BBYCT 137 Solved Guess Paper 2026: Plant Physiology and Metabolism for IGNOU BSCG and BSCM

Water potential and water pathways in the root

Question: (a) Explain water potential and its components. (b) Give a comparative account of the three pathways for the movement of water from the epidermis to the endodermis in a primary root. Support your answer with a labelled sketch.

(a) Water potential

Water potential is the difference between the chemical potential of water at any point in a system and that of pure water at standard temperature and pressure. (Slatyer and Taylor, 1960)

It is written Ψw. Pure water has a Ψw of zero, so any solution has a negative Ψw, and water always moves from higher to lower (more negative) Ψw.

Ψw = Ψs + Ψp + Ψm + Ψg

1. Solute potential (Ψs): dissolved solutes lower the chemical potential of water, so Ψs is always negative.
2. Pressure potential (Ψp): usually positive and equal to turgor pressure; in the xylem, tension makes it negative.
3. Matric potential (Ψm): water held on cell walls, seeds and soil particles; always negative and important in dry seeds and drying soil.
4. Gravitational potential (Ψg): changes by only about 0.01 MPa per metre of height, so it is ignored at cell level.

(b) Three pathways from epidermis to endodermis

1. Apoplastic pathway: water moves through cell walls and intercellular spaces without crossing any membrane, because the wall is hydrophilic (water-loving).
2. Symplastic pathway: water enters the epidermal cell once, then moves cell to cell through the cytoplasm via plasmodesmata (cytoplasmic bridges).
3. Transmembrane pathway: water enters one cell and leaves from the far side, again and again, crossing the plasma membrane twice in every cell.

At the endodermis, the Casparian strip (suberin, a waxy substance) blocks the apoplast, so water must pass through living cells, mostly through aquaporins (water channels).

Conclusion: Ψw decides the direction of water flow, and at the endodermis only the cellular route remains open.

The Z-scheme with cyclic and non-cyclic photophosphorylation

Question: Describe the Z-scheme of photosynthesis involving non-cyclic and cyclic photophosphorylation. Support your answer with a well-labelled diagram.

The two photosystems working in tandem are described through a modern version of Robin Hill's Z or zig-zag scheme. Carriers are arranged by redox potential, and electrons travel from water (low energy) to NADPH (high energy) along a path shaped like the letter Z.

Non-cyclic (linear) electron flow

1. Photolysis at PS II: light excites P680, and the oxygen-evolving complex (Mn and Cl ions) splits water: 2H₂O → 4H⁺ + O₂ + 4e⁻.
2. PS II to plastoquinone: excited P680 passes its electron to pheophytin, then Q_A and Q_B, forming plastoquinol (PQH₂).
3. Cytochrome b6/f complex: PQH₂ hands electrons to Cyt b6/f while protons are pumped into the lumen; this gradient later drives ATP synthesis.
4. Plastocyanin to PS I: plastocyanin, a copper protein, carries electrons to P700 of PS I.
5. PS I to NADP⁺: light excites P700; electrons pass through A₀, A₁, Fe-S proteins and ferredoxin, and FNR reduces NADP⁺ to NADPH + H⁺.

Non-cyclic photophosphorylation therefore yields both ATP and NADPH (assimilatory power), and oxygen is evolved.

Cyclic photophosphorylation

1. Only PS I works: it runs when PS II is absent, NADP⁺ is scarce, or light is above 680 nm.
2. Cyclic path: P700 → A₀, A₁ → Fe-S → ferredoxin → Cyt b6/f → plastocyanin → back to P700.
3. Product: only ATP is made, with no NADPH and no oxygen released, and the process is not inhibited by DCMU.

Conclusion: for every O₂ evolved, about 3 ATP and 2 NADPH are formed; non-cyclic flow gives ATP plus NADPH, while cyclic flow gives ATP alone.

Factors affecting enzyme activity, and enzyme inhibition

Question: (a) Discuss the various factors that affect enzyme activity. (b) Discuss briefly what you understand by competitive and non-competitive inhibition of enzymes.

(a) Factors affecting the rate of enzyme action

Rates of enzyme-catalysed reactions depend on temperature, pH, enzyme and substrate concentration, oxidation and radiation exposure.

1. Temperature: the rate rises up to an optimum, then the enzyme denatures, giving a bell-shaped curve; plant enzymes work best at 20 to 35 °C.
2. pH: each enzyme has an optimum pH, usually near 7.0 ± 1.5, though pepsin works at pH 2 and invertase at 4.5.
3. Enzyme concentration: the rate is directly proportional to the amount of enzyme when other conditions stay the same.
4. Substrate concentration: the rate rises until all active sites are full and V_max is reached; the substrate concentration at half V_max is K_m, the Michaelis constant.

(b) Competitive and non-competitive inhibition

1. Competitive: the inhibitor resembles the substrate and competes for the same active site; adding more substrate reverses it. Malonate inhibits succinate dehydrogenase because it looks like succinate.
2. Non-competitive: the inhibitor binds at a different site, changes the enzyme's shape and spoils the active site; extra substrate cannot restore the rate. L-isoleucine inhibits L-threonine dehydratase, and heavy metal ions, cyanide and fluoride act similarly.

Conclusion: temperature, pH and enzyme and substrate levels set enzyme speed; competitive inhibition can be beaten by more substrate, non-competitive cannot.

Mitochondrial ultrastructure and the respiratory quotient

Question: (a) With the help of a schematic diagram, describe the ultrastructure and organisation of the mitochondrion. (b) What is respiratory quotient? Explain its values with suitable examples.

(a) Mitochondrion

Mitochondria are double membrane-bound organelles present in eukaryotes.

1. Outer membrane: smooth, with porins (protein channels) that let ATP, NAD⁺ and CoA pass easily.
2. Inner membrane: impermeable, with special transporters, folded into cristae that increase surface area.
3. F₁ particles: racquet-shaped oxysomes on the matrix face of the inner membrane that make ATP.
4. Two compartments: the intermembrane space between the membranes, and the matrix inside.
5. Matrix: holds Krebs cycle enzymes, mitochondrial DNA and 70S ribosomes.

(b) Respiratory quotient (RQ)

RQ is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration, measured with Ganong's respirometer.

Conclusion: mitochondria run aerobic respiration, and the RQ value tells you which substrate is being respired.

How auxins and gibberellins affect plant growth

Question: (a) Describe how auxins affect plant growth. (b) Describe how gibberellin hormone affects plant growth.

(a) Auxins

Auxin was the first plant growth hormone discovered and controls basic growth processes such as cell enlargement.

1. Cell elongation: stem and coleoptile cells grow longer as auxin moves downward (basipetally).
2. Root initiation: high auxin forms lateral and adventitious roots; IAA, IBA and phenyl acetic acid are used on cuttings.
3. Vascular differentiation: new xylem and phloem form around wounds; low auxin favours phloem, high auxin favours xylem.
4. Wall loosening: auxin pumps out protons, the acidic wall activates enzymes and expansins, and the wall loosens (acid growth).
5. Lodging and dwarfing: α-naphthyl acetamide stiffens cells and prevents lodging in crops.
6. Flowers and leaves: it controls floral meristem growth and leaf arrangement (phyllotaxy).

(b) Gibberellins

Gibberellins are terpenoid plant hormones.

1. Seed germination: GA breaks dormancy and induces amylases and proteases in cereal grains to digest stored food.
2. Stem elongation: dwarf and rosette plants show dramatic stem growth, and dwarf mutants grow to normal height.
3. Leaf size: leaves of pea, beans, cucumber and cabbage grow larger.
4. Juvenile to adult: GA₃ shortens the juvenile phase in woody plants.
5. Flowering: it promotes flowering in long-day plants and the formation of staminate flowers.
6. Fruits: it aids fruit set and parthenocarpy, and enlarges seedless grapes.

Conclusion: auxin mainly drives cell elongation and rooting, while gibberellins drive stem growth, germination and fruit development.