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Photosynthesis: NEET MCQs with answers

121 questions on this topic of Plant Physiology (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following statements about Blackman's Law of Limiting Factors is correct?

  1. If a chemical process is affected by more than one factor, its rate is determined by the factor nearest to its minimal value
  2. Photosynthetic rate increases indefinitely if any single factor is continuously increased
  3. The factor present in the highest relative quantity governs the rate of the reaction
  4. All environmental factors exert an equal limiting effect simultaneously irrespective of their quantities
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Answer: (A) If a chemical process is affected by more than one factor, its rate is determined by the factor nearest to its minimal value

Blackman's Law of Limiting Factors (1905) states: '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.'

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2. During the light reaction of photosynthesis, which of the following mobile electron carriers transfers electrons from complex III (cytochrome b₆f complex) to photosystem I?

  1. Plastocyanin
  2. Plastoquinone
  3. Pheophytin
  4. Ferredoxin
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Answer: (A) Plastocyanin

Plastocyanin is a small, copper-containing peripheral membrane protein located on the lumenal side of the thylakoid membrane that acts as a mobile carrier transferring electrons from the cytochrome b₆f complex to PS I (P700).

hard

3. In a mutant C3 plant, the carboxylase activity of RuBisCO is completely unaffected, but its oxygenase activity is reduced to zero. When grown in an atmosphere containing 21% O₂ and 380 ppm CO₂ at 35 °C, what physiological consequence would be observed compared to wild-type C3 plants?

  1. The net carbon fixation will increase significantly due to total suppression of photorespiration.
  2. The plant will require 5 ATP molecules per CO₂ fixed instead of the usual 3 ATP molecules.
  3. The photosynthetic efficiency will decrease due to accumulation of hydrogen peroxide in peroxisomes.
  4. The plant will fail to regenerate RuBP because phosphoglycolate cannot be produced.
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Answer: (A) The net carbon fixation will increase significantly due to total suppression of photorespiration.

In C3 plants at higher temperatures (35 °C), RuBisCO binds O₂ and undergoes photorespiration, wasting over 25% of fixed carbon and consuming ATP without sugar production. Eliminating oxygenase activity abolishes photorespiration completely, significantly increasing net carbon fixation.

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4. The large cells around the vascular bundles of C4 plants, which form several layers and have many chloroplasts and thick walls impervious to gaseous exchange, are called:

  1. Companion cells
  2. Mesophyll cells
  3. Guard cells
  4. Bundle sheath cells (Kranz anatomy)
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Answer: (D) Bundle sheath cells (Kranz anatomy)

This 'Kranz' (wreath) anatomy is characteristic of C4 plants; bundle sheath cells have no intercellular spaces.

easy

5. Which of the following photosynthetic pigments appears yellow-orange in a paper chromatogram of leaf pigments?

  1. Chlorophyll a
  2. Chlorophyll b
  3. Xanthophyll
  4. Carotenoids
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Answer: (D) Carotenoids

Paper chromatography shows four distinct pigments: chlorophyll a (bright or blue green), chlorophyll b (yellow green), xanthophylls (yellow), and carotenoids (yellow to yellow-orange). Hence, carotenoids appear yellow-orange.

hard

6. In C4 plants, the bundle sheath cells are rich in RuBisCO but lack:

  1. Chloroplasts
  2. ATP synthase
  3. Mitochondria
  4. PEP carboxylase
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Answer: (D) PEP carboxylase

In C4 plants, mesophyll cells have PEP carboxylase (PEPcase) but lack RuBisCO. Bundle sheath cells receive C4 acids, release CO2 and run the Calvin cycle; they are rich in RuBisCO but lack PEPcase.

NCERT Class 11

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7. Julius von Sachs (1854) provided evidence for:

  1. The role of RuBisCO
  2. The source of oxygen being water
  3. The action spectrum of photosynthesis
  4. Production of glucose when plants grow, which is usually stored as starch, and that chlorophyll is located in special bodies (later called chloroplasts)
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Answer: (D) Production of glucose when plants grow, which is usually stored as starch, and that chlorophyll is located in special bodies (later called chloroplasts)

Sachs showed glucose is produced and stored as starch, and that chlorophyll is located in chloroplasts within cells, where glucose is made.

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8. How many molecules of ATP and NADPH respectively are consumed during the reduction step alone of the Calvin cycle for the synthesis of one molecule of glucose?

  1. 6 ATP and 6 NADPH
  2. 18 ATP and 12 NADPH
  3. 12 ATP and 12 NADPH
  4. 12 ATP and 6 NADPH
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Answer: (C) 12 ATP and 12 NADPH

For each CO₂ molecule fixed, reduction requires 2 ATP and 2 NADPH to convert 3-PGA to G-3-P. Therefore, for 6 turns (producing 1 glucose molecule), the reduction step alone consumes 6 × 2 = 12 ATP and 6 × 2 = 12 NADPH.

easy

9. Who used a prism to split light into its spectral components and illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria to plot the first action spectrum of photosynthesis?

  1. Julius von Sachs
  2. Jan Ingenhousz
  3. T.W. Engelmann
  4. Cornelius van Niel
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Answer: (C) T.W. Engelmann

T.W. Engelmann (1843-1909) split light into its spectral components using a prism and illuminated Cladophora suspended in an aerobic bacterial medium. The bacteria accumulated mainly in regions of blue and red light, producing the first action spectrum.

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10. In chromatography of leaf pigments, chlorophyll a appears as:

  1. Bright or blue-green band
  2. Yellow-green band
  3. Yellow band
  4. Yellow-orange band
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Answer: (A) Bright or blue-green band

Chlorophyll a appears bright or blue-green, chlorophyll b yellow-green, xanthophylls yellow and carotenoids yellow to yellow-orange. Chlorophyll a is the chief pigment of photosynthesis.

NCERT Class 11

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11. In the Calvin cycle, how many NADPH molecules used are needed to synthesise 4 molecules of glucose?

  1. 12
  2. 24
  3. 72
  4. 48
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Answer: (D) 48

One glucose needs 6 CO₂, 18 ATP and 12 NADPH (6 turns of the cycle). For 4 glucose: 12 × 4 = 48.

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12. How many turns of the Calvin cycle and how many molecules of NADPH are required for the net synthesis of one molecule of sucrose (C₁₂H₂₂O₁₁)?

  1. 6 turns and 12 NADPH
  2. 24 turns and 24 NADPH
  3. 12 turns and 18 NADPH
  4. 12 turns and 24 NADPH
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Answer: (D) 12 turns and 24 NADPH

One sucrose molecule contains 12 carbon atoms, requiring the fixation of 12 CO₂ molecules (12 turns of the Calvin cycle). Since each turn requires 2 NADPH during the reduction step, a total of 12 × 2 = 24 NADPH molecules are consumed.

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