101. A chloroplast preparation is able to produce ATP and NADPH in light but cannot convert carbon dioxide into carbohydrate. Which structure is most likely functionally absent or inactive?
ⓐ. Stroma enzymes needed for the dark reaction
ⓑ. Thylakoid pigments needed for light absorption
ⓒ. Chlorophyll molecules of the photosystems
ⓓ. Thylakoid membranes required for the light reaction
Correct Answer: Stroma enzymes needed for the dark reaction
Explanation: If ATP and NADPH are being produced in light, that shows that the light reaction machinery is functioning properly. Therefore, the thylakoid membranes, pigments, and photosystems must still be active enough to support energy conversion. The problem lies in the inability to use carbon dioxide to synthesize carbohydrate. That step belongs to the dark reaction, which occurs in the stroma and requires specific enzymes. If those enzymes are absent or inactive, carbon fixation cannot proceed even though the light reaction continues. This separates the two phases clearly in terms of function. Hence, inactive or missing stroma enzymes are the most likely explanation.
102. Which statement is scientifically correct about the timing of the two major phases of photosynthesis?
ⓐ. Both phases can proceed only in darkness
ⓑ. The light reaction can proceed without light if glucose is available
ⓒ. The light reaction requires light directly, whereas the dark reaction may proceed when its required products are available
ⓓ. The dark reaction must always stop immediately whenever sunlight is present
Correct Answer: The light reaction requires light directly, whereas the dark reaction may proceed when its required products are available
Explanation: The light reaction is directly dependent on light because its main role is to capture and transform light energy. The dark reaction does not directly absorb light and instead depends on ATP and NADPH produced earlier or simultaneously by the light reaction. This means the second phase is controlled more by the availability of these chemical resources than by the simple presence or absence of light itself. The two phases are therefore linked but not identical in their timing requirements. This is why the term light-independent is more accurate than night-only. So the scientifically correct statement is the one given in option C.
103. Which situation best shows the immediate connection between the light reaction and carbon fixation?
ⓐ. Blocking ATP and NADPH formation reduces the plant’s ability to synthesize organic compounds from carbon dioxide
ⓑ. Increasing root hair number directly creates new photosystems in the chloroplast
ⓒ. Closing flowers at night causes direct splitting of water in the thylakoids
ⓓ. Storing starch in seeds activates pigment molecules in mature leaves
Correct Answer: Blocking ATP and NADPH formation reduces the plant’s ability to synthesize organic compounds from carbon dioxide
Explanation: Carbon fixation in the dark reaction depends on ATP and NADPH generated during the light reaction. If the production of these molecules is blocked, the chloroplast loses both the energy source and reducing power needed to convert carbon dioxide into organic compounds. This gives a direct demonstration of how the two phases are functionally linked. The other options involve unrelated plant processes and do not show the specific dependence of carbon fixation on light-reaction products. The relationship is biochemical rather than merely structural. Therefore, blocking ATP and NADPH formation best demonstrates the immediate connection between the two phases.
104. Which comparison between light reaction and dark reaction is correct?
ⓐ. Both occur mainly in the thylakoid membrane and both release oxygen directly
ⓑ. Light reaction occurs mainly in the stroma, whereas dark reaction occurs in the grana lumen
ⓒ. Light reaction is chiefly photochemical, whereas dark reaction is chiefly synthetic
ⓓ. Light reaction forms carbon dioxide, whereas dark reaction uses oxygen as its main substrate
Correct Answer: Light reaction is chiefly photochemical, whereas dark reaction is chiefly synthetic
Explanation: The light reaction is chiefly photochemical because it begins with the absorption of light and the conversion of that energy into ATP and NADPH. The dark reaction is chiefly synthetic because it uses those products to build organic compounds from carbon dioxide. This makes the two phases different in both role and nature. One captures and converts energy, while the other uses that stored energy to synthesize food. They are therefore complementary phases of one overall process. The correct comparison is the one that identifies photochemical versus synthetic roles. Hence, option C is accurate.
105. A student says, “Photosystem I and Photosystem II are just two names for the same structure.” Which response is best?
ⓐ. The statement is correct because both terms refer to the chloroplast envelope
ⓑ. The statement is incorrect because they are two distinct major photosystems that participate in the light reaction
ⓒ. The statement is correct because both names are used interchangeably for chlorophyll b
ⓓ. The statement is incorrect because one belongs to roots and the other belongs to leaves
Correct Answer: The statement is incorrect because they are two distinct major photosystems that participate in the light reaction
Explanation: Photosystem I and Photosystem II are not two labels for the same structure. They are distinct major photosystems present in the thylakoid membrane and both participate in the light reaction of photosynthesis. Each functions as a pigment-protein complex involved in light absorption and energy conversion. Their coordinated activity helps drive the formation of useful chemical products during the light-dependent phase. The misconception usually arises because both belong to the same broad process. However, being part of one process does not make them identical structures. Therefore, the correct response is that they are two distinct major photosystems involved in the light reaction.
106. What is the light harvesting complex in a photosystem?
ⓐ. A group of root cells that absorb minerals for chloroplast activity
ⓑ. A cluster of pigment molecules that captures light and passes energy to the reaction centre
ⓒ. A set of enzymes in the stroma that fixes carbon dioxide into sugars
ⓓ. A membrane channel that releases oxygen from the chloroplast
Correct Answer: A cluster of pigment molecules that captures light and passes energy to the reaction centre
Explanation: The light harvesting complex, also called the antenna complex, is made of many pigment molecules associated with proteins in the thylakoid membrane. Its main role is to absorb incoming light energy efficiently from the environment. The captured energy is not used directly at every pigment molecule but is transferred toward the reaction centre. This arrangement increases the ability of the photosystem to collect light over a wider area. It therefore improves the efficiency of the light reaction. The complex acts as an energy-gathering system for the photosynthetic apparatus. That is why it is correctly described as a pigment cluster that captures light and passes energy to the reaction centre.
107. In a photosystem, the pigments of the antenna complex mainly function to:
ⓐ. absorb light and funnel excitation energy to the reaction centre
ⓑ. transport sucrose from the chloroplast to the phloem
ⓒ. split carbon dioxide into carbon and oxygen atoms
ⓓ. synthesize ATP directly without membrane involvement
Correct Answer: absorb light and funnel excitation energy to the reaction centre
Explanation: The antenna pigments in a photosystem are specialized for capturing light energy and directing it inward to the reaction centre chlorophyll a. This transfer allows light absorbed at many different pigment molecules to contribute to one central photochemical event. Such organization greatly improves the efficiency of photosynthesis because the plant can harvest more of the available light. The pigments themselves do not carry out carbon fixation or long-distance food transport. Their role is specifically related to light absorption and energy transfer. This is why the phrase “funnel excitation energy to the reaction centre” best describes their function. It captures the central idea of how antenna pigments assist photosynthesis.
108. The reaction centre of a photosystem is best described as:
ⓐ. the outer chloroplast membrane where carbon dioxide enters
ⓑ. the stroma region where glucose is stored temporarily
ⓒ. the air space between mesophyll cells for gas movement
ⓓ. a special chlorophyll a molecule that receives excitation energy and starts the primary photochemical event
Correct Answer: a special chlorophyll a molecule that receives excitation energy and starts the primary photochemical event
Explanation: The reaction centre is the core functional part of a photosystem where the absorbed light energy is finally directed. It is formed by a special chlorophyll a molecule that can undergo the primary photochemical event after receiving excitation energy from surrounding pigments. This makes it different from the many other pigments that mainly gather and transfer energy. The reaction centre is therefore the site where energy capture becomes chemically significant. It plays a central role in initiating the light reaction. Without this special chlorophyll a, the absorbed light energy would not be effectively converted into a usable form. Hence, this description correctly defines the reaction centre.
109. The reaction centre chlorophyll of Photosystem I is known as:
ⓐ. $P_{680}$
ⓑ. $P_{760}$
ⓒ. $P_{700}$
ⓓ. $P_{600}$
Correct Answer: $P_{700}$
Explanation: Photosystem I contains a special reaction centre chlorophyll a known as $P_{700}$. The name reflects the wavelength of light at which this reaction centre shows a characteristic absorption maximum, around $700$ nanometres. This pigment is central to the functioning of Photosystem I in the light reaction. It receives excitation energy from the associated light harvesting pigments and participates in the primary photochemical process. The identity of $P_{700}$ helps distinguish Photosystem I from Photosystem II. This difference is one of the most important foundational facts in the study of light reactions. Therefore, $P_{700}$ is the correct name for the reaction centre chlorophyll of Photosystem I.
110. The reaction centre chlorophyll of Photosystem II is called:
ⓐ. $P_{700}$
ⓑ. $P_{750}$
ⓒ. $P_{650}$
ⓓ. $P_{680}$
Correct Answer: $P_{680}$
Explanation: Photosystem II contains the special reaction centre chlorophyll a known as $P_{680}$. This name is based on its characteristic absorption maximum near $680$ nanometres. It serves as the photochemically active centre of Photosystem II and receives energy transferred from the antenna pigments. The distinction between $P_{680}$ and $P_{700}$ is essential for understanding the two major photosystems of higher plants. $P_{680}$ specifically belongs to Photosystem II and not to Photosystem I. This unique identity helps clarify how the two systems differ while still working together in the light reaction. Therefore, $P_{680}$ is the correct answer.
111. Why is the reaction centre of Photosystem I called $P_{700}$?
ⓐ. Because it contains 700 pigment molecules in its antenna complex
ⓑ. Because it is present only in 700 types of plants
ⓒ. Because it absorbs light most effectively near $700,\text{nm}$
ⓓ. Because it is located 700 layers below the leaf surface
Correct Answer: Because it absorbs light most effectively near $700,\text{nm}$
Explanation: The term $P_{700}$ refers to the characteristic wavelength near which the reaction centre chlorophyll of Photosystem I shows maximum absorption. The number does not indicate the number of pigment molecules or the position of the photosystem in the leaf. It is simply a naming convention based on absorption properties. This type of naming helps scientists distinguish reaction centres of different photosystems clearly. Since $P_{700}$ belongs to Photosystem I, it provides a straightforward way to identify that system. Understanding this naming rule is important for clarity in photosynthesis. Hence, it is called $P_{700}$ because it absorbs most effectively near $700$ nanometres.
112. Why is the reaction centre of Photosystem II called $P_{680}$?
ⓐ. Because it is the 680th chlorophyll molecule discovered
ⓑ. Because it absorbs light most effectively near $680,\text{nm}$
ⓒ. Because it contains 680 proteins in one photosystem
ⓓ. Because it occurs only in plants growing at $680,\text{m}$ altitude
Correct Answer: Because it absorbs light most effectively near $680,\text{nm}$
Explanation: $P_{680}$ is named according to the wavelength near which its reaction centre chlorophyll a shows a major absorption maximum. This value is around $680$ nanometres, and that is the origin of the number in its name. The term does not refer to how many molecules are present or where the plant grows. The naming system is based on a functional property of the pigment. This makes the distinction between Photosystem I and Photosystem II easier to remember and understand. The reaction centre of Photosystem II is therefore identified as $P_{680}$ for this reason. Thus, the correct explanation is its absorption maximum near $680$ nanometres.
113. Which statement correctly matches the two major photosystems with their reaction centres?
ⓐ. Photosystem I — $P_{680}$; Photosystem II — $P_{700}$
ⓑ. Photosystem I — $P_{700}$; Photosystem II — $P_{680}$
ⓒ. Photosystem I — chlorophyll b; Photosystem II — xanthophyll
ⓓ. Photosystem I — carotene; Photosystem II — chlorophyll c
Correct Answer: Photosystem I — $P_{700}$; Photosystem II — $P_{680}$
Explanation: The two major photosystems in higher plants are distinguished by the names of their reaction centre chlorophyll a molecules. Photosystem I contains $P_{700}$, while Photosystem II contains $P_{680}$. This difference is one of the most basic and important facts about the organization of the light reaction. It does not mean that the two photosystems lack other pigments, but their defining reaction centres are different. The names are linked with their characteristic absorption maxima. This allows each photosystem to be identified clearly in biological study. Therefore, the correct matching is Photosystem I with $P_{700}$ and Photosystem II with $P_{680}$.
114. In a photosystem, most pigments other than the reaction centre chlorophyll a mainly act as:
ⓐ. electron acceptors only
ⓑ. dark-reaction enzymes
ⓒ. membrane lipids
ⓓ. antenna pigments
Correct Answer: antenna pigments
Explanation: In each photosystem, one special chlorophyll a molecule functions as the reaction centre, while many surrounding pigments mainly serve as antenna pigments. These pigments capture light energy and transfer excitation energy inward to the reaction centre. Their role is therefore supportive but highly important for efficient light harvesting. They do not serve as the main site of carbon fixation or as enzyme systems of the dark reaction. Their organization allows the photosystem to gather light from many molecules instead of relying on a single pigment alone. This makes energy capture much more efficient. Hence, most pigments other than the reaction centre act as antenna pigments.
115. The main advantage of having many pigment molecules in the light harvesting complex is that they:
ⓐ. increase the range and efficiency of light capture
ⓑ. convert starch directly into ATP in sunlight
ⓒ. eliminate the need for a reaction centre chlorophyll
ⓓ. store oxygen safely inside the grana
Correct Answer: increase the range and efficiency of light capture
Explanation: A large number of pigment molecules in the light harvesting complex allows a photosystem to absorb light more effectively. Different pigments can absorb somewhat different wavelengths, so together they widen the range of usable light. They also increase the probability that incoming photons will be captured. The absorbed energy is then passed to the reaction centre where the primary photochemical event begins. This arrangement makes photosynthesis more efficient under natural light conditions. It is one of the major structural advantages of the photosynthetic apparatus. Therefore, the main advantage is increased range and efficiency of light capture.
116. Which pigment is directly responsible for the reaction centre function in both Photosystem I and Photosystem II?
ⓐ. Xanthophyll
ⓑ. Carotene
ⓒ. Chlorophyll a
ⓓ. Chlorophyll b
Correct Answer: Chlorophyll a
Explanation: In both Photosystem I and Photosystem II, the reaction centre is formed by a special chlorophyll a molecule. Although chlorophyll b and carotenoids may be present in the antenna system and help absorb light, the actual reaction centre role belongs to chlorophyll a. This makes chlorophyll a central not only as a major pigment but also as the direct initiator of the primary photochemical event. The reaction centre names $P_{700}$ and $P_{680}$ both refer to forms of chlorophyll a. This is a crucial point in understanding pigment organization within photosystems. The other pigments assist but do not replace this central role. Therefore, chlorophyll a is the correct answer.
117. Light absorbed by an accessory pigment in the antenna complex is usually passed to the reaction centre in the form of:
ⓐ. mineral ions
ⓑ. excitation energy
ⓒ. starch granules
ⓓ. water molecules
Correct Answer: excitation energy
Explanation: When an accessory pigment in the antenna complex absorbs light, it does not usually perform the main photochemical step itself. Instead, it transfers the absorbed energy to nearby pigments and ultimately to the reaction centre chlorophyll a. This transfer occurs as excitation energy. Such pigment interaction allows the entire photosystem to function as a coordinated light-harvesting unit. The process is essential because it ensures that energy captured by many molecules can be used at one reaction centre. This improves the efficiency of photosynthesis without requiring every pigment to act independently. Therefore, the energy is passed in the form of excitation energy.
118. Which statement best describes pigment interaction within a light harvesting complex?
ⓐ. Pigments compete with one another to block light from reaching the reaction centre
ⓑ. Pigments transfer absorbed energy stepwise toward the reaction centre chlorophyll
ⓒ. Pigments synthesize carbon dioxide before the light reaction begins
ⓓ. Pigments move physically through the membrane to deliver electrons
Correct Answer: Pigments transfer absorbed energy stepwise toward the reaction centre chlorophyll
Explanation: Pigment interaction in the light harvesting complex involves the transfer of absorbed energy from one pigment molecule to another. This stepwise movement directs the excitation energy toward the reaction centre chlorophyll, where the primary photochemical event can occur. The pigments do not need to move physically or manufacture carbon dioxide. Instead, their interaction is based on organized energy transfer within the photosystem. This arrangement allows even light absorbed at peripheral pigments to contribute to photosynthetic work. It is one of the main reasons the antenna system is so efficient. Thus, pigment interaction is best described as stepwise transfer of absorbed energy toward the reaction centre.
119. Which statement is correct about the organization of pigments in a photosystem?
ⓐ. Every pigment molecule acts independently as a separate reaction centre
ⓑ. Only carotenoids are present in the antenna complex of all photosystems
ⓒ. Photosystems contain one reaction centre surrounded by many light-harvesting pigments
ⓓ. The reaction centre lies outside the thylakoid membrane and receives no energy transfer
Correct Answer: Photosystems contain one reaction centre surrounded by many light-harvesting pigments
Explanation: A photosystem is an organized unit in which many pigment molecules collect light and one special reaction centre chlorophyll carries out the primary photochemical role. This means the structure is cooperative rather than a set of isolated pigments acting independently. The surrounding pigments form the light-harvesting system and deliver excitation energy to the central reaction centre. Such organization allows photosynthesis to be efficient even when light is absorbed away from the centre. It also explains why multiple pigments are present in the thylakoid membrane. Therefore, the correct statement is that one reaction centre is surrounded by many light-harvesting pigments.
120. Although it is called Photosystem II, this photosystem is named so even though:
ⓐ. it contains fewer pigments than Photosystem I
ⓑ. it was discovered after Photosystem I
ⓒ. it functions only in darkness
ⓓ. it generally acts before Photosystem I in the light reaction sequence
Correct Answer: it generally acts before Photosystem I in the light reaction sequence
Explanation: The numbering of Photosystem I and Photosystem II is historical and does not indicate the exact order in which they act in the overall sequence of light reactions. In the usual functional sequence, Photosystem II acts before Photosystem I. This often confuses students because the name might suggest the reverse order. Understanding this point is important for building a correct conceptual framework of the light reaction. The term “II” therefore should not be interpreted as meaning second in functional operation. It is simply the assigned name of that photosystem. Hence, the correct statement is that Photosystem II generally acts before Photosystem I.
