Plant Physiology and Biochemistry Questions and Answers

Answer: C

The CO₂ acceptor of the C₄ cycle is phosphoenolpyruvate.

In the mesophyll cells of C₄ plants, PEP carboxylase adds bicarbonate (from CO₂) to phosphoenolpyruvate to form the four-carbon oxaloacetate. The C₄ acid is then carried to the bundle sheath, decarboxylated, and the released CO₂ enters the Calvin cycle there.

Oxaloacetate is the product of that fixation, not the acceptor. Ribulose-1,5-bisphosphate is the acceptor of the C₃ (Calvin) cycle, carboxylated by Rubisco. 1,3-Bisphosphoglycerate is an intermediate of glycolysis and of the Calvin cycle's reduction phase and captures no CO₂.

Answer: A

The statement disproven is The source of oxygen produced in photosynthesis in green plants is CO₂.

Van Niel's comparison is the point of the question. Purple sulfur bacteria use H₂S and release sulfur, not oxygen, while green plants use H₂O and release oxygen. Writing both as one general reaction, with H₂A as the electron donor and 2A as its oxidised product, places the released element (S or O) in the donor, not in CO₂. For green plants A is oxygen, so the O₂ must come from water, a conclusion later confirmed with ¹⁸O-labelled water.

That supports, rather than disproves, the statement that the oxygen comes from H₂O. Every equation includes light, so the light statement is consistent with them. The equations write the product as the generic (CH₂O) and say nothing either way about glucose.

Answer: A

The correct match is P-1-a; Q-3-b; R-3-c.

• C₃ cycle: Rubisco fixes CO₂ onto ribulose-1,5-bisphosphate, and the first stable product is the three-carbon 3-phosphoglycerate. Wheat is a C₃ plant.
• C₄ cycle: PEP carboxylase fixes CO₂ onto phosphoenolpyruvate, giving the four-carbon oxaloacetate. Sugarcane is a C₄ plant.
• CAM: the same PEP carboxylase reaction, run at night, also gives oxaloacetate, which is stored as malic acid. Pineapple is a CAM plant.

Glyceraldehyde-3-phosphate is formed later in the Calvin cycle by reduction of 3-phosphoglycerate, so it is no pathway's first stable product. Options assigning wheat to C₄, sugarcane to C₃ or pineapple to anything but CAM are wrong as well.

Answer: A

The path is H₂O → Photosystem II → Cyt b₆f → Photosystem I → NADP⁺.

In the Z-scheme, the oxygen-evolving complex of photosystem II takes electrons from water, releasing O₂. Photosystem II passes them through plastoquinone to the cytochrome b₆f complex, which hands them to plastocyanin. Photosystem I re-energises them and sends them via ferredoxin to ferredoxin-NADP⁺ reductase, which reduces NADP⁺ to NADPH.

Paths that start at photosystem I have the photosystems in the wrong order. Paths that end at O₂ confuse the by-product of water oxidation with the final electron acceptor.

Answer: B

The correct matching is P-3; Q-4; R-2; S-1.

• Photosystem I: its P700 reaction centre, excited by light, takes electrons from plastocyanin and passes them to ferredoxin (3).
• Photosystem II: its oxygen-evolving complex oxidises water and the reaction centre reduces plastoquinone (4).
• Cytochrome b₆f: carries electrons from reduced plastoquinone (plastoquinol) to oxidised plastocyanin while pumping protons (2).
• CF₀-CF₁: the chloroplast ATP synthase, which uses the proton gradient across the thylakoid to make ATP (1).

Each wrong option gives one of these a neighbour's step; for example, assigning the plastoquinone-to-plastocyanin transfer to photosystem I confuses it with cytochrome b₆f.

Answer: D

The correct matching is P-2; Q-3; R-6; S-5; T-4.

• Codeine, from the opium poppy, is a nitrogen-containing benzylisoquinoline alkaloid (2).
• Podophyllotoxin, from Podophyllum, is an aryltetralin lignan, the parent of the anticancer drugs etoposide and teniposide (3).
• Resveratrol, found in grape skins, is a stilbene (6).
• Linamarin, in cassava and flax, is a cyanogenic glycoside that releases hydrogen cyanide when hydrolysed (5).
• Limonene, the citrus peel compound, is a C₁₀ monoterpene (4).

The options that call codeine a flavone, podophyllotoxin a cyanogenic glycoside or resveratrol a cyanogenic glycoside each misclassify a well-known compound; flavones are not used at all in the correct match.

Answer: C

It is substrate level phosphorylation.

In glycolysis ATP is made directly by transferring a phosphate group from a high-energy substrate to ADP: from 1,3-bisphosphoglycerate (by phosphoglycerate kinase) and from phosphoenolpyruvate (by pyruvate kinase). No electron transport chain is involved, which is what substrate level phosphorylation means.

Oxidative phosphorylation takes place on the inner mitochondrial membrane using the electron transport chain, and photophosphorylation occurs in chloroplasts using light. 'Reductive phosphorylation' is not the term for this step.

Answer: B

The hormone is auxin.

Auxins, chiefly indole-3-acetic acid, promote cell enlargement, especially cell elongation in shoots, by loosening the cell wall so that the cell takes up water and grows. They also control phototropism and apical dominance.

Cytosine is a nitrogen base of DNA, not a hormone (the hormone promoting cell division is cytokinin). Gibberellins mainly cause stem elongation and seed germination, and dormin (abscisic acid) is a growth inhibitor.

Answer: D

The key gives thirty eight.

The older textbook account of aerobic respiration counts 2 ATP from glycolysis, 2 from the Krebs cycle and 34 from the respiratory chain, giving 38 net ATP per glucose, and the paper follows that account.

Newer estimates give about 30 to 32 ATP, because less ATP is made per NADH and FADH₂ and some energy is spent moving materials in the cell, which is why thirty (C) is also seen in recent texts. Ten and twenty five are well below either account. Because the question sets 'thirty eight' as the textbook figure, the key is kept.

Answer: A

The light that is not effective in photosynthesis is green light.

Chlorophylls a and b absorb strongly in the blue and red parts of the spectrum and very little in the green, which is mostly reflected or transmitted. That is why leaves look green, and why green light drives the least photosynthesis.

Red and blue light are the most effective wavelengths, and sunlight contains them, so 'Sun light' is effective too.