Plant Phsysiology: MCQ Test

Test your knowledge of photosynthesis with these multiple-choice questions designed for competitive exams, covering every aspect and intricate detail of the topic.

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Showing questions 41 - 48 of 48

41. How does water stress primarily affect the rate of photosynthesis in plants?

It directly inhibits the enzymes of the light reaction.
It causes stomata to close, thereby reducing the availability of CO2.
It increases the rate of photorespiration, reducing efficiency.
It leads to the breakdown of chlorophyll molecules.

Water stress causes the stomata to close hence reducing the CO2 availability. Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.

42. Which component of the Z scheme is responsible for the splitting of water molecules to release oxygen, protons, and electrons?

Photosystem I (PS I)
Photosystem II (PS II)
Electron transport chain (ETS)
NADP reductase enzyme

The electrons that were moved from photosystem II must be replaced. This is achieved by electrons available due to splitting of water. The splitting of water is associated with the PS II; water is split into 2H+, [O] and electrons.

43. The NADP reductase enzyme, crucial for the reduction of NADP+ to NADPH + H+ in the light reaction, is located on which side of the thylakoid membrane?

The lumen side
The inner side
The stroma side
Embedded within the membrane

The NADP reductase enzyme is located on the stroma side of the membrane.

44. Which of the following describes the most abundant enzyme in the world?

PEP carboxylase, found mainly in C4 plant mesophyll cells.
ATP synthase, responsible for generating ATP.
RuBisCO, characterized by its ability to bind to both CO2 and O2.
NADP reductase, involved in NADPH synthesis.

RuBisCO that is the most abundant enzyme in the world is characterised by the fact that its active site can bind to both CO2 and O2 – hence the name.

45. What is the primary reason for cyclic photophosphorylation to occur in plants?

To produce equal amounts of ATP and NADPH.
To provide additional electrons for Photosystem II.
To primarily generate more ATP, which is required in higher amounts than NADPH for the Calvin cycle.
To release excess oxygen from the chloroplast.

For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place. Cyclic photophosphorylation only results in ATP synthesis.

46. Which set of processes correctly describes the light reactions (photochemical phase) of photosynthesis?

CO2 fixation, sugar synthesis, RuBP regeneration.
Light absorption, water splitting, oxygen release, ATP and NADPH formation.
Starch formation, ATP hydrolysis, CO2 release.
Glycolysis, Krebs cycle, oxidative phosphorylation.

Light reactions or the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.

47. The primary acceptor of the electron in Photosystem I (PS I), after excitation by 700 nm light, transfers electrons downhill to which molecule for reduction?

Cytochrome b6f complex
Plastoquinone
NADP+
Water

Electrons in the reaction centre of PS I are excited when they receive red light of wavelength 700 nm and are transferred to another accepter molecule that has a greater redox potential. These electrons then are moved downhill again, this time to a molecule of energy-rich NADP+. The addition of these electrons reduces NADP+ to NADPH + H+.

48. If a plant had high concentrations of chlorophyll b but lacked chlorophyll a, would it be able to carry out photosynthesis, and why do plants have accessory pigments like chlorophyll b?

Yes, because chlorophyll b can act as the primary pigment; it broadens the light spectrum utilised.
No, because chlorophyll a is the chief pigment; accessory pigments help utilise a wider range of wavelengths and protect chlorophyll a.
Yes, but only in green light; accessory pigments absorb excess light.
No, because chlorophyll b is an inhibitor; accessory pigments are for colouration only.

Though chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a. Indeed, they not only enable a wider range of wavelength of incoming light to be utilised for photosyntesis but also protect chlorophyll a from photo-oxidation. Without chlorophyll a, even if chlorophyll b absorbs light, the energy cannot be funnelled to a reaction center for the primary photochemical event. (The explanation in the source implies chlorophyll a is essential as the reaction centre).

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