Plant Biology

Question 1 (12)

(a) (4 marks) Gravity drop over 30 m: $0.01 \times 30 = 0.30\text{ MPa}$. (1) Total pressure change from root ($+0.1$ MPa) to top ($-1.5$ MPa): $\Delta P = 0.1 - (-1.5) = 1.6\text{ MPa}$ (1) Additional (transpirational) drop beyond gravity: $1.6 - 0.30 = 1.3\text{ MPa}$ (1) So transpirational pull must supply $\approx 1.3\text{ MPa}$ of the tension. (1)

(b) (4 marks)

• Xylem consists of many parallel vessels/tracheids, not a single tube (1).
• Water can move laterally through pits/bordered pits into adjacent functional vessels, bypassing the embolised one (1).
• Cohesion (H-bonding) keeps the water columns in other vessels intact so tension is transmitted (1).
• Redundancy of the network means overall flow continues, though slightly reduced (1).

(c) (4 marks)

• Wind normally removes the humid boundary layer at the leaf surface (1).
• When wind stops, water vapour accumulates near stomata, raising local humidity (1).
• This reduces the water-vapour concentration gradient between leaf air spaces and outside air (1).
• Therefore transpiration rate decreases (even though temperature is still high) (1).

Question 2 (10)

(a) (5 marks)

• Batch A (no ABA) shows greater transpiration (1).
• ABA triggers efflux of $K^+$ (and anions/malate) from guard cells (1).
• Solute loss raises guard-cell water potential (makes it less negative) (1).
• Water leaves the guard cells by osmosis (1).
• Guard cells lose turgor → stomata close → transpiration falls in batch B (1).

(b) (5 marks)

• Guard-cell walls are unevenly thickened: inner wall (facing pore) thick/rigid, outer wall thin/elastic (1).
• When turgid, the thin outer wall stretches more, bowing the cells outward and opening the pore (1).
• When turgor is lost, cells become flaccid and straighten (1).
• The paired cells collapse together, closing the pore (1).
• Hence loss of turgor → closed stoma → reduced water loss (1).

Question 3 (10)

(a) (6 marks)

• Source (mature leaf) loads sucrose into phloem sieve tubes actively (1).
• High solute lowers water potential at source → water enters from xylem → high hydrostatic pressure (1).
• Sinks (root tips, fruit) unload sucrose; solute drops, water leaves, pressure falls (1).
• A pressure gradient from source to sink drives bulk (mass) flow of phloem sap (1).
• Mature leaves are sources, not sinks — they export, not import, so no net inflow to them (1).
• Flow is thus directional toward regions of active growth/storage (roots, fruit) that are net sinks (1).

(b) (4 marks)

• Sugar accumulates in the bark/phloem above the ring (near the source) (1) because downward transport is blocked (1).
• Roots below the ring are starved of sugar (1) and eventually die/stop growing; tree may die over time (ringing/girdling effect) (1).

Question 4 (10)

(a) (3 marks)

• Gravitropism (geotropism) operates (1) (phototropism excluded—it is dark).
• Root bends downward → positively gravitropic (1).
• Shoot bends upward → negatively gravitropic (1).

(b) (4 marks)

• Auxin accumulates on the lower side of the horizontal organ (1).
• In the shoot, high auxin promotes cell elongation → lower side grows faster → shoot bends up (1).
• In the root, the same high auxin concentration on the lower side inhibits elongation (roots are sensitive to lower auxin levels) (1).
• So the upper side of the root elongates more → root bends down (1).

(c) (3 marks)

• Control: place a mica/impermeable barrier vertically down the middle of the shoot tip (or remove tip), preventing lateral auxin movement (1).
• Keep everything else (gravity, orientation) identical (1).
• If bending is abolished/altered, the response depends on auxin redistribution, not mere mechanical pull of gravity (1).

Question 5 (8)

(a) (4 marks)

• Photoperiod is measured by phytochrome, which detects continuous night length (1).
• Short-day plants need an uninterrupted long night to convert enough $P_{fr}$ back to $P_r$ (flowering form) (1).
• A red-light flash converts $P_r \to P_{fr}$, effectively breaking the night into two short nights (1).
• Sufficient $P_{fr}$ remains to inhibit flowering, so flowering is prevented (1).

(b) (4 marks)

• (i) Zygote: diploid ($2n$) → develops into the embryo (1).
• (ii) Primary endosperm nucleus: triploid ($3n$) → develops into endosperm (nutritive tissue) (1).
• Ovule → becomes the seed (1).
• Ovary → becomes the fruit (1).

[
  {"claim":"Gravity pressure drop over 30 m is 0.30 MPa","code":"result = (0.01*30 == 0.30)"},
  {"claim":"Total pressure change from root +0.1 to top -1.5 is 1.6 MPa","code":"result = (0.1-(-1.5) == 1.6)"},
  {"claim":"Additional transpirational drop beyond gravity is 1.3 MPa","code":"total=0.1-(-1.5); grav=0.01*30; result = (total-grav == 1.3)"},
  {"claim":"Zygote ploidy 2n and endosperm nucleus 3n differ by one genome set n","code":"n=symbols('n'); zygote=2*n; endosperm=3*n; result = (simplify(endosperm-zygote)==n)"}
]