Plant Physiology Questions and Answers

Answer: D

Nitrogen fixation is the conversion of atmospheric nitrogen to a more usable form for living organisms.

Plants cannot use the nitrogen gas that makes up about 78% of air. Nitrogen-fixing bacteria such as Rhizobium in the root nodules of legumes, some free-living soil bacteria and blue-green algae, and lightning convert it into compounds such as ammonia and nitrates that plants can absorb.

The other options are different steps of the nitrogen cycle: converting nitrogen compounds back into nitrogen gas is denitrification, converting ammonia into nitrates is nitrification, and releasing nitrogen from dead organic matter is ammonification by decomposers.

Answer: C

The correct sequence is (D) → (A) → (B) → (C).

1. (D) Bacteria such as Rhizobium and blue-green algae fix atmospheric nitrogen into compounds in the soil.
2. (A) Plants take up these nitrogen compounds through their roots.
3. (B) Plants use them to build proteins and other compounds, which pass to animals through food.
4. (C) When plants and animals die, bacteria and fungi convert their nitrogenous wastes back into nitrogen compounds.

Option A puts decomposition before uptake, option B starts with uptake before any fixation, and option D has plants making proteins before they absorb nitrogen.

Answer: D

Only A is true.

Insects exchange gases through spiracles, small openings on the body leading into tracheae, and leaves exchange gases through stomata. So A is correct.

B is false: a leaf has a very large number of stomata, thousands in every square centimetre, while an insect has at most about ten pairs of spiracles. C is false: spiracles lie along the sides of the insect's thorax and abdomen, not on a 'lower surface', even though stomata are mostly on the lower surface of a leaf. So every option including B or C is wrong.

Answer: D

The micronutrients are Zinc, Iron, Manganese.

Plants need micronutrients in very small amounts; the usual list is iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel. Zinc, iron and manganese are all on it.

The other sets each contain a macronutrient: potassium, magnesium, phosphorus, calcium and sulphur are needed in large quantities. Aluminium is not an essential plant nutrient at all, so the set with sulphur and aluminium fails twice.

Answer: A

The correct statement is Fertilisers are richer in specific nutrients as compared to manure.

Fertilisers are manufactured chemicals such as urea or NPK mixtures that supply particular nutrients (nitrogen, phosphorus, potassium) in concentrated form. Manure, from decomposed plant and animal waste, contains nutrients in smaller amounts.

The other statements describe manure, not fertilisers: manure adds humus, improves water-holding capacity and supports friendly soil microbes, while heavy fertiliser use tends to reduce soil organic matter.

Answer: A

Both S1 and S2 are true.

Photosynthesis is endothermic: plants absorb light energy to make glucose from carbon dioxide and water, storing that energy in chemical bonds.

Respiration is exothermic: glucose is oxidised to carbon dioxide and water, releasing energy that the cell captures as ATP and partly loses as heat. The two processes are energetically opposite, so the options declaring either one false are wrong.

Answer: B

The source of electrons in photosynthesis is water.

In the light reactions, photosystem II splits water (photolysis), releasing oxygen and protons and supplying electrons that pass along the electron transport chain to reduce NADP⁺ to NADPH:

\[ \mathrm{2H_2O \to O_2 + 4H^+ + 4e^-} \]

CO₂ is the electron acceptor, reduced to sugar in the Calvin cycle, not the donor. Carbohydrates are the product. NADH is the reduced carrier of respiration; photosynthesis uses NADPH, which it makes rather than consumes as a source.

Answer: B

The one that is not an auxin is abscisic acid.

Abscisic acid (ABA) is a separate class of plant hormone, a sesquiterpenoid that signals stress: it closes stomata during drought and maintains seed and bud dormancy. It often acts against growth-promoting hormones rather than as one of them.

The other three are auxins. Indole-3-acetic acid (IAA) is the principal natural auxin, controlling cell elongation, apical dominance and tropisms. Phenylacetic acid is a weaker naturally occurring auxin. Indole-3-butyric acid occurs naturally and is widely used to promote rooting in cuttings.

Answer: D

The correct match is (iv) - (q): nitrogen in NO₂ has oxidation state +4.

Taking oxygen as -2, the oxidation state of nitrogen in each compound is:

• N₂: 0, as in any element.
• NO: \(x - 2 = 0\), so +2.
• N₂O: \(2x - 2 = 0\), so +1.
• NO₂: \(x - 4 = 0\), so +4.

So the full matching is (i)-(r), (ii)-(s), (iii)-(p), (iv)-(q). The options (i)-(s), (ii)-(p) and (iii)-(r) each give a compound the value that belongs to another one.

Answer: D

The statement that is not true is Photosystem I is responsible for water splitting.

Water is split by the oxygen-evolving complex, a Mn₄CaO₅ cluster on the lumen side of Photosystem II. It supplies the electrons that replace those lost from the PSII reaction centre P680 and releases O₂ and protons. Photosystem I receives electrons from plastocyanin and uses light to pass them on to ferredoxin and NADP⁺.

The other statements are correct. PSII is concentrated in the stacked grana membranes and PSI in the unstacked stroma lamellae and grana margins. The PSI reaction centre, P700, absorbs maximally at 700 nm.