observed that during bacterial photosynthesis hydrogen sulphide was broken down to hydrogen and sulphur. The hydrogen reduces carbon dioxide to synthesize carbohydrate and sulphur is accumulated as a waste product.
Hence, the correct answer is 'All the above'.

A specific function of light in the process of photosynthesis is

botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

  1. Reduction of $CO _2$

  2. Activate other molecules

  3. Split water molecules

  4. None of the above.

Show answer
Correct Option: C
Explanation:

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.

So the correct option is 'Split water molecules'.

In photosynthesis, photolysis of water is used in

botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

  1. Reduction of NADP$^+$

  2. Oxidation of NADP

  3. Oxidation of FAD

  4. None of the above

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Correct Option: D
Explanation:

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.

So the correct option is 'None of the above'.

Photolysis of water requires

botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

  1. Light

  2. Chlorophyll

  3. Both A and B

  4. Electron transport

Show answer
Correct Option: C
Explanation:

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

B) Photolysis is the breakdown of water into hydrogen and oxygen using the light during the photosynthesis process. Light energy is absorbed by chlorophyll and is used to regenerate adenosine tri-phosphate or ATP and split water. Photolysis of water results in the release of oxygen, as a by-product, and the release of hydrogen.
C) Both light and chlorophyll are required in the photolysis of water.
D) Electron transport is not required in the photolysis of water.
So the correct answer is 'Both A and B'.

Which of the following provides electrons for reduction reactions in photosynthesis?

botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

  1. NADPH

  2. Chlorophyll

  3. Cytochrome

  4. Water

Show answer
Correct Option: D
Explanation:

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'.

Four electrons produced during photolysis of water will enter

botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

  1. PS I

  2. PQ

  3. PS II

  4. PC

Show answer
Correct Option: C
Explanation:
    • 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'.

    In photosynthesis, splitting of water occurs during

    botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

    1. Cyclic photophosphorylation

    2. Non-cyclic photophosphorylation

    3. Oxidative photophosphorylation

    4. Calvin cycle

    Show answer
    Correct Option: B
    Explanation:
    The other pathway, non-cyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. Being a light reaction, non-cyclic photophosphorylation occurs in the thylakoid membrane. First, a water molecule is broken down into 2H+ + 1/2 O2 + 2e− by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H+and 1/2O2 are left out for further use. Then a photon is absorbed by chlorophyll pigments surrounding the reaction core centre of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, phaeophytin. 

    So, the correct option is 'Option B'.

    Photolysis of a water molecule yields

    botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

    1. 2 electrons and 4 protons

    2. 4 electrons and 4 protons

    3. 4 electrons and 2 protons

    4. 2 electrons and 2 protons

    Show answer
    Correct Option: D
    Explanation:

    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'.

    Minerals involved in photolysis are

    botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

    1. Mg and CI

    2. K and Mn

    3. Mo and Mn

    4. Mn and Cl

    Show answer
    Correct Option: D
    Explanation:

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

    B) K is involved in maintaining the anion-cation balance and activates several enzymes connected with phosphorylation and synthesis of chlorophyll. Mn helps in splitting water to liberate oxygen .
    C) Mo is a component of several enzymes like nitrogenase and nitrate reductase.
    D) Mn and Cl are involved in photolysis.
    So the correct answer is 'Mn and Cl'.

    Extrusion of electron from reaction centre of PS II leaves a hole which is filled by electron released from

    botany photosynthesis modern concept of photosynthesis photosystems photosynthesis in higher plants

    1. $H _2O$

    2. $CO _2$

    3. Chlorophyll

    4. Light

    Show answer
    Correct Option: A
    Explanation:

    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.

    $2H _{2}O$ $\longrightarrow$ $4H^{+}$ + $O _{2}$ + $4e^{-}$