Test your knowledge of photosynthesis with these multiple-choice questions designed for competitive exams, covering every aspect and intricate detail of the topic.
Test Your Knowledge
Showing questions 21 - 40 of 48
21. Cyclic photophosphorylation primarily occurs when only Photosystem I (PS I) is functional. This process mainly leads to the synthesis of:
When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+.
22. According to the chemiosmotic hypothesis for ATP synthesis in chloroplasts, where do protons (hydrogen ions) accumulate, creating a gradient for ATP production?
In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane. Here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. Protons produced by water splitting also accumulate within the lumen.
23. The ATP synthase enzyme, involved in ATP synthesis during chemiosmosis, consists of CF0 and CF1 components. Which component is embedded in the thylakoid membrane and forms the transmembrane channel for proton diffusion?
The ATP synthase enzyme consists of two parts: one called the CF0 is embedded in the thylakoid membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane.
24. The products of the light reaction, ATP and NADPH, are consumed in the biosynthetic phase (dark reactions). What is their primary role in this phase?
ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis, dependent on ATP and NADPH.
25. Melvin Calvin's pioneering work, using radioactive 14C in algal photosynthesis, led to the discovery of the first stable CO2 fixation product in the Calvin cycle. This product was identified as:
The use of radioactive 14C by Melvin Calvin in algal photosynthesis studies led to the discovery that the first CO2 fixation product was a 3-carbon organic acid, 3-phosphoglyceric acid or in short PGA.
26. In the Calvin cycle, the primary acceptor molecule of carbon dioxide is a 5-carbon ketose sugar known as:
The studies very unexpectedly showed that the acceptor molecule was a 5-carbon ketose sugar – ribulose bisphosphate (RuBP).
27. The carboxylation step, the most crucial step of the Calvin cycle, involves the utilization of CO2 for the carboxylation of RuBP. This reaction is catalysed by which enzyme?
Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA, more correctly called RuBisCO.
28. For every CO2 molecule entering the Calvin cycle, how many molecules of ATP and NADPH are required respectively?
For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required.
29. To produce one molecule of glucose (C6H12O6) through the Calvin pathway, how many turns of the cycle are required, and what is the net consumption of ATP and NADPH?
To make one molecule of glucose, 6 turns of the cycle are required, which consumes 18 ATP and 12 NADPH.
30. Which of the following is a characteristic feature that differentiates C4 plants from C3 plants?
C4 plants are special: They have a special type of leaf anatomy (Kranz), they tolerate higher temperatures, they show a response to high light intensities, they lack a process called photorespiration and have greater productivity of biomass.
31. Kranz anatomy, a distinctive feature of C4 plants, involves:
The particularly large cells around the vascular bundles of the C4 plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy. These cells may form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
32. In the Hatch and Slack Pathway (C4 pathway), where does the initial carbon dioxide fixation occur, and what enzyme catalyses this reaction?
The primary CO2 acceptor is phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase. It is important to register that the mesophyll cells lack RuBisCO enzyme.
33. In C4 plants, the Calvin pathway (C3 cycle) takes place exclusively in which type of cells?
In the C4 plants, the Calvin pathway does not take place in the mesophyll cells but does so only in the bundle sheath cells.
34. RuBisCO, the enzyme central to the Calvin cycle, has the unique characteristic of being able to bind to both CO2 and O2. When the CO2:O2 ratio is nearly equal, RuBisCO exhibits a much greater affinity for:
RuBisCO has a much greater affinity for CO2 when the CO2:O2 is nearly equal. This binding is competitive.
35. Photorespiration occurs in C3 plants when O2 binds to RuBisCO instead of CO2. This pathway is considered wasteful because it:
In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO2 with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH.
36. C4 plants effectively avoid photorespiration because they have a mechanism that:
In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site. This takes place when the C4 acid from the mesophyll is broken down in the bundle sheath cells to release CO2 – this results in increasing the intracellular concentration of CO2. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.
37. According to Blackman's (1905) Law of Limiting Factors, if a chemical process like photosynthesis is affected by more than one factor, its rate will be determined by the factor which is:
If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.
38. Light saturation in photosynthesis, where the rate does not increase further despite increased light intensity, typically occurs at what percentage of full sunlight?
Light saturation occurs at 10 percent of the full sunlight.
39. Which environmental factor is considered the major limiting factor for photosynthesis in the atmosphere due to its very low concentration (between 0.03 and 0.04 percent)?
Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO2 is very low in the atmosphere (between 0.03 and 0.04 per cent).
40. At high light intensities, how do C3 and C4 plants differ in their response to increasing CO2 concentration?
At high light intensities, C4 plants show saturation at about 360 µlL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450 µlL-1. Thus, current availability of CO2 levels is limiting to the C3 plants.