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Photosynthesis in Higher Plants

Light Reactions Practice Questions

20 free Light Reactions practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 20 Medium

Which of the following is the primary photosynthetic pigment that forms the reaction center in both Photosystem I and Photosystem II?

  1. A Chlorophyll b
  2. B Chlorophyll a
  3. C Xanthophylls
  4. D Carotenoids

Correct answer: Chlorophyll a

Chlorophyll a is the essential pigment that forms the reaction center, capable of converting light energy into chemical energy. Other pigments like chlorophyll b and carotenoids act as accessory pigments that harvest light and transfer it to chlorophyll a.

Question 2 of 20 Medium

During non-cyclic photophosphorylation, which molecule acts as the ultimate source of electrons to replace those lost by Photosystem II?

  1. A Carbon dioxide
  2. B NADP+
  3. C Water
  4. D ATP

Correct answer: Water

Water undergoes photolysis (splitting) associated with the PS II complex on the inner side of the thylakoid membrane. This process releases electrons, protons, and oxygen, ensuring a continuous supply of electrons to PS II.

Question 3 of 20 Medium

In the Z-scheme of light reactions, the movement of electrons from PS II to PS I occurs through which of the following?

  1. A Cytochrome b6f complex
  2. B ATP synthase complex
  3. C Rubisco enzyme complex
  4. D Ferredoxin-NADP reductase

Correct answer: Cytochrome b6f complex

Electrons from PS II pass through the cytochrome b6f complex in the electron transport chain. This complex facilitates the transfer of electrons onward to PS I via plastocyanin.

Question 4 of 20 Medium

Where is the Oxygen Evolving Complex (OEC) physically located in the chloroplast?

  1. A Outer chloroplast membrane
  2. B Thylakoid stroma lamellae
  3. C Inner thylakoid membrane face
  4. D Chloroplast intermembrane space

Correct answer: Inner thylakoid membrane face

The Oxygen Evolving Complex is located on the inner (lumenal) side of the thylakoid membrane. This positioning ensures that the protons released during water photolysis contribute to the proton gradient within the lumen.

Question 5 of 20 Medium

Cyclic photophosphorylation results in the synthesis of which of the following products?

  1. A ATP and NADPH only
  2. B ATP only
  3. C NADPH and Oxygen only
  4. D ATP, NADPH, and Oxygen

Correct answer: ATP only

Cyclic photophosphorylation involves only PS I and the electron is circulated back to the system. Since PS II and NADP reductase are absent in the stroma lamellae where this occurs, only ATP is synthesized, and no NADPH or oxygen is produced.

Question 6 of 20 Medium

The absorption peak for the reaction center of Photosystem I (P700) occurs at which wavelength?

  1. A 660 nm
  2. B 680 nm
  3. C 700 nm
  4. D 720 nm

Correct answer: 700 nm

Photosystem I is named P700 because its reaction center chlorophyll a molecule has an absorption peak at 700 nm. Similarly, PS II is known as P680 due to its peak at 680 nm.

Question 7 of 20 Medium

What is the primary role of accessory pigments like carotenoids during the light reactions?

  1. A Fixing CO2 into glucose in the stroma
  2. B Protecting chlorophyll a from photo-oxidation
  3. C Splitting water molecules in the thylakoid
  4. D Synthesizing ATP directly from ADP

Correct answer: Protecting chlorophyll a from photo-oxidation

Accessory pigments broaden the spectrum of light absorbed for photosynthesis and transfer that energy to chlorophyll a. They also play a crucial role in protecting chlorophyll a from damage by intense light, known as photo-oxidation.

Question 8 of 20 Medium

According to the Chemiosmotic Hypothesis, what drives the synthesis of ATP in the chloroplast?

  1. A Active transport of glucose across the membrane
  2. B Reduction of NADP+ by ferredoxin
  3. C Proton gradient across the thylakoid membrane
  4. D Breakdown of water in the stroma matrix

Correct answer: Proton gradient across the thylakoid membrane

ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane. The breakdown of this gradient through the CF0-CF1 ATPase complex provides the energy necessary to synthesize ATP from ADP and inorganic phosphate.

Question 9 of 20 Medium

During the light reaction, protons (H+) accumulate in which part of the chloroplast?

  1. A Stroma
  2. B Thylakoid lumen
  3. C Periplasmic space
  4. D Cytoplasm

Correct answer: Thylakoid lumen

Protons accumulate in the lumen of the thylakoids due to the photolysis of water on the inner membrane side and the transport of protons from the stroma by the cytochrome complex. This creates the high H+ concentration needed for chemiosmosis.

Question 10 of 20 Medium

Which enzyme is responsible for the final step of non-cyclic electron transport, reducing NADP+ to NADPH?

  1. A ATP synthase
  2. B NADP reductase
  3. C PEP carboxylase
  4. D Cytochrome oxidase

Correct answer: NADP reductase

NADP reductase is located on the stroma side of the thylakoid membrane. It facilitates the transfer of electrons from ferredoxin to NADP+, along with protons from the stroma, to form NADPH.

Question 11 of 20 Medium

The 'Z-scheme' describes the pathway of electron flow from ________ to ________.

  1. A PS I to PS II
  2. B Water to NADP+
  3. C CO2 to Glucose
  4. D ATP to ADP

Correct answer: Water to NADP+

The Z-scheme illustrates the whole scheme of transfer of electrons, starting from the splitting of water to PS II, up the electron transport chain to PS I, and finally down to NADP+. The characteristic shape is formed when carriers are placed according to their redox potential.

Question 12 of 20 Medium

Stroma lamellae membranes lack which of the following components?

  1. A PS I and PS II reaction centers
  2. B PS II and NADP reductase enzyme
  3. C PS I and the ATP synthase complex
  4. D Only Photosystem I (PS I)

Correct answer: PS II and NADP reductase enzyme

While grana lamellae have both PS I and PS II, the stroma lamellae lack PS II and the enzyme NADP reductase. This is why stroma lamellae are only capable of cyclic photophosphorylation.

Question 13 of 20 Medium

Which metal ion is a vital constituent of the chlorophyll molecule's porphyrin ring?

  1. A Iron
  2. B Manganese
  3. C Magnesium
  4. D Copper

Correct answer: Magnesium

Magnesium (Mg) is the central metal ion in the porphyrin head of the chlorophyll molecule. It is essential for the pigment's ability to absorb light energy effectively.

Question 14 of 20 Medium

In the thylakoid membrane, the CF1 part of the ATP synthase is located on:

  1. A The side facing the lumen
  2. B The side facing the stroma
  3. C Embedded within the lipid bilayer
  4. D The outer chloroplast membrane

Correct answer: The side facing the stroma

The ATP synthase consists of a CF0 portion embedded in the membrane and a CF1 portion that protrudes into the stroma. The synthesis of ATP occurs on the CF1 headpiece facing the stroma.

Question 15 of 20 Medium

Which of the following is NOT a part of the Light Harvesting Complex (LHC)?

  1. A Reaction center
  2. B Antenna molecules
  3. C Accessory pigments
  4. D Proteins

Correct answer: Reaction center

The LHC (or antennae) is composed of hundreds of pigment molecules bound to proteins, but it does not include the single reaction center chlorophyll a molecule. The LHC plus the reaction center together make up a photosystem.

Question 16 of 20 Medium

When only PS I is functional, the electron is cycled back to the PS I complex from which electron carrier?

  1. A Plastoquinone
  2. B Oxygen
  3. C Ferredoxin
  4. D NADP+

Correct answer: Ferredoxin

In cyclic photophosphorylation, excited electrons from P700 are passed to ferredoxin and then cycled back to the cytochrome b6f complex and eventually back to P700, rather than being used to reduce NADP+.

Question 17 of 20 Medium

How many protons are required to be pumped through the ATP synthase to produce one molecule of ATP in the chloroplast?

  1. A 1
  2. B 2
  3. C 4
  4. D 10

Correct answer: 2

According to the standard NCERT interpretation of chemiosmosis in plants, for every ATP synthesized, 2H+ (protons) pass through the CF0 channel from the lumen to the stroma down the electrochemical gradient.

Question 18 of 20 Medium

Which of the following describes the correct sequence of electron flow in non-cyclic photophosphorylation?

  1. A P700 → Pheophytin → P680 → NADP+
  2. B P680 → Electron Transport Chain → P700 → NADP+
  3. C P700 → P680 → Plastoquinone → NADPH
  4. D Water → P700 → Cytochromes → P680

Correct answer: P680 → Electron Transport Chain → P700 → NADP+

Electrons are first excited in PS II (P680), pass through a series of electron carriers (ETS), reach PS I (P700), are excited again, and finally reduce NADP+ to NADPH.

Question 19 of 20 Medium

Manganese and Chloride ions are primarily associated with which process in photosynthesis?

  1. A Carbon dioxide fixation
  2. B Photolysis of water
  3. C Cyclic electron transfer
  4. D ATP hydrolysis

Correct answer: Photolysis of water

Manganese (Mn), Calcium (Ca), and Chloride (Cl-) ions are essential cofactors for the Oxygen Evolving Complex (OEC) associated with PS II, which facilitates the splitting of water into protons, electrons, and oxygen.

Question 20 of 20 Medium

Which pigment appears as blue-green in a chromatogram?

  1. A Chlorophyll a
  2. B Chlorophyll b
  3. C Xanthophyll
  4. D Carotenoid

Correct answer: Chlorophyll a

Chromatographic separation of leaf pigments shows that Chlorophyll a appears blue-green, Chlorophyll b appears yellow-green, Xanthophylls appear yellow, and Carotenoids appear yellow to yellow-orange.

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