Practice important
Chlorophyll Biosynthesis
MCQ questions and answers.
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MCQ Questions
1. A detached leaf incubated with cytokinin remains green longer than a control leaf. Which interpretation most directly links this observation to chlorophyll biosynthesis?
A.
Cytokinin enhances chlorophyll formation and delays the loss of chlorophyll during senescence
B.
Cytokinin converts chlorophyll into carotenoids, which are more stable
C.
Cytokinin prevents chlorophyll from absorbing red and blue light
D.
Cytokinin replaces chlorophyll with anthocyanin pigments in chloroplasts
Correct Answer:
Cytokinin enhances chlorophyll formation and delays the loss of chlorophyll during senescence
Explanation:
Cytokinins promote chloroplast development and chlorophyll biosynthesis while also retarding senescence-associated chlorophyll degradation, causing leaves to remain green.
2. In a cytokinin-treated dark-grown seedling, chlorophyll accumulation occurs only after exposure to light. Which conclusion is most appropriate?
A.
Cytokinin can enhance chlorophyll biosynthetic competence, but light-dependent processes may still be required for full chloroplast development
B.
Cytokinin directly substitutes for light in every step of chlorophyll biosynthesis
C.
Cytokinin prevents the conversion of etioplasts into chloroplasts
D.
Cytokinin causes chlorophyll to accumulate independently of plastid development
Correct Answer:
Cytokinin can enhance chlorophyll biosynthetic competence, but light-dependent processes may still be required for full chloroplast development
Explanation:
Cytokinin supports chloroplast differentiation and chlorophyll formation, but it does not universally replace the requirement for light in light-regulated chloroplast development and pigment accumulation.
3. Two identical leaf discs are supplied with cytokinin. One receives a transcription inhibitor, whereas the other does not. If chlorophyll accumulation is greatly reduced only in the inhibitor-treated disc, what does this suggest?
A.
Cytokinin-mediated chlorophyll biosynthesis depends substantially on altered gene expression
B.
Cytokinin promotes chlorophyll formation exclusively by increasing water uptake
C.
Cytokinin destroys chlorophyll unless transcription is blocked
D.
Cytokinin acts only by increasing the optical reflectance of the leaf
Explanation:
The inhibition of the cytokinin response by a transcription inhibitor indicates that cytokinin stimulation of chlorophyll biosynthesis involves regulation of genes associated with chloroplast development or pigment formation.
4. A cytokinin-treated leaf shows increased chlorophyll content per unit area, but chlorophyll content per chloroplast remains unchanged. Which explanation best fits these data?
A.
Cytokinin increased chloroplast number or chloroplast development without increasing pigment concentration within each chloroplast
B.
Cytokinin converted chlorophyll into a more concentrated form without changing plastid characteristics
C.
Cytokinin eliminated chloroplasts but increased chlorophyll in the cytosol
D.
Cytokinin reduced leaf area while causing complete inhibition of chloroplast formation
Correct Answer:
Cytokinin increased chloroplast number or chloroplast development without increasing pigment concentration within each chloroplast
Explanation:
An increase in total chlorophyll per leaf area with unchanged chlorophyll per chloroplast is consistent with cytokinin-promoted chloroplast development or increased plastid abundance.
5. Which experimental result would provide the strongest evidence that cytokinin stimulates chlorophyll biosynthesis rather than merely slowing chlorophyll degradation?
A.
A cytokinin-treated leaf shows increased incorporation of a labeled chlorophyll precursor into newly formed chlorophyll
B.
A cytokinin-treated leaf remains green for a longer period after detachment
C.
A cytokinin-treated leaf has lower activity of chlorophyll-degrading enzymes
D.
A cytokinin-treated leaf loses chlorophyll more slowly during darkness
Correct Answer:
A cytokinin-treated leaf shows increased incorporation of a labeled chlorophyll precursor into newly formed chlorophyll
Explanation:
Increased incorporation of a labeled precursor directly demonstrates enhanced synthesis of new chlorophyll, whereas prolonged greenness or reduced degradation could occur without increased biosynthesis.
6. A mutant plant fails to increase chlorophyll content after cytokinin treatment but responds normally to light by developing chloroplasts. Which defect is most specifically suggested?
A.
A defect in cytokinin-dependent regulation of chlorophyll biosynthetic pathways
B.
A complete inability to perceive light
C.
A defect in the structural formation of cell walls
D.
A constitutive increase in chlorophyll degradation caused by excess light
Correct Answer:
A defect in cytokinin-dependent regulation of chlorophyll biosynthetic pathways
Explanation:
Normal light-induced chloroplast development but failure of the cytokinin-induced chlorophyll increase indicates a defect in the cytokinin signaling or response pathway connected with chlorophyll biosynthesis.
7. When cytokinin is applied to a senescing leaf, which combination most strongly indicates stimulation of chlorophyll biosynthesis rather than only pigment retention?
A.
Increased expression of chlorophyll biosynthetic genes together with increased chlorophyll precursor incorporation
B.
Reduced leaf respiration together with unchanged chlorophyll precursor incorporation
C.
Reduced chlorophyll-degrading enzyme activity together with unchanged biosynthetic gene expression
D.
Delayed yellowing together with reduced formation of chloroplasts
Correct Answer:
Increased expression of chlorophyll biosynthetic genes together with increased chlorophyll precursor incorporation
Explanation:
Gene activation and enhanced incorporation of precursors into chlorophyll provide direct evidence for increased biosynthesis, whereas delayed yellowing alone may result only from reduced degradation.
8. A cytokinin concentration series produces the following response: chlorophyll content rises from low to intermediate concentrations but falls at the highest concentration. Which conclusion is most scientifically justified?
A.
Cytokinin promotes chlorophyll accumulation within an optimal range, whereas excessive cytokinin may inhibit normal chloroplast or pigment metabolism
B.
Chlorophyll biosynthesis is completely independent of cytokinin concentration
C.
The highest cytokinin concentration must always produce the greatest chlorophyll content
D.
Cytokinin changes chlorophyll into a nonpigmented form only at low concentrations
Correct Answer:
Cytokinin promotes chlorophyll accumulation within an optimal range, whereas excessive cytokinin may inhibit normal chloroplast or pigment metabolism
Explanation:
Hormonal effects are often concentration-dependent. Moderate cytokinin levels can stimulate chloroplast development and chlorophyll biosynthesis, while excessive levels may produce inhibitory or toxic effects.
9. A cytokinin-treated leaf develops more extensive internal chloroplast membrane systems and has a higher chlorophyll content than an untreated leaf. What relationship is best inferred?
A.
Cytokinin-associated chloroplast development is coordinated with increased chlorophyll accumulation
B.
Chlorophyll accumulation necessarily causes chloroplast membranes to disappear
C.
Cytokinin promotes membrane formation only after chlorophyll has been removed
D.
Chloroplast membrane development is unrelated to pigment biosynthesis in the leaf
Correct Answer:
Cytokinin-associated chloroplast development is coordinated with increased chlorophyll accumulation
Explanation:
Chlorophyll is integrated into photosynthetic membrane systems. Cytokinin-promoted chloroplast development commonly involves coordinated development of internal membranes and increased chlorophyll formation.
10. A leaf treated with cytokinin has higher chlorophyll content than a control even though both leaves have identical chlorophyll degradation rates. Which conclusion follows most directly?
A.
The cytokinin-treated leaf must have a higher net rate of chlorophyll synthesis
B.
Cytokinin must have inhibited chloroplast formation in the treated leaf
C.
The control leaf must have synthesized more chlorophyll than the treated leaf
D.
The difference can be explained only by cytokinin-induced conversion of chlorophyll into carotenoids
Correct Answer:
The cytokinin-treated leaf must have a higher net rate of chlorophyll synthesis
Explanation:
If degradation rates are equal but chlorophyll content is higher after cytokinin treatment, the difference is attributable to greater chlorophyll production, consistent with stimulation of chlorophyll biosynthesis.
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